GB2261084A - Control circuit for controlling the heat supplied by a load - Google Patents

Abstract

A controller, for controlling a load (LOAD) connected across an AC mains supply, comprises a control circuit and a low voltage power supply circuit. The power supply comprises two zener diodes (ZD1, ZD2) connected cathode-to-cathode and in series with a resistor (R1) across the AG mains supply. The low voltage is taken from the junction between the zener diodes (ZD1, ZD2) and a square waveform is taken from the junction between zener diodes (ZD1) and the resistor (R1). The square waveform is supplied to the control circuit in order to achieve switching of the load (LOAD) at the zero voltage- crossing points of the mains supply. <IMAGE>

Description

CONTROLLER This invention relates to a controller for controlling an electrical load, and may be arranged to control the temperature of an electrical load or of an article or space heated by the load.

In many applications where low-cost temperature controllers are required, as a cost-saving the low voltage electronic circuitry of the controller is supplied direct from the high voltage AC mains supply, the excess power being simply dissipated as heat in a resistive element. Typically for every 4mA consumed by the controller circuitry from a 240 volts AC supply, 1 watt electrical power is dissipated as heat.

We have now devised a controller which is driven direct from the AC mains supply but which consumes relatively little current and dissipates relatively little power as heat.

In accordance with this invention as seen from one aspect, there is provided a controller comprising a control circuit for a load connected in series with an AC switching element across an AC mains supply, and a low voltage supply circuit for the control circuit, said low voltage supply circuit comprising a resistive element and two opposite-poled zener diodes connected in series across the AC mains supply and providing the low voltage supply between the junction of the zener diodes and the AC mains neutral line and a square wave at the junction between the resistive element and the adjacent zener diode, the control circuitry being arranged to use the square wave in order to switch the AC switching element substantially at the zero voltage crossing points of the AC mains supply.

In the embodiment to be described, the current drawn by the controller is substantially equal on both positive and negative half cycles of the AC mains supply.

The square wave which is generated is used by the control circuitry so that the AC switching element (typically a triac) for the load is switched substantially at the zero voltage crossing points of the AC mains supply. At such points, pulses are applied to the AC switching element to maintain it in conduction.

In closed loop control applications, it is necessary to switch power to the load on and off in accordance with control demand. It is often necessary that little or no power is wasted by this function. Also it is often necessary to be able to determine both the switch on and switch off points.

In accordance with this invention as seen from a second aspect, there is provided a controller which comprises a first comparator having one input receiving successive pulses and a second input connected to the output of a second comparator, the second comparator having its inputs connected respectively to a sensor and a reference point.

In the embodiment of the invention to be described herein, the successive pulses applied to the first comparator coincide with the zero crossing points of the AC mains supply and the first comparator supplies corresponding pulses, when the sensor output signal is below a preset level, to the AC switching element for the load, to maintain this switching element in conduction.

The desired switching point is set by adjusting the reference point, e.g. using a potentiometer. Preferably the output of the second comparator is connected with the potentiometer in such a way that once the sensor signal has fallen below the preset level, the reference point is modified and requires this signal to reach a higher level before the output of the second comparator switches back to its previous level.

Another common cost saving in low-cost temperature controllers is the use of a wide-tolerance potentiometer as a variable resistor to set the desired temperature level.

However this results in a very wide setting difference between production controllers for a given temperature: typically a variation of 70 angular degrees can be found in the setting of the rotary potentiometers of different batches of production units.

In accordance with this invention as seen from a third aspect, there is provided a controller comprising a comparator having one input connected to a sensor and a second input connected to the slider of a potentiometer which has its opposite ends connected by respective resistors across a power supply, the potentiometer setting providing a reference which determines a setting for the controller and the output of the comparator controlling the supply of current to a load.

In this arrangement it is found that the position of the potentiometer slider, for a given setting, is substantially independent of potentiometer tolerance.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawing, the single figure of which is a circuit diagram of a temperature controller in accordance with the invention.

