GB2076996A

GB2076996A - Temperature control circuit

Info

Publication number: GB2076996A
Application number: GB8015909A
Authority: GB
Original assignee: THORN HEATING Ltd
Current assignee: THORN HEATING Ltd
Priority date: 1980-05-13
Filing date: 1980-05-13
Publication date: 1981-12-09

Abstract

A control circuit for a heating or cooling device which is switched alternately on and off with an on/off ratio determined by an actual and a desired temperature is provided with means which ensure a minimum duration for both the on and off periods.

Description

SPECIFICATION Control circuit The present invention relates to a control circuit for a heating or cooling device and in particular to a control circuit for the fuel valve of a fuel burner.

In a simple temperature control system a heating or cooling device is switched on and off by a thermostat placed in the space which it is desired to maintain at a given temperature. This method of control allows the temperature to vary by at least the range of temperature between the on and off switch points of the thermostat. A more uniform temperature can be maintained by switching the heating or cooling device alternately on and off, the ratio of on to off time being adjusted proportionately if the desired temperature is changed or the actual temperature changes relative to the desired temperature.

However, with this time-proportional control it is possible to have the heating or cooling device switched on or switched off for very short periods near the upper and lower temperature limits. This can cause operating difficulties particularly when a fluid fuel burner controlled by a fuel valve is used as the heating device.

According to the present invention there is provided a control circuit for a heating or cooling device, comprising means for generating a control signal which when applied to the device switches it between alternate fully on and fully off states, the relative periods of these two states being controlled by a temperature selector element and a temperature sensor element connected to the control signal generating means, and incorporating means for setting a minimum duration for both the fully on and fully off states.

Preferably the control signal generating means comprises a comparator having two input terminals and an output terminal the voltage at which output terminal depends on the relative I levels of the voltage at the input terminals, ore of the said input terminals being connected to a voltage source capable of producing a cyclically varying voltage at that input terminal and the other said input terminal being connected to the temperature selector element and temperature sensor element so as to produce a voltage at that terminal controlled by those elements, and the cyclically varying voltage remains at its maximum and minimum values for a predetermined minium period in each cycle thereof.

A control circuit constructed in accordance with the present invention will now be described by way of exampie with reference to the accompalying drawings in which: Figure 1 is a circuit diagram of a control circuit constructed in accordance with the invention; Figure 2 is a waveform diagram showing a control signal generated by the control circuit of Figure 1; and Figure 3 is a waveform diagram showing two signals from inside the control circuit of Fig. 1, which produce the control signal shown in Figure. 2.

Referring to Figure 1, the control circuit comprises a voltage comparator 1 connected between power rails 2 and 3 having inverting and non-inverting input terminals 4 and 5 and an output terminal 6. The output terminal is intended for connection to a fuel valve of a fluid burner or any other suitable means for turning a heating or cooling device on and off, and the voltage at this output terminal has two levels depending on whether the voltage at input terminal 4 is greater than that at input terminal 5 or vice versa, one level switching the heating (or cooling) device on and the other switching it off.

The inverting input 4 of comparator 1 is connected to a voltage divider formed by variable resistor 7 and thermistor 8 connected across the power rails 2, 3. Thermistor 8 is a temperature sensor and is positioned in the space to be heated (or cooled) so that its resistance varies with the temperature of that space. Variable resistor 7 is a temperature selector and is adjustable to select a desired temperature for the space. The setting of variable resistor 7 and the resistance of thermistor 8 determine the voltage applied to input 4 of the comparator.

The non-inverting input 5 of comparator 1 is connected to a voltage divider formed by a resistor 9 and resistors 10, 11 and 1 2 connected between power rail 3 and a three stage binary counter 1 3. Input clock pulses to the binary counter are provided by a fixed frequency oscillator 1 4 and resistors 10 11 and 1 2 are connected respectively to the outputs of the successive stages of the counter.

