WO2008035033A1

WO2008035033A1 - A circuit

Info

Publication number: WO2008035033A1
Application number: PCT/GB2007/003375
Authority: WO
Inventors: Nathan James Croft
Original assignee: Dyson Technology Limited
Priority date: 2006-09-21
Filing date: 2007-09-10
Publication date: 2008-03-27
Also published as: GB0618616D0; GB2442021A

Abstract

There is provided a power supply indication circuit which may be adapted for use with a switched mode power supply or other constant voltage power supply. The circuit comprises a component for generating a signal indicating the power drawn from the power supply, a feedback portion for receiving the signal indicating the power drawn from the power supply, the feedback portion includes a signal generation component for generating a feedback signal, the feedback signal being used to adjust the power and current supplied to a component such as a high frequency agitation source drawing current from the power supply. The drive circuit is particularly suitable for use with a high frequency agitation source such as piezoelectric crystals.

Description

A Circuit

The invention relates to a power supply indicating monitoring circuit for a high- frequency agitation source. Particularly, the invention relates to power supply monitoring circuit and a controller for a piezoelectric crystal.

Piezoelectric crystals are well known in the art and are used for a number of purposes. Piezoelectric motors, transformers and linear drives are common. An important use for a piezoelectric crystal is in nebulisation. There are many cases where a fine mist of a substance is required without the application of heat. One example of this is a medical nebuliser, wherein a pharmaceutical compound is nebulised by a piezoelectric crystal in order to be inhaled by a patient. Another use for nebulisers is in the field of water dispersal such as garden water features. In order to disperse a dispersal agent effectively, a high voltage, high frequency drive source is required. Typically, a piezoelectric crystal for use in nebulisation is driven at its resonance frequency. This frequency varies between piezoelectric crystals, however it is usually in the region of 1.6-1.7 MHz.

Drive circuits for piezoelectric crystals are well known in the art. A simple way of generating such a high frequency signal is through the use of a transistor circuit. However, if this is done, a high voltage amplifier or a transformer is required to generate the peak to peak voltages needed to drive a piezoelectric crystal. Typically, these voltages are in the region of 100-150 V. Transformers are the most commonly used components for this purpose. However, transformers are often bulky and expensive. Other types of drive circuit may be used.

A further requirement for an electronic device that will use a mains power supply is that

Electromagnetic Compatibility Standards (EMC) have to be met. These standards define an acceptable level for the harmonic content in the current which electrical equipment draws from a mains AC supply, as well as an acceptable level of voltage distortion. A high- voltage square wave signal may contain an unacceptable level of harmonic content both for efficient driving of a piezoelectric crystal and for meeting the required standards of harmonic content. A common way of solving this problem is to pass the signal through a low-pass filter. If the low-pass filter is tuned to the fundamental driving frequency of the piezoelectric crystal, higher order harmonics can be filtered out, leaving only the fundamental frequency to drive the piezoelectric crystal. Often, a low- pass filter is also used to give a voltage gain. However, in order to drive a piezoelectric crystal at resonance, a relatively high quality factor is required. In order to achieve this with a low-pass filter such as an LC circuit, the capacitances of the system in which the LC circuit is located need to be constant. However, the capacitance of the wiring and the piezoelectric crystal itself may vary with temperature, age, condition and use. Therefore, this often makes an LC circuit unsuitable for driving a piezoelectric crystal at the precise resonant frequency required.

When forming part of a nebuliser, a piezoelectric crystal acts on a head of liquid in order to disperse the liquid into a fine mist. During operation of the piezoelectric crystal, the head of liquid absorbs the vibrational energy and sinks some of the thermal energy of the piezoelectric crystal. This has the effect of cooling the piezoelectric crystal, hi order to operate a piezoelectric crystal efficiently it is important to drive the piezoelectric crystal at a precise frequency providing the greatest resonance and at a frequency corresponding to the greatest rate of dispersal of a head of liquid. Further, it is desirable that unnecessary use or non-optimised use of the piezoelectric crystal is avoided as this can be wasteful of energy, hi some cases, for example, if the piezoelectric crystal continues to operate when all of the liquid has been nebulised and without a head of water the temperature of the crystal will rapidly increase and thermal damage will occur. Thermal damage may also occur if the piezoelectric crystal operates in an inefficient way for a long period of time. US 5,803,362 discloses a device which is capable of varying the power fed to an oscillator circuit depending upon the temperature of a piezoelectric crystal in order to control the temperature of the piezoelectric crystal. This process can prevent the temperature of a piezoelectric crystal from exceeding a maximum temperature. However, varying the power supplied to (and thus the amplitude of oscillation of) a piezoelectric crystal can be an inefficient method of controlling a piezoelectric crystal.

