Heater apparatus, control circuitry, and control methods

Abstract

A hair styling apparatus (1, fig.1a) comprising a plurality of heaters (6a, 6b, fig.1a) preferably with heating zones (642, fig.3a) having heater electrodes (64, fig.3k), for heating hair in contact t

Description

The present invention relates to heating apparatus and methods. The heaters can be used for drying and / or styling hair. Such drying and / or styling of the hair may be performed by a user in respect of their own hair, for example, or by a hair stylist. The invention has particular, but not exclusive, relevance to drying or styling devices comprising one or more low thermal mass heaters. Heated hair styling tools or hair drying tools use heat to increase the temperature of hair to a desired styling or drying temperature. For example, a hair straighter having a heated plate applies heat directly via conduction to heat the hair, which may be either wet or dry, to achieve the desired temperature for styling. The hair may be heated to a temperature that is particularly suitable for styling hair (for example, to or beyond a glass transition phase temperature). At lower temperatures, the user may have to make many passes with the hair straightener over the hair to achieve a desired styling effect, whereas at higher temperatures, there is a risk of causing permanent damage to the hair. Similarly, a heated brush or hair dryer can also be used to style hair by heating the hair to a temperature suitable for styling or drying. Hair may be styled from wet, for example after the user has washed their hair, although the hair could also be styled from dry. Existing hair styling appliances typically use relatively thick heating plates or heating tubes that provide a certain amount of thermal mass to the hair styling appliance. These heating plates or tubes are heated by a heater that is mounted on an inner surface of the heating plate / tube. As a result of the thermal mass, the heating plates / tubes take time to heat up and, once heated, they can take quite a long time to cool down. This thermal mass makes it quite difficult to control the heating of the hair and over heating or under heating of the hair can result. There has been a general desire to move towards hair styling appliances that use heaters that have a lower thermal mass and can therefore heat up and cool down much more quickly. Low thermal mass heaters are more responsive and are easier to control. However, there is a need for improved control of the power delivered to such low thermal mass heaters. For example, there is a need for improved apparatus and control methods for preventing possible overheating of such low thermal mass heaters, as when such heaters overheat there is a risk of damage not only to the heater itself, but also to other parts and components of the hair styling device. The present invention aims to address or at least partially ameliorate one or more of the above problems. Summary of Invention The present invention is set out in the appended independent claims. Optional features are set out in the appended dependent claims. In the following, any examples and embodiments not falling within the scope of the claims do not form part of the invention and are provided for illustrative purposes only. According to a first aspect there is provided a hair styling apparatus comprising a plurality of heaters for heating hair in contact with hair contacting surfaces of the plurality of heaters by conductive heating, a plurality of microcontroller units, MCUs, in communication with one another, each one of the MCUs being configured to control a subset of the plurality of heaters, wherein at least one MCU of the plurality comprises a processor configured to detect malfunctions of the hair styling apparatus; and control functions of the hair styling apparatus in response to the processor detecting malfunctions of the hair styling apparatus. In some aspects, the processor may be configured to determine whether a communication link between the at least one MCU and another MCU of the plurality of MCUs has failed; and trigger a reset of the hair styling apparatus based on determining that the communication link has failed. In some aspects, the processor may determine that the communication link between the at least one MCU and another MCU has failed if the at least one MCU does not receive within a predetermined time period, from the another MCU, a signal indicating normal operation. In some aspects, the signal indicating normal operation is a heartbeat message sent periodically to the at least one MCU. In some aspects, the processor may be configured to determine whether a rate at which power is provided to the plurality of heaters exceeds a threshold; and trigger a reset of the hair styling apparatus based on determining that the rate at which power is provided to the plurality of heaters exceeds the threshold. In some aspects, the hair styling apparatus may further comprise temperature measurement means for measuring a temperature of each one of the plurality of heaters, and wherein to detect malfunctions and control function of the hair styling apparatus in response to the detection, the processor is further configured to: analyse data received from the temperature measurement means, the data indicating a voltage and / or a current value measured across each corresponding heater; determine, for each corresponding heater, whether the measured voltage and / or current value exceeds a threshold; and trigger a reset of the hair styling apparatus based on determining that the measured voltage and / or current value for at 5 least one of the plurality of heaters exceeds the threshold. In some aspects, the threshold is a voltage and / or a current value corresponding to a maximum safe operational temperature of the plurality of heaters. In other aspects, the threshold is a voltage and / or a current value corresponding to a maximum operational 10 voltage and / or a current value of the temperature measurement means. In some aspects, the reset triggered by the processor of the at least one MCU comprises switching off power to the plurality of heaters of the hair styling apparatus by switching a respective transistor switch associated with each one of the plurality of heaters. 15 In some aspects, the processor of the at least one MCU may be configured to: switch a corresponding MOSFET gate drive associated with each heater of the subset of heaters controlled by the at least one MCU to high impedance; and transmit a command message, to the other MCUs of the plurality of MCUs, to instruct them to switch off power to heaters controlled by the other MCUs of the plurality of MCUs. 20 In some aspects, for each one of the other MCUs, in response to receiving the command message, a processor of the MCU may be configured to: switch a corresponding MOSFET gate drive associated with each heater controlled by the MCU to high impedance. 25 In some aspects, the reset triggered by the processor of the at least one MCU comprises switching the hair styling apparatus from an active mode to an idle mode, wherein in the idle mode a power provided to the heaters is provided at a low rate, for example at a rate of 1 Hz, and wherein in the active mode the power provided to the heaters is provided at a greater rate than in the idle mode. 30 In some aspects, the processor may be configured to trigger the reset periodically until the processor determines that the hair styling apparatus is operating correctly. In some aspects, determining that the hair styling apparatus is operating correctly comprises 35 determining that the communication link between the at least one MCU and the other MCU of the plurality of MCUs is established, determining that the rate at which power is provided to the plurality of heaters is below a threshold, and determining that the measured voltage and / or current value across each of the plurality of heaters is below a threshold. According to another aspect there is provided a hair styling apparatus comprising 5 communication means for communicating with at least one external computing device for controlling the hair styling apparatus, a plurality of heaters for heating hair in contact with hair contacting surfaces of the plurality of heaters by conductive heating, a plurality of microcontroller units, MCUs, in communication with one another, each one of the MCUs being configured to control a subset of the plurality of heaters, wherein at least one MCU of 10 the plurality comprises a processor configured to: detect malfunctions of the hair styling apparatus; and control functions of the hair styling apparatus in response to the processor detecting malfunctions of the hair styling apparatus. In some aspects, the communication means may be configured to receive from the at least 15 one external computing device, a command message to apply settings of the hair styling apparatus, and wherein the command message comprises an error detection feature, and the processor of the at least MCU may be configured to process the error detection feature of the command message to check the validity of the command message. 20 In some aspects, in response to the processor determining that the command message is not valid, the communication means may be configured to transmit, to the external computing device, an indication that the settings have not been applied. In some aspects, the processor of the at least one MCU may be configured to determine 25 whether a communication link between the hair styling apparatus and the external computing device has failed and trigger a reset of the hair styling apparatus based on determining that the communication link has failed. In some aspects, the processor may determine that the communication link between the hair 30 styling apparatus and the external computing device has failed if hair styling apparatus does not receive within a predetermined time period, from the external computing device, a signal indicating normal operation. In some aspects, the signal indicating normal operation may be a heartbeat message sent periodically to the hair styling apparatus. In some aspects, the reset triggered by the processor of the at least one MCU may comprise switching off power to the plurality of heaters of the hair styling apparatus by switching a respective transistor switch associated with each one of the plurality of heaters. 5 In some aspects, to switch off power to the plurality of heaters of the hair styling apparatus the processor of the at least one MCU may be configured to switch a corresponding MOSET gate drive associated with each heater of the subset of heaters controlled by the at least one MCU to high impedance, and transmit a command message, to the other MCUs of the plurality of MCUs, to instruct them to switch off power to heaters controlled by the other 10 MCUs of the plurality of MCUs. In some aspects, for each one of the other MCUs, in response to receiving the command message, a processor of the MCU may be configured to switch a corresponding MOSFET gate drive associated with each heater controlled by the MCU to high impedance. 15 In some aspects, the reset triggered by the processor of the at least one MCU may comprise switching the hair styling apparatus from an active mode to an idle mode, wherein in the idle mode a power provided to the heaters is provided at a low rate, for example at a rate of 1 Hz, and wherein in the active mode the power provided to the heaters is provided at a 20 greater rate than in the idle mode. In some aspects, the processor may be configured to trigger the reset periodically until the communication link between the hair styling apparatus and the external computing device is re-established. 25 In some aspects, the processor may be configured to determine whether a communication link between the at least one MCU and another MCU of the plurality of MCUs has failed and trigger a reset of the hair styling apparatus based on determining that the communication link has failed. 30 In some aspects, the processor ,may determine that the communication link between the at least one MCU and another MCU has failed if the at least one MCU does not receive within a predetermined time period, from the another MCU, a signal indicating normal operation. In some aspects, the signal indicating normal operation may be a heartbeat message sent periodically to the at least one MCU. 35 In some aspects, the processor may be configured to determine whether a rate at which power is provided to the plurality of heaters exceeds a threshold and trigger a reset of the hair styling apparatus based on determining that the rate at which power is provided to the plurality of heaters exceeds the threshold. 5 In some aspects, hair styling apparatus may further comprise temperature measurement means for measuring a temperature of each one of the plurality of heaters, and wherein to detect malfunctions and control function of the hair styling apparatus in response to the detection, the processor may be further configured to: analyse data received from the 10 temperature measurement means, the data indicating a voltage and / or a current value measured across each corresponding heater, determine, for each corresponding heater, whether the measured voltage and / or current value exceeds a threshold, and trigger a reset of the hair styling apparatus based on determining that the measured voltage and / or current value for at least one of the plurality of heaters exceeds the threshold. 15 In some aspects, the threshold may be a voltage and / or a current value corresponding to a maximum safe operational temperature of the plurality of heaters. In some aspects, the threshold may be a voltage and / or a current value corresponding to a maximum operational voltage and / or a current value of the temperature measurement means. 20 In some aspects, the reset triggered by the processor of the at least one MCU may comprise switching off power to the plurality of heaters of the hair styling apparatus by switching a respective transistor switch associated with each one of the plurality of heaters. In some aspects, the processor of the at least one MCU may be configured to switch a 25 corresponding MOSFET gate drive associated with each heater of the subset of heaters controlled by the at least one MCU to high impedance, and transmit a command message, to the other MCUs of the plurality of MCUs, to instruct them to switch off power to heaters controlled by the other MCUs of the plurality of MCUs. 30 In some aspects, for each one of the other MCUs, in response to receiving the command message, a processor of the MCU may be configured to switch a corresponding MOSFET gate drive associated with each heater controlled by the MCU to high impedance. In some aspects, the reset triggered by the processor of the at least one MCU may comprise 35 switching the hair styling apparatus from an active mode to an idle mode, wherein in the idle mode a power provided to the heaters is provided at a low rate, for example at a rate of 1 Hz, and wherein in the active mode the power provided to the heaters is provided at a greater rate than in the idle mode. In some aspects, the processor may be configured to trigger the reset periodically until the 5 processor determines that the hair styling apparatus is operating correctly. In some aspects, determining that the hair styling apparatus is operating correctly may comprise determining that the communication link between the at least one MCU and the other MCU of the plurality of MCUs is established, determining that the rate at which power 10 is provided to the plurality of heaters is below a threshold, and determining that the measured voltage and / or current value across each of the plurality of heaters is below a threshold. Brief Description of the Drawings Figure 1a illustrates a handheld (portable) hair styler; 15 Figure 1b illustrates the handheld (portable) hair styler of Figure 1a in use; Figure 2a illustrates an exploded view of a low thermal mass heater; Figure 2b illustrates a perspective see-through assembled view of the low thermal mass heater shown in Figure 2a; Figure 2c illustrates a perspective opaque assembled view of the low thermal mass heater 20 shown in Figure 2a; Figure 2d illustrates a weak link which may be used in a thermal safety circuit; Figure 3a-3k illustrates varies possible arrangement of heating zones that may be implemented in the hair styling device of Figure 1; Figure 4 is a temperature-time plot of a heating response of the low thermal mass heater; 25 Figure 5 is a schematic circuit diagram that illustrates the operation of a thermal safety circuit that is used to isolate a heating circuit of the hair styler; Figure 6 illustrates one form of a thermal safety circuit; Figure 7 illustrates an alternative form of a thermal safety circuit; Figure 8 illustrates a further example of a thermal safety circuit; and Figure 9 illustrates a further example of a thermal safety circuit. Figure 10 is a simplified block diagram of control circuitry of the hair styling device of Figure 1; Figure 11a is a simplified block diagram of the heater control sub-circuitry of the hair styling device of Figure 1; Figure 11b is another simplified block diagram of the heater control sub-circuitry of the hair styling device of Figure 1; Figure 12 shows a schematic view of the microcontroller units, software, and other hardware implemented in the hair styling device of Figure 1; Figure 13 illustrates a tabular example summary of the malfunctions that may occur with the hair styling device of Figure 1; and Figure 14 illustrates the hair styling device of Figure 1 in wireless communication with an external computing device. Overview Figure 1a illustrates a handheld (portable) hair styler 1. The hair styler 1 includes a first movable arm 4a and a second movable arm 4b, which are coupled at proximal ends thereof to a shoulder 2. The first arm 4a bears a first heater 6a at its distal end, and the second arm 4b bears a second heater 6b at its distal end. The first and second heaters 6a, 6b oppose one another and are brought together as the first and second arms 4a, 4b are moved from an open configuration to a closed configuration. As shown in Figure 1b, during use, a tress of hair 40 is sandwiched between the two arms 4a,4b so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 6a, 6b. Therefore, as the user pulls the hair styler 1 along the tress of hair 40, the tress of hair 40 is heated by conductive heating to a suitable temperature to facilitate styling. The heaters 6a, 6b are low thermal mass heaters and are therefore able to heat up and cool down rapidly. Whereas the heaters of a typical conventional hair styler may take, for example, around 30 seconds to reach an operating temperature of about 200°C from ambient temperature, a low thermal mass heater operating at maximum power may take less than one second to reach the same operating temperature. It will be appreciated that the exact operating temperature and heat-up time depend on the particular requirements of the device, and on the maximum power output of the device. By way of example, a low thermal mass heater may heat up at a rate of approximately 410 °Cs'1 and may cool from an operating temperature for styling to a touchable temperature in a timescale of the order of a 5 few seconds. A user interface 11 is provided to allow the user to turn the device on or off and could also be used to enable the user to set user-definable parameters of the device 1 or to output information to the user. For example, a desired operating temperature for the heaters 6a, 6b could be input via the user interface 11. The user interface 11 may have a dial, button 14, or 10 touch display for allowing the user to input information into the device 1 and the user interface 11 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user. The user interface 11 may also comprise an indicator light 15 to indicate whether the device is on. A printed circuit board assembly (not shown) may be provided at any suitable location within 15 the housing of the device 1 and carries the control circuitry for controlling the operation of the device 1 and for controlling the interaction with the user via the user interface 11. In this example, electrical power is provided to the device 1 by means of a power supply located at an end of the device, via a power supply cord 3. The power supply may be an AC mains power supply. However, in an alternative embodiment the power supply may comprise one 20 or more DC batteries or cells (which may be rechargeable, e.g. from the mains or a DC supply via a charging lead), thereby enabling the device 1 to be a cordless product. In use, the device 1 is turned on, energising the heaters 6 to cause them to heat up. The user then opens the first and second arms 4a, 4b and, normally starting from the roots of the hair (i.e. near the scalp), a length or tress of hair 40 (which may be clumped) is introduced 25 between the arms 4a, 4b, transversely across the heaters 6a, 6b. The user then closes the arms 4a, 4b so that the length of hair 40 is held between the first and second arms 4a, 4b and then the user pulls the hair through the closed arms (as illustrated in Figure 1b). The outer (hair contacting) surface of the heaters 6 is flat in this embodiment and so the hair styler 1 can be used to straighten the user’s hair (but alternatively the device 1 could 30 comprise a cylindrical heater 6, or another shape of heater 6 having a curved surface). The hair styling device 1 shown in Figure 1 can also be used to curl the hair by turning the device 1 through approximately 180 degrees or more after clamping the hair between the arms 4a, 4b and before moving the device 1 along the tress of hair 40. Hair has a relatively high thermal mass, and when in contact with the heating surface of the heaters 6 the hair absorbs a significant amount of heat energy. If the temperature of the heaters 6 falls below the glass transition temperature of the hair, or below the temperature required to raise the hair temperature above the glass transition temperature, the hair may 5 not retain the styled shape. However, if the hair is heated to a temperature that is too high, the hair can undergo damage. Therefore, the device 1 is configured to control the temperature of the heaters 6 so that the hair-contacting surfaces remain within a particular temperature range when in use for styling the hair. A hinge may be provided at the shoulder 2. The hinge may comprise a spring for biasing the 10 first and second arms 4a, 4b into the open configuration, such that the user is required to apply pressure to the arms 4a, 4b to close them together (overcoming the effect of the spring). For example, the hinge may include a leaf spring or a coiled spring. The spring itself can be used to couple the first and second arms 4a, 4b together, thereby avoiding the need to provide a separate mechanical hinge and simplifying the overall construction of the 15 device. Alternatively, the first and second arms 4a, 4b may be formed in a unitary manner (e.g. from a plastics material) with a “U” shaped middle part at the elbow 2, the “U” shaped middle part being able to resiliently flex to allow opening and closing of the first and second arms 4a, 4b. Opening and closing of the arms 4a, 4b may be detected using a switch. For example, a 20 microswitch or Hall effect sensor and passive magnet could be used detect closure and opening of the arms 4a, 4b. The present invention is not limited to the type of device illustrated in Figure 1a. For example, a device that additionally transfers heat to hair using heated air could be used. When heated air is used to heat the user’s hair, the device may comprise a heater having an 25 electrically powered heating coil (or any other suitable type of heating element), operable to heat air drawn in by a fan assembly. It will be appreciated that the apparatus and methods described below are applicable to any suitable device (e.g., hair styling or hair drying device) comprising one or more low thermal mass heaters. Low thermal mass heaters 30 The heaters 6a, 6b are low thermal mass heaters and can therefore heat up and cool down quickly. Figures 2a to 2c show an exemplary embodiment of such heaters 6a, 6b, which comprise a stack of thin layers. Referring in particular to Figure 2a, the heaters 6a, 6b include an upper dielectric (electrically insulating) layer 62, an electrode layer 63 that has a plurality of heater electrodes 64, and a lower dielectric layer 66 which electrically insulates the heater electrodes 64 from other components mounted behind