Safety circuitry

Abstract

A safety circuit for a hair drying or styling device comprising a heater with a heat up rate greater than 30 °C per second which is configured to disconnect power within a period between 0.02 and 2 se

Description

Field of the Invention The present invention relates to safety circuitry and methods for use in hair styling appliances. The invention also relates to hair styling appliances having such a safety circuitry for ensuring the safe operation of the hair styling appliance by the user. Background to the Invention There exist a variety of apparatus for styling hair, which use heat to effect a change in the shape of hair. Examples include hair straighteners, curling tongs and hair crimpers. The hair styling apparatus directly heats the hair to above its glass transition temperature, where the hair becomes mouldable. The hair is styled at a temperature above its glass transition temperature; and once cooled, the hair generally remains styled until it next becomes wet. Hair styling appliances, such as hair straighteners (as shown in Figures 1a and 1b), typically comprise heating surfaces (typically defined by heating plates) which heat hair that comes into contact with the heating surfaces. Other styling devices, such as curling tongs, provide a curved heating surface, but the principle remains the same - the hair is heated above its glass transition temperature and styled by the heating surface. 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. As a result of the thermal mass, the heaters 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 overheating 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. Such low thermal mass heaters are therefore more responsive and are easier to control. However, the inventors have realized that the existing safety circuitry and methods typically employed to ensure the safe operation of the hair styling appliance are not suitable for use with hair styling appliances that use such low thermal mass heaters. Accordingly, there is a need for improved safety circuitry and methods that can ensure the safe operation of hair styling appliances that use lower thermal mass heaters. Summary of the 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 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 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 assembly within a period between 0.02 and 2 seconds of the overheat trigger event or the fault detection event. Preferably, the safety circuitry is configured to disconnect the power source from the heater assembly within a period between 0.02 and 1 seconds of the overheat trigger event or the fault detection event. The time period will typically be dependent on the heatup rate and the temperature difference between the trigger temperature and the maximum permitted temperature. In some implementations, the heater may comprise independently operable heater zones, and preferably each heater zone may comprise at least one independently operable heater electrode. The heater may be a heater assembly (and the terms may be used interchangeably). The safety circuitry may be configured to disconnect power from the heater in dependence on detection of the overheat trigger event or the fault detection event in any one (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. In some implementations, the safety circuitry may comprise an array of temperature sensors, and a means for detecting if any one (or more) of the temperature sensors indicates an overheat trigger event. Preferably, at least one of the array of temperature sensors is provided in thermal connection to each heater zone of the heater assembly, preferably wherein at least one of the array of temperature sensors is provided in thermal connection to each heater electrode of the heater assembly. In some implementations, the safety circuitry may comprise at least one weak link configured to melt at a (predetermined) temperature indicative of an overheat trigger event, thereby breaking the circuit. In some implementations, at least one weak link may be provided in thermal connection with each heater zone of the heater assembly such that the weak link is 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 assembly such that the weak link is configured to melt if the corresponding heater electrode exceeds the predetermined temperature. The safety circuitry may be configured to disconnect the power only to the corresponding heater zone and / or heater electrode in dependence on at least one weak link melting. Preferably, the at least one weak link is formed of solder configured to melt within between 0.02 and 2 seconds of the overheat trigger event. Preferably, the mass of the weak link is less than or equal to 1 gram. The mass may preferably be more than or equal to 0.005 milligrams. In some implementations, 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. Preferably, the second solder material forms a connection between the first solder material and electrical conductors of the circuit. This connection is preferably physical (for example, bonding) and electrical connection. The first solder material may be solder wire and / or solder tape and / or solder preform. The second solder material may be solder paste. The second solder material may be configured to form connections (for example, pads). These may preferably have a triangular shape (with the tips pointing towards one another). The electrical conductors are preferable conductive traces, which may for example be provided on a substrate. The substrate may be a dielectric, for example dielectric heater plate. The at least one weak link may be deposited by heating to a temperature intermediate between the first melting temperature and the second melting temperature. The at least one link may be deposited by the method of claims 50 to 56. Preferably, the combined mass of the first solder material and the second solder material is less than or equal to 1 gram. The combined mass is preferably greater than or equal to 0.005 milligrams. In some implementations, a substrate on which the weak link is provided may have a coating of solder resist to facilitate removal of melted solder to ensure a break in electrical connection. The hair drying and / or styling device may further comprise microfluidic structures located adjacent the at least one weak link for channeling molten material away, and thereby break electrical connection. The at least one weak link may be resiliently biased such that electrical connection is broken upon melting. The at least one weak link may be deposited via a printing technique configured to achieve required small dimensional thickness of the weak link. Preferably this thickness may be 0.11 mm. In some implementations, the at least one weak link may be connected to a switch, preferably wherein the switch is configured to disconnect power from the heater assembly in dependence on the at least one weak link melting. In some implementations, the safety circuitry may comprise an array of weak links, wherein the weak links are connected in series to a switch. Preferably the switch is configured to disconnect power from the heater assembly in dependence on at least one weak link of the array of weak links melting. In some implementations, 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 assembly 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. (This is preferably in dependence on the voltage exceeding or falling below a threshold value indicative of an overheat trigger event in the case the thermistors are PTC or NTC thermistors, respectively.) At least one of the array of non-linear thermistors may be provided in thermal contact with each heater zone of the heater assembly for monitoring the temperature of that heater zone. Preferably at least one of the array of non-linear thermistors is provided in thermal contact with each heater electrode of the heater assembly for monitoring the temperature of that heater electrode. Preferably, the array of non-linear thermistors is connected in series. The array may be provided in parallel, but typically may achieve higher sensitivity if provided in series. In some implementations, 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 assembly 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. The safety circuitry may comprise 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 assembly 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. In some implementations, the safety circuitry may comprise 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 assembly in dependence on the sampled 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. In some implementations, the safety circuitry may comprise a first microprocessor and a second