FIELD OF INVENTION

The present invention relates to a haircare appliance.

BACKGROUND

Haircare appliances are generally used to treat or style hair, and some haircare appliances may treat or style hair using airflow along with heat. Such haircare appliances are typically held by a user and moved relative to the hair to obtain desired treatment or styling.

SUMMARY OF INVENTION

According to a first aspect, there is provided a DC power supply circuit comprising: a rectifying circuit for receiving an AC power supply and generating a rectified voltage, the rectifying circuit having first and second outputs for outputting the rectified voltage; a capacitor having first and second terminals, the first terminal being connected to the first output; a switch, a first side of which is connected to the second terminal, and the second side of which is connected to the second output; and a controller having a voltage input for sensing the AC power supply, the controller being connected to control whether the switch is open or closed. The controller is configured to: determine a voltage of the AC power supply based on the voltage input; determine whether the voltage is within an acceptable range; when the voltage is within the acceptable range, close the switch, such that the rectifying circuit generates the rectified voltage.

This may reduce the number of high-voltage capacitors needed in a DC power supply circuit.

The controller may comprise a voltage determining circuit for determining the voltage of the AC power supply.

The voltage determining circuit may comprise an analog-to-digital converter.

The controller may comprise a microcontroller.

The switch may comprise a semiconductor switch. For example, the semiconductor switch may be a MOSFET. The rectifying circuit may comprise a full-wave or half-wave rectifier.

The controller may be configured to: determine at least one future expected zero-crossing point for the voltage of the AC power supply; and perform the step of closing the switch at the expected zero crossing point, thereby to reduce an inrush current to the capacitor.

According to a second aspect, there is provided a method of controlling a DC power supply, the method comprising: supplying an AC power supply to an input of a rectifying circuit; determining a voltage of the AC power supply; determining whether the voltage of the AC power supply is within an acceptable range; and when the voltage is within an acceptable range, closing a switch that is serially connected with a capacitor, the switch and capacitor being connected in series between outputs of the rectifying circuit, such that the rectifying circuit generates the rectified voltage.

Determining the voltage of the AC power supply may comprise performing analog-to-digital conversion of an AC power supply signal based on the AC power supply.

The method may comprise: determining at least one future expected zero-crossing point for the voltage of the AC power supply; and performing the step of closing the switch at the expected zero-crossing point, thereby to reduce an inrush current to the capacitor.

According to a third aspect, there is provided a haircare appliance comprising: an airflow housing; an airflow generator disposed in the airflow housing, for generating an airflow through the airflow housing; a circuitry housing separate to the airflow housing; airflow generator control circuitry disposed in the circuitry housing, for generating a drive current; an electromagnetic capability (EMC) filter disposed in the circuitry housing. The EMC filter is coupled to the airflow generator control circuitry, for: receiving the drive current; and EMC-filtering the drive current to generate an EMC- filtered drive current. A wire connects the circuitry housing to the heater housing, the wire coupling the EMC filter to the airflow generator so that the airflow generator can be driven, in use, by the EMC- filtered drive current.

The EMC filter may comprise a common-mode filter, such as a common-mode choke.

The airflow generator control circuit may comprise an El-bridge motor driver. The airflow generator control circuitry may comprise a switch for controlling current flow to the airflow generator.

The haircare appliance of any preceding claim, wherein the airflow generator may comprise a motor and an impeller driven by the motor, wherein a rotational position of the impeller is calculated using current and/or voltage values communicated to the airflow generator control circuitry over the wire.

The motor may comprise a single-phase motor.

The haircare appliance may be configured to be powered by a mains power supply, and the haircare appliance may comprise a power converter for providing electrical power to the airflow generator control circuitry, the power converter disposed in the circuitry housing.

The haircare appliance may comprise: a heater assembly disposed in the heater housing, for heating the airflow as it moves through the airflow housing; a pair of power supply wires for providing electrical power to the heater assembly, the pair of power supply wires extending from the circuitry housing to the airflow housing; and a relay disposed within the circuitry housing, the relay interrupting a current supply path from a mains power supply to one of the power supply wires.