It is an object of the present invention to provide a power supply indication circuit, capable of indicating current supplied to a component, with feedback to the power supply dependent upon a characteristic of the operation of the component, and further to adjust the operation of the component in response to the feedback signal. It is an object of the present invention to provide a power supply indication circuit and controller, suitable for use with a high-frequency agitation source, able to detect the agitation status of the source at the power supply side and adjust performance of the agitation source accordingly. It is an object of the present invention to drive a piezoelectric crystal at its resonance frequency by monitoring the power requirements of the piezoelectric crystal at the power supply, deducing the resonance status of the piezoelectric crystal from that power and current information and taking action accordingly to maintain and to drive the piezoelectric crystal at its resonance frequency.

The invention provides a power supply indication circuit for a high frequency agitation source, the circuit comprising a component for generating a signal indicating the power drawn from a constant voltage power supply, a feedback portion for receiving the signal indicating the power drawn from a said power supply, the feedback portion including a signal generation component for generating a feedback signal, the feedback signal being used to adjust the power supplied to the high-frequency agitation source from a said power supply.

Preferably the feedback signal is used to adjust the current supplied to the high- frequency agitation source from the constant voltage power supply. Preferably the power indication source circuit is adapted for use with a high frequency agitation source, more preferably the high-frequency agitation source is a piezoelectric crystal. Preferably the feedback part of the circuit indicates the current and also indicates the load drawn from the power supply. By providing an indication of the load drawn from the power supply by the piezoelectric crystal in a feedback loop linked to the piezo controller, the controller including signal generation means for generating a drive signal for the piezoelectric crystal, the piezoelectric crystal can be tuned to its resonant frequency in response to a variation in the load drawn by the agitator. In a nebuliser this allows the piezoelectric crystal to be driven at the most efficient frequency for nebulisation.

Advantageously, the feedback portion comprises a filter stage and an amplification stage. Preferably the filter stage comprises a low-pass filter.

Advantageously, the supply indication circuit is adapted for use with a switched mode power supply.

The invention provides a self-contained feedback control system which is able to complete a nebulisation process efficiently. The power supply monitoring circuit provides a system that can also minimise unnecessary use of the piezoelectric crystal, and ensure that the agitation source is driven efficiently. The invention is particularly suitable to^' drive a nebuliser for use in a hand dryer. '

An embodiment of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a circuit diagram of part of a power supply indication circuit according to the invention;

Figure 2a is a circuit diagram showing further details of a power supply indication circuit according to the invention; Figure 2b is a graph showing plots of actual output voltage signal readings sampled at Ch 1, 2 and 3 points in the circuit of Figure 2a;

Figure 3 is a block diagram of the components and operation scheme of a controller incorporating the indication circuit according to the invention;

Figure 4 shows the measurement periods and the occurrences of temperature measurements made by the controller;

Figure 5a shows a graph of the expected temperature characteristic of a piezoelectric crystal during a typical nebulisation process;

Figure 5b shows a graph of an actual output temperature characteristic of a piezoelectric crystal during a typical nebulisation process; and

Figure 6 shows a hand dryer incorporating a nebuliser controlled by the controller of Figure 3.

Figure 1 shows the power supply indication circuit according to the invention. The circuit is powered by a DC power supply originating from . an AC/DC converter powered by mains electricity supply. The power supply is an isolating Power Supply Unit (PSU) and the power supply is isolated with an opto-isolator (OPI). Specifically, the embodiment shown in Figure 1 is a voltage controlled flyback opto-isolated Switched Mode Power Supply Unit (SMPSU) such as, for example, SMPSU product numbers NCP1216, NCP1216A from ON semiconductor, USA (www.onsemi.com). The values of components of the SMPSU such as Rl - RlO, Cl, C2, C3 and Dl to D4 are set by the system requirements and available from the application notes and product data sheets for the SMPSU. The opto-isolator (OPI) anode side of the SMPSU is supplied by a 5 V power rail and includes Zener diode protection. Diodes Dl and D2 provide a stable forward basis and voltage drop. The voltage drop in conjunction with the forward voltage drop across D3 and D4 ensures a minimum voltage of 2.9 V at the anode side. Thus the PSU regulates the voltage under very low load conditions and achieves good voltage stability and a high efficiency. The power supply indication circuit is provided at the cathode side of the isolator and is shown on Figure 1 with a dashed line. The remaining opto-isolator (OPI) cathode side components are held to ground, or OV. The OPTO_SENSE signal generated is converted to a voltage (ChI on Figure 2b) with Rl and processed by the circuit shown in Figure 2. The resultant output signal S8 is provided to the controller of Figure 3 (Ch3).