the heater 6a, 6b. The three layers 62, 63 and 66 are bonded together and define a heater with very low thermal mass. The upper surface of the layer 62 provides the hair contacting surface, although a 5 non-stick coating may be applied to the upper surface of the layer 62 to facilitate the passage of the user’s hair over the heating surface. The bonded layers 62, 63 and 66 define a flexible heater and rigidity of the heater is provided in the illustrated embodiment by mounting the heater layers 62, 63 and 66 into a rigid support 68 which forms a base. If a flexible heater is desired, then there is no need for the rigid support 68. Typically, the total 10 thickness of across the multiple bonded layers is between 30 microns and 2 mm. In the illustrated embodiment, there are ten heater electrodes 64 that each snake across and back across the width of the heater 6, folding twice such that they each cross the width three times. It may be generally understood that this is a non-limiting arrangement and other serpentine layouts that snake across and back across the width of the heater 6, folding once 15 or more, may be envisioned . The ends of each of the heater electrodes 64 are electrically connected through the base substrate 66 to electrical connections within the rigid support 68, which connect to an electrical connector 70. Drive circuitry (not shown) that is mounted within one of the arms 4 connects to the heater electrodes 64 via the electrical connector 70 and applies current to the individual heater electrodes 64 to control the heat generated by 20 each heater electrode 64. The electrical connector 70 extends from a surface of the rigid support 68 facing away from the surface layer 62 (shown in Figures 2a to 2c as extending directly away from the upper layer 62, but it could also be provided as extending in a perpendicular direction). Each of the series of heater electrodes 64 thus creates an individual heater zone 642, which 25 spans the width (which we shall refer to as the x-direction) of the heater 6 and the heater electrodes 64 are arranged sequentially one after the other along the length (the y-direction) of the heater 6. Figure 2d will be described later. Heating Zones The independently controllable heating zones 642 of the heaters 6a, 6b will now be 30 described with reference to Figure 3a. Figure 3a shows a set of heating zones 642, Z1 to Z7, of the hair styling device 1. Whilst the zones 642 are illustrated for the first heater 6a, a corresponding set of zones is also provided for the second heater 6b. The power output to each heating zone 642, and therefore the temperature of each heating zone, is independently controllable. Advantageously, this enables the temperature distribution along the longitudinal length of the heater 6 to be controllable. Moreover, the use of independently controllable heating zones 642 also enables better mitigation against reductions in temperature of the styling surface due to 5 transfer of heat to the user’s hair 40. For example, when the user places a tress of wet or damp hair on heating zones Z3 and Z4, the power output to heating zones Z3 and Z4 can be independently increased in order to maintain the hair-contacting surface at the styling temperature. As will be described in more detail later, the use of independently controllable heating zones 642 also enables some of the zones 642 to be operated at a temperature 10 below the styling temperature even when the device is in use (e.g. when only some of the heating zones 642 are in contact with the user’s hair), increasing the energy efficiency of the device 1. Each heating zone can be independently controlled towards a respective target temperature. Control is performed to reduce the difference between the actual temperature of the heating zone and the target temperature. The heating zones can be switched off 15 when the device is not in use to style hair, which can be detected, for example, by sensing that the heater arms are in the open position. A more detailed description of the possible arrangement of the independently controllable heating zones 642 will now be described with reference to Figures 3b-3k. Figures 3b and 3c show schematic views of different arrangements of such heating zones 20 642. Figure 3b shows an arrangement corresponding to that of Figures 2a and 2b, in which the heating zones 642-1 to 642-10 are arranged along the y-direction only. Figure 3c shows an alternative arrangement, in which heating zones 642-1 to 642-16 are arranged in both the x- and y-directions. Such an arrangement of heating zones 642 can be provided by arranging two sets of heater electrodes 64 like those shown in Figure 2a side by side in the 25 width (x-) direction. The heaters 6 may be separated in this way into any number of heating zones 642 and may comprise any number of heating zones along the x- and y-directions. In particular, whilst Figure 3c shows two zones along the x-direction, a greater number of zones in the y-30 direction could also be provided, with the heater electrodes and their electrical connections being designed in a suitable manner to fit such an arrangement. The heating zones 642 of the heaters 6a, 6b can be operated (heated) independently, which can help to reduce hot / cold spots when using very low thermal mass heaters 6 such as those shown in Figure 2. It may be generally understood that the arrangement of the heater electrodes 64 may 35 comprise any suitable layout to enable the desired arrangement of heating zones 642. The heating zones illustrated in Figures 3b and 3c are all the same size. Of course, different sized heating zones 642 may be provided, as illustrated in Figure 3d, which shows a heater 6 having seven different sized heating zones (labelled Z1 to Z7). The way in which the 5 heater electrodes 64 would be arranged to define these different sized zones would be understood by the skilled reader and will not be described in detail here. The heating zones 642 described above form part of a heater having a flat hair contacting surface. The heater is not limited to flat hair contacting surfaces and can be configured for 10 use in a tubular form (as illustrated in Figure 3e) for example for use in a hair curler device or in a curved form (as illustrated in Figure 3f) for example for use in a heated hairbrush. The heater surface may have a corrugated or ribbed shape to provide a hair crimping device. The temperature of each heating zone 642 is independently controllable. Each heating zone 15 642 can be set to a target temperature. The target temperature of each heating zone 642 may be different. A separate temperature sensor may be provided for sensing the temperature of each heating zone 642 which is fed back to the microprocessor 29 to allow the microprocessor 29 to control the delivery of power to the heater electrode 64 of the corresponding heating zone 642. Alternatively, if the heater electrodes 64 are formed of a 20 material having a Positive Temperature Coefficient (PTC) or a Negative Temperature Coefficient (NTC) (such that its resistance varies with its temperature), then the temperature of each heating zone 642 can be determined by determining the resistance of the corresponding heater electrode 64. The microprocessor 28 controls the heating in order to reduce the difference between the actual temperature of the heating zone 642 and the target 25 temperature for that heating zone 642. Heating Zone Sizing One issue with low thermal mass heaters 6 is the regulation of hair contacting surface temperature in the locally hair loaded regions of the heater within desired temperature 30 limits, without causing overheating of the unloaded regions at the same time. Specifically, when the user loads a tress of hair 40 onto the heaters 6, some parts of the heater will be loaded with hair whilst other parts will not be loaded with hair. Upon loading with hair, more power is supplied to the heater 6 to ensure that all regions on the hair contacting surface can be retained within and / or recovered back to the desired 35 operating temperature limits. The low thermal mass heaters 6 described above are relatively thin and the dielectric layers are formed of materials with relatively low thermal diffusivities. If there was just a single heating zone, and hence a single continuous heater electrode 64 running across the whole length and whole width of the heater 6, then when more power is supplied to the heater 6 to recover the temperature drop in the locally hair loaded regions, the unloaded regions would undergo overheating, which could cause the heater materials to exceed their maximum operating temperatures, or cause the overheated regions to burn relatively small bundles / strands of hair that come into contact with them. This overheating can be prevented by using materials with higher thermal diffusivities in the layers that constitute the heater, and / or by increasing the thicknesses of the layers that constitute the heater and / or by dividing the heater 6 into multiple separately powered and controlled heating zones 642 across its length or its length and width. Increasing the thickness of the layers increases the thermal mass of the heater 6 which is undesired and there are limited materials that have the required dielectric strength and high thermal diffusivity (and which are available for use in mass produced consumer products). Therefore, the inventors have divided the heaters 6 up into plural heating zones. These heating zones can be equally and / or unequally sized and can be arranged regularly and / or irregularly across the width and length of the heater. However, overheating can still occur within a single heating zone. For example, if half of the heating zone is loaded with hair (which is assumed to be the realistic worst case scenario during operation) and the other half is not loaded with hair, then the half that is loaded with hair will cause the temperature of that part of the heating zone to drop which will cause more power to be applied to that heating zone in its entirety. That applied power will bring the average temperature of the heating zone back up to the desired operating temperature, but the unloaded part of the heating zone will be above the average temperature of the heating zone. This temperature increase may be sufficient to cause the unloaded part to overheat. At the same time the loaded part of the heating zone will be below the average temperature causing a reduction in heat transfer and reduced styling performance. This situation is illustrated in Figure 3g, which shows a tress of hair 40 overlying heating zones Z2, Z3 and Z4, with heating zone Z3 being fully loaded with hair and heating zones Z2 and Z4 being only partially loaded with hair. This problem can be reduced by making the heating zones very small - but that is costly due to all the connections needed to connect each heater electrode 367 for each heating zone back to the drive circuitry 23 as well as the number of control switches in the drive circuitry 23 needed to control the powering of each heater electrode 64. The inventors have found that for a given permitted maximum temperature within the heater, a maximum size of the heating zones can be defined which depends on the maximum power density to hair that can be extracted from the heating zone and the material characteristics and thicknesses of the layers forming the heating zone. Specifically, if it is assumed that only one half of a heating zone 642 is loaded with hair, upon loading with hair, the maximum temperature that occurs in the unloaded half of a heating zone 467 can be defined with the equation below: q.W2 7 -- *7^ “1“ ............................................................ 'max 1 Tar ~ y 16t. k where, TMax = maximum temperature (°C) on the surface of the heater which would occur in the unloaded half (worst case) of an individual heating zone; TTar = target operational temperature or average temperature (°C) of individual heating zones; q = power density (VW2) required to heat hair passing over the surface to the desired temperature for styling; W = width of a heating zone measured perpendicular to the motion of hair over the surface; t = total thickness of the layers that constitute the heating zone; and k = the thickness averaged thermal conductivity of the layers that constitute the heating zone. If it is assumed that the thickness averaged thermal conductivity of the constituent layers of a heating zone 642 and the total thickness of the layers that form the heating zone 642 are known and fixed (for any given device), then the above equation can be used to determine the required zone width (W) and hence a number of divisions along the length of the heater that will prevent overheating of the unloaded halves, when their other halves are loaded with hair, and more power is supplied to maintain and / or recover the hair contacting surface temperatures back to the desired operating limits. Consequently, for a given surface area that must be covered with the considered heater technology, the equation above can be used to determine the number of heating zones that should be positioned along the length of the given surface area, so that each heating zone 642 can be operated without exceeding the maximum operating temperature of the heater materials and without causing the temperature of the unloaded part of a heating zone 642 to exceed the maximum temperature (^Tmax) that could cause burning of relatively small bundles / strands of hair that come in contact with such overheated regions of the heating zone. Specifically, the required divisions along the length can be determined from: nL > (^Max ~ Trar) Where, L = length of the heater plate (perpendicular to the direction that hair typically travels across the surface); nL = number of zonal divisions along the length of the heater plate; TMax = maximum permitted temperature (°C) on the surface of the heater (which would occur in the unloaded half (worst case) of an individual heating zone) needed to avoid damage to hair or the heater; TTar = target operational temperature or average temperature (°C) of individual heating zones; k = the thickness averaged thermal conductivity (Wm'1°C1) of the layers that constitute the heating zone; t = total thickness of the layers that constitute the heating zone; and q = power density (Wm'2) required to heat hair passing over the surface to the desired temperature for styling. For a hair styling device, the inventors have found the following suitable ranges for these parameters: - Power density required for styling (q) is greater than 40,000 W / m2 and less than 100,000 W / m2. - The average thermal conductivity of the layers forming the heating zone (k) (averaged through the depth of the various layers) is between 80 and 200 W / m.K. - The maximum permitted temperature of a heating zone to manage (ideally avoid) hair damage is less than 250°C, more preferably less than 220°C and most preferably less than 200°C. - The total thickness of the layers (t) which make up the heater is less than 300pm but no less than 75pm due to manufacturing limitations. - The target operational temperature of the heater (TTar) is between 150°C and 230°C. Operating within these ranges, the inventors have found that the required number of heating zones per unit length (cm) along the length of the heater is between 0.6 and 2.5 per cm which is equivalent to a zone width (in the lengthwise direction of the heater) of between 0.4 cm and 1.7 cm. Of course, this is for the case of there not being multiple zones in the width direction of the heater as well (e.g. this is for the single row case shown in Figure 3b). If multiple rows of heating zones 642 are provided along the length of the heater (such as is shown in Figure 3c), then each row of heating zones 642 should meet the limits defined above if the above described overheating problem is to be avoided. Alternative Heater Arrangement An alternative flexible heater 6’ is illustrated in Figure 3h, which shows on the left-hand side an exploded cross-sectional view of the heater 6’ and substrate 68’ and on the right-hand side a perspective view of the heater 6’ and substrate 68’. As shown in Figure 3h, the heater 6’ has curved edges 72-1 and 72-2 that are shaped to match the shape of an upper surface 74 of the rigid support substrate 68’ so that the flexible heater 6’ can be bonded securely using an adhesive or diffusion bonding of the underlying materials to the upper surface of the rigid substrate 68’. The curved edges of the heater 6’ can be formed, for example, using a heat forming process. Figure 3h also illustrates that one or more surface mounted electronic components 76 may be attached to an underside of the heater 6’. These components may be, for example thermistors for sensing the temperature of the heating zones 642 of the heater 6’. Figure 3h also shows a control printed circuit board (PCB) 78 that carries the drive and control electronics 15 illustrated in Figure 10 that controls the heating of the different heating zones 642 of the heater 6’. As before, the heater 6’ is formed from a number of discrete layers that are mechanically or chemically bonded together. Each layer has a thickness between about 1 pm and 150 pm. The different layers forming part of the heater 6’ are shown in exploded cross-sectional and perspective views in Figure 3i. A description of each layer is given below. 5 Low Friction Coating 81 (Optional) This is an optional layer and can be added to create a smooth, low friction surface to enhance the user experience by making the heater 6’ feel less grippy against the hair. This layer would be as thin as possible (for example, between 1 and 150 pm) to reduce the 10 thermal resistance from the heater 6’ to the hair, whilst still being sufficiently durable and scratch resistant. This layer would typically be applied last, possibly as a spray coating (e.g. Cerasol), after the rest of the heater 6’ has been produced and assembled around the rigidifying substrate 68’. 15 This is needed because the coating is prone to cracking when flexed, and once applied the coating will reduce the natural flexibility of the heater, and so it should be applied once the heater 6’ has been formed into its final shape. Alternatively, this coating may itself comprise multiple layers including, for example, a 20 primer layer (of about 6pm), a base coat layer (of about 25pm) and a topcoat layer (of about 10pm). Heat Spreading Layer 82 (optional) This is also an optional layer and, when provided, helps to spread the heat within each 25 heating zone 642 to ensure that the temperature of individual heating zones 642 is able to maintain an acceptable degree of homogeneity during typical use. As discussed above, if a heating zone 642 was to be partially loaded with hair and was sufficiently large, the unloaded portion of the heating zone 642 could develop an unacceptably high temperature, whereas the loaded region would be too cold, as heat could not adequately flow from the hot region to 30 the cold region. This problem is exacerbated by the anisotropic thermal characteristics of the serpentine like heater electrodes 64, and by the fact the control electronics 15 would typically work to maintain an “average” temperature within the heating zone 64 based on the overall resistance of the heater electrode that forms the heating zone 642 - from the perspective of the control electronics 15, the heating zone 642 would be at the “correct” 35 temperature despite having hot and cold regions. Each heating zone 642 would have its own heat spreader 91, which is thermally separated (there is a high thermal impedance / low thermal conductivity) from the heat spreaders for adjacent zones. This is desirable to prevent heating zones 642 from heating neighbouring heating zones 642 which might otherwise increase power consumption, reduce warm up 5 time, and complicate algorithms based on zonal power consumption by adding crosstalk. Figure 3j illustrates an example form of the heat spreader layer 82. As shown, in this example there are 20 heat spreaders 91-1 to 91-20, each formed of a relatively high thermal conductivity material (such as copper). Each heat spreader 91 is separated from its neighbouring heat spreaders 91 and in effect forms an island of thermally conductive 10 material over the corresponding heating zone that substantially does not touch neighbouring heat spreaders to reduce heat spreading from one heating zone to an adjacent heating zone. This also helps signal to noise for sensing and the independent control of the different heating zones. Alternatively, those islands may be electrically interconnected but thermally decoupled by minimising the area of contact between neighbouring islands (as opposed to 15 ensuring they do not touch each other at all). For example, it may be desirable to provide an electrical connection between neighbouring heat spreaders - for example to ground the heat spreading layer. In this case the individual heat spreaders may have some conducting material connecting them with at least some of their neighbouring heat spreaders. Even though an electrical connection is provided between adjacent heat spreading elements, as 20 long as the connection is relatively small (for example less than 1 / 10th of the length / width of the heat spreader), there will still be, in effect, a thermal break or decoupling between neighbouring heat spreaders. The heat spreaders 91 may be separated from each other by a solid material having a 25 thermal conductivity lower than 35 W / mK or they may be separated by air. The heat spreaders 91 may be formed, for example, by taking a planar layer of metal (such as a layer of copper) that is bonded onto the layer below and then etching this layer of copper to physically separate the individual heat spreaders 91 (so that they do not touch each other). Provided there is a break between neighbouring heat spreaders 91, it is difficult for heat from 30 one heating zone 642 to pass into neighbouring heating zones 642. The solid material that is provided in the gap between adjacent heat spreaders 91 may be provided by a coating or a wash that is applied to the heat spreading layer 82 after the etching process has formed the gaps between adjacent heat spreaders 91 and may be the coating layer 81 described above. Alternatively other suitable methods may be used to form substantially or fully 35 physically separated individual heat spreaders 1591 and any suitable method may be used to provide solid material in the gaps between individual heat spreaders 1591, such as masking and vapour deposition, etc. This layer 82 can provide mechanical integrity to the overall heater 6’, providing some protection from damage to the hair contacting surface that might otherwise expose the underlying heater electrodes 64, which in turn could lead to short circuits or loss of functionality. Polyimide Separator layer 83 The polyimide separator layer 83 provides electrical isolation between the hair contacting surface of the heater 6’ (which may be the upper surface of this layer 83 if the optional layers 81 and 82 are not provided) and the main heater electrode layer. This layer 83 would have as low thermal resistance as possible whilst still achieving the dielectric requirements of the layer. As the name suggests, this layer is formed of polyimide, although other dielectric materials could be used. Because this layer is relatively thin, the in-plane thermal diffusivity or thermal conductivity of this layer (in a plane perpendicular to its thickness) is quite low (less than 35 W / mK). This helps to prevent heat spreading from one heating zone 642 to an adjacent heating zone 642. Main Heater Electrode &Sensing Layer 84 This layer 84 is where heat is created by dissipating electric power from the power source (e.g. a power supply unit (PSU) or one or more batteries). This layer 84 comprises a number of independently controllable heater electrodes 64 each defining a corresponding heating zone 642. Figure 3k illustrates in more detail the form that this layer 84 takes in this example heater 6’. As shown, in this example, there are twenty independently controllable heater electrodes 64-1 to 64-20 that each defines a corresponding heating zone 642. Each heater electrode 64 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material is specified in order to achieve the desired resistance and peak power requirements for the relevant power source. Each heater electrode 64 is formed into a serpentine pattern using, for example, chemical etching as a manufacturing process. In more detail, a solid layer of conductive material is provided and then etched to form the different heater electrodes 64. The straight lines shown in Figure 3k are the etched parts of the layer 84 and the white parts of the figure show the serpentine conductor paths that form the heater electrodes 64. In this illustrated example, adjacent heater electrodes 64 share a common positive terminal (although in other embodiments they may share a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control board 78 and the heater 6’. This common positive terminal is connected to the different heater electrodes at suitable vias 5 65-1 to 65-5, which connect through to connection circuitry below (not shown) that connects to the drive and control board 78. The other end of each heater electrode connects through a respective switch (not shown) to the drive and control board 78 to allow independent control of current flow through each heater electrode 64. As those skilled in the art will appreciate, it is not essential to have such a common positive (or ground) terminal, each 10 heater electrode 64 may be physically separate from all other heater electrodes 64 in which case, each end of each heater electrode 64 would be connected separately back to the drive and control board 78. Alternatively, other processes such as printing, thick film printing, physical vapour 15 deposition and the like could be used to form the heater electrodes 64. As schematically illustrated in Figure 3k, the end of each heater electrode 64 that is connected to the switch is provided at the side of the heater and the direction of the serpentine tracks changes in this edge portion (which corresponds to the portion of the 20 heater which is curved over the upper surface 74 of the rigid support substrate 68’). The inventors have found that this arrangement helps heat generated in the heater electrodes 64 in these edge portions to pass up to the top surface of the heater which is more likely to come into contact with the user’s hair. However, if the device is twisted in use such that the user’s hair comes into contact with the curved edge portion, then the hair will still be heated 25 as this curved edge portion is heated. The conductive material used in the layer 84 is preferably a PTC or an NTC material (such as stainless steel or copper) so that the resistance of the heater electrode 64 depends upon its temperature - and so the temperature of the heating zone 642 can be determined by 30 measuring a parameter that varies with the resistance of the corresponding heater electrode 64. Poly imide Separator (Optional) 85 When an auxiliary heater electrode layer is provided, this layer is required to provide the 35 required electrical separation between that auxiliary heater electrode layer and the main heater electrode layer 84 described above. This polyimide layer 85 would have a low thermal resistance in the thickness direction whilst still achieving the dielectric requirements. Due to this layer being relatively thin, it will have a low thermal conductivity in the plane perpendicular to its thickness of less than about 35 W / mK. Other dielectric materials could be used instead of polyimide. 