The haircare appliance may comprise a thermal protection circuit, wherein the relay is controlled by the thermal protection circuit.

Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a perspective view of an embodiment of a haircare appliance;

Figure 2 is a schematic view illustrating internal components of the haircare appliance of Figure 1 ; Figure 3 is a schematic longitudinal section of a heater housing of the haircare appliance of Figures 1 and 2;

Figure 4 is a fdter forming part of the haircare appliance of Figures 1-3;

Figure 5 is a circuit forming part of the haircare appliance of Figures 1-3; Figure 6 is a further filter forming part of the haircare appliance of Figures 1-3;

Figure 7 shows voltage sensing circuit, forming part of the haircare appliance of Figures 1-3; and Figure 8 shows waveforms related to the voltage sensing circuit of Figure 7.

DETAILED DESCRIPTION

A haircare appliance, generally designated 10, is shown schematically in Figures 1 and 2. The haircare appliance 10 in the embodiment of Figures 1 and 2 is a hairdryer, although it will be appreciated that some of the teachings discussed herein may be applied to other types of haircare appliance, for example hair straighteners or hair curlers or the like.

The haircare appliance 10 comprises a circuitry housing 12, a heater housing 14, and an electrical cable 16 extending from the circuitry housing 12 to the heater housing 14. The circuitry housing 12 defines an enclosure that houses a number of electronic components as will be described hereinafter, and the electronic components within the circuitry housing 12 are coupled to corresponding electronic components within the heater housing 14 by wires held within the electrical cable 16. Whilst referred to as wires, it will be appreciated that each wire may comprise more than one electrically conducting filament, for example as is the case with a braided wire, with the overall structure of multiple filaments being considered a wire. A power connector 15 in the form of a plug is coupled to the opposite side of the circuitry housing 12 to the electrical cable 16. The power connector 15 is configured to interact with an AC mains power supply, for example via a mains socket (not shown), to provide electrical current to the haircare appliance 10 in use.

The heater housing 14 defines a hollow, generally elongate, handle that is intended to be grasped by a user in use. As seen in Figure 1, the heater housing 14 comprises a conical end portion 18 and a wall 20 extending upwardly from the conical end portion 18, such that a first end 22 of the heater housing 14 is generally cylindrical in form. The heater housing 14 has a second end 24 distal from the first end 22, and the heater housing 14 is curved such that the second end 24 is angled relative to the first end 22. An air inlet 26 is located at the first end 22 of the heater housing 14 on the wall 20, and takes the form of a plurality of apertures, for example in a mesh-like structure. An air outlet 28 is located at the second end 24, and comprises an aperture through which air may flow in use.

A user interface 30 is formed on the wall 20, and may take the form of a plurality of buttons, a touchscreen, or a combination thereof. User interface 30 allows a user to input various desired settings that are used by the other components of the haircare appliance 10 to control heat and fan settings, and may also provide visual and/or audible feedback, as described in more detail below.

A heater 32 and an airflow generator 34 are disposed within the heater housing 14.

Turning to Figure 2, the internal components and functional features of the haircare appliance 10 will be described in more detail. The skilled person will appreciate that the components and features are set out in schematic form, and that the relative positions and sizes of those components and features of the actual appliance may vary from what is illustrated in Figure 2.

Power connector 15 supplies, in use, AC mains power into circuitry housing 12. The AC mains power is supplied to a mains fdter 36, which filters the AC mains power. Operation of the mains filter 36 is described in more detail below.

The AC output of the mains filter 36 is supplied to the input of an AC-to-DC converter 38. The AC- to-DC converter 38 converts the incoming AC voltage to DC, outputting various DC voltages as required by different components of the haircare appliance 10, as described in more detail below.

