GB2128374A - Temperature controller for heating apparatus - Google Patents

Abstract

Heating apparatus has at least two electrical heating elements 2, 3 and a temperature controller which responds to a temperature sensor TH1 to vary the duty cycle of each of the elements in a sequential manner. For example, at an upper sensed temperature level, the first element is supplied with power at a first-duty cycle; at a mid-range level the first element is supplied with more power at a second duty cycle; and at a lower level the first element is provided with maximum power whilst the second element is supplied with power at a third duty cycle. The controller may employ zero voltage switches connected to respective triacs T1, T2 in order to control burst firing whereby the first and second duty cycles are controlled at different sensor temperatures within a proportional control band. <IMAGE>

Description

SPECIFICATION Heating apparatus This invention relates to heating apparatus which includes at least two electrical heating elements and a thermostatic power controller having a temperature sensor and means capable of controlling the amount of power supplied to the elements in accordance with a variation in the temperature of the sensor. The power controller is capable of varying the duty cycle of each of the elements in a sequential manner, so that the power supplied to the elements is gradually decreased as the temperature of the sensor increases, and vice versa. An embodiment of the invention may be advantageously applied to a domestic electrical heating appliance for heating a room in which the sensor is situated, the heating element being supplied with power in accordance with variation in the room temperature.In such an application, besides the thermostatic control, the power controller is capable of reducing the power supplied to the heating appliance without a noticeable ioss in the appearance, i.e. the 'redness' or 'glow', of one or more of the heating elements.

A user may thereby conserve power without entirely losing the cheerful appearance of the fire, which would be the case if one or more of the heating elements were switched off for a long enough period to cool down and thereby lose their glow. This advantage will become more apparent from the following description.

Thermostatic power controllers are well known in the field of domestic electrical heating appliances. For example, there is the conventional bimetallic type of thermostat which responds to a change in temperature to make or break associated contacts which are connected in series with an electrical heating element. Thermostats of this type either turn the heating element fully on, or fully off and the duration of the 'off' periods is such that the heating element cools down and loses its glowing effect before it is turned on again. Moreover, this type of thermostat does not provide very accurate control of room temperature because the temperature may cycle continually over a wide range, i.e. between 'on' and 'off' points, whereby only a rough average control temperature is provided.

Another well known form of thermostatic power control employs an energy regulator which may include, for example, an accelerated bimetallic strip. This form of control provides a duty cycle in which a heating element is switched on and off more rapidly than with the former bimetallic (non-accelerated) thermostat to provide a closer control over the average heating temperature. This known type of energy regulator has been used for many years as a form of 'simmer' control on electric cookers.

It is known from GB 2077418 to use an energy regulator to control a heating element so as to provide a duty cycle which ranges from 1 2 seconds ON/18 seconds OFF at a minimum thermostatic setting of 40 seconds ON/12 seconds OFF at a maximum setting. However, the off periods in these duty cycles is still comparatively long having regard to the rapid fall in temperature of an electrical heating element when switched off, and a visible cooling of the heating element is clearly apparent when the current is turned off. In the particular arrangement disclosed in this reference, a pair of parallel connected elements are switched on and off to vary the heating effect, but the duty cycle is such that the elements can be seen to be cycling under thermostatic control.

Moreover, the radiating temperature of the individual elements is reduced, thereby giving them a dull appearance. Furthermore, both elements are controlled simultaneously from one and the same regulator whereby no sequential phased increase in the duty cycles of two or more heating elements is possible for smoothiy accommodating any variation in room temperature.

Burst firing techniques are also known in the art. According to this technique, a triac is controlled by an integrated circuit which is responsive to a temperature sensor such as a thermistor. For example, a zero voltage switch (such as a Plessey SL441 C) may be used as the integrated circuit for controlling the power supplied to a heating element. The integrated circuit generates burst firing pulses which are supplied to a triac in series with a heating element.The triac thereby supplies cycles of mains current, within a fixed time period, to the heating element A thermistor which acts as a temperature sensor, is connected in series with a potentiometer whereby a variable voltage signal, i.e. dependent on the temperature of the thermistor and on the setting of the potentiometer, is supplied to the integrated circuit so as to vary the number of complete cycles supplied in the fixed time period within a proportional control band. (within a predetermined voltage range). Outside this voltage range, the triac either passes every cycle of AC (e.g. when the voltage is below the predetermined control range), or it remains switched off and no power is supplied to the heating element (e.g. when the voltage exceeds the predetermined control range).

The voltage control range may be adjusted, to suit particular requirements, by using a conventional voltage-dividing network.

Burst firing techniques are preferred, because the use of phase control (wherein the amount of power supplied in each half cycle of AC is varied) is prohibited for controlling the power supplied to heating elements of thermal appliances (see BS 5406.EN 50.006). The length of the burst period which will meet the requirements of the Standard can be calculated from the Standard and varies with the load. The bigger the load, the longer the burst period.

Whilst the latter zero voltage switch may be used to form the basis of an electronic thermostat for a room heater, and to provide the advantages of accurate temperature control, it is still limited to the control of a single heating element (or to a group of heating elements which are controlled in unison).

