HAIR STYLING APPLIANCE

Abstract

A hair styling apparatus includes a plurality of heater electrodes which heat one or more hair styling heaters, a power source for powering the plurality of heater electrodes and a controller configured to control powering of the plurality of heater electrodes from the power source. The plurality of heaters includes a first subset and a second subset of heaters. The controller is configured to, in a first mode of operation; control the power source so that the first and second subsets are not simultaneously powered.

Claims

1. A hair styling apparatus comprising: a plurality of heater electrodes which heat one or more hair styling heaters, the plurality of heater electrodes comprising a first subset and a second subset; a power source for powering the plurality of heater electrodes and a controller configured to control powering of the plurality of heater electrodes from the power source; wherein, in a first mode of operation, the controller is configured to control the power source so that the first and second subsets of the plurality of heater electrodes are not simultaneously powered and are powered in a time interleaved manner multiple times per second.

2. (canceled)

3. A hair styling apparatus according to claim 1, wherein the controller is configured to select the first and second subsets so that a total current drawn by each of the first and second subsets is below a predetermined current threshold.

4. A hair styling apparatus according to claim 1, wherein no heater electrodes are selected for both the first and second sub sets.

5. A hair styling apparatus according to claim 4, wherein the controller is configured to switch the power source between the first and second subsets.

6. A hair styling apparatus according to claim 5, wherein the controller is configured to switch between each subset multiple times per second.

7. A hair styling apparatus according to claim 1, wherein the controller is configured to alternate between the first mode of operation and a second mode of operation in which the first and second subsets of the plurality of heater electrodes are simultaneously powered.

8. A hair styling apparatus according to claim 7, wherein the controller is configured to switch from the second mode of operation to the first mode of operation in response to a control signal.

9. A hair styling apparatus according to claim 8, wherein the control signal is that a predetermined amount of time in the second mode of operation has elapsed.

10. A hair styling apparatus according to claim 7, wherein the power source is a battery source and the hair styling apparatus further comprises a battery temperature sensor which senses the temperature of the battery source and which sends a battery temperature sense signal to the controller.

11. A hair styling apparatus according to claim 10, wherein the battery temperature sense signal is compared to a battery temperature threshold and the control signal is generated when the battery temperature sense signal is greater than the battery temperature threshold.

12. A hair styling apparatus according to claim 11, wherein the battery temperature threshold is in the range of 60 to 80 degrees C., more preferably 70 degrees C.

13. A hair styling apparatus according to claim 7, further comprising a temperature sensor sensing ambient temperature; and said control signal is generated when the sensed ambient temperature is below a threshold ambient temperature.

14. A hair styling apparatus according to claim 13, wherein the ambient temperature threshold is in the range of 25-35 degrees C.

15. A hair styling apparatus according to claim 1, comprising a first arm having a first contacting surface and a second arm having a second contacting surface, wherein the arms are moveable between a closed position in which the first and second contacting surfaces are adjacent and an open position in which the first and second contacting surfaces are spaced apart.

16. A hair styling apparatus according to claim 15, wherein the first arm comprises a first hair styling heater having a plurality of heater electrodes.

17. A hair styling apparatus according to claim 15, wherein the first arm comprises a first hair styling heater and the second arm comprises a second hair styling heater and each hair styling heater comprises at least one heater electrode.

18. A hair styling apparatus according to claim 1, comprising a touch sensitive switch configured to enable or disable the hair styling apparatus.

19. A hair styling apparatus according to claim 18, comprising a first arm having a first contacting surface and a second arm having a second contacting surface, wherein the arms are moveable between a closed position in which the first and second contacting surfaces are adjacent and an open position in which the first and second contacting surfaces are spaced apart, wherein the touch sensitive switch is located on the first or second contacting surface.

20. A hair styling apparatus according to claim 18, wherein when the arms are in the closed position, the touch sensitive switch is deactivated.

21. A hair styling apparatus according to claim 18, wherein the controller is configured to determine when a user has activated said touch sensitive switch for at least a predetermined duration of time and enable or disable the hair styling apparatus responsive to said determining.

22. A hair styling apparatus according to claim 1, wherein the or each heater comprises a heater plate which is mounted on a thermally insulating support structure.

23. A hair styling apparatus according to claim 22, wherein the heater plate comprises at least one recess which cooperates with a corresponding projection on the thermally insulating support structure.