One output of the converter 38 is a DC power supply to a fan motor controller 40. Fan motor controller 40 receives control signals as described in more detail below, and outputs a fan motor drive signal to an electromagnetic compatibility (EMC) filter 42. The EMC filter 42 outputs the filtered fan motor drive signal to a fan motor 44. The EMC filter filters out harmonics generated by the fan motor 44, in use. The fan motor 44 forms part of the airflow generator 34, as described in more detail below.

Fan motor 44 may take the form of, for example, a V9 Dyson Digital Motor by Dyson Technology Limited. The V9 Dyson Digital Motor is a single-phase motor. Use of a single-phase motor may reduce the number of wires required to extend from circuitry housing 12 to heater housing 14 compared to, for example a similar arrangement where a three-phase motor is utilised by the airflow generator 34 within heater housing 14. Alternatively, a three-phase motor may be used to obtain a smaller and/or lighter heater housing 14. The output of mains filter 36 also supplies the filtered AC mains power to heater housing 14 via electrical cable 16. Circuitry housing 12 also includes a relay 46, for selectively switching live circuit 48 of the AC mains power. Control of relay 46 is described in more detail below.

Several of the features and components of the haircare appliance 10 are implemented in a microcontroller unit (MCU) 48, as described in more detail below. The MCU 48 includes a processor, memory, and other components necessary to implement the features and components described herein. Although the example describes the use of a haircare appliance 10 having a single MCU 48 disposed within heater housing 14, it will be appreciated that the MCU 48 may be located within circuitry housing 12. Alternatively, implementation of the features and components of the haircare appliance 10 may be distributed across two or more processors, located within circuitry housing 12, heater housing 14, or both. Additional supporting circuitry, such as communications and power, are omitted for clarity. An example of a suitable MCU is the ARM Cortex-M0+.

User interface 30 allows a user to set a target temperature 50 and a fan speed 52. Target temperature 50 and fan speed 52 may each be selectable from a relatively small number of options (e.g., high, medium and low settings for each of target temperature 50 and fan speed 52). Alternatively, either or both of the target temperature 50 and fan speed 52 may be selected in a more granular way. For example, target temperature may be chosen as a specific temperature in degrees C or F, to a resolution of, say, 5, 10 or 20°. Similarly, fan speed 52 may be chosen as a specific airflow, such as in litres per second.

Target temperature 50 and fan speed 52 are stored within MCU 48. They may be stored persistently or reset to default values at each start-up of the haircare appliance 10.

Fan speed 52 is provided as an input to fan motor controller 40 and to a control block 54 within MCU 48, and target temperature 50 is supplied to control block 54, as described in more detail below.

As described in more detail below in relation to Figure 3, heater housing 14 includes an air pressure sensor in the form of a barometer 58. Barometer 58 provides a raw pressure signal to pressure processing circuitry 60, which includes scaling and filtering circuitry that processes the raw pressure signal before outputting it as a pressure value. Such circuitry is well known to the skilled person and so will not be described in more detail. The pressure value is provided as an input to MCU 48, where it is used as described below. The pressure value may be, for example, the instantaneous pressure value based on the raw pressure signal from barometer 58. Alternatively, the pressure value may be low-pass fdtered to reduce the impact of, for example, noise or spurious short-term pressure changes.

As described in more detail below in relation to Figure 3, heater housing 14 includes an air exit temperature (AET) sensor 84. AET sensor 84 provides a raw temperature signal to AET processing circuitry 86, which includes scaling and filtering circuitry that processes the raw temperature signal before outputting it as a temperature value. Such circuitry is well known to the skilled person and so will not be described in more detail. The temperature value may be, for example, the instantaneous temperature value based on the raw signal from AET sensor 84. Alternatively, the temperature value may be low-pass filtered to reduce the impact of, for example, noise or spurious short-term temperature changes.

The temperature value is provided as an input to an AET buffer 88, as described in more detail below. AET buffer 88 provides a buffered temperature value as an input to MCU 48, where it is used as described in more detail below.