microprocessor, both configured to run software measuring temperature of the heater assembly. Preferably, the safety circuitry comprises means for comparing outputs from the first microprocessor and the second microprocessor to detect a fault detection event. The first microprocessor and second microprocessor may be configured to run identical software measuring heater assembly temperature. The safety circuitry may comprise 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 assembly. Preferably, 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. In some implementations, the safety circuitry may comprise 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 assembly if the first microprocessor and the second microprocessor disagree for any one (or more) of the heater electrodes. The first microprocessor and the second microprocessor may be configured to feed into an XOR gate, such that if the outputs of first microprocessor and the second microprocessor agree (and / or or, for example, are the same) the XOR gate outputs a logical low (0). In some implementations, the safety circuitry comprises a series of XOR gates, each XOR gate corresponding to a heater electrode of the heater assembly, wherein the first microprocessor and the second microprocessor are configured to feed into each of the series of XOR gate in respect of the relevant heater electrode. The outputs of the series of XOR gates may be output into one or more OR gates such that if the outputs of the first microprocessor and the second microprocessor for any one (or more) of the heater electrodes disagree, the safety circuit outputs a logical high (1). The outputs of the series of XOR gates may be 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). The outputs of the series of XOR gates may be output into one or more OR gates such that if the outputs of the first microprocessor and the second microprocessor disagree (for example, are not the same) for any one (or more) of the heater electrodes are the same, the safety circuit outputs a logical low (0). In some implementations, the first microprocessor runs control software and the second microprocessor runs safety software. The safety circuitry may preferably comprise means for sending temperature measurements to the first microprocessor and the second microprocessor. The first microprocessor may be configured to control the heater in dependence on the temperature measurements. The safety circuitry may preferably comprise a master connection and disconnection means for disconnecting power to heater assembly, wherein the second microprocessor controls the master connection and disconnection means in dependence on the temperature measurements. In some implementations, the heating assembly may comprise at least one thermistor and the means for sending temperature measurements comprises a means for measuring resistance. The heater assembly may be 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. In some implementations, each heater electrode of the heater is connected to first microprocessor via a first connection and disconnection means controlled by the first microprocessor, and the heater assembly is connected to the second microprocessor via a second connection and disconnection means controlled by the second microprocessor. In some preferable implementations, the first processor is 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 forthat heater electrode. In some preferable implementations, a master connection and disconnection means may be provided for disconnecting power to the heater assembly in dependence on an overheat trigger event and / or fault detection event. The connection and disconnection means may comprise a switch, preferably a MOSFET switch. The hair drying and / or styling device may preferably further comprise 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 occurred. In some preferable implementations, the safety circuitry may comprise a comparator for comparing a voltage to a threshold value indicative of an overheat trigger event or a fault detection event. The safety circuitry may comprises a switch for controlling power supply to the heater assembly, preferably a MOSFET switch and / or preferably wherein the output of the comparator controls the switch. This switch may be the same as the master connection and disconnection means. There may also be provided a computer program product for implementing the software (e.g. safety and / or control), for example the software run on the first and / or second processor. Additionally, there may be provided a (tangible) non-transitory computer-readable product including instructions that, when executed by a processor, implements the software (e.g. safety and / or control). Additionally, there may be provided a signal for implementing the software (e.g. safety and / or control). According to a further aspect of the invention, there is provided safety circuitry for a hair styling appliance comprising a heater assembly 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 assembly within a period between 0.02 and 2 seconds of an overheat trigger event or a fault detection event. The safety circuitry may preferably further comprise any of the safety circuitry features as described above. According to a further aspect of the invention, there is provided 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 (preferably, thereby to connect the second solder material to the first conductor and the second conductor via the first connector and second connector, respectively). The method may preferably further comprise a cooling step. The first solder material may be solder tape and / or solder ribbon and / or solder preform and / or a solid block. The method may further comprise depositing solder resist material in the region between the end of the first conductor and the end of the second conductor. The method may further comprise depositing solder resist material in region of the second solder material. The method may further comprise depositing solder resist material on a substrate to which the first and second solder material are applied. The first conductor and second conductor may be conductor traces, preferably conductor traces on a dielectric layer. The weak link may be provided adjacent to a heater, for example a heater electrode. The may further comprise connecting the weak link to a switch, preferably a control switch. In preferable implementations of the method, the steps may 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. The invention extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings. Any apparatus feature as described herein may also be provided as a method feature, and vice versa. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and / or all features in one aspect can be applied to any, some and / or all features in any other aspect, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and / or supplied and / or used independently. Brief Description of the Drawings One or more aspects will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which: Figure 1a illustrates a hair styler; Figure 1 b illustrates a user using the hair styler on their hair; 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 shown in Figure 2a; Figure 2d illustrates a weak link which may be used in a thermal safety circuit; Figure 3a schematically illustrates a first arrangement of heater zones provided on the heating surface of a hair styling heater; Figure 3b schematically illustrates a second arrangement of heater zones provided on the heating surface of a hair styling heater; Figure 4 is a temperature-time plot of a heating response of the low thermal mass heater; 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. Detailed Description of Preferred Embodiments The embodiments described below represent the best ways known to the inventors of putting the invention into practice. However, they are not the only ways in which this can be achieved. Overview of hair styler Figure 1a illustrates a 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. Hair can be inserted between and in contact with a heating surface of the heaters 6a, 6b, in which position the heaters 6a, 6b impart conductive heat to the hair to facilitate styling. 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 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 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. 