The feedback circuit of Figure 2b comprises two stages. The first stage comprises an initial filter capacitor, C2, and first stage amplification with amplifier, Amp 1 (an operational amplifier, LM358).

The second stage of the feedback circuit includes a filter comprising resistor, R7 and capacitor C3 and a second stage of amplification with signal amplifier, Amp 2 (an operational amplifier, LM358). The Capacitor C4 and resistor R9 provide integration of the signal.

In operation the current through the opto-isolator, OPI, provides an indication of the current load on the power supply in the system i.e. the current drawn by the high frequency agitator, in this embodiment. The SMPSU uses the current flow through the opto-isolator to hold the power supply to the correct voltage for the system using the ratio of resistor, R4 to the sum of resistors, R2 and R3. The voltage signal

OPTO_SENSE from the opto-isolator, OPI, provides an indication of the current load on the power supply. Figure 2b shows the OPTO_SENSE voltage level sampled at

Ch2. The indication signal is processed and passed through a low pass filter section with

C2 to reduce noise in the signal. Amp 1 amplifies the AC and DC components of the signal and provides a DC offset. Figure 2b shows the OPTO_OP voltage level sampled at Ch2. The DC offset component of the signal is filtered by R7 and C3, the filter removes low frequencies, up to 1 Hz, any fluctuations and changes in the indication signal are searched for and monitored above 1 Hz. C4 and R9 provide an integration circuit for the signal. Next the signal is amplified by Amp 2 to produce a voltage signal output, S8, with a voltage range, relative to the mean, of between 0 and 2 Volts, the voltage (Ch 3) produced is suitable for use with a microprocessor or controller. The circuit design provides a strong, clean indication signal of the current load.

In the above illustrated embodiment the values of the resistors, capacitors and other components of the indication circuit are listed below. Other values of the components could be envisaged and could also provide a circuit with characteristics suitable for use in the present invention. The components have units of resistance of Ohms, Ω, or kΩ (10³ Ohms) and units of capacitance of Farads, F, or μF (micro Farads i.e. 10^'6 Farads) or nF (nano Farads i.e. 10^"9 Farads).

Details of the component parts in the present embodiment, illustrated in Figures 1 and 2, are set out below;

Cl = I nF

C2 = l nF

C3 = 22 μF

Rl = 100 Ω R2 = 2200 Ω

R3 = 10 kΩ

R4 = 100 kΩ

R5 = 8200 Ω

R6 = 100 kΩ R7 = 100 kΩ

R8 = 100 kΩ

R9 = 390 kΩ

R1O = 33O Ω

Diodes Dl, D2, D3 (diode internal to the opto-isolator or opto-coupler) and D4, a shunt or voltage regulator. The operation of the indication circuit of Figures 1 and 2 will now be described in use in a controller for a high frequency agitation source, such as a piezoelectric crystal. Figure 3 shows a controller incorporating the indication circuit according to the invention. In the embodiment shown in Figure 3 the high frequency agitation source is a piezoelectric crystal.

The control system of Figure 3 includes a 24 V power supply unit 10 including a power indication unit 11. The power supply unit is an opto-isolated SMPSU. The output signal S 7 is connected to the piezo drive 5 and the output from power indication unit 11 is connected to the controller. The controller 1 includes a signal generator 3. The signal generator 3 generates a synchronisation signal Sl at a specified frequency, for example 1.66 IdHz. A phase locked loop (PLL) 4 is connected to the signal generator 3. The PLL multiplies the synchronisation signal Sl by a specified amount to give a signal S2 at a higher frequency, for example 1.699 MHz. The synchronisation signal Sl has a frequency that is variable in order to drive the piezoelectric crystal 2 at an optimum frequency.

The output S2 from the PLL 4 is connected to the piezo drive 5. The piezo drive 5 comprises switching means such as a Power Metal Oxide Field Effect Transistor (Power MOSFET). The piezo drive 5 converts the signal S2 to a drive signal S3. The drive signal S3 is a sinusoidal waveform of an appropriate voltage to drive the piezoelectric crystal 2. In this embodiment the signal S3 is a high voltage signal with a drive frequency of 1.699 MHz. The signal S3 is a sine wave with peak-to-peak; voltage of 100- 140 V. The components and functioning of the piezo drive 5 are not material to the present invention and will not be discussed further here. A modulator 6 is connected to the piezo drive 5 and provides a modulation signal S4 to control the piezo drive 5 as required. The modulator 6 can be used to provide a pulse train with a variable duty cycle.

