Safety circuitry

Abstract

Hair drying and / or styling devices are disclosed comprising a calibrated thermistor 1000. The calibrated thermistor includes a first main thermistor line 1001 having a first impedance; a second main thermistor line 1002 having a second impedance; and a plurality of thermistor calibration lines 1010a-g each having their own impedance. The plurality of thermistor calibration lines are electrically connected in parallel to each other, and in series with the first main thermistor line and the second main thermistor line. At least one of the plurality of thermistor calibration lines is broken to provide a calibrated impedance of the calibrated thermistor.

Description

Field of the Invention The present invention relates to a thermistor and its use in hair styling appliances. The invention also relates to hair styling appliances having such a thermistor for temperature sensing or as part of safety circuitry. 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. Existing hair styling appliances typically use thermistors for temperature sensing to control the heating of the heaters or as part of safety to disconnect power to the heaters in the event of them overheating. However, especially when producing thermistors for low thermal mass heaters it is difficult to mass produce and accurately calibrate such thermistors, leading to increased inefficiency and greater potential for waste. The present invention aims to address or at least partially ameliorate one or more of the above problems. 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. In a first aspect there is provided a hair drying and / or styling device including a calibrated thermistor, wherein the calibrated thermistor includes: a first main thermistor line having a first impedance; a second main thermistor line having a second impedance; and a plurality of thermistor calibration lines each having their own impedance and wherein the plurality of thermistor calibration lines are electrically connected in parallel to each other, and in series with the first main thermistor line and the second main thermistor line, wherein at least one of the plurality of thermistor calibration lines is broken to provide a calibrated impedance of the calibrated thermistor. Advantageously, this provides a calibrated thermistor for the hair drying and / or styling device. In one example, there is provided a hair drying and / or styling device including a calibrated thermistor, wherein the calibrated thermistor includes: a first main thermistor line having a first impedance; a second main thermistor line having a second impedance; and a plurality of thermistor calibration lines each having a first end, a second end, and their own impedance, wherein the plurality of thermistor calibration line first ends are electrically connected in to each other and the first main thermistor line, and the plurality of thermistor calibration line second ends are electrically connected to each other and the second main thermistor line, wherein at least one of the plurality of thermistor calibration lines is broken to provide a calibrated impedance of the calibrated thermistor. Optionally, wherein the first main thermistor line, the second main thermistor line and at least one thermistor calibration line of the plurality of thermistor calibration lines are printed on a surface of the hair drying and / or styling device or other substrate. Advantageously, this means that no additional substrate is required to fabricate the thermistor, and that the thermistor is fabricated by printing rather than using traditional surface mount components. Optionally, wherein the first main thermistor line and second main thermistor line have the same impedance. Optionally, wherein at least two of the plurality of thermistor calibration lines each have the same impedance when unbroken. Advantageously, this means that where there are multiple calibration lines with the same impedance, and one needs to be broken, any of them can be broken rather than selecting a specific line. Optionally, wherein at least two of the plurality of thermistor calibration lines each have a different impedance when unbroken. Advantageously, this means that there can be multiple calibration lines that can provide differing amounts of change to the overall thermistor impedance, so there is greater choice during the calibration process. Optionally, wherein at least one of the first main thermistor line and second main thermistor line have an impedance lower than at least one of the plurality of thermistor calibration lines. Advantageously, this means the calibration lines provide a larger amount of possible calibration relative to the overall impedance of the thermistor. Optionally, wherein at least one of the first main thermistor line and second main thermistor line have an impedance higher than at least one of the plurality of thermistor calibration lines. Advantageously, this means that the calibration lines provide fine tuning relative to the overall impedance of the thermistor. Optionally, wherein the plurality of thermistor calibration lines are individually breakable. Advantageously, this means that each calibration line can be broken, and the overall impedance checked each time, rather than having to break multiple at a time and overshoot the calibration process. Optionally, wherein the calibrated thermistor forms part of a thermal safety circuit for a heater of the hair styling device, or a sensor circuit for sensing the temperature of the heater of the hair styling device. In a second aspect there is provided a method of making a hair drying and / or styling device having a calibrated thermistor, the method comprising: providing a first main thermistor line having a first impedance; providing a second main thermistor line having a second impedance; providing a plurality of thermistor calibration lines each having their own impedance and wherein the plurality of thermistor calibration lines are electrically connected in parallel to each other, and in series with the first main thermistor line and the second main thermistor line; calibrating the impedance of the thermistor to be within a desired tolerance range of a desired impedance; and incorporating the calibrated thermistor into the hair drying and / or styling device. Optionally, wherein calibrating the impedance of the thermistor to be within a desired tolerance range of a desired impedance comprises measuring the impedance of the thermistor at a set temperature, and comparing the measured impedance with the desired impedance. Optionally, wherein calibrating the thermistor further comprises selecting, based on the comparison of the measured impedance with the desired impedance, at least one of the plurality of thermistor calibration lines; and breaking the selected at least one thermistor calibration line. Optionally, wherein breaking the selected at least one thermistor calibration line includes using a laser to break the at least one thermistor calibration line. Advantageously, this means that no physical contact with the thermistor or device may be necessary to calibrate the thermistor. Optionally, wherein calibrating the impedance of the thermistor to be within a desired tolerance range of a desired impedance further comprises determining, based on the comparison of the measured impedance with the desired impedance, to provide at least one additional thermistor calibration line; and printing the at least one additional thermistor calibration line in series with the first main thermistor line and the second main thermistor line. Advantageously, this means that the impedance of the thermistor may also be lowered during the calibration process. 