5 Auxiliary Heater Electrode Layer (Optional) 86 Some embodiments of the heater 6’ may benefit from the presence of an additional heating element layer 86. This additional layer 86 could be used to dissipate power (create heat) from a secondary power source that operates at a different voltage to the main power source 21, for example the main power source could be a power supply and the power source for 10 the auxiliary heater electrode layer 86 could be one or more batteries. In other embodiments the primary source could be one or more batteries and the auxiliary one or more supercapacitors. Alternatively, the conductors on this auxiliary layer 86 could be used for temperature sensing, in which case, the heater electrodes 64 in the main layer 84 may only be used for heating. 15 The heater electrodes on the auxiliary layer 86 will typically have the same form as the heater electrodes 64 used in the main heater electrode layer 84 - so that they will define the same heating zones 642 as the heating zones 642 defined by the heater electrodes 64 on the main heater electrode layer 84. The path taken by the heater electrodes on the auxiliary 20 layer 86 do not need to follow the same path as the corresponding heater electrodes 64 formed on the main heater electrode layer 84. For example, whilst the main part of each heater electrode 64 on the main heater electrode layer 84 (ignoring the edge part of each heater electrode 64) serpentines in the longitudinal direction of the heater 6’ in Figure 3k, the corresponding heater electrodes of the auxiliary heater electrode layer 86 could be arranged 25 to serpentine in the width direction of the heater 6’. Such an arrangement may help to spread the heat flow within the heating zone 642 particularly if the heating zone 642 is only partially loaded with hair. Polyimide backing 87 30 This layer encapsulates the bottom heating layer (either the main or the auxiliary heating layer) so as not to allow its accidental exposure and to prevent moisture ingress. This backing layer 87 electrically separates the bottom heating layer from any surface mounted components that are mounted in the surface mounting layer 88 (discussed below) on the bottom of the heater 6’. If desired, this dielectric layer 87 can be made thicker than the 35 upper dielectric layers to provide enhanced structural integrity of the flexible part of the heater system. As with the other dielectric layers, this backing layer 87 does not need to be a polyimide layer and other dielectric materials could be used. Rear Side Surface Mount Components (Optional) 88 This layer is used to mount components on to the rear of the flexible heater 6’. These components may be temperature sensors (e.g. thermistors) or other components involved in 5 providing fusing functionality for the heater (e.g. solder links). This layer may be produced using standard chemical etching methods from the PCB manufacturing process or other suitable methods such as those described above. Additional surface mount components would be added later. 10 High Temperature Adhesive 89 The function of this layer is to enable bonding of the flexible heater 6’ to the rigid substrate 68 (shown in Figure 3h) that forms the final shape of the overall heater. Various types of adhesives could be used such as a pressure activated adhesive (PAA) or a heat activated 15 adhesive (HAA). It could also be a thermoplastic film which sets after heat and pressure have been applied in a forming tool. Thermal Safety Circuit In order to ensure user safety, it is important to limit the maximum temperature that the heaters 6, and / or the flexible heaters 6’ and / or the heating zones 642 described above can 20 reach, in the case of an electronics or firmware failure. In the description that follows most examples refer to heaters 6. It will nevertheless be appreciated that the examples apply equally to a scenario where the heaters are the flexible heaters 6’ described above. By way of illustration, Figure 4 is a plot of temperature against time for a heater 6 operating 25 at maximum power. In this example, the operating temperature, Toperating, is set at 230 °C, and the maximum permitted temperature at which the heater 6 can safely operate, Tmax, is defined as 300°C. A trigger temperature, Ttngger, is defined as the temperature above which a safety fuse mechanism is triggered to cut the electrical power to the heater 6. In this example, Ttngger is set at 240°C. If Ttngger is set too low, there is a risk that the safety fuse 30 may be triggered by small fluctuations around the operating temperature. However, if Ttngger is set too high, then the fuse may not be able to react sufficiently quickly to prevent the temperature rising beyond Tmax. This can be a particular issue for very low thermal mass heaters 6 like those shown in Figure 2 and / or the flexible heater 6’ like those shown in Figure 3h. For the example as shown in Figure 4, if the heater has a maximum heat-up rate of 200 °Cs'1, then a response time of 0.3 seconds of a thermal safety fuse is required to prevent the temperature from rising above Tmax. It is known to implement thermal safety fuses in hair styling appliances to prevent overheating and guarantee safety. However, the fastest response rate of commercially 5 available thermal fuses is typically only 40 °Cs’1. Therefore, if such a thermal fuse is used with a low thermal mass heater, the high heat-up rate can cause the heaters 6a, 6b to reach extremely high temperatures before the thermal fuse can cause the electrical power to be cut from the heater electrodes 64. This could pose a safety risk to users and the present invention seeks to provide a thermal safety cut-out system which can react to failures 10 sufficiently fast to prevent such unsafe operation when using very low thermal mass heaters 6, 6’. The mechanisms of the invention provide an independent cut-out circuit. These will typically be provided in addition to functionality of the main controller CPU also routinely monitoring for faults in normal operation. 15 As will be described in more detail below, the increased cut-out speed of the thermal safety cut-out systems embodying the present invention can be implemented by providing intentionally weak parts, by electronic means or via microprocessor-based safety strategies. The thermal safety cut-out of the present invention may disconnect power to the gate of a leader Metal Oxide Semiconductor Field Effect Transistor (MOSFET), which controls the 20 ability of the drive circuit to be able to provide electrical power to the heaters 6. Figure 5 shows an exemplary block diagram of the connection between a thermal safety circuit 102 and the heating circuit 104 (which comprises the very low thermal mass heaters 6). As shown, the thermal safety circuit 102 controls the voltage that is applied to the gate of a MOSFET switch 106 that isolates the heating circuit 104 from ground thereby preventing 25 current from being able to flow through the heater electrodes 64 of the heaters 6. A number of different thermal safety circuits 102 will now be described. Embodiment 1 - Utilizing intentionally weak parts In a first embodiment, as shown in Figure 5, intentionally weak parts are provided in the thermal safety circuit. In an exemplary implementation, the intentionally weak parts are 30 solder links which are designed to melt at a set temperature, thereby breaking an electrical link of the thermal safety circuit which removes the voltage from the gate of the MOSFET 106, which disconnects the heaters 6 in the heating circuit 104 from ground and thereby prevents current flowing through the heater electrodes 64. In some implementations, the weak links may be located at or near the hair contacting surface of the upper layer 62, but the weak links are preferably positioned at a distance from the hair surface. A suitably thermally conductive (but preferably electrically insulating) material can be positioned over and / or around the weak link to ensure that it is sufficiently responsive to temperature increases. Preferably these weak parts are mounted close to the heater electrodes 64 so that they experience as much of the heat flux generated by the heater electrodes 64 as possible. In one embodiment, these weak links are mounted on the back of the dielectric layer 66 (on the surface facing towards the rigid support 68). A separate weak link may be provided adjacent each heater electrode 64 that is designed to melt if the corresponding heater electrode 64 gets too hot. This weak link may be designed to cut the electrical power to only the corresponding heater electrode 64 or to cut power to all the heater electrodes 64. The weak link may be formed of solder material. A preferred arrangement of the weak link is illustrated in Figure 2d. In particular, Figure 2d illustrates part of the back of the dielectric layer 66, with two conductor traces 71-1 and 71-2 mounted thereon. The ends of the two conductor traces 71 closest to each other are connected to a respective solder pad 72-1 and 72-2 for receiving a solder paste having a first melting temperature. Once the solder paste has been applied to the solder pads 72, a solid block of solder 74 (for example, a solder wire or ribbon, sometimes referred to as a ‘solder preform’) having a melting temperature that is higher than the melting temperature of the solder paste 72 is placed between the two solder pads 72 - thereby making an electrical connection between the two conductor traces 71. The weak link is then heated in an oven to a temperature that is between the melt temperature of the solder paste and the melt temperature of the block of solder 74 to melt the solder paste (but not the block of solder 74). The heater is then removed from the oven so that the solder paste solidifies to bond the block of solder 74 to the solder pads 72. The weak link (fusible solder link) therefore consists of both the solder preform (wire / ribbon) and the solder paste. The surface of the dielectric layer 66 around the weak link (at least in the gap between the solder pads 72-1 and 72-2) is covered with a solder resist material 76 that repels solder. The melt temperature of the block of solder 74 is chosen so that, during use, if a heater 6 (or a flexible heater 6’) overheats, the block of solder 74 melts within the permitted response time. The melted solder is repelled by the solder resist 76 which ensures that the solder is drawn away quickly from the gap between the two conductor traces 71 thereby breaking the connection between the conductor traces 71. A weak link like the one shown in Figure 2d may be provided on the underside of the dielectric layer 66 adjacent each heater 6. The weak links may each be connected back to a respective switch 106 or the weak links may be connected in series and connected back to a common control switch 106. Therefore, the control switch(es) 106 can detect if any of the weak links melt 5 and remove power from the heaters 64 accordingly. In order to ensure that the weak link cuts the electrical power supply within the required response time, each fusible solder link is to be made of a combination of solder paste and solder wire / ribbon whose combined mass can range between 0.005 milligrams and 1 gram (inclusive). 10 It can be beneficial to ensure that the circuit is reliably broken above a set temperature. In a further exemplary implementation to that outlined above, this can be achieved via the use of microfluidic structures (such as channels) configured to encourage the molten solder to flow away from the connection points, thereby breaking the circuit reliably. In a yet further exemplary implementation, the solder link is resiliently biased (for example, spring-loaded), 15 to ensure the circuit is reliably completely broken when the solder melts at the set temperature. In some implementations, the weak link may comprise a solder link of very small thickness. Such an implementation requires accurate and precise manufacturing methods. Hot air solder level (HASL) processes typically result in non-uniform solder thicknesses and so are 20 usually unsuitable. In some exemplary implementations of this embodiment of the invention, the weak parts such as the solder links are deposited onto the PCB via a printing technique. This methodology can be useful in achieving the required small dimensional thickness. Although the MOSFET switch 106 used in this embodiment is an enhancement mode MOSFET, a depletion mode MOSFET switch may be used instead. In such an instance, a 25 potential divider and / or comparator may be provided between the circuit containing the weak part and the MOSFET to control the opening and closing of the MOSFET switch. Embodiment 2 - Utilizing sensors having non-linear temperature coefficients In a second embodiment, as shown in Figure 6, a thermal safety circuit is provided which comprises components which react in a non-linear manner to changes in temperature. 30 Figure 6 shows an exemplary circuit diagram comprising a thermal safety circuit 202 which controls the power supply to the heating circuit 204 via a MOSFET switch 206. The thermal safety circuit 202 comprises an array of non-linear thermistors 208, in this example four thermistors 208a to 208d, that are connected in series. The thermistors 208 are again arranged close to the heater electrodes 64, preferably mounted on the back of the dielectric layer 66 (on the surface facing towards the rigid support 68). The thermistors 208 may be arrayed over the heating area defined by the heater electrodes 64. In this exemplary embodiment, there are only four thermistors 208 whilst there are ten heater electrodes 64 in 5 the heater design shown in Figure 2. In other embodiments, one or more thermistors 208 may be provided adjacent each heater electrode 64. The non-linear thermistors, 208a to 208d, may be non-linear Positive Temperature Coefficient (PTC) thermistors, which undergo a sharp increase in resistance above a critical temperature, Tc. Preferably, the critical temperature, Tc, for the PTC thermistors used is set 10 to correspond to, or at least be related to, the above-described trigger temperature, Ttrigger. A DC source 209 (for example the battery of the hair styler) applies a voltage to the network of series connected thermistors 208. This network of series connected thermistors 208 defines a potential divider circuit with the resistor R. A comparator, 210, is connected to this potential divider circuit and compares the voltage dropped across the resistor R with a 15 reference voltage, Vref. When the temperature of the heaters 6 is low or is within their normal operating temperature range (i.e. below the critical temperature Tc), the voltage dropped across the resistor R will be greater than the reference voltage and the output from the comparator 210 will be a logical high value which maintains the MOSFET switch 206 ON; and therefore, the heating circuit 204 is connected to ground and heating of the heater 20 electrodes 64 can continue in the normal way. However, if the temperature of one or more of the heater electrodes 64 exceeds the trigger temperature, then the resistance of the thermistor(s) 208 closest to that (those) heater electrode(s) 64 will rise significantly which will increase the resistance of the thermistor network which in turn will reduce the voltage dropped across the resistor R. When the voltage dropped across the resistor R falls below 25 the reference voltage Vref, the comparator output will go from high to low which turns OFF the MOSFET switch 206, thereby isolating the heating circuit 204 from ground. As a result, current flow through the heater electrodes 64 will stop which will reduce the temperature of the heaters 6 and prevent them from reaching the above defined maximum permitted temperature Tmax- 30 As shown in Figure 6, latching circuitry 214 is provided between the comparator 210 and the MOSFET 206 to maintain the MOSFET 206 in the OFF state once the comparator 210 switches the MOSFET OFF. This ensures that when the heater cools down slightly the comparator 210 doesn’t turn the MOSFET 206 back ON. Specifically, the latching circuitry 214 acts to introduce a delay or a semi-permanent disconnection between the comparator 35 210 and the MOSFET 206. The latching circuitry 214 may include counter circuitry to define the delay that is introduced or the latching circuitry 214 may need to be reset by the main controller or by a power cycling event before the MOSFET 106 is allowed to be turned back ON. In an alternative implementation, instead of using PTC thermistors, Negative Temperature Coefficient (NTC) thermistors could be used, which undergo a sharp decrease in resistance as their temperature increases above a critical temperature, Tc. Again, the critical temperature, Tc, corresponds to, or is at least related to, the above trigger temperature, Tigger of the heater electrodes 64. In such an implementation, when the temperature of the heaters 6 is low or is within their normal operating temperature range (i.e. below the critical temperature Tc), the voltage dropped across the resistor R will be lower than the reference voltage and in this case, the comparator 210 is configured to output a high voltage which maintains the MOSFET switch 206 ON; and therefore, the heating circuit 204 is connected to ground and heating of the heater electrodes 64 can continue in the normal way. This may be achieved by swapping the inputs on the comparator 210 so that the reference voltage is applied to the positive input of the comparator 210 and the voltage dropped across the resistor R is applied to the negative input of the comparator. When the temperature of one or more of the heater electrodes 64 exceeds the trigger temperature, then the resistance of the thermistor(s) 208 closest to that (those) heater electrode(s) 64 will drop significantly which will decrease the resistance of the thermistor network which in turn will increase the voltage dropped across the resistor R. When the voltage dropped across the resistor R rises above the reference voltage Vref, the comparator 210 will output a low voltage which turns OFF the MOSFET switch 206 thereby isolating the heating circuit 204 from ground. As a result, current flow through the heater electrodes 64 will stop which will reduce the temperature of the heaters 6 and prevent them from reaching the above defined maximum permitted temperature Tmax. Although the MOSFET switch 206 used in the above examples is an enhanced mode MOSFET, a depletion mode MOSFET switch 206 could be used instead, in which case the comparator 210 would be configured to output a low value when the MOSFET is to be ON and a high value when the MOSFET is to be OFF. The MOSFET could be placed in the positive supply (‘high side switch’) or as shown here in the negative supply (‘low side switch’), with level-shifting gate drive circuitry as appropriate. In the embodiments described above, the thermistors 208 were connected in series with each other. In another implementation, the network of non-linear thermistors, 208a to 208d, are connected in parallel with each other. For example, if the thermistors are non-linear Negative Temperature Coefficient (NTC) thermistors, a decrease in the resistance of at least one of the thermistors will decrease the overall resistance of the parallel thermistor network, thereby increasing the voltage dropped across the resistor R that is input to the comparator 210. The comparator 210 then operates in the same way as described above to control the state of the MOSFET switch 206. 5 It is preferable to utilize PTC thermistors and / or NTC thermistors, which have a high temperature sensitivity (percentage change per degree C) so that the thermal safety circuit 202 has good responsivity to the temperature rising above the trigger temperature, Tigger. This can aid in ensuring a quick response time to any over-heating of the heaters 6. Nonlinear thermistors 208 can also provide this high temperature sensitivity. 