The optimum frequency of synchronisation signal Sl can be deteπnined by measurement of the operational characteristics of the piezoelectric crystal 2 and by transmission of this information to the controller 1. As a high frequency agitator, such as a piezoelectric crystal, approaches resonance the amplitude of modulation or agitation will increase and this requires more electrical energy and power from the drive source. Thus the current drawn from the power supply by the piezoelectric crystal is a measure of the operational state of the piezoelectric crystal and monitoring an indication of the power drawn from the power supply by the piezoelectric crystal can provide information about how close the piezoelectric crystal is to resonance. This information can be used in a successive approximation technique to adjust the frequency of the drive signal supplied to the piezoelectric crystal to tune the frequency towards the optimum (resonance) frequency.

It is to be noted that the use of the feedback and current indication system of the invention, using a signal indicating the power drawn from the power supply at the power supply side with the opto-isolator, OPI and circuit of Figures 1 and 2 provides a low voltage signal. This is particularly advantageous in a system having a requirement to conform to EMC regulations and a system in which a user will have, and will require, access to the controller and other system components.

It is possible to measure the current passing through the piezo directly by sampling S3 signal power through a circuit having resistor/capacitor/inductor circuit. In the embodiment represented in Figure 3 the signal S3 is a high voltage signal and a power resistor monitoring the current supplied to the piezo would dissipate heat. The dissipated heat may affect the operation of other components in the circuit, adding impedence to the drive circuit. The tuned resonance of the piezo will be affected and leads to inefficient operation of the system. The voltage (potential difference) across the power resistor and sampling at that point instead of the external monitoring of the present invention described here may cause voltage distortion, electrical noise and disturbance in the system and on the signal S3. Furthermore, the associated current measurement connected and fed back to a controller in a feedback loop is undesirable and causes heat dissipation. The use of a signal indicating the power drawn from the power supply at the power supply side with the opto-isolator, OPI and the circuit of Figures 1 and 2 provides a measurement solution that addresses the risks and disadvantages of a direct current measurement feedback system.

The piezoelectric crystal 2 comprises a ceramic material (which is responsive to an electric field) and electrical contacts. Piezoelectric crystals are well known in the art and any suitable piezoelectric crystal can be used. A negative temperature coefficient (NTC) thermistor 7 is connected to the piezoelectric crystal 2 by a thermal link 7a. The thermal link 7a is a thermally conductive and malleable material which is in conformal contact with both the NTC thermistor 7 and the piezoelectric crystal 2. The NTC thermistor has a resistance that is dependent upon temperature. A thermistor conditioning block 8 converts a signal S 5 from the NTC thermistor 7 into a temperature signal S6 which is suitable for the controller 1. An analogue input 9 forming part of the controller 1 receives the temperature signal S6 from the thermistor conditioning block 8. The controller 1 may use the temperature signal S6 to determine the status of the piezoelectric crystal 2 and to control the drive signal S3.

It is common to drive a piezoelectric crystal at a range of frequencies and the magnitude and frequency of the drive source is varied to drive a piezoelectric crystal at, or close to, its resonant frequency. For most piezoelectric crystals this frequency lies in the range between 1.5 to 2 MHz. A preferred driving frequency is close to 1.7 MHz.

In operation, the signal generator 3 generates a synchronisation signal Sl of a particular frequency. The synchronisation signal Sl is then supplied to the PLL 4. The controller uses the feedback signal S 8 from power indicator SIl to determine the status of operation of the piezo and how close the piezo is to its resonant frequency. The controller then adjusts the synchronisation signal Sl to optimise the piezo operation (maximum current drawn). The PLL 4 multiplies the synchronisation signal by 1024 to generate a signal S2. The piezo drive 5 converts the signal S2 into a drive signal S3. The drive signal S3 has a sinusoidal waveform with a frequency equal to the signal S2. The drive signal S3 also has a peak to peak voltage in the region of 100-140 V. The drive signal S3 is supplied to the piezoelectric crystal 2 in order to drive the piezoelectric crystal 2 in the required manner.