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 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 layer62, 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, which 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. Figures 3a and 3b show schematic views of different arrangements of such heating zones. Figure 3a shows an arrangement corresponding to that of Figures 2a to 2c, in which the heating zones 642 are arranged along the y-direction only. Figure 3b shows an alternative arrangement, in which the heating zones 644 are arranged in both the x- and y-directions. Such an arrangement of heating zones 644 can be provided by arranging two sets of heater electrodes 64 like those shown in Figure 2a side by side in the width (x-) direction. The heaters 6 may be separated in this way into any number of zones and may comprise any number of zones along the x- and y-directions. In particular, whilst Figure 3b shows two zones along the x-direction, a greater number of zones in the x-direction could also be provided. The zones 642, 644 of the heaters 6a, 6b can be operated (heated) independently, which can help to reduce cold spots when using very low thermal mass heaters 6 such as those shown in Figure 2. Since the heaters 6 have a very low thermal mass, the heaters 6 will heat up much more quickly and cool down much more quickly compared to prior art heaters that employ relatively thick heater plates. Whereas typical conventional hair stylers from ambient may take 30 seconds to reach their operating temperature of about 200°C, a low mass heater at maximum power may exceed the same temperature in 1 second (or less), but the exact operating temperature and heat-up time are defined by requirements, thermal mass and maximum power. For the sake of illustration, we describe a heater which, at maximum power, may heat up at a rate of 200 °Cs V In order to ensure user safety, it is necessary to limit the maximum temperature that the heaters 6 can reach, in the case of an electronics or firmware failure. By way of illustration, Figure 4 is a plot of temperature against time for a heater 6 operating 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, Tigger, 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, Trigger is set at 2400 C. If Tigger is set too low, there is a risk that the safety fuse may be triggered by small fluctuations around the operating temperature. However, if Tigger is 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. 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 available thermal fuses is typically only 40 °Cs1. 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 sufficiently fast to prevent such unsafe operation when using very low thermal mass heaters 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. As will be described in more detail below, the increased cut-out speed of the thermal safety cutout 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 cutout of the present invention may disconnect power to the gate of a master Metal Oxide Semiconductor Field Effect Transistor (MOSFET), which controls the 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 current from being able to flowthrough 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, intentionally weak parts are provided in the thermal safety circuit. In an exemplary implementation, the intentionally weak parts are 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 64 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 64. 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 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). 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), to ensure the circuit is reliably completely broken when the solder melts at the set temperature. In some implementations, the weak link may comprises 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 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 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, a thermal safety circuit is provided which comprises components which react in a non-linear manner to changes in temperature. 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 nonlinear 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 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 to correspond to, or at least be related to, the above described trigger temperature, Tigger. 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 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 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 thermistors) 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 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. 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 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, Trigger 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 To), 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. 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. 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, Ttngger. This can aid in ensuring a quick response time to any over-heating of the heaters 6. Non-linear thermistors 208 can provide this high temperature sensitivity. Embodiment 3 - Analogue OR gate of maximum zone temperature sensing In a third embodiment, 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 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 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 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 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 dropped 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). 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 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. 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 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 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 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 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. In a fourth embodiment, a safety strategy is implemented by running the same software directly measuring heater electrode temperature on two identical 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 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 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 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 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 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 fora 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 disagree, 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 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 “master” 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 master MOSFET switch thereby removing the power from all of the heater electrodes 64. Alternatively, for redundancy, two serially connected master 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 master 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 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 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 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 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 may be different for each heater electrode 64). Embodiment 5 - Dedicated safety microprocessor In a fifth embodiment, 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. 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 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 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 master MOSFET 516 (which is normally ON and will be described in more detail below) to 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 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 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 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 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 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 above described trigger temperature Tigger- 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 master 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 fora 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. Alternatives and modifications Various embodiments have been described above by way of example. As those skilled in the art will appreciate, various alternatives and modifications may be made to the above embodiments. Some of those alternatives and modifications will now be described. 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, 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). 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 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 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 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, 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 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, Ttngger. By way of example, for a heat up rate of 410°Cs-1, a Tmax of 250 °C and a Ttngger 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 Ttngger 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: Az= Lp Where Lz 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.