10 Embodiment 3 - Analogue OR gate of maximum zone temperature sensing In a third embodiment, as shown in Figure 7, there is provided a thermal safety circuit comprising an array of separate temperature sensors, which feed into a parallel array of diodes that act as an analogue OR gate such that if the sensed temperature from any one of the separate temperature sensors exceeds a threshold, then the thermal safety circuit 15 isolates the heating circuit from ground. Figure 7 shows an exemplary implementation, in which is provided a thermal safety circuit 302 comprising an array of temperature sensors 308a to 308d provided in parallel. Each branch of the parallel circuit is connected to the DC supply 309 and comprises a temperature sensor, 308a to 308d, and a resistor, 314a to 314d, connected in series. The temperature 20 sensors 308a to 308d are implemented as thermistors. A diode 312a to 312d is connected to each branch of the circuit, between the corresponding temperature sensor, 308a to 308d, and resistor, 314a to 314d. The outputs of the diodes 312 are connected together and feed into a comparator 310. The output of the comparator 310 then controls a MOSFET switch 306 as before, in order to control the ability to supply power to the heater electrodes 64 of 25 the heating circuit 304. As in Embodiment 2, the output from the comparator 310 is input to latching circuitry 314 that ensures that the MOSFET 306 cannot be turned ON again once the heater cools down a little and the output from the comparator 310 changes back to a logical high value. Indeed, all embodiments will typically include some form of latching circuitry to ensure that the device cannot start heating again (for a defined period of time or 30 until the device is reset) once the thermal safety circuitry has removed power from the heaters. The thermistors may be PTC thermistors or NTC thermistors. In the case of PTC thermistors, if any one (or more) of the thermistors, 308a to 308d, detects a temperature above the trigger temperature, then its resistance will increase significantly causing the voltage drop across that thermistor 308 to increase above the reference voltage, Vref (which may be a different reference voltage than the one used in the other embodiments described herein). It will be appreciated that the value of Vref will vary depending on the type of 5 thermistor used (i.e., temperature sensor used), the size of the resistor to which it is connected, and the desired switching temperature at which the thermal safety circuit isolates the heating circuit from ground. The highest voltage input to the diodes 312 will pass through the diodes 312 to the comparator 310 causing the comparator to change state, in this case from a high state to a 10 low state causing the MOSFET switch 306 to turn OFF and isolate the heating circuit 304 from ground. In this way, the diodes 312 are acting as an analogue “OR” gate that will output a high voltage if any of the input voltages to the diodes 312 is high. A similar arrangement can be provided for NTC type thermistors and when using depletion mode MOSFET devices as discussed above for Embodiment 2. 15 As an alternative embodiment, the diodes forming the analogue “OR” gate may be replaced by a scanned analogue multiplexer, which sequentially samples the thermistor / resistor outputs and applies them to the comparator. This embodiment offers the advantage of eliminating any voltage drop and leakage currents which may be contributed by the diodes. The thermistors 308 are again arranged close to the heater electrodes 64, preferably 20 mounted on the back of the dielectric layer 66 (on the surface facing towards the rigid support 68). The thermistors 308 may be arrayed over the heating area defined by the heater electrodes 64. In this exemplary implementation, there are only four thermistors 308 whilst there are ten heater electrodes 64 in the heater design shown in Figure 2. In other embodiments, one or more thermistors 308 may be provided adjacent to each heater 25 electrode 64 (heater zone). In further embodiments, a single thermistor may be ‘shared’ by multiple zones e.g. placed so that it bridges across two adjacent zones, or at the corners of four adjacent zones, if arranged in a suitable pattern. An array of thermistors may each span multiple zones. As alternatives to the use of thermistors as the temperature sensing components, printed 30 thermocouples or other temperature sensing components may be used. In some implementations, combinations of different sensing components may be used, for example different types of temperature sensors for the different zones may be used and / or some or each of the heating zones may implement a combination of different temperature sensing components. Embodiment 4 - Dual microprocessor Overheating of the heater electrodes 64 could potentially happen due to a number of 5 different faults. One fault that could potentially cause heater electrodes 64 to overheat would be if the microprocessor (that is used to control the heating of the heater electrodes 64) developed a fault and output the wrong control signals for controlling the heating of the heater electrodes 64. Ina fourth embodiment, a safety strategy is implemented by running the same software directly measuring heater electrode temperature on two identical 10 microprocessors. If the two software circuits disagree, then the system ‘trips’ and cuts power to the heater electrodes 64. An exemplary implementation of such an embodiment is shown in Figure 8. As shown, there is provided an array of n heater electrodes 64, four of which are shown and labelled 64a, 64b, 64c and 64n. These heater electrodes 64 are arrayed over the area of the heater 6 to 15 be heated (as shown in Figure 2). There is also provided a first microprocessor 412 and a second microprocessor 414. Each of the heater electrodes 64 is connected at one end to a DC source 409 which is typically a battery used to power the hair styling device, and at the other end to two serially connected MOSFET switches, 404 and 406 (for example, heater electrode 64a is connected to MOSFET gates 404a and 406a). The gates of the first set of 20 MOSFETs, 404 are connected to and controlled by the first microprocessor 412 (for example, the gate of MOSFET 404a is connected to output 5 of the first microprocessor 412; MOSFET 404b is connected to output 6, and so on). The gates of the second set of MOSFETs, 406 are connected in a similar manner to the outputs of the second microprocessor 414 (for example, MOSFET 406a is connected to output 5 of the second 25 microprocessor 414; MOSFET 406b is connected to output 6 of the second microprocessor 414, and so on). The source terminals of the MOSFETs 406 are connected together and connect to ground through a resistor R. Therefore, in operation, when current is to be applied to a heater electrode 64, the first microprocessor 412 must output an appropriate control signal to turn ON the corresponding MOSFET switch 404 and the second 30 microprocessor 414 has to output a corresponding control signal to turn ON the corresponding MOSFET switch 406 so that current can flow through the heater electrode 64, MOSFET switch 404 and MOSFET switch 406 and to ground through the resistor R. If the microprocessors 412 and 414 output different control signals such that only one of the MOSFETs 404 and 406 is turned ON, then the corresponding heater electrode 64 will not be 35 connected to ground and so no current will flow through the heater electrode 64. This arrangement acts as a failsafe operation of the circuitry in case of a fault with one of the microprocessors. In addition, the output from the first microprocessor 412 and the output from the second microprocessor 414 for a given heater electrode 64 also feed into a corresponding XOR gate 408. For example, output 5 of the first microprocessor 412 and output 5 of the second microprocessor 414 are input to a first XOR gate 408a. For each of the XOR gates 408, if the software running on the first microprocessor 412 agrees with the parallel software running on the second microprocessor 414, then the inputs to the corresponding XOR gate will be the same in which case, the output from the XOR gate will be a logical low (0). However, if the software running on the two microprocessors 412 and 414 ever disagrees, then their control outputs will disagree and the corresponding XOR gate 408 will output a logical high (1). The outputs from the XOR gates 408 are passed through the OR gates 410 (OR gates 410a to 410c are shown in Figure 8), so that if there is any disagreement in the control signals output from the two microprocessors 412 and 414, this will be flagged as a high logic level on input terminal 3 of each microprocessor. On the other hand, if all the corresponding control signals match each other, then the output from the OR gates will be a logic low (0) level that is fed into the input terminal 3 of each microprocessor. If a logical high signal is received at the input terminal 3 of the microprocessors 412 and 414, then both microprocessors 412 and 414 may be programmed to change its output control signals to turn off the MOSFETs 404 and 406, in order to prevent any current flowing through any of the heater electrodes 64. Alternatively, a separate “leader” MOSFET switch (not shown) may be provided between the DC supply 409 and the heater electrodes 64 that is controlled by one of the microprocessors, so that in the event of a disagreement in the control signals, one of the microprocessors can turn OFF that leader MOSFET switch thereby removing the power from all of the heater electrodes 64. Alternatively, for redundancy, two serially connected leader MOSFET switches may be provided between the DC power supply 409 and the heater electrodes 64, with each one being controlled by a different one of the two microprocessors. In this way, if one of the microprocessors is faulty then the other one can still be relied upon to turn off the corresponding leader MOSFET switch that will remove the power from all of the heater electrodes 64. Of course, such redundancy is already provided by switching off the MOSFET switches 404 and 406. In the circuit shown in Figure 8, both the first microprocessor 412 and the second microprocessor 414 receive temperature sensor signals for each zone of the heater 6. It does this using the resistor R and the operational amplifier 415. In particular, in this embodiment, each heater electrode 64 is formed of a material whose resistance changes depending on the temperature of the heater electrode 64. For example, the heater electrodes 64 may be formed from a PTC type material such that the resistance of the heater electrode 64 increases as its temperature increases. Of course, NTC type materials could be used as 5 well. As shown in Figure 8, each heater electrode 64 forms a potential divider with the resistor R. Therefore, the voltage at the positive terminal of the amplifier 415 will depend on the temperature of the heater electrode 64. To obtain a temperature sensor signal for a desired heater electrode 64, then the corresponding switches 404 and 406 are switched ON and the other heater electrodes 64 are isolated from the resistor R by ensuring that their 10 corresponding MOSFET switches 404 and 406 are switched OFF. The output signal from the amplifier 415 will vary with the temperature of the desired heater electrode 64 which signal is fed back to input terminal 2 of each microprocessor 412 and 414. The microprocessors 412 and 414 can then cycle through each of the heater electrodes 64 one at a time, connecting each heater electrode 64 to the resistor R, to thereby obtain a 15 temperature sensor signal for each heater electrode 64. If desired, the temperature sensor signals from the amplifier 415 can be converted into actual temperature measurements of the heater electrodes 64 via a suitable equation or look up table. Alternatively, the control loops may use these temperature sensor signals directly in the control loop calculations. Such temperature sensor signals can be obtained at the same time as or interleaved with 20 the powering of the heater electrodes 64 for heating the heater 6 for styling the user’s hair. The microprocessors 412 and 414 then use the temperature sensor signals or the converted temperatures for the different heater electrodes 64 as part of a feedback loop to control the powering of the heater electrodes 64 to maintain a desired temperature for each heater electrode 64 (which desired temperature may be the same for each heater electrode 64 or it 25 may be different for each heater electrode 64). Embodiment 5 - Dedicated safety microprocessor In a fifth embodiment, as shown in Figure 9, a dedicated safety microprocessor is used to run safety critical software in order to prevent overheating in addition to a control microprocessor that controls the normal operation of the hair styling device. 30 Figure 9 shows an exemplary implementation of this embodiment. An array of heater electrodes 64 is provided. In this case four heater electrodes, 64a to 64d, are provided and are each connected at one end to a DC power source 509 that provides electrical power to heat the heater electrodes 64. The other end of each heater electrode 64 is connected to two MOSFET switches 504 and 506. For example, heater electrode 64a is connected to 35 MOSFET switch 504a and MOSFET switch 506a; heater electrode 64b is connected to MOSFET switch 504b and MOSFET switch 506b, and so on. The MOSFET switches 504 are controlled by a control microprocessor 512. For example, output 5 of the control microprocessor 512 controls the MOSFET switch 504a; output 6 of the control microprocessor 512 controls the MOSFET switch 504b, and so on. On the other hand, the 5 MOSFET switches 506 are controlled by a safety microprocessor 514. For example, output 5 of the safety microprocessor 514 controls the MOSFET switch 506a; output 6 of the safety microprocessor 514 controls the MOSFET switch 506b, and so on. The source terminals of the MOSFET switches 504 are connected together and connect through the resistor R1 and a leader MOSFET 516 (which is normally ON and will be described in more detail below) to 10 ground. The source terminals of the MOSFET switches 506 are also connected together and connect through the resistor R2 (which may be the same as or different to resistor R1) to ground. Therefore, the heater electrodes 64 each forms a potential divider circuit with resistor R1 and a second potential divider circuit with resistor R2. The control microprocessor 512 runs standard software for performing all temperature 15 control and sensing functions. Specifically, the control microprocessor 512 controls the MOSFET switches 504 to control current flow through the heater electrodes 64. In this embodiment, each heater electrode 64 is formed of a material whose resistance changes with temperature. As before this may be a PTC type of material or an NTC type of material. When the temperature of a desired heater electrode 64 is to be sensed, the control 20 microprocessor 512 turns ON the corresponding MOSFET switch 504 whilst isolating the other heater electrodes 64 from the resistor R1 by ensuring that their corresponding MOSFET switches 504 are switched OFF. Since each heater electrode 64 forms a potential divider with the resistor R1, the voltage at the positive terminal of the operational amplifier 515-1 will depend on the temperature of the heater electrode 64. Therefore, the signal 25 output from the comparator 515-1 will vary with the temperature of the desired heater electrode 64 which sensor signal is fed back to an input terminal of the control microprocessor 512. The control microprocessor 512 can then cycle through each of the heater electrodes 64 one at a time, connecting each heater electrode 64 to the resistor R1, to thereby obtain a temperature sensor signal for each heater electrode 64. As before, each 30 temperature sensor signal from the amplifier 515-1 can be converted into a corresponding temperature measurement via a suitable equation or via a suitable look up table or it can be used directly as an input in the control loop calculations performed by the control microprocessor 512. Such temperature sensor signals can be obtained at the same time as or interleaved with the powering of the heater electrodes 64 for heating the heater 6 for 35 styling the user’s hair. The control microprocessor 512 then uses the temperature sensor signals or the converted temperatures for the different heater electrodes 64 as part of a feedback loop to control the powering of the heater electrodes 64 to maintain a desired temperature for each heater electrode 64 (which desired temperature may be the same for each heater electrode 64 or it may be different for each heater electrode 64). The safety microprocessor 514 runs a similar temperature sensing process on the heater electrodes 64 except using the potential divider formed between the respective heater electrode 64 and the resistor R2. When the safety microprocessor 514 wishes to determine the temperature of a desired heater electrode 64, the safety microprocessor turns ON the corresponding MOSFET 506 whilst isolating the other heater electrodes 64 from the resistor R2 by ensuring that their corresponding MOSFET switches 506 are switched OFF. Since each heater electrode 64 forms a potential divider with the resistor R2, the voltage at the positive terminal of the amplifier 515-2 will depend on the temperature of the heater electrode 64. The output signal from the operational amplifier 515-2 will therefore vary with the temperature of the desired heater electrode 64 which output signal is fed back to an input terminal of the safety microprocessor 514. The safety microprocessor 514 can then cycle through each of the heater electrodes 64 one at a time, connecting each heater electrode 64 to the resistor R2, to thereby obtain a temperature sensor signal for each heater electrode 64. Such temperature measurements can be obtained at the same time as or interleaved with the powering of the heater electrodes 64 for heating the heater 6 for styling the user’s hair and can be performed at the same time as or interleaved with the temperature sensing performed by the control microprocessor 512. The safety microprocessor 514 then uses the sensed temperatures to monitor for any heater electrodes 64 whose temperature sensor signal exceeds a threshold corresponding to the abovedescribed trigger temperature Ttngger. If the safety microprocessor 514 detects that the temperature of any of the heater electrodes 64 has gone above the trigger temperature, then the safety microprocessor outputs a control signal to turn OFF the leader MOSFET 516 (which is usually turned ON during normal operation). By doing this, the safety microprocessor 514 disconnects the heater electrodes 64 from ground via the resistor R1. The safety microprocessor 514 also switches OFF the MOSFETs 506 to disconnect the heater electrodes 64 from ground via resistor R2. Because the safety microprocessor is not running any other software, as soon as a determination is made that one or more of the heater electrodes 64 has reached the trigger temperature, the safety microprocessor 514 can swiftly take action to prevent current flowing through the heater electrodes 64 which will therefore cool down and will avoid any of the heater electrodes 64 from getting hotter than the maximum permitted temperature Tmax. In summary, safety circuits are described for a low thermal mass hair styling appliance. The safety circuits can rely on intentionally weak links or temperature transducers, and electronic circuits that can remove power from the heater electrodes, and dual microprocessor designs. It will be appreciated that the safety circuits described above may, by way of example only, be implemented in device control circuitry of the hair styler. Device Control Circuitry Figure 10 is a simplified block diagram of control circuitry 15 (e.g., a microcontroller unit) that controls the operation of the hair styler device 1 shown in Figure 1. As shown, the control circuitry 15 comprises a power supply 21 that, in this embodiment, may derive power from a battery power source (e.g., a battery having a voltage between 3 V and 42 V, although any other suitable voltage battery could alternatively be used). A mains power supply input may be provided to charge the battery via an AC to DC converter (not shown), which may be external or internal to the device 1. Alternatively, the power supply 21 may derive power from an AC mains supply input. In this example, power is provided to the heaters 6 for heating the user’s hair. The power supplied to the heaters 6 is controlled by a controller 28 having a microprocessor 29. The power supplied to the heaters 6 is controlled by drive circuitry 23 (which may include one or more power semiconductor switching devices (triacs)) which controls the application of an AC mains voltage, or a DC voltage derived from the AC mains or from a battery to the heaters 6 in accordance with instructions from the microprocessor 29. The heaters 6 and drive circuitry 23 may be collectively referred to as the heater control sub-circuitry 32 of the control circuitry 15. It will be appreciated that the heater control sub-circuitry 32 of the control circuitry 15, may include the heating circuit and safety circuitry shown in Figures 5-9. The microprocessor 29 is coupled to a memory 30 (which is typically a non-volatile memory) that stores processor control code for implementing one or more control methods that control the heating of the heaters 6 in accordance with a desired operating temperature of the heaters 6 (for each of the independently controllable heating zones) and sensed temperatures of the heaters obtained from temperature measurement circuitry 25. The memory 63 may store, for example, one or more operating profiles or parameters (e.g., a target temperature for each of the heating zones, a maximum current limit, or a maximum power limit). Software stored in the memory 63 may include, for example, an operating system and a heater control module suitable for implementing one or more of the methods described below. The temperature measurement circuitry 25 may comprise temperature sensors such as thermistors or may use circuitry that senses the resistance of heater electrodes that are used to heat the heaters 6, which resistance depends on the temperature of the heater electrode. The temperature measurement circuitry 25 may comprise a non-contact type of temperature sensor (for example, an infrared sensor) for sensing the temperature of the user’s hair, and / or for sensing the temperature of a part of the device 1 (for example the temperature of the hair-contacting surface of the heater 6). The temperature sensor(s) could be provided inside a body portion of the device 1 or could be provided on an exterior surface of the device 1. A temperature measurement of a part of the device 1 can be measured directly or indirectly. For example, a heat pipe could be used to transfer heat via conduction from the part whose temperature is to be measured to an internal sensor. Figure 10 also shows a user interface 11 that is coupled to the microprocessor 29, for example to provide one or more user controls and / or output indications such as a visual indication or an audible alert. The output(s) may be used to indicate to the user, for example, if they have inserted too much hair between the heaters 6, if they are moving the device 1 too quickly along the hair tress 40, or simply if the device is on or off. In this example, the control circuitry 15 also comprises communications circuitry 27 to allow the device to communicate with a remote sensor, a remote server, or a remote application (e.g. on a mobile telephone). The communications circuitry 27 may use, for example, Bluetooth, Wi-Fi and / or 3GPP communication protocols to communicate with the remote device. The communications circuitry 27 could be used to receive an input parameter from a remote device, for example a desired operating temperature for the heaters 6 for styling hair. As those skilled in the art will appreciate, the device 1 does not necessarily need to have all of the blocks illustrated in Figure 10. For example, if the device 10 is a hair straightener, then there is not necessarily a need for the communications circuitry 27. As previously indicated, it will be appreciated that the heater control sub-circuitry 32 of the control circuitry 15, may include the heating circuit and safety circuitry shown in Figures 5-9. Alternatively, or additionally to the heating circuit and safety circuitry shown in Figures 5-9, the heater control sub-circuitry 32 of the control circuitry 15 may also include other components for monitoring the temperature of the heating zones and controlling the power output to the heating zones. These will now be described with reference to Figure 11. Heater Control Sub-circuitry Figure 11a is an example schematic view of the heater control sub-circuitry 32 of the control circuitry and the way in which the heater electrodes 64 may be connected together and controlled. As shown in Figure 11a, each heater electrode 64 is connected at one end to the power supply 21 and at the other end to a respective switch (in this case a MOSFET switch) 95-1 to 95-20. The switches 95 are controlled by the microprocessor 29. When a heater electrode 64 is to provide heat, the corresponding switch 95 is closed thereby connecting the heater electrode 64 to ground through the resistor R. As a result, current flows from the power supply 21 to ground causing the heater electrode 64 to heat up. The microprocessor 29 can control the position of each switch 95 independently thereby allowing each heater electrode 64 to be powered independently. When the temperature of a selected heating zone 642 is to be determined, the switch 95 of the corresponding heater electrode 64 is closed and all other switches 95 are opened. In this way, the selected heater electrode 64 is provided in series with the resistor R. Since the heater electrodes 64 are formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, by measuring