GB2072887A

GB2072887A - Control of electrical heating elements

Info

Publication number: GB2072887A
Application number: GB8009805A
Authority: GB
Original assignee: Kenwood Manufacturing Co Ltd
Current assignee: Electrical and Musical Industries Ltd
Priority date: 1980-03-24
Filing date: 1980-03-24
Publication date: 1981-10-07

Abstract

A circuit for controlling the power supplied to a heating element (1), e.g. of a domestic cooker, operates by generating electrical signals indicative of the resistance of the heating element, comparing these signals with a reference signal (VREF) indicative of a desired resistance and hence temperature of the heating element, and varying the power supplied to the heating element accordingly. In one example this variation is achieved by controllably gating a triac (2) using a burst fire controller (3). The resistance measurements are effected in the intervals when the triac (2) is OFF, the voltage at the junction of resistors (5, 6) then being a fraction of the mains voltage which varies in dependence on heater temperature. This voltage is fed via an electronic switch (8) and a buffer (7) to a comparator (9) which adjusts the controller (3). The arrangement may be used to set a maximum temperature for the heater (1), or to adjust the mark-space ratio of the controller to keep the temperature at a set value. <IMAGE>

Description

SPECIFICATION Control of electrical heating elements The present invention relates to the control of the energisation of electrical heating elements particularly, but not exclusively, electrical heating elements of domestic electric cookers.

In electric cookers and other devices using electrical heating elements it is frequently desired to measure the temperature of a heating element in order to carry out closed loop energisation of the heating element in accordance with a set temperature and/or to provide a thermal trip function in order to prevent the element overheating. This has been achieved in the past in a number of ways, for example by the use of bimetallic switch elements and analog temperature measuring devices such as thermistors of which a characteristic such as electrical resistance changes with temperature.

According to the present invention we measure the temperature of a heating element having temperature dependent electrical resistance by measuring its own resistance. Thus in one embodiment of the invention, an electrical heating element is connected to a power controller for controlling the power supplied to the heating element and the power controller includes circuitry arranged to produce an electrical signal representing the temperature of the element. This signal may be used to implement closed loop temperature control and/or a thermal tripping function.

In order to avoid the need to measure the resistance of the element while it is being energised, it is convenient to energise the element according to a burst fire scheme, i.e. a scheme in which the mains supply voltage is periodically applied across the element for an integral number of half cycles of the supply waveform and to sample the resistance of the element during the periods while it is not being energised.

The invention will be further described by way of example with reference to the accompanying drawing in which: Figure 1 is a somewhat schematic circuit diagram of one embodiment of the present invention; and Figure 2 shows the variation of the mean power supplied to the heater in the embodiment of Figure 1 as a function of time for one particular setting of the user-operable control.

As shown in Figure 1, an electrical heating element 1 of a domestic electric cooker is connected in series across the electrical mains supply with a triac 2 which is gated on and off by means of a burst fire controller 3 in accordance with the setting of a user-operable control 4. The burst fire controller may be placed on a well known type in which the voltage picked off from the potentiometer 4 is compared with a slowrunning ramp waveform having a time period of several seconds by a comparator and the comparator is arranged to gate the triac 2 so that it is gated on in each cycle of the ramp until the ramp voltage exceeds the voltage from potentiometer 4. The mean power supplied to the heating element thus varies as indicated in Figure 2 with the "mark" portions of the graph being of a length dependent on the setting of the potentiometer 4.

The heating element is made of a wire such as nichrome which has a positive temperature coefficient so that its resistance increases markedly with temperature.

Connected across the triac 2 is a potential divider comprising resistors 5 and 6 so that during the periods when the triac 2 is turned off the voltage at the junction of resistors 5 and 6 is a fraction of the mains voltage, this fraction being determined by their resistance and that of the element 1 and thus varying with the temperature of the element 1.

The voltage occurring junction of the resistors 5 and 6 is applied to a buffer amplifier 7 via an electronic switch 8 and rectifying and smoothing circuitry (not referenced). The state of switch 8 is controlled by a signal derived from the burst fire controller 3 so that the switch closes during those periods when the triac 2 is off (the period T1 to T2 in Figure 2). This may be achieved by inverting the gating signal for the triac although it might be preferable to ensure that the switch is not closed until some time after the triac turns off at T1 and opens again some time before the triac is turned on again at T2 so that transients possibly generated by the switching of the triac 2 do not cause incorrect operation.The output of buffer amplifier 7 is thus representative of the temperature of the element and is applied to a comparator 9 together with a reference voltage representing the desiredmaximum temperature of the element. When the temperature of the element exceeds the desired maximum, the comparator 9 produces an output signal, which is applied to the burst fire controller 3 to inhibit further gating on of the triac 2 until the temperature of the element has returned to below the tripping level. This thermal tripping function serves both to prevent damage to the element and to save power when there is only a small thermal load on the element.

The above arrangement may be modified so that as well as, or instead of, the thermal tripping function, closed loop control of the temperature of the element is provided. For this purpose, the voltage from potentiometer 4 and the output of the buffer amplifier 7 may be applied to a differencing amplifier in order to produce an error signal and it is this error signal which is compared with the ramp to determine the proportion of the time that the triac 2 is gated on.

It will be appreciated that in the above arrangement the voltage at the junction of resistors 5 and 6 is also a function of the mains supply voltage and it may be preferred, therefore, to provide an arrangement in which the temperature-representing signal is derived by passing a known current from a stable reference source through the element 1 while the triac 2 is turned off. It would, of course, also be possible to apply a known reference voltage across the heating element and measure the resulting current through it.

Claims

1. A circuit for controlling the power supplied to a heating element comprising means for generating first electrical signals indicative of the resistance of the heating element, means for comparing a said first signal with a preselectable signal, representative of a desired temperature of the heating element, to generate an error signal indicative of a difference in the magnitude of said signals, and means responsive to an error signal for regulating the power supplied to the heating element in a sense appropriate for reducing a difference in magnitude.

2. A circuit according to Claim 1 wherein the regulating means comprises a first switching means for connecting the heating element to a source of power, and control means responsive to an error signal for changing the state of said switching means.

3. A circuit according to Claim 2 wherein the control means is capable of generating, at a preselectable frequency, control signals suitable for rendering the switching means electrically conductive and responds to an error signal, applied thereto, to inhibit generation of a said control signal.

4. A circuit according to Claim 2 wherein the control means generates control signals suitable for rendering the switching means electrically; conductive at a frequency which depends on the magnitude of an error signal applied thereto.

5. A circuit according to any one of Claims 2 to 4 including further switching means for preventing generation of said first signals whenever power is supplied to the heating element.

6. A circuit according to Claims 2 to 5 wherein the means for generating said first signals comprises a potential divider connected in series with the heating element.

7. A circuit substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.