24. A hair styling apparatus according to claim 22, wherein the thermally insulating support structure is resiliently mounted within an arm of the hair styling apparatus.

25. A hair styling apparatus according to claim 1, wherein the power source is a battery source.

26. A hair styling apparatus according to claim 1, wherein, in the first mode of operation, the controller is configured to control the powering of the heater electrodes using predefined heating cycles during one or more of which power is provided, at different times, to the first and second subsets of the plurality of heaters, and preferably wherein the heating cycles have a duration of between 100 s and 500 ms.

27. A hair styling apparatus according to claim 1, wherein the controller is configured to use pulse width modulation, (PWM), power control to control the delivery of power to the first and second subsets such that, during the first mode of operation, the controller is configured to generate power control signals for the first and second subsets to cause power to be delivered to the first and second subsets at different times within each of one or more heating cycles of the PWM power control.

28. A method of controlling a hair styling apparatus comprising a plurality of heater electrodes which heat one or more hair styling heaters, the plurality of heaters comprising a first subset and a second subset; the method comprising: controlling powering of the heater electrodes so that the first and second subsets of the plurality of heaters are not simultaneously powered and are powered in a time interleaved manner multiple times per second.

29. A controller for a hair styling appliance, wherein the controller configured is configured to implement the method of claim 28.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

[0028] FIG. 1 shows a first example of a hair styling appliance in which embodiments of the invention may be employed;

[0029] FIG. 2 shows a schematic block diagram of a hair styling appliance of the type illustrated in FIG. 1;

[0030] FIG. 3 shows a plan view of an embodiment of a hair styling heater for use in the hair styling appliance of FIG. 1;

[0031] FIGS. 4a and 4b show timing diagrams illustrating example duty cycles of heating electrodes driven, for example, by the control system in FIG. 2;

[0032] FIG. 5 shows a variant of the schematic block diagram of FIG. 2;

[0033] FIGS. 6a to 6d show timing diagrams illustrating example duty cycles of heating electrodes driven, for example, by the control system in FIG. 5; and

[0034] FIG. 7 shows a cross sectional view of the hair styling appliance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] FIG. 1 depicts an example of a typical hair straightener 1. The hair straightener 1 includes first and second arms each comprising an arm member 4a, 4b and heatable plates 6a, 6b coupled to heaters (not shown) in thermal contact with the heatable plates. The heatable plates are substantially flat and are arranged on the inside surfaces of the arms in an opposing formation. During the straightening process, hair is clamped between the hot heatable plates and then pulled under tension through the plates so as to mould it into a straightened form. The hair straightener may also be used to curl hair by rotating the hair straightener 180 towards the head prior to pulling the hair through the hot heatable plates.

[0036] Also shown in FIG. 1 is touch sensitive switch 5070 which is used to power the hair styling appliance on and off. The switch may be implemented as a capacitive touch switch comprising an electrode placed behind the plastic casing of the hair styling appliance. This obviates the need for a mechanical switch. In variants a resistive touch sensitive switch may also be used, or a piezo touch switch. As shown in FIG. 1, the touch sensitive switch 5070 is positioned on the inside of an arm facing the other arm. This means that the switch can only be pressed when the arms are spaced apart to prevent a user accidentally touching the switch and unintentionally turning the hair styling appliance on. Further, should the hair styling appliance be placed in luggage, such as a handbag, if a user rummages around in the bag for an item, it also prevents any accidental pressing of the switch. As a further safety mechanism, in embodiments the switch may also be deactivated (or the power supply configured to prevent activation of the appliance) when the arms of the hair styling appliance are closed together.

[0037] It will be appreciated that a hair straightener is just one example of a hair styling appliance and a skilled person would implement the various embodiments of the invention without difficult into other hair styling appliances such as a crimping iron for crimping hair or a hair styling appliance for curling hair.

[0038] FIG. 2 shows a block diagram of a power/control system 500 for a hair styling appliance incorporating a heater 300. The system comprises a low voltage power supply 504 deriving power from a 12v lithium ion battery 505 and/or a mains power supply input 502, which is used to charge the battery 505 via an AC to DC converter 503 which may be external or internal to the appliance. Power supply 504 may be configured to provide approximately 100 watts per heater; the heater resistance when hot may be selected accordinglyfor example at 12v a current in the range 5-10 amps may be delivered to a heater with a resistance in the range 1-2 ohms. The resistance may be scaled accordingly as the design voltage increases or decreases (changing as the inverse square of the voltage).