Heater housing 14 includes a heater temperature sensor 90, positioned to sense a temperature of heater 32. Heater temperature sensor 90 provides a raw temperature signal to heater temperature processing circuitry 92, which includes scaling and filtering circuitry that processes the raw temperature signal before outputting it as a temperature value. Such circuitry is well known to the skilled person and so will not be described in more detail. The temperature value is provided as an input to a heater temperature buffer 94, as described in more detail below. Heater temperature buffer 94 provides a buffered temperature value as an input to MCU 48, where it is used as described in more detail below.

Pressure processing block 62 within control block 54 processes the pressure and temperatures value as described in more detail below, and outputs control information to a power controller 64 within control block 54. Target smoothing block 66 processes the target temperature 50 as described in more detail below, and outputs power target information to power controller 64. Based on the received control information and power target information, power controller 64 outputs a power control signal to a current controller 68. The AC power supply provided from circuitry housing 12 to heater housing 14 via electrical cable 16 is supplied as an input to a voltage sensing circuit 70. Voltage sensing circuit 70 may comprise, for example, an analogue to digital converter for sampling a voltage of the AC power supply and converting it to a numerical value that can be used by MCU 48. The output voltage sensing circuit 70 is provided to an RMS voltage calculator 72, which determines an RMS voltage of the AC power supply, as described in more detail below.

The calculated RMS voltage is provided as an input to current controller 68. The calculated RMS voltage is also provided as an input to a voltage check block 73. Voltage check block 73 determines whether the calculated RMS voltage is above a threshold (or, alternatively, below a threshold, or between two thresholds), and outputs a gate control signal to the converter 38, as described in more detail below.

Current controller 68 uses the RMS voltage and power control signal to determine a desired power output, which may be in the form of an instantaneous or average desired current, power, or combination thereof. The desired output is provided as an input to a TRIAC pattern calculator 74. TRIAC pattern calculator 74 uses the desired output to generate a suitable TRIAC drive pattern. For example, the TRIAC pattern calculator 74 may use a burst-fire control scheme, a phase angle control scheme, or a combination thereof, to generate a TRIAC drive pattern to control current flow to heater elements, as described in more detail below. The TRIAC drive pattern is provided as an input to TRIAC drive signal generator 76.

The AC power supply provided from circuitry housing 12 to heater housing 14 via electrical cable 16 is also supplied as an input to a zero-crossing detection circuit 78. The zero-crossing detection circuit 78 determines zero-crossing points of the AC power supply, as described in more detail below, and provides them as an input to a delay compensation block 80 within MCU 48. The delay compensation block 80 determines a suitable delay compensation value and provides this as an input to TRIAC drive signal calculator 76.

TRIAC drive signal calculator 76 converts the TRIAC drive pattern into suitable TRIAC drive signals and applies appropriate delay compensation, as described in more detail below. The TRIAC drive signals are output from MCU 48 and provided as an input to TRIAC drive circuit 82. The AET temperature and heater temperature are also provided as inputs to overheat protection circuitry 96. Overheat protection circuitry 96 operates to determine when the AET temperature and/or heater temperature exceed various thresholds, and provides overheat control signals to TRIAC drive circuit 82 so that appropriate action can be taken, as described in more detail below.

TRIAC drive circuit 82 outputs TRIAC drive signals to TRIACs 98, and the TRIACs 98 are also coupled to receive the AC power supply, as described in more detail below. Although three TRIACs are illustrated, the skilled person will appreciate that any suitable number of TRIACs may be driven by the TRIAC drive signals.

Each TRIAC 98 drives a heater element 100 within heater 32. Each heater element 100 may take the form of, for example, a resistive trace on a heat-resistant substrate, such as a resistive wire wound around an insulating scaffold. Alternative types of heater can use a heater track printed onto a polyamide sheet such as Kapton or a ceramic heater coupon having an embedded heater track formed from a trace made from an electrically conductive material such as but not limited to tungsten. In order to dissipate heat from the ceramic heater coupon cooling fins may be provided. Each heater element 100 is exposed to air flowing through an air flow path of the haircare appliance 10, as described in more detail below.