Claims

1. 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; andsafety 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.

2. The hair drying and / or styling device of claim 1, wherein the heater comprises independently operable heater zones, and wherein each heater zone comprises at least one independently operable heater electrode.

3. The hair drying and / or styling device of claim 2, wherein the safety circuitry is configured to disconnect power from the heater in dependence on detection of the overheat trigger event or the fault detection event in any one (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.

4. The hair drying and / or styling device of any of claims 1 to 3, wherein the safety circuitry comprises an array of temperature sensors, and a means for detecting if any one (or more) of the temperature sensors indicates an overheat trigger event.

5. The hair drying and / or styling device of claim 4, 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.

6. The hair drying and / or styling device of any preceding claim, 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.

7. The hair drying and / or styling device of claim 6, wherein at least one weak link is provided in thermal connection with each heater zone of the heater such that the weak link is configured to melt if the corresponding heater zone exceeds the predetermined temperature, preferablywherein 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.

8. The hair drying and / or styling device of claim 7, 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.

9. The hair drying and / or styling device of any of claims 6 to 8, 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.

10. The hair drying and / or styling device of any of claims 6 to 9, wherein the at least one weak link has a mass less than or equal to 1 gram.

11. The hair drying and / or styling device of any of claims 6 to 10, 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.

12. The hair drying and / or styling device of claim 11, 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.

13. The hair drying and / or styling device of claim 11 or 12, 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.

14. The hair drying and / or styling device of any of claims 6 to 13, 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.

15. The hair drying and / or styling device of any of claims 6 to 14, further comprising microfluidic structures located adjacent the at least one weak link for channeling molten material away, and thereby break electrical connection.

16. The hair drying and / or styling device of any of claims 6 to 15, wherein the at least one weak link is resiliently biased such that electrical connection is broken upon melting.

17. The hair drying and / or styling device of any of claims 6 to 16, 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.

18. The hair drying and / or styling device of any of claims 6 to 17, 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.