[0039] Power from power supply 504 is provided to a power control module 514, which in turn powers the one or more heaters 516. Power control module 514 may employ one or more power semiconductor switching devices to provide pulse width modulation control of the (DC) voltage from power supply 504 to heaters 516. Thus a high percentage on-time duty cycle may be employed during the initial, heating phase and afterwards the on-time duty cycle may be reduced and controlled to control the temperature(s) of the heaters 516.

[0040] Power from power supply 504 is also provided to a microcontroller/control means 506 coupled to non-volatile memory 508 storing processor control code for a temperature control algorithm, and to RAM 510. The skilled person will appreciate that any of a wide range of different control algorithms may be employed including, but not limited to, on-off control and proportional control. Optionally the control loop may include a feed-forward element responsive to a further input parameter relating to the hair styling appliance, for example to use the operation of the apparatus to improve the temperature control. An optional user interface 512 is also coupled to microcontroller 506, for example to provide one or more user controls and/or output indications such as a light or audible alert. The output(s) may be employed to indicate, for example, when the temperature of the heating plate has reached an operating temperature, for example in a region 140 C.-185 C.

[0041] Microcontroller 506 is also coupled to one or more optional temperature sensors such as thermistors 340. However, as previously mentioned, the temperature of a heating element may be sensed from its resistance and thus embodiments of the system include a current sense input 515 to microcontroller 506 sensing the current provided to a heater, for example via a current-sense resistor connected in series with the electrode. A predetermined calibration of resistance against temperature for an electrode may be stored in non-volatile memory 504 and in this way the printed track may be employed as a temperature sensor.

[0042] In the illustrative embodiment of FIG. 2, the touch sensitive switch 5070 is shown coupled to the low voltage PSU 504 to remove the need for the microcontroller to be permanently powered up. When off, the low voltage PSU 504 monitors for a change in capacitance of the touch sensitive switch indicating that a user has pressed the switch. The low voltage PSU then powers up the hair styling appliance. When on, the low voltage PSU 504 again monitors for a change in capacitance of the switch, then powering down the hair styling appliance. To power up and down, it may be necessary for a user to press on the touch sensitive switch for a minimum period of time before the PSU fully registers the press as a valid request to power on/off. This eliminates any accidental power up, or when styling, any accidental touching of the touch sensitive switch. A user may, for example, be required to press the touch sensitive switch for two, three, four or five seconds, or longer. In variants two touch sensitive switches may be used: one for turning the styling appliance on, another for turning the styling appliance off.

[0043] Providing such a touch sensitive switch on a hair styling appliance provides several advantages. Firstly, for battery powered products, if the hair styling appliance is carried around in luggage, it prevents the appliance being accidentally knocked on by other items in the luggage. Further, it also improves the aesthetic appearance of the product, eliminating the need for additional components on the surface of the hair styling appliance.

[0044] In this embodiment the touch sensitive switch is shown coupled to the low voltage PSU. In variants such a switch may be coupled to the microcontroller, although in such variants it will then be appreciated that the microcontroller may then need to be permanently powered to permit detection of a press of the touch sensitive switch. In other variants a dedicated circuit may also be used.

[0045] This touch sensitive power switch may be applied to any of the embodiments described herein and also as an adaption to otherwise standard devices.

[0046] Each heater plate may be powered by a heater electrode. Depending upon the thickness of the heater plate, lateral conductivity within the plate may not be sufficient to give the desired results with a single heater electrode. Accordingly, an example of a heater plate is illustrated in FIG. 3 which may form the heatable plates 6a, 6b of the hair straightener of FIG. 1. The heater plate 300 may be provided with a plurality of separately controllable heating zones 300a, b, each with a respective electrode 330a, b and thermistor 340a, b. Connections to these are brought out, for convenience, to one edge of the heater plate; a broadened track region 332 is provided for the electrode further from the connection point to reduce heating in the connection path. Each of the electrodes is provided with a separate control loop controlled by the temperature sensed by the respective thermistor. In embodiments more than 3 zones may be provided.