The operation of the piezo drive 5 is controlled by the modulator 6. The modulator 6 controls the piezo drive 5 with a modulation signal S4. The modulation signal S4 can take the form of a pulse train having a duty cycle. The duty cycle of the modulation signal S4 is determined by the controller 1. The duty cycle may be determined on the basis of the temperature signal S6. The modulation signal S4 is supplied to the piezo drive 5 and modulates the drive signal S3. Therefore, the modulator 6 is able to control the drive signal S3 by switching it on or off. Under the action of the modulator 6, the drive signal S3 takes the form of a series of wave "packets" or pulses (on state), with a "dead time" (off state) in between. The dead time is determined by the duty cycle which is the ratio of the pulse width to the period.

When the piezoelectric crystal 2 is operating, thermal energy will be generated. This thermal energy will change the resistance of the NTC thermistor 7. This is because the NTC thermistor 7 is in thermal contact with the piezoelectric crystal 2 by means of the thermal link 7a. The change in resistance of the NTC thermistor 7 causes a change in the signal S5. The signal S5 is converted by the thermistor conditioning block 8 into a temperature signal S6 suitable for the analogue input 9 of the controller 1. The temperature signal S6 contains the same information as the signal S5.

When the analogue input 9 receives the signal S6, the controller 1 evaluates the temperature signal S6 by a successive approximation technique. In this embodiment, the temperature signal S6 is sampled at regular intervals. It is advantageous that the temperature signal S6 is sampled when the piezoelectric crystal 2 is not in operation. This is to reduce the background noise and temperature variations which may be introduced by the operation of the piezoelectric crystal 2. Figure 4 shows a schematic diagram illustrating the points at which the temperature signal S6 is sampled. The sample points Pl, P2, P3, P4 are uniformly spaced and occur in the "dead time" between pulses of the drive signal S3. The pulses of the drive signal S3 have a pulse width a and a period b. Therefore, in this case the duty cycle D is equal to a/b. The "dead time" in between pulses is the optimum time for sampling the temperature signal S6. The value of the temperature signal S6 is related to and representative of the actual temperature so that the controller 1 can determine the actual temperature of the piezoelectric crystal 2.

In a typical nebulisation process without any temperature control, the temperature of the piezoelectric crystal 2 will rise at different rates depending on the state of operation of the piezoelectric crystal 2. If the piezoelectric crystal 2 is broken (line Cl), there will not be any significant temperature rise. However, when the piezoelectric crystal 2 is operating correctly, the rate of temperature rise can reveal important information about the environment of the piezoelectric crystal 2, as shown on Figures 5 a and 5b.

The operation of the piezoelectric crystal 2 through a power cycle will now be described with reference to Figure 5a. Initially, the duty cycle is set to a maximum so that the average power delivered to the piezoelectric crystal 2 is high. Therefore, operation of the piezoelectric crystal 2 will cause the piezoelectric crystal 2 to heat up. It has been shown by experimental analysis that, during a nebulisation process, the temperature of a piezoelectric crystal follows a characteristic profile. Initially, the temperature is seen to increase (first stage). Once the system reaches thermal equilibrium, the energy imparted by the piezoelectric crystal is used to nebulise the liquid. Therefore, the rate of change of temperature with time is seen to decrease (second stage). The value of the temperature may remain constant or even decrease in this stage. Finally, when the liquid has been completely nebulised (i.e. the piezo is dry), the rate of change of temperature with time is again seen to increase (third stage). Figure 5b is a graph of an actual measurement sequence showing the temperature profile described above. The temperature change can be used to detect when the nebulisation process has finished. The controller 1 can vary the power delivered to a piezoelectric crystal acting on a head of liquid in order to prevent the temperature of the piezoelectric crystal exceeding a pre-determined maximum value. In this embodiment, the pre-determined maximum value is 45°C. This maximum allowable temperature is chosen to prevent the build up of limescale. By preventing the build-up of limescale, the life of the piezoelectric crystal 2 can be extended.

The above-described embodiment of the invention is a low-cost, safe solution for a power supply indication circuit, capable of indicating current supplied to a component, with feedback to the power supply dependent upon a characteristic of the operation of the component, and further to adjust the operation of the component in response to the feedback signal. The invention may be used in any situation where a source such as a high frequency agitation source is required to be driven reliably and effectively with a feedback loop and where it is not desirable to take the feedback signal directly from the component, or agitator, side. This is of benefit to applications where users have access to the component for example, household appliances or medical devices.