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; Figure 9 illustrates a further example of a thermal safety circuit; and Figure 10 illustrates an example for a printed and laser calibratable ladder thermistor. 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-1t 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, 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 may be triggered by small fluctuations around the operating temperature. However, if Trigger 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 °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 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 contacting 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 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 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, 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. 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, Tigger. 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 Ttrigger. 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. Embodiment 6 - printed and laser calibratable ladder thermistor In a sixth embodiment, there is provided a novel thermistor arrangement which comprises components which can be printed and calibrated to create linear and non-linear PTC and NTC thermistors that may be used and work as described in place of any thermistor described in the other embodiments, examples, and alternatives. Specifically, the thermistor can be used as a temperature sensor in circuits for sensing the temperature of the heater and / or the temperature of the hair of the user of the hair drying and / or styling device, where the temperature is fed back to a controller that controls the power supplied to the heater; and / or it can be used as part of the safety (fusing) circuitry that disconnects the heater from the power circuitry in the event that the heater overheats. The measurement values obtained from the thermistor can be equated to the temperature of different things - such as the temperature of the heater, the temperature of the hair contacting surface or even the temperature of the user’s hair, through the use of different calibration data (determined by experiment in advance) that relates the signal values obtained from the thermistor to the temperature being sensed. Having a well calibrated thermistor allows the same calibration data to be used by the controllers of different hair styling devices. That is, each hair styling device (with its own thermistor) does not need to be separately calibrated to relate the measurements obtained from its thermistor to the desired temperature values. In addition or alternatively, the position of the thermistor within the layered heater stack may depend on what temperature is being sensed. So for example, when the thermistor forms part of a circuit that is used to sense and / or control the temperature of the user’s hair, the thermistor may be placed in a layer closer to the hair contacting surface, and when the thermistor forms part of a circuit to sense and / or control the temperature of the heater, the thermistor may be placed in a layer closer to the heater track. Figure 10 shows an exemplary implementation, in which is provided a main thermistor line 1001 being the positive terminal of the thermistor 1000, a main thermistor line 1002 being the negative terminal of the thermistor 1000, and a number of thermistor calibration lines 1010 which are connected in parallel to the other thermistor calibration lines 1010 and in series with the two main thermistor lines 1001 and 1002. In this example there are seven thermistor calibration lines 1010a-1010g, but the minimum required number of thermistor calibration lines 1010 is two. As a result of the ladder arrangement, the overall impedance of the thermistor 1000 at a given temperature can be calculated by the following equation: / n \ 1 , ( 1 \ | — Rthermistor line 1001 T I / ) I T Rthermistor line 1002 \ '^thermistor calibration line 1010n' I \ n=l / Where RT is the total impedance of the thermistor 1000 at a given temperature, Rthermistor une 1001 is the impedance of the main thermistor line 1001 at the given temperature, Rthermistor une 1002 is the impedance of the main thermistor line 1002 at the given temperature, and ^thermistor calibrationtine wwn is the impedance of each thermistor calibration line that is being used. Each of the main thermistor lines 1001 and 1002, and the thermistor calibration lines 1010 may be printed, including screen printed, or deposited onto a surface of the hair styling appliance or other suitable substrate to fabricate the thermistor 1000. For example, each part of thermistor 1000 can be printed and cured with conductive or resistive pastes, inks, and other printable substances in the configuration shown in Figure 10. Particularly, the thermistor calibration lines 1010 are printed and cured, or otherwise produced, in a way that makes them susceptible to being severed in some way. Once fabricated, the thermistor 1000 can then be tested under known conditions to determine whether the impedance of the thermistor 1000 at a given temperature is within a desired tolerance range of a desired value. If the impedance of the thermistor 1000 is not within the desired tolerance range, a laser or other suitable tool may then be used to break one or more of the thermistor calibration lines 1010 as required to increase the impedance of the thermistor at that temperature. If the impedance is too large, then further calibration lines 1010 may be printed and cured and connected between the main thermistors lines 1001 and 1002 as required to lower the impedance of the thermistor into the desired tolerance range of the desired value. Using this method of cutting and creating calibration lines 1010, it is possible to calibrate the impedance of every thermistor 1000 that is fabricated to be within a desired tolerance of a desired value. When a