The controller which is shown comprises a control circuit for controlling a load LOAD via a triac connected in series with the load across an AC mains supply, in accordance with the temperature sensed by a thermistor TH1. The controller further comprises a combined square wave generator and low voltage regulated supply for the control circuit, formed by the series circuit of resistor R1 and two oppositelypoled zener diodes ZD1 and ZD2 connected across the 240 volts mains supply, and a rectifying diode D5 connecting the junction between the two zeners to the DC supply rail Vcc. A smoothing capacitor Cl is connected between the supply rail Vcc and mains neutral.A full-wave rectifier bridge formed by diodes D1 to D4 has one node connected to mains neutral and the junction between resistor R1 and zener ZD1 is connected to its opposite node through a resistor R2. A resistor R3 is connected across the other diagonal of the rectifier bridge and across the baseemitter junction of a transistor TR1, the collector of which is connected by a resistor R4 to Vcc. The collector of TR1 delivers narrow pulses, as will be explained, to the noninverting input of one comparator (comparator A) of an integrated circuit IC1. The output of comparator A is connected to Vcc by a resistor R6, and to the base of an emitter follower TR2 via a resistor R7. TR2 controls the triac via its emitter resistor R15.

The inverting input of comparator A is connected to Vcc via a resistor R5 but is also connected to the output of a second comparator (comparator B) of IC1 via a resistor R8. The non-inverting input of comparator B is connected to the slider of a potentiometer RV1, whilst its inverting input is connected to the junction between a resistor R14 and the thermistor TH1 which are connected in series between Vcc and mains neutral.

Potentiometer RV1 is connected at one end through resistors R12, R13 to Vcc, and at its other end through resistor Rll to mains neutral. A resistor R10 is connected from the one end of the potentiometer RV1 to its slider. A resistor R9 is connected between the output of comparator B and the junction between resistors R12 and R13.

In order to minimise the generation of radio frequency interference when switching the AC load, it is necessary to switch the AC switching element (the triac) at the zero voltage points of the AC supply, which at 50 Hz occur every 10 milliseconds. Resistor R1 and zener diodes ZD1 and ZD2 are used to generate the square wave used to accomplish zero voltage switching. Normally both zener diodes would reverseconduct equally one during the positive half cycle and one during the negative half cycle to provide a square wave whose mid-point is zero voltage. Typically this current would be 5mA in both half cycles. If a further 10 mA of power were required to supply the low voltage temperature controller electronics, then the overall current drain from the mains supply would be 15ma during the positive half cycle and 5mA during the negative half cycle.This unsymmetrical load could possibly trip Earth Leakage Circuit Breakers usually installed to protect the AC supply.

In the circuit shown, the arrangement of resistor R1 and zeners ZD1, ZD2 and diode D5 form both a square wave generator and low voltage regulated power supply (typically 6.5 volts). The design of the circuit achieves equality between the load current, taken from this power supply by the other electronic circuitry on the positive half cycles, and the negative half cycle current taken by zener ZD1, resulting in zener diode ZD1 being forced into switch-off during the positive half cycle. The result is a symmetrical load, with no DC component, on the mains supply and the maximum use of the available current to supply the remaining control circuitry.

The mains frequency square wave (described above) is passed through resistor R2 and full-wave rectified by diodes bridge D1 to D4. The output holds transistor TR1 in saturation almost continuously except for the short duration that the square wave voltage drops to zero at the zero-crossing of the original AC waveform. At this point the output from transistor TR1 rises rapidly to Vcc.

A reference voltage Von is obtained from the potential divider formed by R13, R12, RV1 and Rll. This voltage is taken from the slider of potentiometer of RV1 and fed to the noninverting input of comparator B. A temperature-dependent sense voltage V2 is obtained by the potential divider action of resistor R14 and the negative temperature coefficient thermistor TH1. This voltage is fed to the inverting input of comparator B. Whilst V2 is less than Von, comparator B will remain off and its output high. However, when the temperature falls, V2 rises and if it becomes equal to Von, comparator B turns on and its output goes low. This is defined as the ON temperature.

