GB2040121A - Power control circuits - Google Patents

Abstract

A circuit for controlling the power supplied to a load 1 e.g. a shower heating element, connected in series with at least one controlled semiconductor device 2, e.g. a triac or parallel connected semiconductor switches in each half cycle of an A.C. supply voltage, has a control device 3 e.g. a thermistor possessing a characteristic dependent on a monitored variable, and means 8-12 for generating trigger pulses at twice the frequency of the supply, said means incorporating or being associated with a phase shifting arrangement 4-7 for causing each trigger pulse to be applied to the semiconductor device or devices prior to or after the a.c. supply voltage crosses zero, in dependence upon the state of the control device 3; by this means only a slight change of phase of the trigger pulses will result in substantially zero or substantially full power being supplied to the load during respective half cycles of the supply voltage. Discharge of capacitor 7 via anode-controlled and cathode-controlled thyristors 8, 11 produces the trigger pulses. <IMAGE>

Description

SPECIFICATION Improvements in or relating to control circuits This invention relates to control circuits for controlling the supply of electric power to a load operating on an alternating current supply, and relates especially though not exclusively, to control circuits for controlling the power supplied to an electric resistive heating element such as that employed for heating the water flowing through an instantaneous water heater, for example a shower unit.

It is known to control the supply of electrical power to a load operated on an alternating current supply by varying the periods of conduction of a controlled semiconductor device connected in series with the load. This is achieved by controlling the instants at which trigger pulses are applied to the device in respective half cycles of the supply voltage, and by utilising one or more devices which can permit cur rent to pass through the load in both directions each half cycle of the supply voltage can be utilised.

An object of the present invention is to provide a novel form of control circuit which makes use of such devices, and which provides a highly sensitive form of control.

According to the invention a control circuit, for controlling the supply of electric power to a load operating on an alternating current supply of the kind incorporating at least one controlled semiconductor device in series with the load and capable of controlling the power to the load in each half cycle of the supply voltage, comprises a control device having a characteristic variable in response to change in a condition required to be monitored and means for generating trigger pulses at approximately twice the frequency of the supply, (and) said means incorporating or being associated with a phase shifting arrangement, the latter being dependent upon the state of the control device to cause each trigger pulse to be applied to the semiconductor device or devices priorto orafterthe a.c. supply voltage becomes zero in dependence upon the state of the control device.

In use of such a circuit the application of a trigger pulse to a semiconductor device which is capable of controlling the power to the load in a respective half cycle, just after the supply voltage becomes zero at the commencement of the half cycle, will cause the device to conduct, and the load to be supplied with current, for substantially the whole of that half cycle, whereas if the phase of the trigger pulse is advanced so that it is applied to the device just before the sup ply voltage becomes zero and terminates prior to the commencement of a half cycle it will no longer cause the device to conduct during that half cycle, but merely for a very short period during the preceding half cycle when the amplitude of the voltage waveform is very small.

Thus only a slight change of phase of the trigger pulses will result in substantially full or substantially zero power being supplied to the load during respective half cycles of the supply voltage.

Control of the power supplied to the load is conveniently effected by a triac or other similar controlled semiconductor device which can be triggered to pass current in both directions, although in some cases at least two controlled semiconductor devices capable of passing current in one direction only and connected in reverse parallel could alternatively be employed.

Conveniently the trigger pulses are generated by discharging a capacitor through the gate or gates of the semiconductor device or devices at appropriate instants determined by the phase shifting arrangement.

The latter is preferably provided by a resistivecapacitive combination, the control device forming part of the combination and having a resistance or a capacitance which changes in response to the condition required to be monitored.

For example, where temperature is required to be monitored, the control device conveniently comprises a temperature sensitive resistor, commonly termed a thermistor; the load in such a case may, for example be an electrical heating element, the thermistor being arranged to vary the phase of the trigger pulse applied to the controlled semiconductor device to effectively reduce the power supplied to the heating element when the temperature of a body or a medium heated by the element attains a predetermined value.

Thus in the case of an electrically operated instantaneous water heater, such as a shower unit, the thermistor is conveniently located so as to monitor the temperature of water passing through the heater after its passge past the heating element.

The invention has the further advantage that the components forming the control circuit are comparatively small and can be constructed as a compact unit taking up very little space.

One control circuit in accordance with the invention, for controlling the power supplied to an electric resistive heating element employed in an instantaneous water heater such as a shower unit, will now be described by way of example with reference to Figures 1 to 3 of the accompanying schematic drawing, in which Figure 1 illustrates the circuit in diagrammatic form, and Figures 2 and 3 show load voltage waveforms under different conditions for illustrating the operation of the circuit.

In Figure 1 of the drawings 1 represents a resistive heating element of an instantaneous water heater designed to operate on an alternating current electrical supply, the element being arranged, when energised, to heat water flowing through the heater.

Electric power to the heating element is controlled by a triac 2 connected in series with it between input terminals T1, T2 which are arranged to be connected in use of the heater to the alternating current supply.

Control of the triac 2 is achieved by means of trigger pulses generated at a frequency twice that of the supply, underthe control of a phase shift network The drawing(s) originally filed was/were informal and the print here reproduced is taken from later filed formal copy.

consisting of resistive elements 3,4 and 14 and capacitors 5, 6, 7, connected in series between the input terminals T., T2.