Figure 3 shows a simplified schematic and partially sectioned view of the heater housing 14 and some of the components it contains. Many of the features and components described in relation to Figures 1 and 2, including the bend in heater housing 14, have been omitted for clarity.

Figure 3 shows schematically an air flow path 102 defined within heater housing 14. Airflow generator 34 comprises the fan motor 44 and an axial impeller 104. Fan motor 44 drives impeller 104 to generate airflow. Air is initially sucked through air inlet 26, as shown by an air-in arrow 106. The air passes through an inlet filter 108, which filters particles such as dust and hair from the air before it passes into the airflow path 102. In use, resistance offered by inlet filter 108 causes a reduced pressure region 110 between inlet filter 108 and impeller 104. Pressure sensor 58 is disposed within this region, to allow sensing of pressure changes as described in more detail below.

Air moves through impeller 104 and past motor 44, cooling motor 44 as it passes. The air is then heated as it passes through heater 32. The temperature of heater 32 is monitored by heater temperature sensor 90, as described in more detail below. Air then passes AET sensor 84, before exiting outlet 28 as shown by an air-out arrow 110.

Haircare appliance 10 may be used with one or more optional detachable accessories, such as a flow- accelerating accessory 160 as shown in Figure 3. Accessory 160 may be releasably attached at or adjacent air outlet 28 to control the shape, direction and speed, for example, of the airflow. Haircare appliance 10 includes a sensor or scanner, such as ID sensor 162, that allows an attached airflow accessory to be identified.

An attached airflow accessory 160 may be identified in any of a number of ways. For example, the accessory 160 may include an identifier that can be sensed or scanned by corresponding ID sensor 162.

The identifier may take the form of a circuit that can communicate the identifier. For example, the identity can be encoded by an identifier in the form of an RFID or near field communication (NFC) device 163 disposed in or on a portion of the accessory. In that case, ID sensor 162 takes the form of a corresponding RFID/NFC scanner provided on or in the haircare appliance 10.

The identifier may, alternatively or in addition, take the form of a scannable image, that may be printed, embossed, engraved, 3-D-printed, or otherwise disposed in a scannable form onto or into a surface of the airflow accessory 160. The scannable image may take the form of, for example, a QR-code, a barcode, alphanumeric text, or any other suitable form of machine-readable image. The ID sensor 162 takes the form of a corresponding sensor or scanner, located in or on the heater housing 14 (or a main housing, in the event the haircare accessory is not formed of separate circuitry housing 12 and heater housing 14). The ID sensor 162 may operate optically (whether or not in the visible spectrum), ultrasonically, or electromagnetically, or based on any other suitable technology, or combination of such technologies.

The identifier may alternatively, or in addition, take the form of a physical shape or shapes that encode an identifier. For example, one or more raised ribs, lands, fingers, or recessed portions on the accessory can interact with a corresponding tongue, pin, tang, lever, or other physical element that is connected to, for example, a switch on heater housing 14, such as a physical, optical or electromagnetic switch. The identifier may alternatively, or in addition, take the form of a magnetic or electromagnetic portion that can be sensed by a corresponding magnetic- or electromagnetic-sensitive switch or sensor.

Whatever approach is taken to identifying an attached airflow accessory, in general, installation of the accessory 160 onto the heater housing 14 results in the ID sensor 162 or scanner being positioned adjacent the identifier 163 (or the structure or mechanism by which the identifier is encoded). The scanner or sensor can then sense or scan the identifier value, allowing the haircare appliance 10 to identify the attached accessory 160.