19. The hair drying and / or styling device of any preceding claim, 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.

20. The hair drying and / or styling device of claim 19, wherein at least one of the array of nonlinear 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.

21. The hair drying and / or styling device of claim 19 or 20, wherein the array of non-linear thermistors is connected in series.

22. The hair drying and / or styling device of any preceding claim, 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.

23. The hair drying and / or styling device of claim 22, 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.

24. The hair drying and / or styling device of claim 22 or 23, 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 any one (or more) of the parallel array of temperature sensors exceeding or falling below a threshold value indicative of an overheat trigger event.

25. The hair drying and / or styling device of any preceding claim, wherein the safety circuitry comprises a first microprocessor and a second microprocessor, both configured to run software measuring temperature of the heater.

26. The hair drying and / or styling device of claim 25, wherein the safety circuitry comprises means for comparing outputs from the first microprocessor and the second microprocessor to detect a fault detection event.

27. The hair drying and / or styling device of claim 25 or 26, wherein the first microprocessor and second microprocessor are configured to run identical software measuring heater temperature.

28. The hair drying and / or styling device of any of claims 25 to 27, 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.

29. The hair drying and / or styling device of any of claims 25 to 28, 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 disconnectionmeans 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.

30. The hair drying and / or styling device of any of claims 25 to 29, 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 any one (or more) of the heater electrodes.

31. The hair drying and / or styling device of any of claims 25 to 30, wherein the first microprocessor and the second microprocessor are configured to feed into an XOR gate, 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).

32. The hair drying and / or styling device of any of claims 25 to 31, 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 configured to feed into each of the series of XOR gate in respect of the relevant heater electrode.

33. The hair drying and / or styling device of claim 32, 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 any one (or more) of the heater electrodes disagree, the safety circuit outputs a logical high (1).

34. The hair drying and / or styling device of claim 32 or 33, 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).

35. The hair drying and / or styling device of claim 25 or 26, wherein the first microprocessor runs control software and the second microprocessor runs safety software.

36. The hair drying and / or styling device of claim 35, wherein the safety circuitry comprises means for sending temperature measurements to the first microprocessor and the second microprocessor.

37. The hair drying and / or styling device of claim 36, wherein the first microprocessor is configured to control the heater in dependence on the temperature measurements.

38. The hair drying and / or styling device of claim 36 or 37, wherein the safety circuitry comprises a master connection and disconnection means for disconnecting power to heater, wherein the second microprocessor controls the master connection and disconnection means in dependence on the temperature measurements.

39. The hair drying and / or styling device of any of claims 36 to 38, wherein the heating assembly comprises at least one thermistor and the means for sending temperature measurements comprises a means for measuring resistance.

40. The hair drying and / or styling device of any of claims 35 to 39, wherein 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.

41. The hair drying and / or styling device of claim 40, wherein each heater electrode of 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.

42. The hair drying and / or styling device of claim 41, wherein the first processor is 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.

43. The hair drying and / or styling device of any preceding claim, comprising a master connection and disconnection means for disconnecting power to the heater in dependence on an overheat trigger event and / or fault detection event.

44. The hair drying and / or styling device of any of claims 28 to 43, wherein the connection and disconnection means comprise a switch, preferably a MOSFET switch.

45. The hair drying and / or styling device of any preceding claim, 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 occurred.

46. The hair drying and / or styling device of any preceding claim, 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.

47. The hair drying and / or styling device of any preceding claim, 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.

48. 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.

49. The safety circuitry of claim 48, further comprising any of the safety circuitry features of any of claims 1 to 47.

50. 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; andheating to a temperature between the first melting temperature and the second melting temperature.

51. The method of claim 50, further comprising depositing solder resist material in the region between the end of the first conductor and the end of the second conductor52. The method of claim 50 or 51, further comprising depositing solder resist material in region of the second solder material.

53. The method of any of claims 50 to 52, wherein the first conductor and second conductor are conductor traces, preferably conductor traces on a dielectric layer.

54. The method of any of claims 50 to 53, wherein the weak link is provided adjacent to a heater.

55. The method of any of claims 50 to 54, further comprising connecting the weak link to a switch, preferably a control switch.

56. The method of any of claims 50 to 55, wherein the steps are performed on a heater for a hair 5 drying and / or styling appliance, preferably on a dielectric layer of a heater for a hair drying and / or styling appliance.

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