GB2150771A - Heated airflow delivery apparatus - Google Patents

Abstract

Apparatus, such as a hair dryer or a fan heater, for delivering a heated airflow includes an electric motor (6, Fig. 1) coupled to a fan and connected by way of two parallel connected heating elements (2) to load terminals 4 which are connectable to power supply terminals 10 via a double pole switch 12 and a thyristor 25. A pulse generating circuit has a variable resistor 22 arranged to vary the frequency of the pulses which are fed to trigger the thyristor and hence connect the motor and the heating elements to the power supply terminals 10. The power supplied to both the motor and the heating element is thus dependent upon the frequency of the pulses and can be varied by adjustment of the resistor 22. The pulse generator comprises a multivibrator 19 triggered at zero crossings of the A.C. supply by a gate 1850 that thyristor 25 turns on and off at zero crossings. In an alternative circuit (Fig. 3), switching of the thryristor (125) is not controlled to occur at zero crossings. The power output is indicated by the flashing rate of an LED 24. The fan motor may have a flywheel. <IMAGE>

Description

SPECIFICATION Heated airflow delivery apparatus The present invention relates to apparatus for delivering a heated airflow, for example, to an electric hair dryer or a fan heater.

Known apparatus for delivering a heated airflow comprises an airflow inducing means such as a fan driven by an electric motor and one or more heating elements arranged to heat the airflow induced.

It is known to provide means for varying eitherthe heat output or the airflow. Thus, hair dryers are traditionally provided with a multi-position switch which is arranged to selectively connect one or more of the heating elements to the power supply such that the heat output can be increased.

For safety reasons it is necessary that the airflow, and hence the motor speed, be increased as the heating power is increased and it is common for a hair dryer to also be provided with means to increase the power fed to the motor such that the motor is increased when the heat output is increased.

Known hair dryers generally include two heating elements and have a switch with two ON positions.

In one position, one of the heating elements is connected to the power supply and the motor is arranged to be run at half speed. In the second position both heating elements are connected to the power supply and the motor is driven at full speed.

Heretofore, there have been no hair dryers available for which the heat output and the airflow are variable over a range.

According to the present invention there is provided apparatus for delivering a heated air flow comprising an electric motor, airflow inducing means arranged to be driven by said motor, an electrical heating device arranged to heat the air flow induced, and a circuit for controlling the supply of power to both said motor and said heating device, the control circuit comprising means for generating a series of pulses, means for varying the frequency of said pulses, and a triggerable switching device responsive to said pulses for connecting and disconnecting said motor and said heating device to power supply terminals such that the power supplied to both said motor and said heating device depends upon the frequency of said pulses.

In the apparatus of the present invention the power fed to both the motor and the heating element is dependent upon the frequency of the pulses. Thus, as the power fed to the heating element is increased so is the power fed to the motor. This means that as the heat output is increased the air flow is increased. The single control circuit, which can be on a single chip within the hair dryer is thus controlling the relationship between the heat output and the airflow.

As the power fed to the motor is periodically cut off, a fly wheel is preferably connected to the motor such that the motor is kept running at a constant speed.

As the power is being periodically cut off the circuit could generate radio frequency interference.

Accordingly, in one embodiment, interference suppression means are provided. For example, such interference suppression means could be constituted by a low pass filter.

Rather than allowing radio frequency interference to be generated and then suppressing it, it is also possible to prevent the generation of such interference. Thus, in one embodiment, means are provided to ensure that the power is only switched on or off at the zero crossing point of the power supply.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows the interconnection of the heating elements and the motor of a hair dryer, Figure 2 shows a circuit diagram of a first embodiment of a control circuit for controlling the power supply to the load of Figure 1, and Figure 3 shows a second embodiment of a control circuit for supplying power to the load of Figure 1.

Figure 1 shows the load of a hair dryer, the load comprising two heating elements 2 connected in parallel to load terminals 4. A motor 6 of the hair dryer is connected by way of a rectifier circuit 8 and one of the heating elements 2 to the terminals 4.

As can be seen from Figure 2, the terminals 4 are connected to mains supply terminals 10 by way of a double pole, three position switch 12. It can be seen from Figure 2 that the switch 12 has an OFF position; a first ON position in which the load terminals 4 are connectable to the supply terminals 10 by way of a diode 14; and a second ON position in which the load terminals 4 are connectable directly to the supply terminals 10. It will thus be appreciated that in the second ON position full mains power can be delivered to the load terminals 4 whereas in the first ON position only one half of the cycle of the input can be delivered to the load terminals 4.

Figure 2 also shows one embodiment of a circuit for controlling the power supplied to the load terminals 4. This control circuit is a DC circuit and is therefore connected to the supply terminals 10 by way of a rectifying circuit comprising a resistor 15, a diode 16 and a capacitor 17. The rectified current is then fed by way of a NAND gate 18 which acts as an inverter to a multivibrator circuit made up of two NAND gates 19, a capacitor 20, a resistor 21, and a variable potentiometer 22. The output of th is multivibrator circuit is fed by way of a NAND gate 23, which acts as a buffer and a light emitting diode 24 to the gate of a thyristor 25.

It will be seen that the thyristor 25 is connected between the switch 12 and the terminals 4. Thus, power can only be connected from the supply terminals 10 to the load terminals 4 when the thyristor 25 conducts.