the voltage dropped across the resistor R (using the operational amplifier 97), the microprocessor 29 can determine the resistance of the selected heater electrode 64 and hence can determine the temperature of the corresponding heating zone 642. If the determined temperature is above the desired temperature for that heating zone 642, then the microprocessor 29 can reduce the power applied to that heater electrode 64; or if the heating zone 642 is at a lower temperature than that desired, then the microprocessor 29 can increase the power applied to the corresponding heater electrode 64. Any suitable ON / OFF control or PWM (pulse width modulation) control can be used to vary the power applied to the different heater electrodes 64. The microprocessor 29 can select each heater electrode 64 in turn in order to determine the temperature of each heater electrode 64 / heating zone 642. Figure 11b shows another example schematic view of the heater control sub-circuitry 32 of the control circuitry and the way in which the heater electrodes 64 may be connected together and controlled. As shown in Figure 11b, each heater electrode 64 is connected at one end to a power supply and at the other end to a respective switch (in this case a metal-oxide-semiconductor field-effect transistor, MOSFET, switch) 95-1 to 95-3. The switches 95 are controlled by the microprocessor 29. When a heater electrode 64 is to provide heat, the corresponding switch 95 is closed thereby connecting the heater electrode 64 to ground through the resistor R. As a result, current flows from the power supply 21 to ground causing the heater electrode 64 to heat up. The microprocessor 29 can control the position 5 of each switch 95 independently thereby allowing each heater electrode 64 to be powered independently. When the temperature of a selected heating zone is to be determined, the switch 95 of the corresponding heater electrode 64 is closed and all other switches 95 are opened. In this way, the selected heater electrode 64 is provided in series with the resistor R. 10 Since the heater electrodes 64 are formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, the temperature of the heating zone can be determined based on the resistance of the heater electrode 64. The resistance of the heater electrode 64 at room temperature may be, for example, between 0.5 D and 40 Q, although it will be appreciated that the heater electrodes 64 may have any other suitable 15 resistance at room temperature. The heater electrodes 64 may be formed from, for example, copper or stainless steel, although any other suitable electrically conductive material could alternatively be used. As described in more detail below, the resistance (and therefore temperature) of the heater electrode 64 can be determined based on measurements of the attenuated power supply potential, VSUppiy, and the voltage at the output 20 of the current sense amplifier, VCUrrent. If the determined temperature is above the desired temperature for that heating zone, then the microprocessor 29 can reduce the power applied to that heater electrode 64; or if the heating zone is at a lower temperature than that desired, then the microprocessor 29 can increase the power applied to the corresponding heater electrode 64. Any suitable ON / OFF control or PWM (pulse width modulation) control can be 25 used to vary the power applied to the different heater electrodes 64. The microprocessor 29 can select each heater electrode 64 in turn in order to determine the temperature of each heater electrode 64 / heating zone 642. The microprocessor 29 may be a pre-programmed microcontroller unit with in-built timing references, digital outputs, and analogue-to-digital converter (ADC) inputs (e.g., from the 30 operational amplifier 97). The microprocessor 29 may also receive an input signal from a microswitch, or Hall effect sensor and passive magnet, (not shown in the figure) to detect closure / opening of the arms 4a, 4b of the device. A DC-to-DC converter arranged between the microprocessor and the heater electrodes 64, and a VSUppiy monitor input for monitoring the voltage provided by the power supply, are also illustrated. Whilst all of the heater electrodes could be controlled by a single microprocessor 29 via the corresponding set of switches 95, this need not necessarily be the case. Alternatively, a plurality of microprocessors could be provided, each for controlling the operation of a respective set of heater electrodes 64. Advantageously, the use of a plurality of 5 microprocessors 29 (and a corresponding plurality of resistors, R, and operational amplifiers 97) enables the resistance (and therefore temperature) of a plurality of heater electrodes 64 to be measured independently and simultaneously. Advantageously, use of such low thermal mass heaters in hair styling apparatuses as described above allows the temperature of the hair styling apparatus to change extremely 10 quickly when powered; room temperature to normal operating temperature is expected to be achieved in approximately 1s. However, such rapid changes in temperature of the low thermal mass heaters, if not controlled carefully, can cause damage both to the heaters themselves, and the hair styling apparatus overall. For example, without appropriate control of the heating of the low thermal mass heaters they may heat too quickly causing the 15 emission of smoke. It may be understood that the speed of warm up may vary depending on the thermal mass of the low thermal mass heater arrangement. The lower the thermal mass, the faster the warm-up (conversely the higher the thermal mass, the slower the warm-up). It may further be understood that depending on the speed of warm-up, different types of fuses may be appropriate. 20 By way of example only, in the case where very low thermal mass heaters are used in hair styling apparatuses as described above, the time taken for the hair contacting surface to warm-up from an ambient temperature to the normal operating temperature may be in the range of between 0.5 seconds and 5 seconds. In this scenario, a fast-responding fuse such as a melting link-based fuse or a thermistor may be required to provide appropriate safety 25 control of the heater. In the case where low thermal mass heaters are used in hair styling apparatuses as described above, the time taken for the hair contacting surface to warm-up from an ambient temperature to the normal operating temperature may be in the region of between 5 seconds and 15 seconds. In this scenario, a conventional fuse may be used, or fast-responding fuse 30 such as a melting link-based fuse or a thermistor may be used to provide appropriate safety control of the heater. In yet another example, in the case where high thermal mass heaters are used in hair styling apparatuses as described above, the time taken for the hair contacting surface to warm-up from an ambient temperature to the normal operating temperature may be in the region of between 15 seconds and 60 seconds. In this scenario, a conventional fuse may be used to provide appropriate safety control of the heater. Additionally, such rapid increases in their temperature may cause the heaters to burn out after only a small interval of use (e.g., 2-3 s). Furthermore, if the heaters are allowed to 5 overheat, they may also cause damage to the hair styling apparatus itself, for example it may cause melting and / or warping of external parts of the apparatus (i.e., cosmetic damage to the hair styling apparatus may occur). Beneficially, the circuitry used for providing power to the heaters may be designed such that each heater has its own fuse that burns out if the heater overheats, thereby preventing 10 catastrophic damage. However, relying on fuses is nevertheless sub-optimal as each fuse will need changing when it burns out which is fiddly and time consuming for an end user. Furthermore, such fuses only prevent catastrophic damage and only address malfunctions of the hair styling apparatus in the most extreme overheating scenarios. The present invention disclosed herein provides a hair styling apparatus with microcontroller 15 units with processors that are configured to collect status data of the hair styling apparatus (e.g., heater temperatures, voltages / currents across heaters, data associated with the status of communication links of the apparatus, and the like). That collected status data is in turn used to determine when different functions of the hair styling apparatus are malfunctioning and responding accordingly. For example, in response to determining that a function of the 20 hair styling apparatus is malfunctioning power to the heaters may be removed to prevent damage. Furthermore, the heaters may not be powered again until the malfunction has been addressed. Beneficially, by providing the hair styling apparatus with means to obtain and assess status data of the hair styling apparatus to identify malfunctions and respond accordingly, the low 25 thermal mass heaters in hair styling apparatus can be controlled to prevent them overheating and causing catastrophic damage to the hair styling apparatus. There now follows a description of the means to obtain and assess status data of hair styling apparatuses to identify malfunctions and how the apparatus responds to prevent instances of catastrophic damage. 30 Heating Control Modes As previously described, any suitable ON / OFF control such as PWM or PDM control can be used to vary the power applied to the different heater electrodes 64 (and thus heaters 64-1, 64-2, 64-3). The microprocessor 29 can select each heater electrode 64 in turn in order to determine the temperature of each heater electrode 64 / heating zone 642, Z1 to Z7. Where any suitable ON / OFF control such as PWM or PDM control is used to vary the power applied to the different heater electrodes 64, the hair styling device 1 may be configured to 5 be operable in different heating modes. For example, the hair styling device 1 may be configured to be operable in an ‘Active’ / ’ON’ mode, or an ‘Idle’ mode. In the Active / ON mode, the suitable ON / OFF control or PWM / PDM control may switch the power applied to the different heater electrodes 64 on and off at a rate of 1 Hz for a predetermined period of time. Following that pre-determined period of time, the suitable 10 ON / OFF control or PWM / PDM control is then modulated such that the power applied to the different heater electrodes 64 is switched on for a larger ‘firing’ window (e.g., for a period of approximately 50 ms) such that the heater electrodes 64 are on for most of the time. This may be repeated indefinitely while in the Active / ON mode. It will be appreciated however that in such an Active / ON mode there is a risk of the heater 15 zones 642, Z1 to Z7, of the hair styling device 1, which are heated by the heater electrodes 64, overheating, especially if faults occur in the operation of the suitable ON / OFF control or PWM / PDM control mechanism. Such overheating may cause damage to the heaters, and / or the hair styling device 1. Furthermore, there is also a risk that such overheating may be dangerous to a user. 20 In the Idle mode, the suitable ON / OFF control or PWM control may only switch the power applied to the different heater electrodes 64 on and off at a rate of 1 Hz. In the Idle mode the rate at which power is switched on and off to the heater electrodes 64 is such that the heater electrodes 64 cannot overheat. Control mechanisms of the heaters 64-1, 64-2, 64-3 in the heater zones 642, Z1 to Z7, of the 25 hair styling device 1 will now be described with reference to Figure 12. Heating Control Mechanisms Internal Malfunctions and Control Figure 12 shows a schematic view of the microcontroller units (MCU), software, and other hardware that is implemented in the hair styling device 1 to allow control of the heaters 64-1, 30 64-2, 64-3 in the heater zones 642, Z1 to Z7, of the hair styling device 1. By way of example only, as shown in Figure 12, there is provided a leader MCU 602a and a follower MCU 602b that are in communication with one another via communication links 604a, 604b. It will be appreciated that although Figure 12 only depicts one follower MCU 602b, there may be any number of follower MCUs that are in communication with the leader 5 MCU. Hereafter components of Figure 12 related to the leader MCU 602a will be referred to as ‘Arm O’, while components of Figure 12 related to the follower MCU 602b will be referred to as ‘Arm T. As shown in Figure 12, in Arm 0 there is also provided an MCU watchdog 608a in communication with the leader MCU 602a. The MCU watchdog 608a is configured to run a 10 Communications (‘Comms’) activity watchdog function 606a. The MCU watchdog function 608a includes, but is not limited to, a Zone temperature (‘temp’) hard limit monitor subfunction 620. Additionally, the leader MCU 602a is configured to run a Set_heater() function 610a and / or a Kill_heater() function 612a. The Set_heater() function 610a includes a zone refresh timeouts 15 function 614a that triggers the operation of digital input / output (DIO) lines 616a to control MOSFETs 95-1 and thus heaters 64-1. The Kill_heater() function 612a also include the zone temperature (‘temp’) hard-limit monitor sub-function 620, as well as an analogue out-of-range monitor function 622. Furthermore, as shown in Figure 12, in Arm 1 there is provided an MCU watchdog 608b in 20 communication with the follower MCU 602b, and the MCU watchdog 608b is configured to run a Communications (‘Comms’) activity watchdog function 606b. Additionally, the follower MCU 602b is configured to run a Set_heater() function 610b and / or a Kill_heater() function 612b. The Set_heater() function 610b includes a zone refresh timeouts 614b function that triggers the operation of DIO lines 616b to control MOSFETs 95-25 2 and thus heaters 64-2. Each component and function in the schematic shown in Arm 0 in Figure 12 will now be described in more detail. The leader MCU 602a, which is a microcontroller, may include a processor, memory, and input / output (I / O) peripherals on a single chip. The leader MCU 602a is configured to govern 30 specific operations of the hair styling device 1. In particular the leader MCU 602a is configured to directly control heaters 64-1 of the hair styling device 1. The leader MCU 602a may also be configured to indirectly control other heaters that are directly controlled by a follower MCU 602b e.g., heaters 64-2. It will be appreciated that there may be any number of follower MCUs controlling any number of heaters, and the follower MCUs and leader MCU 602a will be in communication with one another via an appropriate communication link 604a, 604b to allow indirect control of those heaters by the leader MCU 602a. The leader MCU 602a is configured to receive from a MCU watchdog function 608a, status 5 indications of the hair styling device 1. For example, the MCU watchdog function 608a may comprise a watchdog timer, sometimes called a ‘computer operating properly’ timer (COP timer), which may be an electronic or software timer that is used to detect and recover from malfunctions. Example 1: Communications malfunction 10 In one example, the malfunction may be a failure in communication between the leader MCU 602a and follower MCUs 602b. The communications between the leader MCU 602a and follower MCUs 602b may, for example, be monitored by the Comms activity watchdog function 606a. Upon Comms activity watchdog function 606a detecting that communication between the leader MCU 602a and follower MCUs 602b is experiencing issues, or has 15 completely failed, then the Comms activity watchdog function 606a may send an appropriate message to the MCU watchdog function 608a. The MCU watchdog function 608a may in turn send an appropriate message to the leader MCU 602a, which may trigger a reset of both the leader MCU 602a and follower MCUs 602b. For example, the leader MCU 602a may send a command message to the follower MCU 20 602a to trigger the reset at the follower MCU 602b. The reset may for example be a hardware reset, a software reset, or a combination of the two to completely reset the hardware and software of the hair styling device 1 to re-establish communication between the leader MCU 602a and follower MCUs 602b. As part of the reset the leader MCU 602a and / or the follower MCUs 602b may be configured 25 to trigger their respective Kill_heater() functions 612a, 612b to ‘kill’ (i.e., switch off) the heaters 64-1,64-2 to prevent damage to the heaters 64-1, 64-2 and the hair styling device 1. For example, once triggered, kill_heater() functions 612a, 612b may be configured to trigger the Set_heater() functions 610a, 610b respectively to switch the impedance of the DIO lines 616a, 616b (i.e., MOSFET gate drives) of the MOSFETs 95-1, 95-2 to high impedance. By 30 switching the DIO lines 616a, 616b to high impedance a current is prevented from flowing to the MOSFET gate drives thereby switching off the MOSFET gates. As the MOSFET gates are switched off, current cannot flow between the sources and drains of the MOSFETs, and thus a current cannot flow to the heaters 64-1, 64-2. It will be appreciated that the DIO lines 616a, 616b may each be provided with their own Pull-down (PD) resistor that draws voltage of the DIO lines 616a, 616b to ground when the MOSFET gates are switched off. Additionally, or alternatively, as part of the reset, the heating control mode of the hair styling device 1 may be switched from Active mode to Idle mode. 5 Furthermore, the reset may be performed repeatedly on a loop (e.g., a ‘while’ loop) until the MCU watchdog 606a indicates to the leader MCU 602a that communication between the leader MCU 602a and follower MCUs 602b is re-established and operating correctly. It will be appreciated that the follower MCU 602b may also be configured to transmit to the leader MCU 602a, status indications of the hair styling device 1 where appropriate. For 10 example, the MCU watchdog function 608b, like the MCU watchdog function 608a, may comprise a watchdog timer which may be an electronic or software timer that is used to detect and recover from malfunctions. For example, the communications between the leader MCU 602a and follower MCUs 602b may, for example, also, or alternatively, be monitored by the Comms activity watchdog function 606b of Arm 1, which may be a sub-15 function of the MCU watchdog function 608b of Arm 1. Upon Comms activity watchdog function 606b detecting that communication between the leader MCU 602a and follower MCUs 602b is experiencing issues, or has completely failed, then the Comms activity watchdog function 606b may send an appropriate message to the MCU watchdog function 608b. The MCU watchdog 608b may in turn send an appropriate 20 message to the follower MCU 602b, which may trigger a reset of both the leader MCU 602a and follower MCUs 602b. For example, the follower MCU 602b may send a command message to the leader MCU 602a to trigger a reset. The reset may for example be a hardware reset, a software reset, or a combination of the two as previously described above. In both of the examples above, the Comms activity watchdog function 606a and / or the 25 Comms activity watchdog function 606b may detect that communication between the leader MCU 602a and follower MCUs 602b is experiencing issues, or has completely failed, through the use of a ‘heartbeat’ messaging protocol. For example, the follower MCUs 602b may be configured to send a ‘heartbeat’ message ( / .e., a periodic signal generated by follower MCU 602b to indicate normal operation) periodically (e.g., every 0.1 s) to the leader 30 MCU 602a. Upon receipt, the ‘heartbeat’ message may be processed by the leader MCU 602a to determine that communication between the leader MCU 602a and follower MCUs 602b is maintained and operating correctly. The processing by the leader MCU 602a may merely comprise determining that communication between the MCUs is operating correctly by virtue of the message being received. Alternatively, the processing by the leader MCU 602a, may involve processing information contained in the message. If however the message is not received by and / or processed by leader MCU 602a within a predetermined period (e.g., 500 ms), then the leader MCU 602a may determine that 5 communication between the MCUs has failed. In response to determining that the communication between MCUs has failed, the leader MCU 602a may be configured to run the Kill_heater() function 612a described in more detail above to kill the heaters of the hair styling device 1. It will be appreciated that the leader MCU 602a may also be configured to trigger the follower MCU 602b to run its corresponding Kill_heater() function 612b. 10 Additionally, or alternatively the Comms activity watchdog function 606b may detect that communication between the leader MCU 602a and follower MCUs 602b is experiencing issues, or has completely failed, through the use of a ‘heartbeat’ messaging protocol. For example, the leader MCUs 602a may be configured to send a ‘heartbeat’ message periodically (e.g., every 0.1 s) to the follower MCU 602b. Upon receipt, the ‘heartbeat’ 15 message may be processed by the follower MCU 602b to determine that communication between the leader MCU 602a and follower MCUs 602b is maintained and operating correctly. The processing by the follower MCU 602b may merely comprise determining that communication between the MCUs is operating correctly by virtue of the message being received. Alternatively, the processing by the follower MCU 602b, may involve processing 20 information contained in the message. Alternatively, or additionally the leader MCUs 602a may be configured to send instruction messages to the follower MCU 602b periodically (e.g., every 0.5 ms), which may, by way of example only, be a single-byte instruction message. The instruction message may be an appropriate command message that instructs the follower MCU 602b to turn specific heaters 25 under the control of the follower MCU 602b on / off by running the Set_heater() 610b. The instruction message may also include a request for a response message from the follower MCU 602b. For example, the command message periodically sent to the follower MCU 602b may include a request for a floating message that reports to the leader MCUs 602a, the voltage and / or current data associated with heaters under the control of the follower 30 MCU 602b. If however the ‘heartbeat’ message or instruction message is not received by and / or processed by follower MCU 602b within a predetermined period (e.g., 600 ms), then the follower MCU 602b may determine that communication between the MCUs has failed. In response to determining that the communication between MCUs has failed, the follower MCU 602b may be configured to run the Kill_heater() function 612b to kill the heaters of the hair styling device 1. It will be appreciated that the follower MCU 602b may also be configured to trigger the leader MCU 602a to run its corresponding Kill_heater() function 5 612a e.g., by sending the leader MCU 602a an appropriate message indicating that it should run its corresponding Kill_heater() function 612a. Additionally, or alternatively, where the instruction message includes a request for a response message from the follower MCU 602b, in the event that the leader MCU 602a does not receive the requested response message from the follower MCU 602b at least 10 once, the leader MCU 602a may assume that communication between the MCUs has failed. Alternatively, in the event that the leader MCU 602a does not receive the requested response message from the follower MCU 602b after a pre-configured number of instruction message have been sent to the follower MCU 602b, the leader MCU 602a may assume that communication between the MCUs has failed. 