[0047] The heater used in the various embodiments described herein may be formed as described in WO2014/001769 and GB2503521 which are incorporated by reference. Thus, the heater may comprise an aluminium heater plate of thickness of order 1 mm, bearing a plasma electrolytic oxide (PEO) coating of aluminium oxide of thickness less than 100 m, for example in the range 5-15 m.

[0048] The hair styling appliance comprises a plurality of electrodes. As shown in FIG. 1, there may two heater plates, each with their own electrode and thus there are two electrodes. Alternatively, only one arm of the appliance shown in FIG. 1 may comprise a heater plate but this heater plate comprises at least two, possibly more electrodes. There may also be multiple heater plates each having multiple electrodes.

[0049] The power to the plurality of heater electrodes may be independently controlled. For example, for an appliance having two arms, each with a heater and one heater electrode for each heater, FIGS. 4a and 4b show the Voltage against Time for each heater electrode. In this example illustration, the microcontroller 506 uses pulse width modulation (PWM) power control to control the supply of power to the two heater electrodes using the power control block 514. In pulse width modulation power control, the on time of the power signal within a sequence of PWM periods (here labelled d and also referred to in other parts of this document as heater cycles) is varied in order to vary the amount of power delivered to each heater electrode. Typically, the PWM period / heater cycle may be between 100 s and 500 ms. It will be observed from FIGS. 4a and 4b that initially both heater electrodes are powered simultaneously until the desired operating temperature is reached. A high percentage on-time duty cycle within any given period d may be employed during the initial heating phase; afterwards the on-time duty cycle may be reduced for each heater so as to retain the hair styling heater at a desired operating temperature. Each heater may be controlled independently to stabilise the temperature of each heater at the desired operating temperature. Accordingly, the draw is not exactly the same for both heater electrodes. Nevertheless, it can be observed from FIGS. 4a and 4b that there may be sustained periods of a maximum current draw as a result of both heaters being powered simultaneously.

[0050] When powered by battery, a high current draw, such as from driving both heaters simultaneously, may lead to the battery power source heating up to an unacceptably high temperature. This may be exacerbated if both heaters are simultaneously driven for extended periods of time. The current draw when simultaneously driving the heaters, combined with a desire to conceal the battery power source means that heat dissipation may become an important factor in the construction of such a hair styling appliance.

[0051] FIG. 5 shows a variant of the schematic block diagram of FIG. 2 with modified microcontroller/control means and power control module 514b. The reference numbers in common are used in both systems and thus any description applies equally to both.

[0052] The microcontroller switching control signal 708, labelled temp/power control in FIG. 5 may comprise multiple outputs, one for each heater power switch to be controlled. The embodiment shown in FIG. 5 comprises two heater electrodes, one on each of the two heater plates. Two outputs, one to activate the first heater electrode via the first power switch 702 and the second to activate the second heater electrode via the second power switch 704 are present on the power control module. In FIG. 5, further heater electrodes may also be driven. These may be present, for example, in a multi-zoned heater variant. Dotted arrow lines to the heaters 516 show optional connections to such additional heater elements.

[0053] In variants the switching control signal may be a serial data connection or encoded such that the control system can scale to independently control multiple heater elements. This may be particularly useful in embodiments having multiple heating zones (two or three per heater for example) and where the number of outputs from the microcontroller may be limited. Alternatively the microcontroller may have multiple outputs, one for each power switch. The optional decode block 706 in the power control module 514b decodes the signal received from the microcontroller and splits this out into separate drive signals to activate the power switches 702 and 704. In variants incorporating multiple heating zones on each heater plate the signal may be decoded into more outputs, one for each zone.

[0054] Battery power source 505 in FIG. 5 may further incorporate a battery temperature sensor 5050, such as a thermistor. The battery temperature sensor 5050 provides a battery temperature sense signal 5051 coupled to the microcontroller 506. The battery temperature sense signal may be factored into the temperature control algorithm and powering of the heater plates. It will be appreciated however that such a feature is optional.

[0055] In the embodiment shown in FIG. 5, the battery temperature sensor many also be used as part of the safety shutdown 520. As set out above, the styling appliance may incorporate one or more safety shut down circuits 520 coupled to the one or more heater electrodes and/or temperature sensors 340 to monitor the heater temperature and electronically shut down the power supply to the heater should overheating be detected. In embodiments, this may be extended to also prevent overheating of the battery. In embodiments safety shut down circuit 520 controls a guard transistor 522, as illustrated a power MOSFET, which removes power from the power control block on detection of a potential fault. Guard transistor 522 may be provided either before or after power control block 514a. In normal operation this device is always on; the device may be selected such that when power is removed from the transistor it switches off, thus failing safe, for example by employing an enhancement-mode device. Such control and safety shut down is applicable to all the embodiments described herein.