As soon as the output of comparator B goes low, the reference voltage is modified by the potential divider action of R13, R9, R12, RV1 and RV11 to produce a reduced reference voltage at the slider of potentiometer RV1. The temperature now has to rise to a higher level for the voltage at the inverting input of comparator B to fall below this new reference voltage Voff. This is defined as the OFF temperature.

This arrangement and the choice of thermistor for a particular operating temperature allow this band of control and reference voltages to be approximately one third of the supply voltage Vcc. Once this is achieved the relative value of R13 plus R12 becomes several times that of potentiometer RV1 and the voltage at the centre of travel of RV1 slider becomes essentially independent of the manufacturing tolerance of the potentiometer.

To this end the ON temperature, OFF temperature and the consistency of the control setting can be precisely defined.

Additionally the arrangement and the use of a very high value resistor Rll ensure that the circuit will shutdown in the event of a short circuit failure of thermistor TH1 or revert to the minimum control temperature should the slider of potentiometer RV1 become inadvertently disconnected.

The output of comparator B also controls the voltage at the inverting input of comparator A. When the output of comparator B is high (temperature greater than the OFF temperature) the voltage at the inverting input of comparator A is at Vcc (supply) and comparator A remains on and with its output low. In this condition no voltage is supplied via emitter follower TR2 to energise the triac and switch on the load.

When the comparator B is on and its output low (temperature less than the ON temperature) the voltage at the inverting input of comparator A is less than Vcc and defined by the potential divider action of R5 and R8. This allows the short duration zero crossing triggering pulses (described above) at the non-inverting input of comparator A to turn this comparator off and the output voltage at R6 rises to Vcc. The resulting output pulses from comparator A are supplied to the triac via emitter follower TR2 and hold the triac in conduction.

The output of comparator A can be forced low at any time by shorting the INHIBIT terminal to neutral with a switch or active device (e.g. transistor), in a WIRED OR scheme, to switch off the triac.

The temperature controller which has been described has a very low component count. Its total current consumption is less than 5mA and can therefore be directly driven from the AC mains supply with the minimum of generated heat in resistor R1 (typically less than 1 Watt at 240 V AC). The temperature controller provides zero voltage switching of AC loads to minimise the generation of radio frequency interference. The on and off temperature switching points can be defined for any particular application and the centre scale adjustment point is essentially independent of potentiometer tolerance. The temperature controller has a simple inhibit input able to shut down the control action. The temperature controller will automatically shutdown should the sensing thermistor become short circuited, and will revert to minimum-setting operation should the slider of the potentiometer become disconnected.

Whilst the controller has been described for use in controlling a load in accordance with temperature, it may be used for controlling a load in accordance with other sensed parameters.

Claims (6)

1) A controller comprising a control circuit for a load connected in series with an AC switching element across an AC mains supply, and a low voltage supply circuit for the control circuit, said low voltage supply circuit comprising a resistive element and two opposite-poled zener diodes connected in series across the AC mains supply and providing the low voltage supply between the junction of the zener diodes and the AC mains neutral line and a square wave at the junction between the resistive element and the adjacent zener diode the control circuitry being arranged to use the square wave in order to switch the AC switching element substantially at the zero voltage crossing points of the AC mains supply.

2) A controller as claimed in any preceding claim, arranged such that the current taken from the mains supply is substantially equal on both positive and negative half cycles.

3) A controller comprising a first comparator having one input for receiving successive pulses and a second input connected to the output of a second comparator, the second comparator having its inputs connected respectively to a sensor and a reference point.

4) A controller as claimed in claim 5, in which the output of the second comparator is connected with a potentiometer for adjusting the reference point in such a way that once a signal from the sensor has fallen below a preset level, the reference point is modified and requires the sensor signal to reach a higher level before the output of the second comparator switches back to its previous level.

5) A controller comprising a comparator having one input connected to a sensor and a second input connected to the slider of a potentiometer which has its opposite ends connected by respective resistors across a power supply, the potentiometer setting providing a reference which determines a setting for the controller and the output of the comparator controlling the supply of current to a load.

6) A controller substantially as herein described with reference to the accompanying drawing.