thermistor calibration line 1010 is broken, this may mean that the entire thermistor calibration line is removed from the circuit. However, total removal of a thermistor calibration line 1010 is unnecessary. It is enough to separate the thermistor calibration line into two or more parts so that the two main thermistor calibration lines 1001 and 1002 are not electrically connected to each other through that broken thermistor calibration line 1010. In other words, this introduces a ‘break’ in the thermistor calibration line. This process effectively gives the thermistor calibration line a far greater impedance that it had before, so even if there is an end of a part of the thermistor calibration line still in connection with a thermistor main line, the other end will not be in connection with any other part of the circuit, so is just an open circuit. Therefore, a broken thermistor calibration line can be thought of as effectively removing the thermistor calibration line from the circuit. It is also possible to use other cutting, abrasive, or severing methods to create a break in a thermistor calibration line 1010 to calibrate the thermistor 1000. For example, while a laser may be used as part of an automated calibration process, a razor or other suitable scraping tool may be used to break a thermistor calibration line 1010. When designing a printed and laser calibratable ladder thermistor, there are a number of arrangements that the main thermistor lines 1001 and 1002 and the thermistor calibration lines 1010 can take. For example, the main thermistor lines 1001 and 1002 can each have a greater impedance than the thermistor calibration lines 1010, which can allow for finer tuning over the calibration process. Alternatively, the main thermistor lines 1001 and 1002 can have a smaller impedance than the thermistor calibration lines 1010, which can allow for the calibration process to change the impedance more dramatically if desired. Of course, it is possible and sometimes desirable to have a combination of greater and smaller impedance main thermistor lines 1001 and 1002 as well as a combination of greater and smaller impedance thermistor calibration lines 1010 to give the possibility of both fine tuning and coarse tuning of the thermistor impedance. This may be desired in a scenario where the printing process is not always reliable or precise. In this way, a printed and laser calibratable ladder thermistor can be fabricated and calibrated without needing a high-cost printing solution. For simplicity, the following three examples are given of printed and laser calibratable ladder thermistors: Impedance (Q) at 20°C Example number Example 1 Example 2 Example 3 Main thermistor line 1001 5 0.1 5 Main thermistor line 1002 5 0.1 5 Thermistor calibration line 1010a 0.1 9.9 0.1 Thermistor calibration line 1010b 0.2 10 0.1 Thermistor calibration line 1010c 0.3 10.1 0.1 Thermistor calibration line 1010d 0.4 10.2 0.1 Thermistor calibration line 1010e 0.5 10.3 0.1 Thermistor calibration line 101 Of 0.6 10.4 0.1 Thermistor calibration line 1010g 0.7 10.5 0.1 Example 1 and Example 3 are both examples of thermistors having an initial nominal impedance of around 10 Q, but with different arrangements of calibration lines 1010 to enable different ways to calibrating the thermistor 1000. However, in each case, the variability in impedance is less than 1 Q, meaning both can be fine-tuned. Example 2 is an example arrangement where the majority of the impedance of the thermistor is within the thermistor calibration lines 1010a-g. The overall impedance with all the calibration lines 1010 intact is around 1.5 Q, but if all but 1 calibration line 1010 is broken, the impedance could be varied up to around 10 Q. Therefore Example 2 illustrates an example arrangement that can be used with a high variability printing setup, as the overall impedance of the thermistor of example 3 can be varied much more than the overall impedance of the thermistor of examples 1 and 3. As a worked example, if a thermistor was printed with two thermistor main lines each having an impedance of 5 □ at 20°C, and 8 thermistor calibration lines each having an impedance of 10 Q at 20°C, this would provide an overall impedance of 11.25 Q at 20°C. However, if the desired impedance was 12 Q at 20°C with a desired tolerance range of ±0.5 Q at 20°C, then it would be necessary to break a number of thermistor calibration lines. To fit within the tolerance range, it would be reasonable to break between 2-4 thermistor calibration lines, to give an impedance of 11.7-12.5 Q at 20°C. In this way, it is possible to use the same printing process to provide a thermistor that can be calibrated to provide an impedance of 10.8-20 Q at 20°C.There may also be cases where the printing process itself does not always reliably produce a thermistor in line with the intended specifications. Taking Example 1 from the above table, the expected impedance at 20°C would be 10.04 Q. However, if the impedance was actually measured to be 9.7 Q, and the desired impedance was 10 ± 0.1 Q, then thermistor calibration lines 1010a-d may be determined to be broken to raise the impedance. The impedance would then be tested again to check whether any further calibration is necessary. Alternatively, or in addition, it would also be reasonable to break thermistor calibration lines one at a time and re-check the measured impedance. In this way it would also be possible to calculate the impedance of each line that has been broken, allowing some method to check the accuracy of the printing process, and thus further determine if and which thermistor calibration line to break or print next. 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, 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. In embodiment 6, the main thermistor lines and the thermistor calibration lines are described as being printed and cured. Alternatively, all or some of the main thermistor lines and / or the calibration lines may be formed by other physical components (e.g. thermistors) connected as otherwise described in embodiment 6, and calibrated by removing the physical components from the thermistor. 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 including a calibrated thermistor, wherein the calibrated thermistor includes:a first main thermistor line having a first impedance;a second main thermistor line having a second impedance; anda plurality of thermistor calibration lines each having their own impedance and wherein the plurality of thermistor calibration lines are electrically connected in parallel to each other, and in series with the first main thermistor line and the second main thermistor line, wherein at least one of the plurality of thermistor calibration lines is broken to provide a calibrated impedance of the calibrated thermistor.