It will be appreciated that the invention is not limited to the embodiment illustrated in the drawings. The drive source may be varied depending upon the required application. The power supply source may be varied depending upon the required application. For example, the drive source may be a linear power supply, a current controlled SMPSU or another type of isolated power supply unit. The invention may also be adapted for use with a switched mode power supply. In that case the feedback signal is used to adjust the power supplied to a component drawing current from the switched mode supply.

Further, the physical quantities of the described electronic components also may be varied in value. This could be done, for example, to change the resonant point of the filter stages. The number of filter stages could also be varied. Further, other forms of signal generator could be used. What is important is that the feedback circuit and signal is taken from the power supply side of the driven component system.

The invention may be used in any situation where a high frequency agitation source is required to be driven reliably and effectively, for example in an automatic system without user control or in a nebulisation system without water level monitoring. TMs is of benefit to applications such as, for example, household appliances or medical devices.

The above-described embodiment of the invention is particularly suited for use in a hand diyer such as that shown in Figure 6. The hand dryer 200 includes a cavity 210. The cavity 210 is open at its upper end 220 and the dimensions of the opening are sufficient to allow a user's hands (not shown) to be inserted easily into the cavity 210 for drying. A high-speed airflow is generated by a motor unit having a fan (not shown). The high-speed airflow is expelled through two slot-like openings 230 disposed at the upper end 220 of the cavity 210 to dry the user's hands. A drain (not shown) for draining the water removed from a user's hands from the cavity 210 is located at the lower end of the cavity 210. A nebuliser 240 is located downstream of the drain. The nebuliser 240 is shown partially removed from the hand dryer 200 in Figure 5. The nebuliser 240 is partially cut away to show the location of the above-described drive circuit 250. The nebuliser 240 includes a collector (not shown) for collecting waste water and a piezoelectric crystal (not shown) for nebulising the waste water. The piezoelectric crystal is driven by a drive circuit 250 which includes, and is controlled by, the controller 1. The use of the induction circuit of the present invention allows the nebulisation system to be more efficient and reliable in operation. This will result in lower operating and maintenance costs for a consumer.

It will be appreciated that the invention is not limited to the embodiment illustrated in the drawings. The above-described embodiment of the invention with a controller 1 for controlling a piezoelectric crystal forming part of a nebulisation system is also suitable for use in other dryers such as laundry dryers. Other forms of drying apparatus could be envisaged by the skilled reader, for example, other forms of domestic or commercial drying apparatus such as washer-dryers, ventilation-type laundry dryers or full-length body dryers.

In addition any number of piezoelectric crystals and controllers could be implemented. For example, a single controller could control several piezoelectric crystals, for example if the volume of liquid to be nebulised is great. Alternatively, several controllers could be present to handle different types of liquid or operate at different times.

Claims

L A power supply indication circuit for a high frequency agitation source, the circuit comprising a component for generating a signal indicating the power drawn from a constant voltage power supply, a feedback portion for receiving the signal indicating the power drawn from a said power supply, the feedback portion including a signal generation component for generating a feedback signal, the feedback signal being used to adjust the power supplied to the high-frequency agitation source from a said power supply.

2. A power supply indication circuit according to claim 1, wherein the feedback signal is used to adjust the current supplied to the high-frequency agitation source from a said constant voltage power supply.

3. A power supply indication circuit according to claim 1 or 2, wherein the high frequency agitation source is a piezoelectric crystal.

4. A power supply indication circuit according to any one of claims 1, 2 or 3, wherein the component for generating the signal indicating the power drawn from a power supply is an opto-isolator.

5. A power supply indication circuit according to claim 4, wherein the feedback portion comprises a filter stage adapted to filter a dc component from the signal indicating the power drawn from a power supply.

6. A power supply indication circuit according to any preceding claim, wherein the feedback portion indicates the current and load drawn from the power supply.

7. A power supply indication circuit according to claim 5 or 6, wherein the filter stage comprises a low pass filter.

8. A power supply indication circuit according to any preceding claim, wherein the feedback portion comprises an amplification stage.

9. A power supply indication circuit according to any preceding claim, wherein the circuit forms part of a switched mode power supply.

10. A power supply indication circuit according to claim 9, wherein the component drawing current from the power supply is a high frequency agitation source.

11. A power supply indication circuit as hereinbefore described with reference to the accompanying drawings.

12. A controller for a high frequency agitation source incorporating the power supply monitoring circuit of any one of the preceding claims.

13. A nebuliser incorporating the power supply monitoring circuit of any one of the preceding claims.

14. A hand dryer incorporating the nebuliser as claimed in claim 13.