[0056] The battery temperature sensor 5050 (or another battery temperature sensor) may additionally or alternatively be used to control power on/off of the hair styling appliance. The generated signal B.sub.Tsense 5051 is fed into the microcontroller 506 and may be used to provide battery temperature information for use in a safety mechanism to shut down the styling appliance, or stop power delivery to one or more heater elements/plates if the battery temperature exceeds a battery threshold temperature. Thus, the microcontroller/control means may be arranged such that power is only supplied to the heating elements/plates when the temperature sensed by the battery temperature sensor is below a battery threshold temperature. In embodiments, this threshold temperature may be a value in the range of 60-100 C., for example 70 C. However it will be appreciated that the operational threshold temperature may be dependent on the particular construction (packaging, chemical formulation for example) of the particular battery used. Techniques such as active cooling of the battery pack, or heat transfer means such as a heat sink, may often be insufficient to retain the battery pack within its preferred safe operating range.

[0057] Following deactivation of the heater plates, the control system may prevent the hair styling appliance from being used again until the battery temperature has fallen below either the battery threshold temperature at which power down was previously initiated, or below a lower reactivation temperature which would be set to a temperature below the battery threshold temperature.

[0058] Another technique that may be used to prevent heat build-up in the battery power source is to slow the rate of heating by throttling the maximum current delivered or using higher resistance heating elements. However, adopting such a technique may mean that the temperature of a heater plate cannot be changed very rapidly, which may lead to a poor transient response.

[0059] Further, an ambient temperature sensor, such as temperature sensor 5060 in FIG. 5 may be used to monitor the ambient temperature (i.e. the temperature surrounding the hair styling appliance) and prevent power delivery to the heater plates if an ambient temperature threshold is exceeded. An ambient temperature sense signal A.sub.Tsense 5061 is then generated and fed into the microcontroller 5061. In embodiments, this ambient threshold temperature may be a value in the range of 25-35 degrees C., for example 25 degrees C. or 33 degrees C. Such ambient temperature sensing may be used as a further safety mechanism to protect against overheating of the hair styling appliance.

[0060] This is particularly useful in warmer environments in which the battery which may heat up too fast (such as outside in hot climates, or in a hot indoor environment). A user may then be preventing from turning on the hair styling appliance until the ambient temperature has reduced. Thus, the microcontroller/control means may be arranged such that power is only supplied to the heating elements/plates when the sensed ambient temperature is below an ambient threshold temperature.

[0061] In one or both instances above, if either the ambient or battery threshold temperatures are exceeded; visual or audio feedback may be provided to the user to indicate that the device has entered a safety mode or indicate the temperature status.

[0062] In a first control mode the microcontroller/control means in FIG. 5 is configured to lower the maximum current draw by operating the heaters in anti-phase. This means that in an embodiment having two heater electrodes (one for each heater), only one heater element may be powered at a time in the embodiment in FIG. 5. Operating in anti-phase, there may also be periods where both heaters electrodes are off, such as when both heater plates are at a desired operating temperature. Given a current draw of I for one heater arranged to heat an entire heater plate, when two heaters are powered simultaneously to heat two heater plates, the current draw may be approximately 2I. Using this convention, the maximum current draw is limited to the current draw for driving one heater (i.e. I, the current draw for one heater arranged to heat an entire heater plate). This means that the controller may be configured to prevent all (both in the embodiment of FIG. 5) heater electrodes being powered at the same time.

[0063] FIGS. 6a and 6b show a graph for each heater in an appliance having two heaters and operating according to the preceding paragraph. As before, the microcontroller 506 uses pulse width modulation power control to control power delivery to the two heaters. It will be observed from FIGS. 6a and 6b that both heater electrodes are now not powered at the same time within each heating cycle (here labelled c). The dotted lines between the Figures show instances of one heater starting or stoppingnote there is no overlap. In a heating cycle (c), each of the heaters below temperature are powered in a sequence which may be fixed or determined by the microcontroller, but only one at a time. As before, the heating cycle may be between 100 s and 500 ms and thus the controller rapidly switches the delivery of the power between the two heaters such that, as far as the user is concerned, both heaters appear to be heating up simultaneously.