The identifier can be, for example, an index, the haircare appliance 10 having a memory that stores a table mapping each index to information that allows airflow calculations to take into account the attached accessory. For example, the information may comprise a correction factor related to the identified accessory, allowing the airflow calculations to be suitably corrected for the impact of the attached accessory. Alternatively, the identifier may directly encode the information. For example, the identifier may store a number representative of a correction factor related to the accessory.

The identifier can encode multiple bits, representing several potential accessories and/or indices, allowing a correction factor to be used that best corresponds to a particular accessory that is attached. Alternatively, the identifier may effectively encode a single bit of information, allowing the haircare appliance 10 to identify the simple presence or absence of an accessory. This enables a single correction factor to be applied for all accessories, if attached. Although this may be limiting where multiple possible accessories are possible, simple presence/absence detection has the benefit of simplicity and potentially higher reliability.

The signal supplied to drive heater 32 may generate unwanted electromagnetic emissions. This is particularly the case where the heater 32 is phase angle controlled. Phase angle control allows a heater to be turned on at any phase of the AC mains voltage. This results in high di/dt of the input current, which can result in large amounts of undesirable electromagnetic interference. Control of such interference in consumer products is a matter of legal compliance in many jurisdictions. Phase angle control has the advantage of preventing flickering when the haircare appliance 10 is used in a low- voltage region, where the current is higher compared to a high-voltage region. EMC filter 42 reduces the impact of undesirable interference. As shown in Figure 4, EMC filter 42 comprises first and second X capacitors (capacitors that are designed to fail-open) 112 connected between the live and neutral circuits, and a differential mode choke 114, connected in a C-L-C (or pi- filter) configuration.

The skilled person will appreciate that other forms of EMC filtering may be provided. For example, other EMC filters can use different combinations of capacitors and differential or common-mode chokes. Alternatively, or in addition, EMC filtering may be performed by the use of a shielded cable between fan motor controller 40 and fan motor 44, the shield being grounded at both ends.

Also, although the EMC filter is shown as being within circuitry housing 12, it may also be positioned within heater housing 14, or in a single housing of the appliance where the appliance does not use a separate circuitry housing 12 and heater housing 14.

Turning to Figure 5, AC-to-DC converter 38 takes the form of a DC power supply circuit that includes a rectifying circuit 116 that rectifies the AC mains supply voltage to a high-voltage DC. As described in more detail below, this high-voltage DC is used as a supply to fan motor 44.

The rectifying circuit 116 includes a bridge rectifier 118 coupled to receive the AC power supply and generate a rectified voltage across first and second outputs 120 and 122. Rectifying circuit 116 also includes a capacitor 124 for smoothing the rectified voltage, the capacitor 124 including a first terminal and a second terminal. The first terminal of the capacitor 124 is connected to the first DC output 120.

A switch in the form of an NMOS switch 126 is provided in series with capacitor 124. A first side of the switch 126 is connected to the second terminal of the capacitor 124, and a second side of the switch 126 is connected to the second DC output 122.

MCU 48 is connected to control whether switch 126 is open or closed, by way of the gate control signal from voltage check block 73 in MCU 48 (see Figure 2) being connected to the gate of the switch 126. As described above in relation to Figure 2, when the voltage check block 73 confirms that the voltage of the AC power supply is within an acceptable range, the gate control signal causes the switch 126 to close, such that the rectifying circuit 116 generates the rectified voltage. The combination of smoothing capacitor 124 and switch 126 ensures safe operation of the haircare appliance 10, irrespective of whether it has been plugged into the correct AC mains supply voltage. A prior art approach to this problem was to provide two serially connected capacitors (optionally with balancing resistors), instead of the single capacitor 124 in series with switch 126 shown in in Figure 5. The voltage ratings of such serially connected capacitors in the prior art were selected to exceed twice the anticipated AC mains supply voltage. However, such capacitors are relatively expensive components in such a circuit, and the lack of a switching component meant that the protection provided was passive.

While a full-wave bridge rectifier is illustrated, the rectifying circuit may comprise any other form of rectifier, including a half-wave rectifier.