The values of the resistors 10, 11 and 1 2 are in the ratio of 4:2:1 and are large compared to the value of resistor 9 so that the current in each of these resistors when the output voltage of the corresponding stage of the binary counter is applied to it is in the ratio 1:2:4. These currents are combined in eight different ways as the outputs of the binary counter go through their eight possible configurations giving a total current through resistor 9 and hence a voltage at input 5 which increases from zero to a maximum in eight equal steps.The counter then resets to zero and the sequence is repeated giving a cyclic stepped voltage waveform at the input 5 of a period equal to eight times the period of the oscillator 1 4. This waveform is shown in Fig. 3 aiong with constant voltage waveform applied to the inverting input 4 at the same time.

The voltage waveform appearing at the output terminal 6 of the comparator 1 when the waveforms shown in Fig. 3 appear at the inputs is shown in Fig. 2. This control signal is a cyclic waveform of the same period as the waveform at input 5, each cycle of which is divided into two sections having different voltage levels which determine the on and off periods of the heating (or cooling) device. The relative lengths of these on and off periods determine the rate of heating (or cooling) and are fixed by the points of intersection of the two waveforms shown in Fig. 3. If the constant voltage waveorm is completely above or below the stepped voltage waveform so that there is no point of intersection the heating (or cooling) device is either continually on or off and this proportional control no longer operates.

Changes in the desired temperature or the actual temperature will change the voltage level of the constant voltage waveform, via resistor 7 or thermistor 8, and so change the points of intersection, causing an appropriate increase or decrease in the rate of heating (or cooling) as required. However, because of the steps in the stepped waveform, the points of intersection can change only by discrete amounts equal to the length of each step and this ensures that the minimum duration of both the on and off periods is the length of one step. The problem of very short duration on or off periods is thus avoided.

Since the rate of heating increases or decreases by discrete amounts the actual temperature may oscillate slightly. The maximum oscillation is determined by the step height and the number of steps used. With eight steps covering a normal proportional control bandwidth of 1-2"C the effect is negligible.

Increasing the number of steps would further reduce this temperature oscillation but would also reduce the minimum duration of the on and off periods. This could be avoided by having the steps at the ends of the stepped waveform longer than those in between but this would require a more complex waveform generator than the simple binary counter arrangement described.

Claims

1. A control circuit for a heating or cooling device, comprising means for generating a control signal which when applied to the device switches it between alternate fully on and fully off states, the relative periods of these two states being controlled by a temperature selector element and a temperature sensor element connected to the control signal generating means, and incorporating means for setting a minimum duration for both the fully on and fully off states.

2. A control circuit as claimed in claim 1, wherein the control signal generating means comprises a comparator having two input terminals and an output terminal the voltage at which output terminal depends on the relative levels of the voltages at the input terminals, one of said input terminals being connected to a voltage source capable of producing a cyclically varying voltage at that input terminal and the other said input terminal being connected to the temperature selector element and temperature sensor element so as to produce a voltage at that terminal controlled by those elements, and the cyclically varying voltage remains at its maximum and minimum values for a predetermined minimum period in each cycle thereof.

3. A control circuit as claimed in claim 2, wherein the cyclically varying voltage moves between its maximum and minimum values in a series of voltage steps of length equal to the predetermined minimum period.

4. A control circuit as claimed in claim 3 wherein the voltage source comprises a first resistor the current through which is controlled by a plurality of resistors each connected between the output terminal of a binary counter and the said first resistor, the binary counter being connected to a fixed frequency oscillator.

5. A control circuit as claimed in claim 4, wherein each resistor connected to an output terminal of the binary counter has a resistance half that of the resistor connected to the preceding output terminal, so as to give equal voltage steps in the cyclically varying voltage.

6. A control circuit as claimed in any of claims 2 to 5, wherein the temperature selector element is a variable resistor and the temperature sensor element is a thermistor, the resistor and thermistor form a voltage divider across a fixed voltage.

7. A control circuit substantially as herein described with reference to and as illustrated by the accompanying drawings.