The resistive element 3 is a thermistor which is located so as to monitorthetemperatureofwater passing through the water heater after is has passed the heating element 1. The other resistive element 4 is a manually adjustable resistor the purpose of which will be subsequently described. Capacitors 5 and 6 have capacities which are small compared with that of the capacitor 7.

The circuit also includes an anode controlled thyristor 8 and a cathode controlled thyristor 11 connected in reverse parallel with each other, and each in series with a respecitve diode 9, 12. The anode and cathode of the thyristors 8, 11 respectively are connected to a common point on the phase shift network between the capacitors 6 and 7, and the cathode and the anode of the diodes 9, 12 respectively are connected to the gate of the triac 2 through a low value resistor 10.

The gates of the thyristors 8, 11 are both connected to the phase shift network between the capacitors 5, 6through a resistor 14.

In use trigger pulses for firming the triac 2 are generated by charging the capacitor 7 through the phase shift arrangement and discharging it rapidly through either the thyristor 8, diode 9, resistor 10 and the gate circuit of the triac 2 on one half cycle of the supply, and through the thyristor 11, diode 12, resistor 10 and the gate circuit of the triac in the opposite half cycle.

The thermistor is chosen so that an increase in the temperature of the water being monitored advances the phase of the trigger pulses relative to the a.c. supply voltage waveform; when the temperature is below a predetermined value the trigger pulses are arranged to be generated shortly after the zero crossing point of the a.c. voltage waveform V as indicated at P1 in Figure 2, so that the triac 2 permits power to be supplied to the heating element 1 for the major part of each half cycle as represented by the waveform Z1. However when the water temperature reaches the predetermined value the trigger pulses are advanced beyond the zero crossing point X of the a.c. voltage waveform V as indicated at P2 in Figure 3.Consequently the triac 2 is not fired at or near the commencement of each half cycle of the supply voltage, and the power supply to the heating element 1 is effectively reduced to a very low value as represented byZ2 until the water temperature falls again below the predetermined value.

The adjustable resistor4 enables the temperature at which the trigger pulses are advanced past the zero crossing point of the a.c. voltage waveform to be varied within a range of values, for example from 30 to 50"C.

The arrangement is very sensitive and gives a rapid response to changes in water temperature, enabling the water leaving the heating element to be maintained at a temperature within 1" or 2"C of the set value.

Fluctuations in the temperature of the water leaving the heater can be virtually eliminated by arranging for the water to pass into a mixing chamber before being finally discharged from a spray nozzle.

In the control circuit the diodes 9, 12 effectively prevent reverse leakage current being passed through the thyristors 8, 11 which could prevent the capacitor 7 from charging, but may in some cases be dispensed by utilising thyristors having suitable characteristics.

The resistor 13 serves to limit the discharge time of the capacitor 7, and ensures that the trigger pulses applied to the triac are of very short duration. This can prove necessary in view of the wide tolerances on the gate characteristics of triacs, as otherwise a pulse starting just before the end of a half cycle could continue into the next half cycle, and in the event of the triac 2 having different input characteristics on the two half cycles then this could result in one half cycle switching off before the other and imposing a half wave load on the supply. The resistor 14 constitutes part of the phase shift network as well as to supply trigger current to the thyristors 8 and 11.

It will be understood that the invention includes within its scope instantaneous electric water heaters, more especially shower units incorporating water temperature control circuits as above described.

It will be further appreciated that the control unit may incorporate alternative forms of phase shifting networks and/ortriggerpulse generators if desired.

Furthermore a control unit in accordance with the invention may be arranged to control other forms of load circuits, the control device employed being of any suitable kind appropriate to the particular application of the circuit.

Claims (8)

1. A control circuit for controlling the supply of electric power to a load operating on an alternating current supply, of the kind incorporating at least one controlled semiconductor device in series with the load and capable of controlling the power to the load in each half cycle of the supply voltage, the central device having a characteristic variable in response to change in a condition required to be monitored and means for generating trigger pulses at approximately twice the frequency of the supply, said means incorporating or being associated with a phase shifting arrangement, the latter being dependent upon the state of the control device to cause each trigger pulse to be applied to the semiconductor device or devices prior to or after the supply voltage becomes zero.

2. A circuit according to Claim 1 in which said at; least one controlled semiconductor device is a triac or similar device, arranged so that it can be triggered to pass current in both direction.

3. A circuit according to Claim 1 in which said trigger pulses are generated by discharging a capacitor through the gate or gates of said semiconductor device or devices at instants determined by the phase shifting arrangement.

4. A circuit according to any preceding claim in which the phase shifting arrangement is provided by a resistive-capacitive combination, the control device having a resistance or capacitance which changes in response to the condition required to be monitored.

5. A circuit according to any preceding claim in which the control device is a temperature sensitive resistor arranged to vary the phase of the trigger pulse applied to the controlled semiconductor device to effectively reduce the power supplied to a heating element when the temperature of a body or medium heated by the element attains a predeter mined value.

6. A circuit according to Claim 5 for controlling the supply of electric power to an instantaneous water heater in which said temperature sensitive resistor is located to monitor the temperature after its passage through the heating element.

7. A circuit according to any preceding claim substantially as hereinbefore described with refer ence to Figures 1 to 3 of the accompanying draw ings.

8. An electric instantaneous water heater incor porating a control circuit according to any preceding claim.