15 In response to determining that the communication between MCUs has failed because no response message has been received from the follower MCU 602b, the leader MCU 602a may be configured to run the Kill_heater() function 612a to kill the heaters of the hair styling device 1. It will be appreciated that the leader MCU 602a may also be configured to trigger the follower MCU 602b to run its corresponding Kill_heater() function 612b e.g., by sending 20 the follower MCU 602b an appropriate message indicating that it should run its corresponding Kill_heater() function 612b. Example 2: Malfunction of the heaters In another example, the malfunction may be that heaters 64-1, 64-2 of the hair styling device 1 are not switched off at the correct time. For example, the MCU watchdog function 608a 25 may be configured to monitor the heaters 64-1 periodically (e.g., less than every 0.5 seconds) to determine whether the PWM operation of the heaters 64-1 is being performed correctly (e.g., power to the heaters 64-1 is being PW modulated such that the heaters 64-1 are able to heat up / maintain a specific temperature, without overheating and / or experiencing a rapid change in electrical resistance). If the MCU watchdog function 608a detects that the 30 heaters 64-1 are not being operated correctly, then the MCU watchdog function 608a may send an appropriate message to the leader MCU 602a, which may trigger a reset procedure. For example, the leader MCU 602a may trigger the Kill_heater() function 612a to ‘kill’ (i.e., switch off) the heaters 64-1 to prevent damage to the heaters 64-1 and the hair styling device 1. Additionally, the leader MCU 602a may also send an appropriate message over communication link 604a to the follower MCU 602b to instruct the follower MCU 602b to trigger its Kill_heater() function 612b to ‘kill’ (i.e., switch off) the heaters 64-2 under the control of the follower MCU 602b. The reset may for example be a hardware reset, a software reset, or a combination of the two as previously described above. Additionally, or alternatively, as part of the reset, the heating control mode of hair styling device 1 may be switched from Active mode to Idle mode. Furthermore, the reset may be performed repeatedly on a loop (e.g., a ‘while’ loop) until the MCU watchdog 606a indicates to the heaters 64-1 are operating correctly. It will be appreciated that the MCU watchdog function 608b may also be configured to monitor the heaters 64-2 periodically in a similar manner and may also send an appropriate message to the follower MCU 602b if the MCU watchdog function 608b detects that the heaters 64-2 are not being operated correctly. Similarly, to the scenario described above, the follower MCU 602b may then trigger its Kill_heater() function 612b, and where appropriate send a message to the leader MCU 502a to request it also trigger is corresponding Kill_heater() function 612a. Example 3: Malfunction of the temperature measurement circuitry In another example, the malfunction may be that the temperature measurement circuitry 25 of the control circuitry 15 of the hair styling device 1 is malfunctioning such that an accurate temperature of the heaters 64-1, 64-2 cannot be determined. For example, as described above, the temperature measurement circuitry 25 may comprise temperature sensors such as thermistors or, it may use circuitry that senses the resistance of heater electrodes that are used to heat the heaters 64-1, 64-2 whose resistance depends on the temperature of the heater electrode. Part of the determination of the temperature of the heaters 64-1, 64-2 includes analogue outputs such as resistance and electrical power measurements that are passed through an ADC (e.g., an operational amplifier 97) which outputs digital signals that may be passed on to the microprocessor 29 and / or leader MCU 602a for analysis and to determine the temperature of the heaters 64-1, 64-2. The determination of the temperature of the heaters 64-1, 64-2 may be performed by a calc_temp function(). For example, upon the digital signals being passed on to the leader MCU 602a for analysis, the leader MCU 602a may be configured to trigger a calc_temp function() (not shown) that converts voltage and / or current signals / data associated with heaters 64-1, 64-2 to temperature values. The calc_temp function() may use voltage and / or current signals / data to determine temperatures of individual heaters 64-1, 64-2. Alternatively, or additionally, the calc_temp functionQ may use voltage and / or current signals / data to determine temperatures of specific heating zones 642, Z1 to Z7, of the hair styling device 1 associated with specific heaters. If a voltage and / or a current reaches the ADC’s limit and the voltage and / or the current are out-of-range and cannot be measured by the hardware, any temperature calculations made may be invalid i.e., the temperature measurement circuitry 25 malfunctions. To detect such malfunctions of the temperature measurement circuitry 25, an analogue out-of-range monitor function 622, or some other appropriate function, may periodically monitor the voltage and / or current levels passing through the heaters 64-1, 64-2, and the ADC. Where it is detected that the voltage and / or current levels reach the voltage and / or current level limits of the ADC, an error flag may be triggered and the analogue out-of-range monitor function 622 or some other appropriate function, may send an appropriate message directly to the Kill_heater() function 612a to trigger the function and kill the heaters 64-1 to prevent damage to the heaters 64-1 and the hair styling device 1. Where analogue out-of-range monitor function 622 or some other appropriate function, sends an appropriate message directly to the Kill_heater() 612a, the analogue out-of-range monitor 622 or some other appropriate function may additionally send the same, or another appropriate message to the leader MCU 602a, which may trigger a reset procedure. That reset may, for example, be a hardware reset, a software reset, or a combination of the two to completely reset the hardware and software of the hair styling device 1 as previously described. For example, as part of the reset, once triggered, Kill_heater() 612a, 612b functions may be configured to trigger the Set_heater() 610a, 610b functions respectively to switch the impedance of the DIO lines 616a, 616b (i.e., MOSFET gate drives) of the MOSFETs 95-1, 95-2 to high impedance. By switching the DIO lines 616a, 616b to high impedance a current is prevented from flowing to the MOSFET gate drives thereby switching off the MOSFET gates. As the MOSFET gates are switched off, current cannot flow between the sources and drains of the MOSFETs, and thus a current cannot flow to the heaters 64-1, 64-2. It will be appreciated that the DIO lines 616a, 616b may each be provided with their own PD resistor that draws voltage of the DIO lines 616a, 616b to ground when the MOSFET gates are switched off. Additionally, or alternatively, as part of the reset, the heating control mode of hair styling device 1 may be switched from Active mode to Idle mode. Furthermore, the reset may be performed repeatedly on a loop (e.g., a ‘while’ loop) until the analogue out-of-range monitor 622, or some other appropriate function, indicates that the voltage and / or a current levels are below the voltage and / or current level limits of the ADC. It will be appreciated that the analogue out-of-range monitor function 622 may be configured to monitor all heaters 64-1, 64-2 of the hair styling device 1 irrespective of the Arm in which it is shown in Figure 12. For example, there may be an analogue out-of-range monitor 622 function in each Arm (not shown), alternatively there may be a single analogue out-of-range 5 monitor 622 function that is in communication with the Kill_heater() 612a, 612b function of each Arm. Example 4: Malfunction of the drive circuitry In another example, the malfunction may be that the drive circuitry 23 of the control circuitry 15 of the hair styling device 1 is malfunctioning such that heaters 64-1, 64-2 begin to 10 overheat and / or become dangerously hot. For example, the PWM signal used to switch the heater electrodes on and off in a cyclic manner may fail, and instead the heater electrodes may be turned permanently on, leading to overheating of the heaters 64-1, 64-2. Such overheating made cause damage to the heaters 64-1, 64-2 as well as the hair styling device 1 in general. Additionally, if the heaters 64-1, 64-2 become too hot a user is at risk of being 15 burnt. To detect such malfunctions of the heaters 64-1, 64-2, the zone temp hard-limit monitor function 620 may periodically monitor the voltage and / or current levels passing through the heaters 64-1, 64-2 (or some other measure such as electrical power) to determine whether the heaters 64-1, 64-2 are operating at too high a temperature to be safe. 20 The determination of the temperature of the heaters 64-1, 64-2 may be performed by a calc_temp function(). For example, upon the digital signals being passed on to the leader MCU 602a for analysis, the leader MCU 602a may be configured to trigger a calc_temp function() that coverts voltage and / or a current signals / data associated with heaters 64-1, 64-2 to temperatures values. The calc_temp function() may use voltage and / or current 25 signals / data to determine temperatures of individual heaters 64-1, 64-2. Alternatively, or additionally, the calc_temp function() may use voltage and / or current signals / data to determine temperatures of specific heating zones 642, Z1 to Z7, of the hair styling device 1 associated with specific heaters 64-1, 64-2. Where it is detected that the voltage and / or current levels (or some other measure such as 30 electrical power) reach a level such that the heaters 64-1, 64-2 are considered to be operating at too high a temperature to be safe (e.g., 260 °C), the Zone temp hard-limit monitor function 620 may send an appropriate message directly to the Kill_heater() function 612a to trigger the function and kill the heaters 64-1 to prevent damage to the heaters 64-1 and the hair styling device 1, and to prevent a user from being burnt Where appropriate the Zone temp hard-limit monitor function 620 may also send an appropriate message to the MCU watchdog 608a, which may in turn inform the leader MCU 602a of the malfunction. It will be appreciated that the leader MCU 602a may in turn communicate with the follower 5 MCU 602b to trigger its corresponding Kill_heater() function 612b. Alternatively, or additionally, where it is detected that the voltage and / or current levels (or some other measure such as electrical power) reach a level such that the heaters 64-1, 64-2 are considered to be operating at too high a temperature to be safe (e.g., 250 °C), the Zone temp hard-limit monitor function 620 may send an appropriate message the leader MCU 10 602a, which may trigger a reset procedure such as the reset procedure previously described. Additionally, or alternatively, as part of the reset, the heating control mode of hair styling device 1 may be switched from Active mode to Idle mode. Furthermore, the reset may be performed repeatedly on a loop (e.g., a ‘while’ loop) until the Zone temp hard-limit monitor 15 620 function, and / or the MCU watchdog 608a, or some other appropriate function, indicates that the voltage and / or a current level have reached / returned to a level such that the heaters 64-1, 64-2 are considered to be operating at a safe temperature.lt will be appreciated that the Zone temp hard-limit monitor function 620 may be configured to monitor all heaters 64-1, 64-2 of the hair styling device 1 irrespective of the Arm in which it is shown in Figure 12. For 20 example, there may be a Zone temp hard-limit monitor function in each Arm (not shown), alternatively there may be a single Zone temp hard-limit monitor 620 function that is in communication with the Kill_heater() 612a, 612b function of each Arm. Example 5: Time-out In both Arm 0 and Arm 1, when heaters of the hair styling device 1 are to be switched on, the 25 leader MCU 602a and the follower MCU 602b are configured to trigger a respective Set_heater() 610a, 610b function. Each respective Set_heater() 610a, 610b function may be configured to trigger a zone refresh timeouts sub-function 614a, 614b which may comprise a watchdog timer, sometimes called a COP timer (e.g., an electronic or software timer). Each zone refresh timeouts sub-function 614a, 614b may monitor, periodically (e.g., every 30 millisecond), the on / off status of heaters 64-1, 64-2 for which it is responsible up to a maximum time out period (e.g., 100 ms) to determine whether the corresponding heaters 64-1, 64-2 are on or off. Where it is determined that a heater is off, the corresponding zone refresh timeouts sub-function 614a, 614b may be reset (‘kicked’) such that the watchdog timer is restarted. If, however, the zone refresh timeouts sub-function 614a, 614b continuously determines that its corresponding heaters are on for a pre-defined maximum period (e.g., 100 ms), then the respective zone refresh timeouts sub-functions 614a, 614b may trigger to switch the heaters 64-1, 64-2 off to prevent overheating and damage of the 5 heaters and / or the hair styling device 1. Figure 13 illustrates a tabular example summary of the malfunctions that may occur with the hair styling device 1 and the possible mitigations that may be implemented. The malfunctions and possible mitigations listed therein are described in more detail above with reference to Figure 12. 10 External Malfunctions and Control In addition to the control mechanism described above, where the hair styling device 1 is configured to communicate with external computing devices 702 such as a smart device (e.g., a smart phone), or other such devices with computer processing capabilities, control mechanisms are needed in the event communication malfunctions between the hair styling 15 device 1 and the external computing devices 702. Figure 14 illustrates a hair styling device 1 in wireless communication with an external computing device 702 (e.g., a smart phone). As shown in Figure 14, there is a hair styling device 1 in wireless communication 704 with an external computing device 702, however it will nevertheless be appreciated that the 20 communication 704 between the hair styling device 1 and the external computing device 702 may be via a wired communication link. The external computing device 702 may, for example, be in communication with the hair styling device 1 to apply settings of the hair styling device 1 and / or to facilitate software and firmware updates of the hair styling device 1. 25 Additionally, the external computing device 702 may be used by users of the hair styling device 1 to control and adjust settings of the hair styling device 1. For example, in some scenarios, rather than using a user interface of the hair styling device 1, a user may use an interface of the computing device 702 to input settings and to control the hair styling device 1. 30 In such scenarios, it will be appreciated that it is important to maintain communication between the hair styling device 1 and the external computing device 702, and to provide mitigations in the event that such communication is lost. In one example, as shown in Figure 14, the communication between the hair styling device 1 and the external computing device 702 may include the transmission of an appropriate message or command 704a from the external computing device 702 to the hair styling device 1 to configure, and set settings of, the hair styling device 1. The message or 5 command 704a may be processed by the leader MCU 602a, or any other appropriate processing unit of the hair styling device 1. The message or command 704a may include a ‘SETTINGS’ component comprising a signature (e.g., a pre-defined set of characters, or bits) that correspond to a specific setting that the external computing device 702 wishes to set for the hair styling device 1. Additionally, the message or command 704a may include any 10 appropriate error detection component (e.g., a Checksum component) to assist the hair styling device 1 in detecting whether the ‘SETTINGS’ component is correct ( / .e.. it relates an appropriate setting for the hair styling device 1). In the event that the hair styling device 1 detects that the ‘SETTINGS’ component is correct, then the hair styling device 1 will apply the setting being requested in the message or 15 command 704a. On the other hand, in the event that the hair styling device 1 detects that the ‘SETTINGS’ component is incorrect, the hair styling device 1 will not apply the setting being requested in the message (MsgO) or command 704a. Additionally, where the hair styling device 1 detects that the ‘SETTINGS’ component is incorrect, the hair styling device 1 may send an appropriate response message to the external computing device 702 indicating 20 that the settings have not be applied. In another example, as shown in Figure 14, the communication between the hair styling device 1 and the external computing device 702 may include the transmission of an appropriate message (Msg1, 2, 3 etc.,) 704b from the external computing device 702 to the hair styling device 1 that is used to assess the connection between the hair styling device 1 25 and the external computing device 702. The external computing device 702 may send a message 704b to the hair styling device 1 periodically (e.g., every 0.1 s) to assess whether the communication link between the hair styling device 1 and the external computing device 702 is functioning correctly. For example, the message 704b may be a ‘heartbeat’ message (i.e., a periodic signal generated by external computing device 702 to indicate normal 30 operation). Upon receipt, the message 704b may be processed by the leader MCU 602a, or any other appropriate processing unit of the hair styling device 1 to determine that the communication link 704 between the hair styling device 1 and the external computing device 702 is maintained and operating correctly. The processing by the leader MCU 602a, or any other 35 appropriate processing unit may merely comprise determining that the communication link 704 is operating correctly by virtue of the message 704b being received. Alternatively, the processing by the leader MCU 602a, or any other appropriate processing unit may involve processing information contained in the message 704b. If however the message 704b is not received by the hair styling device 1 and / or processed by leader MCU 602a, or any other appropriate processing unit of the hair styling device 1 within a predetermined period (e.g., 500 ms), then the leader MCU 602a, or the other appropriate processing unit may determine that the communication link 704 between the hair styling device 1 and the external computing device 702 has failed. In response to determining that the communication link 704 between the hair styling device 1 and the external computing device 702 has failed, the leader MCU 602a may be configured to run the Kill_heater() function 612a described above to kill the heaters of the hair styling device 1. Modifications and Alternatives Detailed embodiments have been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. By way of illustration only a number of these alternatives and modifications will now be described. For the purposes of simplicity, the above description focuses on hair care products and particularly a hair styling device. However, it will be appreciated that the above-described concepts may be applied widely to any hair styling product and / or other beauty product I devices in the beauty industry, including, for example: hair dryers, curling tongs / wands, hair straighteners, nail gel / varnish curers (such as UV lamp systems for the curing of nail varnish), skin epilators, hair colouring devices, crimpers, etc. In the above embodiments, the beauty product device communicated with a smart processing device. This is not essential - the beauty product device may be provided with all the processing functionality of the processing device 3. Further, the processing of the sensor data could also be communicated to a central server system or cloud for processing with the result being fed back to the devices for user direction and communication. In the above examples, the feedback messages were transmitted I provided to the user by way of the user interface on the beauty product device or on the processing device. This is not essential. The feedback messages may be provided by any suitable user interface of any nearby device. For example, they may be sent to an Amazon Echo speaker device for playout to the user as voice messages or displayed to the user on a television screen or the like. The method of communication between the hair styler and the processing device could be via a cable or wireless means. Examples of applicable wireless communications include 5 Bluetooth, Wi-Fi, LoRa, ZigBee, 802.15 standard, NFC, or optical means - both visible and IR. In the examples given above, various specific temperatures and power levels were discussed. As those skilled in the art will appreciate, all these specific values are clearly not essential to the invention and the particular values used in a given product will depend on 10 the treatment to be given, the voltage sources used etc. An important aspect of determining how the product is being used and then feeding back usage improvements to the user is the processing of the sensor data by the microprocessor of the controller. In the above examples, this processing was based on pure deterministic comparison algorithms and rule-based logic or inference. Instead, such algorithms could be 15 based upon artificial intelligence (Al) techniques, such as forward and back propagation neural networks, deep learning, fuzzy logic etc. In this case feedback from the user as to whether the feedback messages helped to create the style the user wanted to achieve can be used to train the Al model. In the above embodiments, a number of software modules were described. As those skilled 20 in the art will appreciate, the software modules may be provided in compiled or un-compiled form and may be supplied to the hair styler or the corresponding processing device (mobile telephone and / or the like) as a signal over a computer network, or on a recording medium. Further, the functionality performed by part or all of this software may be performed using one or more dedicated hardware circuits. However, the use of software is preferred as it 25 facilitates the updating of the beauty product device (and the processing device). Various other modifications will be apparent to those skilled in the art and will not be described in further detail here. The invention has been described above by way of implementation in a hair styling device for straightening hair (‘hair straighteners’) which employ flat hair styling heaters 6. However, 30 it could alternatively be implemented in any form of hair styling device, such as (but not limited to) crimpers, curlers, or heated brushes. The heaters 6 may define a heating surface that is flat, curved, ridged or in the shape of a barrel. The hair styling device may have two arms like the device illustrated in Figure 1 or it may be a single armed device. In the above embodiments, MOSFET switches were used to control powering and sensing of the heater electrodes. As those skilled in the art will appreciate, other switches could be used instead. For example, Field Effect Transistors (FETs) could be used, such as Gallium Nitride FETs or bipolar junction transistors (BJTs). 5 The switching device can be placed in the high or low side of the power supply to the heaters. In Embodiments 4 and 5 described above, the heater electrodes 64 were used for heating and temperature sensing. In alternative embodiments, separate temperature sensors may be provided for sensing the temperature of each heating zone defined by the individual 10 heater electrodes 64. For example, referring to Figure 2, a separate layer of temperature sensors may be provided under dielectric layer 66 or on top of the layer 62. In the above embodiments, a DC power source was used to provide electrical power for heating the heater electrodes 64. This DC power source will typically be a battery, although DC supplies that derive their power from a mains power AC signal may be used. In 15 embodiments where separate temperature sensors are provided, then AC mains power may be used to heat the heater electrodes. Thicker dielectric layers may be provided in this case between the heater electrodes 64 and the hair contacting surface of the hair styler. In an alternative to Embodiment 4 (implementing a dual microprocessor system), the second, separate microprocessor may be chosen to be not identical to the first one. This 20 can be a deliberate choice to avoid the possibility of duplicated errors. In some possible implementations, the firmware might also be developed by separate teams, to minimize further the possibility of any duplicated firmware errors. The embodiments described above may be used alone or in any combination. For example, 25 a safety solution provided by electronic means (such as Embodiments 2 and 3) may be provided in combination with microprocessor-based safety strategies (such as Embodiments 4 and 5). In the hardware-only Embodiments 1-3, the protection circuitry output can be monitored by the main CPU, so although software is not involved directly in the over-temperature 30 protection, firmware can provide additional desired functionality e.g. logging of the fault to non-volatile memory for later retrieval, or if the manufacturer desires an over-temperature fault to cause permanent disconnection of the heaters, a logged over-temperature fault stored in non-volatile memory can cause the MCU firmware to go into permanent fault mode even after power cycling. It should be understood that the permitted response time is dependent on the heat-up rate, maximum permitted temperature, Tmax, and trigger temperature, Ttrigger. By way of example, for a heat up rate of 410°Cs’1, a Tmax of 250 °C and a Ttrigger set at 240°C, a (maximum) response time of 0.024 seconds is required. By way of a further example, for up rate of 400°Cs'1, a Tmax of 300 °C and a Ttrigger set at 240°C, a (maximum) response time of 0.015 seconds is required. It should also be noted that the solder link may be defined by its thickness rather than weight. For example, the required thickness of the solder link can be determined from the equation: Where Az is the solder film thickness; Q is the energy flux; t is the permitted response time; L is the latent heat of fusion; and p is the density. In an example, the available heater flux is 20 W / cm2, and the permitted response time is 0.3 s (as discussed above). If a tin-based solder is used, the latent heat of fusion is 60 kJ / kg and the density is 7,300 kg / m3. Accordingly, for this example, a solder film thickness, Az, of less than 0.011 mm might be required to achieve the required response time. In order to account for the possibility that not all of the available heater flux passes into the solder link, an even smaller thickness of solder link is preferably implemented. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims. No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers, or steps. EXAMPLES 1. A hair drying and / or styling appliance comprising a multilayer heater comprising a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer; wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm; and wherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness is less than 15 W / m.Kand greater than 0.1 W / m.K. 2. The appliance according to any of examples 1, wherein the heater has a power density that is greater than 2 W / cm2 and less than 100 W / cm2, preferably greater than 8 W / cm2. 3. The appliance according to any of examples 1 to 2, wherein the heater electrode layer comprises a plurality of independently powerable heater electrodes that define a corresponding plurality of heating zones on a heating surface of the multilayer heater. 4. The appliance according to example 3, wherein the combined thermal conductivity of the multilayer heater in the plane perpendicular to the thickness is measured along a line that passes through adjacent heating zones. 5. The appliance according to example 3 or 4, wherein a maximum size of each heating zone depends upon the power density of the multilayer heater, the thickness of the multilayer heater and a lateral conductivity of the multilayer heater. 6. The appliance according to example 5, wherein the maximum size of each heating zone further depends on a maximum permissible temperature difference between different parts of the heating zone in the case where a heating zone is partially loaded with hair. 5 7. The appliance according to any of examples 3 to 6, wherein the multilayer heater further comprises a heat spreading layer provided over the upper dielectric layer, the heat spreading layer comprising a plurality of heat spreaders that regularise the heating provided within the heating zones. 10 8. The appliance according to example 7, wherein each heat spreader is formed as an island that does not touch neighbouring heat spreaders to reduce heat spreading from one heating zone to an adjacent heating zone. 9. The appliance according to example 7 or 8, wherein each heat spreader is formed of 15 a metal. 10. The appliance according to any of examples 7 to 9, wherein the heat spreaders are separated from each other in the plane perpendicular to the thickness by a solid or semisolid material, whose thermal conductivity is lower than 35 W / mK and most preferably lower 20 than 0.3 W / mK. 11. The appliance according to any of examples 1 to 10, wherein the layers of the multilayer heater are bonded together to have a peel strength of at least 0.35 Newtons per mm. 25 12. The appliance according to any of examples 1 to 11, wherein the multilayer heater further comprises one or more of: i) a low friction coating an upper surface of which provides a hair contacting surface of the multilayer heater; 30 ii) a lower dielectric layer provided under the heater electrode layer; and iii) an auxiliary heater electrode layer comprising one or more heater electrodes provided below the heater electrode layer and a dielectric layer provided between the heater electrode layer and the auxiliary heater electrode layer. 35 13. The appliance according to any of examples 1 to 12, wherein one or more layers of the multilayer heater are bonded together using an adhesive or using heat bonding or using physical vapour deposition or using screen printing or another coating process. 14. The appliance according to any of examples 1 to 13, wherein one or more of the dielectric layers comprises polyimide. 15. The appliance according to any of examples 1 to 14, wherein the multilayer heater is flexible and is bonded to a rigid structure to provide the multilayer heater with rigidity. 16. The appliance according to any of examples 1 to 15, wherein the multilayer heater has a flat, curved and / or ribbed heating surface. 17. The appliance according to any of examples 1 to 16, wherein the multilayer heater provides a flat heating surface and has curved edges that provide a curved heating surface. 18. The appliance according to any of examples 1 to 17, further comprising a controller configured to control the application of power to the multilayer heater to control the heat produced by the multilayer heater. 19. The appliance according to any of examples 1 to 18, wherein the appliance is a single arm or a two arm device. 20. A method of making a hair drying and / or styling appliance, the method comprising: providing a multilayer heater having a plurality of functional layers that are bonded together; mounting the multilayer heater in the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating; wherein providing the multilayer heater includes: providing a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and providing at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer from the hair contacting surface; wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm; and wherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness that is less than 15 W / m.K and greater than 0.1 W / m.K. FURTHER EXAMPLES 1a. A hair drying and / or styling device comprising: a heater for providing heat for drying and / or styling hair, the heater having a heat up rate greater than 30 °C per second; a power source for providing power to the heater; and 5 safety circuitry for disconnecting the power source from the heater in response to an overheat trigger event or a fault detection event; wherein the safety circuitry is configured to disconnect the power source from the heater within a period between 0.02 and 2 seconds of the overheat trigger event or the fault detection event. 10 2a. The hair drying and / or styling device of example 1a, wherein the heater comprises independently operable heater zones, and wherein each heater zone comprises at least one independently operable heater electrode. 3a. The hair drying and / or styling device of example 2a, wherein the safety circuitry is configured to disconnect power from the heater in dependence on detection of the overheat 15 trigger event or the fault detection event in anyone (or more) of the heater zones, preferably to disconnect power to all heater electrodes in dependence on detection of the overheat trigger event or the fault detection event in any one (or more) of the heater zones. 4a. The hair drying and / or styling device of any of examples 1a to 3a, wherein the safety circuitry comprises an array of temperature sensors, and a means for detecting if anyone (or 20 more) of the temperature sensors indicates an overheat trigger event. 5a. The hair drying and / or styling device of example 4a, wherein at least one of the array of temperature sensors is provided in thermal connection to each heater zone of the heater, preferably wherein at least one of the array of temperature sensors is provided in thermal connection to each heater electrode of the heater. 25 6a. The hair drying and / or styling device of examples 1a to 5a, wherein the safety circuitry comprises at least one weak link configured to melt at a predetermined temperature indicative of an overheat trigger event, thereby breaking the circuit. 7a. The hair drying and / or styling device of example 6a, wherein at least one weak link is provided in thermal connection with each heater zone of the heater such that the weak link is 30 configured to melt if the corresponding heater zone exceeds the predetermined temperature, preferably wherein at least one weak link is provided in thermal connection with each heater electrode of the heater such that the weak link is configured to melt if the corresponding heater electrode exceeds the predetermined temperature. 8a. The hair drying and / or styling device of example 7a, wherein the safety circuitry is configured to disconnect the power only to the corresponding heater zone and / or heater electrode in dependence on at least one weak link melting. 9a. The hair drying and / or styling device of any of examples 6a to 8a, wherein the at least one weak link is configured to melt within a period between 0.02 and 2 seconds of the overheat trigger event. 10a. The hair drying and / or styling device of any of examples 6a to 9a, wherein the at least one weak link has a mass less than or equal to 1 gram. 11a. The hair drying and / or styling device of any of examples 6a to 10a, wherein the at least one weak link comprises a first solder material having a first melting temperature and a second solder material having a second melting temperature, preferably wherein the second melting temperature is lower than the first melting temperature and the second solder material forms a connection between the first solder material and electrical conductors of the circuit, preferably a physical and electrical connection. 12a. The hair drying and / or styling device of example 11a, wherein the at least one weak link is deposited by heating to a temperature intermediate between the first melting temperature and the second melting temperature. 13a. The hair drying and / or styling device of example 11a or 12a, wherein the combined mass of the first solder material and the second solder material is less than or equal to 1 gram, and preferably greater than or equal to 0.005 milligrams. 14a. The hair drying and / or styling device of any of examples 6a to 13a, wherein a substrate on which the weak link is provided has a coating of solder resist to facilitate removal of melted solder to ensure a break in electrical connection. 15a. The hair drying and / or styling device of any of examples 6a to 14a, further comprising microfluidic structures located adjacent the at least one weak link for channelling molten material away, and thereby break electrical connection. 16a. The hair drying and / or styling device of any of examples 6a to 15a, wherein the at least one weak link is resiliently biased such that electrical connection is broken upon melting. 17a. The hair drying and / or styling device of any of examples 6a to 16a, wherein the at least one weak link is connected to a switch, preferably wherein the switch is configured to disconnect power from the heater in dependence on the at least one weak link melting. 18a. The hair drying and / or styling device of any of examples 6a to 17a, wherein the safety circuitry comprises an array of weak links, wherein the weak links are connected in series to a switch, preferably wherein the switch is configured to disconnect power from the heater in dependence on at least one weak link of the array of weak links melting. 19a. The hair drying and / or styling device of any of examples 1a to 18a, wherein the safety circuitry comprises an array of non-linear thermistors in thermal contact with the heater assembly, and a means for detecting a voltage across the array of non-linear thermistors, wherein the safety circuitry is configured to cut power supply to the heater in dependence on the voltage across the array of non-linear thermistors exceeding or falling below a threshold value indicative of an overheat trigger event. 20a. The hair drying and / or styling device of example 19a, wherein at least one of the array of non-linear thermistors is provided in thermal contact with each heater zone of the heater for monitoring the temperature of that heater zone, preferably wherein at least one of the array of non-linear thermistors is provided in thermal contact with each heater electrode of the heater for monitoring the temperature of that heater electrode. 21a. The hair drying and / or styling device of example 19a or 20a, wherein the array of nonlinear thermistors is connected in series. 22a. The hair drying and / or styling device of any of examples 1a to 21a, wherein the safety circuitry comprises a parallel array of temperature sensors, and a means for detecting if the voltage of any one (or more) of the parallel array of temperature sensors exceeds or falls below a threshold value indicative of an overheat trigger event, wherein the safety circuitry is configured to cut power supply to the heater in dependence on detection of a voltage of any one (or more) of the parallel array of temperature sensors exceeding or falling below a threshold value indicative of an overheat trigger event. 23a. The hair drying and / or styling device of example 22a, wherein the safety circuitry comprises an array of diodes, wherein each of the array of diodes is connected across one of the parallel array of temperature sensors, and a means for detecting the combined voltage output from the array of diodes, wherein the safety circuitry is configured to cut power supply to the heater in dependence on the combined voltage output from the array of diodes exceeding or falling below a threshold value indicative of an overheat trigger event. 24a. The hair drying and / or styling device of example 22a or 23a, wherein the safety circuitry comprises a multiplexer connected across the parallel array of temperature sensors for sampling the voltages, wherein the safety circuitry is configured to cut power supply to the heater in dependence on the sampled voltage of anyone (or more) of the parallel array of temperature sensors exceeding or falling below a threshold value indicative of an overheat trigger event. 25a. The hair drying and / or styling device of any of examples 1a to 24a, wherein the safety circuitry comprises a first microprocessor and a second microprocessor, both configured to run software measuring temperature of the heater. 26a. The hair drying and / or styling device of example 25a, wherein the safety circuitry comprises means for comparing outputs from the first microprocessor and the second microprocessor to detect a fault detection event. 27a. The hair drying and / or styling device of example 25a or 26a, wherein the first microprocessor and second microprocessor are configured to run identical software measuring heater temperature. 28a. The hair drying and / or styling device of any of examples 25a to 27a, wherein the safety circuitry comprises a first connection and disconnection means in series with a second connection and disconnection means, wherein the first microprocessor controls the first connection and disconnection means and the second microprocessor controls the second connection and disconnection means, such that the outputs of the first microprocessor and second microprocessor must agree for power to be supplied to the heater. 29a. The hair drying and / or styling device of any of examples 25a to 28a, wherein the safety circuitry comprises a first array of connection and disconnection means, wherein the first microprocessor controls each of the first array of connection and disconnection means, and a second array of connection and disconnection means, wherein the second microprocessor controls each of the second array of connection and disconnection means, and wherein the heater comprises an array of heater electrodes, each heater electrode being connected to a connection and disconnection means of the first array of connection and disconnection means in series with a connection and disconnection means of the second array of connection and disconnection means, such that the outputs of the first microprocessor and second microprocessor must agree for each heater electrode for that heater electrode and the power source to be in electrical connection. 30a. The hair drying and / or styling device of any of examples 25a to 29a, wherein the safety circuitry comprises means for comparing outputs of the first microprocessor and the second microprocessor for each heater electrode of the heater, wherein a connection and disconnection means is configured to disconnect the power source from the heater if the first microprocessor and the second microprocessor disagree for anyone (or more) of the heater electrodes. 31a. The hair drying and / or styling device of any of examples 25a to 30a, wherein the first microprocessor and the second microprocessor are configured to feed into an XOR gate, 5 such that if the outputs of first microprocessor and the second microprocessor agree (or / for example, are the same) the XOR gate outputs a logical low (0). 32a. The hair drying and / or styling device of any of examples 25a to 31a, wherein the safety circuitry comprises a series of XOR gates, each XOR gate corresponding to a heater electrode of the heater, wherein the first microprocessor and the second microprocessor are 10 configured to feed into each of the series of XOR gate in respect of the relevant heater electrode. 33a. The hair drying and / or styling device of example 32a, wherein the outputs of the series of XOR gates are output into one or more OR gates such that if the outputs of the first microprocessor and the second microprocessor for anyone (or more) of the heater 15 electrodes disagree, the safety circuit outputs a logical high (1). 34a. The hair drying and / or styling device of example 32a or 33a, wherein the outputs of the series of XOR gates are output into one or more OR gates such that if the outputs of the first microprocessor and the second microprocessor for the heater electrodes agree, the safety circuit outputs a logical low (0). 20 35a. The hair drying and / or styling device of example 25a or 26a, wherein the first microprocessor runs control software and the second microprocessor runs safety software. 36a. The hair drying and / or styling device of example 35a, wherein the safety circuitry comprises means for sending temperature measurements to the first microprocessor and the second microprocessor. 25 37a. The hair drying and / or styling device of example 36a, wherein the first microprocessor is configured to control the heater in dependence on the temperature measurements. 38a. The hair drying and / or styling device of example 36a or 37a, wherein the safety circuitry comprises a leader connection and disconnection means for disconnecting power to heater, wherein the second microprocessor controls the leader connection and 30 disconnection means in dependence on the temperature measurements. 39a. The hair drying and / or styling device of any of examples 36a to 38a, wherein the heating assembly comprises at least one thermistor and the means for sending temperature measurements comprises a means for measuring resistance. 40a. The hair drying and / or styling device of any of examples 35a to 39a, wherein the 5 heater is connected to first microprocessor via a first connection and disconnection means controlled by the first microprocessor, and the heater is connected to the second microprocessor via a second connection and disconnection means controlled by the second microprocessor. 41a. The hair drying and / or styling device of example 40a, wherein each heater electrode of 10 the heater is connected to first microprocessor via a first connection and disconnection means controlled by the first microprocessor, and the heater is connected to the second microprocessor via a second connection and disconnection means controlled by the second microprocessor. 42a. The hair drying and / or styling device of example 41a, wherein the first processor is 15 configured to control the first connection and disconnection means power to each heater electrode individually in order to determine temperature measurements for that heater electrode and / or the second processor is configured to control the second connection and disconnection means power to each heater electrode individually in order to determine temperature measurements for that heater electrode. 20 43a. The hair drying and / or styling device of any examples 1a to 42a, comprising a leader connection and disconnection means for disconnecting power to the heater in dependence on an overheat trigger event and / or fault detection event. 44a. The hair drying and / or styling device of any of examples 28a to 43a, wherein the connection and disconnection means comprise a switch, preferably a MOSFET switch. The 25 MOSFET switch may be inserted in the positive supply (‘high side switch’) or the ground connection (‘low side switch’). 45a. The hair drying and / or styling device of any of examples 1a to 44a, further comprising latching circuitry configured to introduce a delay or a semi-permanent disconnection of the power source from the heater after an overheat trigger event or a fault detection event has 30 occurred. 46a. The hair drying and / or styling device of any of examples 1a to 45a, wherein the safety circuitry comprises a comparator for comparing a voltage to a threshold value indicative of an overheat trigger event or a fault detection event. 47a. The hair drying and / or styling device of any of examples 1a to 46a, wherein the safety circuitry comprises a switch for controlling power supply to the heater, preferably a MOSFET switch and / or preferably wherein the output of the comparator controls the switch. 48a. Safety circuitry for a hair styling appliance comprising a heater with a heat up rate greater than 30 °C per second, wherein the safety circuitry is configured to disconnect (or cut) power supply to the heater within a period between 0.02 and 2 seconds of an overheat trigger event or a fault detection event. 49a. The safety circuitry of example 48a, further comprising any of the safety circuitry features of any of examples 1a to 47a. 50a. A method of preparing a weak link for safety circuitry, comprising: applying a first solder material to form a first connector at an end of a first conductor and to form a second connector at an end of a second conductor, wherein the first solder material has a first melting temperature; applying a second solder material such that it connects the first connector and the second connector, wherein the second solder material has a second melting temperature, the second melting temperature being higher than the first melting temperature; and heating to a temperature between the first melting temperature and the second melting temperature. 51a. The method of example 50a, further comprising depositing solder resist material in the region between the end of the first conductor and the end of the second conductor 52a. The method of example 50a or 51a, further comprising depositing solder resist material in the region of the second solder material. 53a. The method of any of examples 50a to 52a, wherein the first conductor and second conductor are conductor traces, preferably conductor traces on a dielectric layer. 54a. The method of any of examples 50a to 53a, wherein the weak link is provided adjacent to a heater. 55a. The method of any of examples 50a to 54a, further comprising connecting the weak link to a switch, preferably a control switch. 56a. The method of any of examples 50a to 55a, wherein the steps are performed on a heater for a hair drying and / or styling appliance, preferably on a dielectric layer of a heater for a hair drying and / or styling appliance.