2. The device of claim 1, wherein the first main thermistor line, the second main thermistor line and at least one thermistor calibration line of the plurality of thermistor calibration lines are printed on a surface of the hair drying and / or styling device or other substrate.

3. The device of any of the previous claims, wherein the first main thermistor line and second main thermistor line have the same impedance.

4. The device of any of the previous claims, wherein at least two of the plurality of thermistor calibration lines each have the same impedance when unbroken.

5. The device of any of the previous claims, wherein at least two of the plurality of thermistor calibration lines each have a different impedance when unbroken.

6. The device of any of the previous claims, wherein at least one of the first main thermistor line and second main thermistor line have an impedance lower than at least one of the plurality of thermistor calibration lines.

7. The device of any of the previous claims, wherein at least one of the first main thermistor line and second main thermistor line have an impedance higher than at least one of the plurality of thermistor calibration lines.

8. The device of any of the previous claims, wherein the plurality of thermistor calibration lines are individually breakable.

9. The device of any of the previous claims, wherein the calibrated thermistor forms part of a thermal safety circuit for a heater of the hair styling device, or a sensor circuit for sensing and / or controlling the temperature of the heater of the hair styling device, or a sensor circuit for sensing / and or controlling the temperature of a user’s hair.

10. A method of making a hair drying and / or styling device having a calibrated thermistor, the method comprising:providing a first main thermistor line having a first impedance;providing a second main thermistor line having a second impedance;providing a plurality of thermistor calibration lines each having their own impedance and wherein the plurality of thermistor calibration lines are electrically connected in parallel to each other, and in series with the first main thermistor line and the second main thermistor line;calibrating the impedance of the thermistor to be within a desired tolerance range of a desired impedance; andincorporating the calibrated thermistor into the hair drying and / or styling device.

11. The method of claim 10, wherein calibrating the impedance of the thermistor to be within a desired tolerance range of a desired impedance comprises measuring the impedance of the thermistor at a set temperature, and comparing the measured impedance with the desired impedance.

12. The method of claim 11, wherein calibrating the thermistor further comprises selecting, based on the comparison of the measured impedance with the desired impedance, at least one of the plurality of thermistor calibration lines; and breaking the selected at least one thermistor calibration line.

13. The method of claim 12, wherein breaking the selected at least one thermistor calibration line includes using a laser to break the at least one thermistor calibration line.

14. The method of claim 11, wherein calibrating the impedance of the thermistor to be within a desired tolerance range of a desired impedance further comprises determining, based on thecomparison of the measured impedance with the desired impedance, to provide at least one additional thermistor calibration line; andprinting the at least one additional thermistor calibration line in series with the first main thermistor line and the second main thermistor line.

Images