[0064] As will be apparent to those of ordinary skill in the art from FIG. 5, the controller 506 controls this delivery of power to the first and second heaters by generating control signals 708 that cause the decode/drive enable unit 706 to open and close the switches 702 and 704. In some cases, the control signals 708 may be directly used to control the switching of the switches 702 and 704.

[0065] The microcontroller may implement a control algorithm configured to allocate equal percentages of a heating cycle c to each heater, for example 50% of the time. Typically this may be the case when a user powers on the appliance to heat both heater plates to the desired operating temperature evenly and as fast as possible. However, in the event that one heater plate heats up slower than the other, a higher portion of time in any given period/heating cycle c may be allocated to the cooler heater plate. Furthermore, in the event one heater plate cools faster than the other when placed about a quantity of hair, the microcontroller, in response to a temperature dependent sense signal, may act accordingly to allocate a higher portion of heating time in any one heating cycle to power the heater in the cooler heating plate.

[0066] In some embodiments of the hair styling appliance, there may be multiple heating zones on each heater plate, as shown in FIG. 3 for example and also shown in GB2477834, herein incorporated by reference. Each heating zone may comprise a separate heater electrode arranged to heat a portion of the heater plate. In such embodiments, it may then be permissible to simultaneously heat multiple heating zones in many different configurations. The controller may therefore be configured to prevent all the heater electrodes distributed across one or more plates being powered at the same time (or for only short periods of time).

[0067] As previously discussed, we generally consider a current draw I to correspond to the current draw necessary to power a heater electrode heating an entire heater plate. Thus, in embodiments having multiple heating zones on a heater plate, one electrode in each zone may be considered to draw (for the purposes of comparison only), a portion of current draw I. In an embodiment having two heating zones on each heater plate, i.e. four heating zones in total, each zone may be considered to draw a current of 0.5I (presuming the resistances are generally the same). A maximum preferred current draw I may therefore correspond to powering two zones simultaneously. Any two zones: both on the same plate, or one on each plate may be simultaneously powered. This means that in the event a quantity of hair is placed on only one section of the heater plates, such that only one zone needs to be powered to retain the desired operating temperature, then opposing zones on two heater plates may be simultaneously powered whilst staying within the preferred current draw limit (I) to prevent the battery source overheating.

[0068] It will be appreciated that the maximum preferred current draw to prevent the battery source overheating may not be I, it may instead be higher or lower than this, Therefore, in some embodiments it may then be possible to power different combinations and numbers of heating zones simultaneously without the battery source overheating. By way of example, in an embodiment having two heater zones on each of two heater plates, given a preferred maximum current draw of 1.5I, it may then be possible to power three heater zones simultaneously whilst staying within the preferred current draw limit.

[0069] Table 1 below shows exemplary combinations of the maximum zones that may be powered at any one time. The nominal current draw column provides examples of the nominal current draw limit, defined in multiples of the current draw of one heater arranged to heat an entire heater plate. Accordingly, for the purposes of this illustrative example, the current draw of two heater electrodes, each heating half of a heater plate, is deemed the same as one heater element powering an entire heater plate. It will however be appreciated that in practice the current draw may be different.

TABLE-US-00001 Zones per Number of plates Nominal Zones powered at plate in appliance draw I any one time 1 2 1 1 2 2 1 2 3 2 1 3 1 2 1.5 1 2 2 1.5 3 3 2 1.5 3

[0070] Returning now to the embodiments shown in FIG. 5 having one heater electrode in each of two plates, in a second control mode the microcontroller may allow periods of overlap in which both heaters are powered simultaneously to heat up both heater plates at the same time. Given the nominal preferred current draw of I, limited periods of a higher current draw may be permitted, so long as these higher current draw periods are interleaved with rest periods in which the nominal preferred current draw is not exceeded. So as to prevent overheating, the duration of overlap may be limited by the microcontroller/control means. The microcontroller/control means may be configured to limit this overlap to a predetermined duration within a fixed period of time based on predetermined characteristics of the battery. The microcontroller may permit, for example, simultaneous heating to only take place during the initial heat up from power on, then revert to the first mode of operation. In other words, the controller may be configured to limit a duration in which the two heater electrodes are simultaneously powerable by the power source. This may also be dependent on the battery temperature or ambient temperature.