Claims

1. A hair styling apparatus comprising:a plurality of heaters for heating hair in contact with hair contacting surfaces of the plurality of heaters by conductive heating;a plurality of microcontroller units, MCUs, in communication with one another, each one of the MCUs being configured to control a subset of the plurality of heaters,wherein at least one MCU of the plurality of MCUs comprises a processor configured to:detect malfunctions of the hair styling apparatus; andcontrol a function of the hair styling apparatus in response to the processor detecting malfunctions of the hair styling apparatus.

2. The hair styling apparatus of claim 1, wherein to detect malfunctions and control a function of the hair styling apparatus in response to the detection, the processor is configured to:determine whether a communication link between the at least one MCU and another MCU of the plurality of MCUs has failed; andtrigger a reset of the hair styling apparatus based on determining that the communication link has failed.

3. The hair styling apparatus of claim 2, wherein the processor is configured to determine that the communication link between the at least one MCU and the another MCU has failed if the at least one MCU does not receive within a predetermined time period, from the another MCU, a signal indicating normal operation.

4. The hair styling apparatus of claim 3, wherein the signal indicating normal operation is a heartbeat message sent periodically to the at least one MCU.

5. The hair styling apparatus of any preceding claim, wherein to detect malfunctions and to control a function of the hair styling apparatus in response to the detection, the processor is configured to:determine whether a rate at which power is provided to the plurality of heaters exceeds a threshold; andtrigger a reset of the hair styling apparatus based on determining that the rate at which power is provided to the plurality of heaters exceeds the threshold.

6. The hair styling apparatus of any preceding claim, further comprising:temperature measurement means for measuring a temperature of each one of the plurality of heaters, and wherein to detect malfunctions and to control a function of the hair styling apparatus in response to the detection, the processor is further configured to:5 analyse data received from the temperature measurement means, the dataindicating a voltage and / or a current value measured across each corresponding heater;determine, for each corresponding heater, whether the measured voltage and / or current value exceeds a threshold; and10 trigger a reset of the hair styling apparatus based on determining that themeasured voltage and / or current value for at least one of the plurality of heaters exceeds the threshold.

7. The hair styling apparatus of claim 6, wherein the threshold is a voltage and / or a current 15 value corresponding to a maximum safe operational temperature of the plurality of heaters.

8. The hair styling apparatus of claim 6, wherein the threshold is a voltage and / or a current value corresponding to a maximum operational voltage and / or a current value of the temperature measurement means.

9. The hair styling apparatus any of claims 2 to 8, wherein the reset triggered by the20 processor of the at least one MCU comprises:switching off power to the plurality of heaters of the hair styling apparatus by switching a respective transistor switch associated with each one of the plurality of heaters.

10. The hair styling apparatus of claim 9, wherein to switch off power to the plurality of25 heaters of the hair styling apparatus, the processor of the at least one MCU is configured to:switch a corresponding MOSFET gate drive associated with each heater of thesubset of heaters controlled by the at least one MCU to high impedance; andtransmit a command message, to the other MCUs of the plurality of MCUs, to instruct them to switch off power to heaters controlled by the other MCUs of the plurality of MCUs.3011. The hair styling apparatus of claim 10, wherein, for each one of the other MCUs, in response to receiving the command message, a processor of the MCU is configured to:switch a corresponding MOSFET gate drive associated with each heater controlled by the MCU to high impedance.3512. The hair styling apparatus any of claims 2 to 8, wherein the reset triggered by the processor of the at least one MCU comprises:switching the hair styling apparatus from an active mode to an idle mode, wherein in the idle mode a power provided to the heaters is provided at a rate of 1 Hz, and wherein in 5 the active mode the power provided to the heaters is provided at a greater rate than in the idle mode.

13. The hair styling apparatus any of claims 2 to 12, wherein the processor is configured to: trigger the reset periodically until the processor determines that the hair styling apparatus is operating correctly.1014. The hair styling apparatus of claim 13, wherein determining that the hair styling apparatus is operating correctly comprises:determining that the communication link between the at least one MCU and the other MCU of the plurality of MCUs is established;15 determining that the rate at which power is provided to the plurality of heaters isbelow a threshold; anddetermining that the measured voltage and / or current value across each of the plurality of heaters is below a threshold.20 15. A hair styling apparatus comprising:communication means for communicating with at least one external computing device for controlling the hair styling apparatus;a plurality of heaters for heating hair in contact with hair contacting surfaces of the plurality of heaters by conductive heating;25 a plurality of microcontroller units, MCUs, in communication with one another, eachone of the MCUs being configured to control a subset of the plurality of heaters, wherein at least one MCU of the plurality comprises a processor configured to: detect malfunctions of the hair styling apparatus; and control a function of the hair styling apparatus in response to the processor30 detecting malfunctions of the hair styling apparatus.

16. The hair styling apparatus of claim 15, wherein the communication means is configured to:receive, from the at least one external computing device, a command message to35 apply settings of the hair styling apparatus, and wherein the command message comprises an error detection feature, and wherein the processor of the at least MCU is configured to:process the error detection feature of the command message to check the validity of the command message.

17. The hair styling apparatus of claim 16, wherein in response to the processor determining that the command message is not valid, the communication means is configured totransmit, to the external computing device, an indication that the settings have not been applied.

18. The hair styling apparatus of any of claims 15 to 17, wherein the processor of the at least one MCU is configured to:determine whether a communication link between the hair styling apparatus and the external computing device has failed; andtrigger a reset of the hair styling apparatus based on determining that the communication link has failed.

19. The hair styling apparatus of claim 18, wherein the processor determines that the communication link between the hair styling apparatus and the external computing device has failed if the hair styling apparatus does not receive within a predetermined time period, from the external computing device, a signal indicating normal operation.

20. The hair styling apparatus of claim 19, wherein the signal indicating normal operation is a heartbeat message sent periodically to the hair styling apparatus.

21. The hair styling apparatus any of claim 18 to 20, wherein the reset triggered by the processor of the at least one MCU comprises:switching off power to the plurality of heaters of the hair styling apparatus by switching a respective transistor switch associated with each one of the plurality of heaters.

22. The hair styling apparatus of claim 21, wherein to switch off power to the plurality of heaters of the hair styling apparatus the processor of the at least one MCU is configured to: switch a corresponding MOSFET gate drive associated with each heater of the subset of heaters controlled by the at least one MCU to high impedance; andtransmit a command message, to the other MCUs of the plurality of MCUs, to instruct them to switch off power to heaters controlled by the other MCUs of the plurality of MCUs.

23. The hair styling apparatus of claim 22, wherein, for each one of the other MCUs, in response to receiving the command message, a processor of the MCU is configured to:switch a corresponding MOSFET gate drive associated with each heater controlled by the MCU to high impedance.

524. The hair styling apparatus any of claim 18 to 20, wherein the reset triggered by the processor of the at least one MCU comprises:switching the hair styling apparatus from an active mode to an idle mode, wherein in the idle mode a power provided to the heaters is provided at a rate of 1 Hz, and wherein in 10 the active mode the power provided to the heaters is provided at a greater rate than in the idle mode.

25. The hair styling apparatus any of claims 18 to 24, wherein the processor is configured to trigger the reset periodically until the communication link between the hair styling apparatus and the external computing device is re-established.15 26. The hair styling apparatus of any of claims, 15 to 25, wherein to detect malfunctions andto control a function of the hair styling apparatus in response to the detection, the processor is configured to:determine whether a communication link between the at least one MCU and another MCU of the plurality of MCUs has failed; and20 trigger a reset of the hair styling apparatus based on determining that thecommunication link has failed.

27. The hair styling apparatus of claim 26, wherein the processor determines that the communication link between the at least one MCU and the another MCU has failed if the at25 least one MCU does not receive, within a predetermined time period, from the another MCU, a signal indicating normal operation.

28. The hair styling apparatus of claim 27, wherein the signal indicating normal operation is a heartbeat message sent periodically to the at least one MCU.3029. The hair styling apparatus of any of claims, 15 to 25, wherein to detect malfunctions and to control a function of the hair styling apparatus in response to the detection, the processor is configured to:determine whether a rate at which power is provided to the plurality of heaters35 exceeds a threshold; andtrigger a reset of the hair styling apparatus based on determining that the rate at which power is provided to the plurality of heaters exceeds the threshold.

30. The hair styling apparatus of any of claims, 15 to 25, further comprising:5 temperature measurement means for measuring a temperature of each one of theplurality of heaters, and wherein to detect malfunctions and to control a function of the hair styling apparatus in response to the detection, the processor is further configured to:analyse data received from the temperature measurement means, the data indicating a voltage and / or a current value measured across each corresponding10 heater;determine, for each corresponding heater, whether the measured voltage and / or current value exceeds a threshold; andtrigger a reset of the hair styling apparatus based on determining that the measured voltage and / or current value for at least one of the plurality of heaters15 exceeds the threshold.

31. The hair styling apparatus of claim 30, wherein the threshold is a voltage and / or a current value corresponding to a maximum safe operational temperature of the plurality of heaters.20 32. The hair styling apparatus of claim 30, wherein the threshold is a voltage and / or acurrent value corresponding to a maximum operational voltage and / or a current value of the temperature measurement means.

33. The hair styling apparatus of any of claims 15 to 25, wherein the reset triggered by the processor of the at least one MCU comprises:25 switching off power to the plurality of heaters of the hair styling apparatus byswitching a respective transistor switch associated with each one of the plurality of heaters.

34. The hair styling apparatus of claim 33, wherein to switch off power to the plurality of heaters of the hair styling apparatus the processor of the at least one MCU is configured to:30 switch a corresponding MOSFET gate drive associated with each heater of thesubset of heaters controlled by the at least one MCU to high impedance; andtransmit a command message, to the other MCUs of the plurality of MCUs, to instruct them to switch off power to heaters controlled by the other MCUs of the plurality of MCUs.

35. The hair styling apparatus of claim 34, wherein for each one of the other MCUs, in response to receiving the command message, a processor of the MCU is configured to:switch a corresponding MOSFET gate drive associated with each heater controlled by the MCU to high impedance.

536. The hair styling apparatus of any of claims 15 to 25, wherein the reset triggered by the processor of the at least one MCU comprises:switching the hair styling apparatus from an active mode to an idle mode, wherein inthe idle mode a power provided to the heaters is provided at a rate of 1 Hz, and wherein in10 the active mode the power provided to the heaters is provided at a greater rate than in the idle mode.

37. The hair styling apparatus of any of claims 26 to 36, wherein the processor is configured to:trigger the reset periodically until the processor determines that the hair styling15 apparatus is operating correctly.

38. The hair styling apparatus of claim 37, wherein determining that the hair styling apparatus is operating correctly comprises:determining that the communication link between the at least one MCU and the other20 MCU of the plurality of MCUs is established;determining that the rate at which power is provided to the plurality of heaters is below a threshold; anddetermining that the measured voltage and / or current value across each of the plurality of heaters is below a threshold.

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