The inverter 18 is arranged to trigger the gate 19 of the multivibrator when the input power on the terminals 10 changes direction, that is, its waveform crosses the zero axis. Thus, the multivibrator is arranged to generate a series of pulses whose leading edges are all coincident with cross over points of the input power. The pulses are fed to the gate of the thyristor 25 to trigger the thyristor to conduct. Thus, the thyristor connects the terminals 4 to the input power at a time when the input power is reversing. As the power is switched on at the cross over point no radio frequency interference arises.

The duration of the pulses and their separation can be varied by way of the potentiometer 22.

However, the resistor 21 and capacitor 20 have a time constant which ensures that the pulses have a minimum duration of 10 msec. In this way it is ensured that the pulses terminate at the null points of the input power supply.

It will be appreciated that the light emitting diode 24 will be illuminated whenever a pulse is fed to the thyristor and hence whenever the thyristor is triggered to conduct. The light emitting diode will therefore flash, and the frequency of the flashes will indicate the amount of power being fed to the load.

Thus, the circuit shown in Figure 2 enables the power fed to the load to be varied over a range.

However, the power supplied cannot be continuously varied as the thyristor 25 is only switched off when the input power reverses at a null point. Thus, the circuit enables a range of predetermined power settings to be selected.

It will be appreciated that the control circuit is effective to connect and disconnect the terminals 4 of the load from the power supply. Thus, it is switching the power supply to the load on and off at a frequency which can be varied. When the pulses are produced at high frequency the thyristor will be continuously conducting and 100% of the power will be fed to the load. At other frequencies the power will be switched on and off. To ensure that the motor continues to rotate at a continuous speed a flywheel will be connected thereto.

Figure 3 shows a second embodiment of a control circuit in which the load terminals 4 are connected to the power supply terminals 10 by way of the switch 12 and a thyristor 125. The circuit shown in Figure 3 is similar to that of Figure 2 except that no attempt is made to ensure that the pulses generated are coincident with the null point of the input power.

Accordingly, as the load is inductive the switching on and off of the power supply thereto could cause radio frequency interference. The circuit shown in Figure 3 therefore includes a capacitor 110 and a coil 112 which together comprise a low pass filter for suppressing the interference.

The control circuit shown in Figure 3 is again a DC powered circuit and is connected to the power supplied atterminals 10 bya rectifying circuit comprising a Zener diode 114 and a resistor 116.

Three NAND gates 118 are connected in parallel and arranged to supply a series of pulses to the gate of thyristor 125. The frequency of these pulses is set by the time constant of a variable potentiometer 120 and a capacitor 122 connected to the gates 118 by way of a further gate 124 which acts as an inverter. It will be appreciated that the time constant can be varied by varying the setting of the potentiometer 120.

Thus, the circuit shown in Figure 3 is arranged to connect the load to the input power for a smaller or greater proportion of the time dependent upon the frequency of the pulses. Hence, the heat output and the motor speed are varied in dependence upon the frequency of the pulses.

As in the previous circuit, the switch 12 has two ON positions, the first position connecting the supply terminals 10 to the load terminals 4 by way of the diode 14 such that only one half of the power cycle is fed to the terminals 4, and the second position of the switch 12 connecting the supply terminals 10 directly to the load terminals 4 such that all the power is availabie. Thus, in the circuit shown in Figure 3 the power supplied to the load is infinitely variable from nothing to 100%

Claims (15)

CLAIMS:

1. Apparatus for delivering a heated air flow comprising an electric motor, airflow inducing means arranged to be driven by said motor, an electrical heating device arranged to heat the air flow induced, and a circuit for controlling the supply of power to both said motor and said heating device, the control circuit comprising means for generating a series of pulses, means for varying the frequency of said pulses, and a triggerable switching device responsive to said pulses for connecting and disconnecting said motor and said heating device to power supply terminals such that the power supplied to both said motor and said heating device depends upon the frequency of said pulses.

2. Apparatus as claimed in Claim 1, wherein a flywheel is connected to said motor whereby the speed of rotation of the motor can be kept constant.

3. Apparatus as claimed in Claim 1 or 2, wherein said triggerable switching device is a thyristor.

4. Apparatus as claimed in any preceding claim, wherein the time constant of said pulse generating means can be varied by way of a potentiometer whereby the frequency of the pulses can be selected.

5. Apparatus as claimed in any preceding claim, wherein a light emitting diode is connected to the trigger of said switching device.

6. Apparatus as claimed in any preceding claim wherein said heating device and said motor are connected to common input terminals connectible to the power supply terminals by way of a double pole, three position switch.

7. Apparatus as claimed in Claim 6 wherein in one position of the switch the common input terminals are connectible-directly to the power supply terminals, and wherein in a second position of the switch the common input terminals are connectible to the power supply terminals by way of rectifying means.

8. Apparatus as claimed in any preceding claim, wherein the control circuit further comprises an interference suppression circuit.

9. Apparatus as claimed in Claim 8, wherein said interference suppression circuit is a low pass filter.

10. Apparatus as claimed in Claim 8 or Claim 9, wherein said interference suppression circuit is arranged to suppress radio frequency signals.

11. Apparatus as claimed in any preceding claim, wherein said pulse generating means is controlled to generate pulses such that the leading edge of each pulse is coincident with a null of the power supply.

12. Apparatus for delivering a heated air flow substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

13. A hair dryer including apparatus for delivering a heated air flow as claimed in any preceding claim.

14. Apparatus for delivering a heated air flow comprising an electric motor, airflow inducing means arranged to be driven by said motor, an electrical heating device arranged to heat the air flow induced, and a circuit for controlling the supply of power to both said motor and said heating device.

15. Apparatus as claimed in Claim 14, wherein the control circuit further comprises an interference suppression circuit.