[0071] In an enhancement to the second control mode the overlap control may be variable, being controlled, for example, in response to feedback from a battery temperature sense signal 5051 as depicted in FIG. 5. In this variant, the microcontroller may then actively monitor the temperature of the battery source, controlling the permissible overlap in which the nominal preferred current draw may be exceeded in response to the temperature of the battery source. This may be useful to allow both heaters (based on an embodiment have one heater element in each heater plate) to be driven simultaneously from cold at power on, with the microcontroller then disabling any overlap in heating once the heaters are first up to temperature.

[0072] By way of example, FIGS. 6c and 6d show a graph for each heater in an appliance having two heaters where overlap is permitted. In the first phase, the battery temperature is within the preferred operating range and so the microcontroller is configured to operate in the second control mode with periods in which both heaters are heated simultaneously. In the second phase, the battery temperature sensor may sense the temperature approaching (or exceeding) a threshold temperature which results in the microcontroller changing to the first control of operation in which the heaters are powered in anti-phase. The microcontroller may then optionally return to the second control mode when the temperature of the battery source drops. The dotted lines between the Figures show regions of overlap in heaters being powered in the first phase.

[0073] The second technique may also be implemented for embodiments having multiple heating zones on one or more of the heater plates. Incorporating the second technique, the microcontroller may then permit various combinations of zones to be heated simultaneously as previously described, with periods in which the nominal preferred current draw is exceeded by powering further heating zones for a limited period of time. This means that the controller may be configured to limit a duration in which at least two or more of the heater electrodes are simultaneously powerable by the battery power source.

[0074] FIG. 7 shows a cross-sectional view of an illustrative embodiment of an arm 700 of a hair styling appliance. The arm 700 comprises an outer casing 712 to which other components of the hair styling appliance are secured. A heater element 704 is positioned on heater plate 702 to form a hair styling heater assembly. The hair styling heater assembly is then retained on the arm by the use of a thermally insulating support structure 714.

[0075] The heater plate 702 comprises a styling surface 715 on one side that contacts the hair to be styled during use. On the other side of the heater plate two L-shaped recesses 709a, 709b provide sockets for securely fixing the hair styling heater assembly to the thermally insulating support structure 714.

[0076] The thermally insulating support structure 714 is formed from insulating material and may, for example, be constructed from a similar material to the casing. The support structure 714 comprises a pair of L-shaped projections 708a, 708b arranged to fit into the 709a, 709b recesses in the heater plate 702 and couple the heater plate and support structure together. To allow the projections to fit into the recesses, they may have a small degree of flex such that then can snap-fit into the recesses, thereby securely fixing the heater plate and support structure together. It will be appreciated however that other means for coupling the hair styling heater assembly and the support structure are possible, and the example shown in FIG. 15 is purely illustrative of one way of doing so.

[0077] To secure the support structure 714 to the casing, sprinted members 710a and 710b are used. These are secured at one end to the casing and at the other end to the support structure. In the illustrative embodiment shown in FIG. 15, compression springs are used which bias the heater assembly and support structure away from the arm. These allow the heater plate to move relative to the casing during styling, allowing the plates to retain contact with varying thicknesses and changes in the profile of hair clamped between opposing pairs of styling surfaces on the heater plates. It will be appreciated that various other arrangements may be used that provide allow for movement of the heater plates.

[0078] This heater assembly arrangement provides several advantages: [0079] 1. Firstly, it reduces the width of the outer casing needed to retain the hair styling heater assembly as no retaining lugs or fixings are now needed at the sides of the heater assembly. [0080] 2. Secondly, with no protrusions extending to one or more sides of the heater plate 702, the widest part of the heater plate is the styling surface 715. Such an arrangement is particularly advantageous during manufacturing as it allows the heater plates to be closely packed, with no or minimal gap between them. This allows a large number of styling surfaces to be screen printed, as if they were one large surface, improving the efficiency of the printing process.

[0081] The skilled person will appreciate that the techniques we have described above may be employed for a range of hair styling appliances including, but not limited to, a hair straightener, a hair crimping device, and a hair curler. The skilled person would also appreciate that features from many of the embodiments are interchangeable and not limited to the specific embodiment they are described in relation to.

[0082] 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 spirit and scope of the claims appended hereto.