As described in more detail below, the gate of switch 126 may be controlled in accordance with an anticipated zero-crossing point for the voltage of the AC mains power supply, thereby to reduce an inrush current to the capacitor.

Figure 5 also illustrates an implementation of a method of controlling a DC power supply. The method comprises supplying the AC power supply to an input of a rectifying circuit (e.g., rectifying circuit 116), determining a voltage of the AC power supply, determining whether the voltage of the AC power supply is within an acceptable range, and, when the voltage is within an acceptable range, closing a switch (e.g., switch 126) that is serially connected with a capacitor (e.g., capacitor 124), the switch and capacitor being connected in series between outputs (e.g., 120 and 122) of the rectifying circuit, such that the rectifying circuit generates the rectified voltage. The method may optionally include determining at least one future expected zero-crossing point for the voltage of the AC power supply, and performing the step of closing the switch at the expected zero-crossing point, thereby to reduce an inrush current to the capacitor.

Converter 38 also includes DC-to-DC conversion functions to generate different DC voltages, as required by the various components of haircare appliance 10. DC-to-DC conversion is known to the skilled person, and so will not be described in detail.

Where airflow generator control circuitry is disposed remotely from the motor, there may be a risk of increased transmission of electromagnetic interference. For example, in the haircare appliance 10 of Figures 1 to 3, the electrical cable 16 may be relatively long, such as 2-3 metres (approximately 6-9 feet). The drive current supplied to fan motor 44 by fan motor controller 40 is rich in harmonics, which will generate large amounts of common mode noise.

As shown in Figures 2 and 6, to mitigate the effect of such noise, EMC fdter 42 is disposed in circuitry housing 12. It receives drive current from fan motor controller 40, and EMC-fdters it to generate an EMC-filtered drive current. The EMC-fdtered drive current is supplied over electric cable 16 to drive the fan motor 44. EMC fdter 42 may take the form of, for example, a common-mode fdter, such as a common-mode choke 128 as shown in Figure 6.

The EMC-fdtered drive current offers considerably reduced noise compared to the unfdtered drive current output by fan motor controller 40, which reduces the amount of interference generated when haircare appliance 10 is in use.

Operation of voltage sensing circuit 70 and RMS voltage calculator 72 will now be described with reference Figures 2, 7 and 8. While the following description assumes a class-2 appliance that is not connected to an external ground, the skilled person will appreciate that a device using a grounded power supply can implement the described RMS voltage calculation method and apparatus, with suitable modifications that will be apparent to the skilled person.

Figure 7 shows voltage sensing circuit 70. Voltage sensing circuit 70 is connected to the live and neutral circuits provided to heater housing 14 via electrical cable 16, as described above. Voltage sensing circuit 70 includes a live sensing circuit 352 and a neutral sensing circuit 354. Live sensing circuit 352 comprises a voltage divider comprising a first resistor 356, a second resistor 358, and a third resistor 360, serially connected between the live circuit and 0V. The voltage divider has a live output 362 at the junction between second resistor 358 and third resistor 360. A capacitor 364 is connected between live output 362 and 0V. The function of live sensing circuit 352 is to provide a scaled-down (i.e., lower voltage) version of the signal on the live circuit.

Neutral sensing circuit 354 has the same components as live sensing circuit 352, but instead is connected between the neutral circuit and 0V. Neutral sensing circuit 354 uses the same reference signs as those in live sensing circuit 352 where appropriate. However, instead of live output 362, neutral sensing circuit 354 has a neutral output 366. The function of neutral sensing circuit 354 is to provide a scaled-down (i.e., low-voltage) version of the signal on the neutral circuit.

Figure 8 shows a neutral circuit waveform 368 corresponding to neutral output 366 and live circuit waveform 370 corresponding to live output 362. It will be noted that the waveforms include some distortion due to the effects of filtering capacitors between live/neutral and 0V to filter common-mode noise. The live circuit waveform 370 has a positive peak in phase with the positive peak of the AC power supply signal, while neutral circuit waveform 368 has a positive peak 180° out of phase with the positive peak of the AC power supply signal.

Live output 362 and neutral output 364 are provided as inputs to MCU 48. An analogue to digital converter (not shown) within MCU 48 samples the voltage of the AC mains power supply by sampling the live output 362 (i.e., live circuit waveform 370) and neutral output 364 (i.e., neutral circuit waveform 368), thereby to generate a sequence of samples. In this case, the sequence of samples includes samples relates to the live output 362 and neutral output 364.

Based on the sequence of samples, a sequence of values is generated if required, each of the values being based on a magnitude of at least one sample from the sequence of samples.

In the example shown in Figure 8, it will be noted that there is some distortion in the live circuit waveform 370 and neutral circuit waveform 368. This distortion arises due to capacitance caused by noise-filtering capacitors between the live/neutral circuits and 0V. The distortion is common-mode, and so can largely be cancelled by taking a difference between the live circuit waveform 370 and neutral circuit waveform 368, the result of which is summed waveform 372. Despite the apparent distortion of live circuit waveform 370 and neutral circuit waveform 368, summed waveform 372 closely resembles the sinewave of the input AC power supply signal.

In this case, MCU 48 generates the sequence of values by calculating a difference between neutral circuit waveform 368 and live circuit waveform 370 at each sampling point of the two waveforms, and then taking an absolute value of the difference. Accordingly, each value in the sequence of values is based on a magnitude of a corresponding sample within the sequence of samples. Next, a moving window function is applied to the sequence of values. For example, a simple moving average may be applied to the sequence of values. The length of the moving average may be selected to suit the circumstances, but may be several tens or hundreds of samples long.

Moving window function may take the form of any type of moving average, averaging function, or other low-pass fdter function suitable to the circumstances.

A voltage of the AC mains power supply may then be estimated, based on an output of the moving window function. For example, the voltage may be estimated based on a mapping between potential outputs of the moving window function and AC mains power supply voltages. If necessary, MCU 48 may apply a correction factor to the output of the moving window function.

Alternatively, the sequence of values may take the form of the sequence of samples, or a subset thereof. For example, live circuit waveform 370 and/or neutral circuit waveform 368 may be sampled, and a moving window function applied directly to those samples. Where applied to just live circuit waveform 370 or neutral circuit waveform 368, a correction factor or mapping can be applied to take into account the fact that the waveform will be around 0V for about half of the time (i.e., between positive excursions).

Where both live circuit waveform 370 and neutral circuit waveform 368 are sampled, their values may be added to generate the sequence of values, or moving window function may be applied to both waveforms separately, and the result averaged.

Although not shown in Figure 2, circuitry housing 12 can also include a voltage sensing circuit similar to voltage sensing circuit 70, and a further microprocessor, for sensing AC mains supply voltage at the circuitry housing. The sensed voltage can be used, for example, to prevent the haircare appliance from turning on if the voltage is outside an acceptable range. Alternatively, or in addition, the voltage can be monitored for temporary power loss, such as in a brown-out or similar situation. If, in use, the voltage drops below a critical level for more than some predetermined period, operation of the haircare appliance 10 may be halted for safety. An example period would be 20ms, although other periods may be selected based on circumstances. More generally, the skilled person will appreciate that there may be a different distribution of components between circuitry housing 12 and heater housing 14 than that shown. Optionally, circuitry housing 12 can incorporate power connector (i.e., plug) 15, such that circuitry housing 12 can be directly plugged into a power socket. Alternatively, all of the components may be disposed within a heater housing, without the use of a separate circuitry housing such as circuitry housing 12.

While haircare appliance 10 has been described as a hairdryer, it will be appreciated that many of the teachings discussed herein may be applied to other types of haircare appliance, such as hair straighteners, hair curlers, and the like, for example.