Electrical radiant fires often have two or more elements, sometimes having different power ratings, whereby the user may select, for example, one, two or three elements depending on variations in the weather or personal comfort requirements. For example, all of the elements may be initially switched on in very cold conditions. If the room temperature then became too hot and/or ambient conditions changed (e.g. it became warmer outside), it would be necessary to switch the elements on and off individually, in order to achieve a desirable room temperature. If the burst firing technique was used to control one of these elements, there may still be a need to switch one of the other two elements on and off depending on the demand for heat, e.g. due to variations in ambient temperature.It would, of course, be possible to provide respective electronic thermostats for each element, but this would add to the expense of the appliance and random switching of the separate elements is undesirable.

The problem facing the invention is to provide a thermostatic power controller which will operate two or more heating elements in such a manner as to achieve smooth and accurate temperature control (e.g. within a room) under variable ambient conditions (e.g. variations in outdoor temperature).

The invention solves this problem by providing heating apparatus comprising at least two electrical heating elements, namely, a first and a second heating element, and a thermostatic power controller, said power controller having a temperature sensor and means capable of controlling the amount of power supplied to said elements, in accordance with a variation in the temperature of said sensor, such that; (a) the first element is periodically provided with said power, at a first duty cycle, when the temperature of said sensor is at a predetermined upper level; (b) the first element is periodically provided with said power, at a second duty cycle, when the temperature of said sensor is at a predetermined mid-range level, said second duty cycle having a mark space ratio which is larger than that of the first duty cycle; and (c) the first element is provided with a maximum amount of power, which is greater than that delivered in the second duty cycle, when the temperature of said sensor is at a predetermined lower level, the second element then being periodically provided with said power at a third duty cycle.

Both heating elements are controlled by the same thermostatic power controller in accordance with the temperature of a common temperature sensor. Therefore, the cost of an additional power controller is avoided. However, the heating elements appear to function independently since they are supplied with current in accordance with a corresponding demand for heat.

Preferably, each duty cycle is such that at least for most of the time, any visible flicker is substantially avoided. However, the burst firing period is within the minimum permitted by BS5406 to avoid any electrical interference.

In a preferred embodiment of the invention, the thermostatic power controller comprises first and second zero voltage switches for burst firing first and second triacs associated with the first and second heating elements respectively, said zero voltage switches being connected to a common voltage dividing network which includes a temperature sensor for transducing a change in temperature to a change in resistance, first and second voltage control means commonly connected to said sensor for adjusting the duty cycles of said first and second heating elements respectively, the duty cycles of the first and second heating elements being controlled at different sensor temperatures, and third voltage control means for adjusting the response of said first and second zero voltage switches to the temperature of said sensor.

One way of carrying out the invention will now be described with reference to the accompanying schematic drawing which represents a circuit diagram of an electronic thermostat for controlling the power supplied to electrical heating elements in an electrical radiant fire.

Referring to the drawing, an electrical radiant fire (not shown) is provided with three electrical heating elements 1 2 and 3. Element 1 is permanently connected across the mains supply lines L and N and hence is always ON when a main control switch SW1 is turned on. The other two elements 2 and 3 are connected in series with respective triacs T1 ,T2 which are controlled by integrated circuits IC1 and IC2. Each of the integrated circuits IC2, IC1, IC2 are zero voltage switches, such as those sold by Plessey under serial No. SL441 C. Since these zero voltage switches are of known construction, only a brief description of their general operation will be given.

Considering either one of the integrated circuits shown in the drawing (since their combined effect in the illustrated circuit will be different, as explained later) the integrated circuit is capable of generating burst firing pulses which are supplied from pin 4 to the gate of a corresponding triac (e.g. T1). Variation in a voltage signal (within a predetermined range) applied to pin 8 causes a variation in the timing of the burst firing pulses so as to vary the number of cycles of AC supplied by the associated triac within a fixed period. This variation occurs over a proportional control band and the position of this band depends on the values of resistors in an associated voltage dividing network connected to pin 8. This voltage dividing network may include, for example, a fixed resistance, a potentiometer and a thermistor.

The burst fire control voltage supplied to pin 8 is varied automatically by means of a change in the resistive value of the thermistor and manually by adjusting the potentiometer. In a typical application, the triac is fully on when the burst fire control voltage is less than 4.25 volts. The associated heating element then receives maximum power. When the control voltage is greater than 4.35 volts, no burst firing pulses are supplied to the triac and hence, it does not supply any power to the associated heating element. In the intermediate or proportional control band, i.e.

for control voltages lying within the range of 4.25-4.35 volts, burst firing pulses are supplied to the gate of the triac. The number of AC cycles supplied by the triac, depends on the value of the control voltage.

Hence, in the proportional band, the heat output of the associated heating element varies in proportion to variation in the control voltage and the heating element is controlled in accordance with the temperature of the thermistor.

Turning now to thermostatic power controller shown in the drawing, thermistor TH 1 acts as a common temperature sensing element for two zero voltage switches IC1 and IC2. Thermistor TH 1 is situated in a room heated by elements 1, 2 and 3. A fixed resistor R4 is connected in parallel with thermistorTHl.This parallel network is connected to a series network including a fixed resistor R5 and variable resistors or potentiometers RV1 and Rc. Potentiometer Rye is mounted on the electrical radiant fire or heating appliance so that it may be adjusted by the user to adjust the setting of the thermostatic power controller. In this way, the user may adjust the room temperature.The values of the fixed resistors R4, R5 and the potentiometer RV1 are selected such that variation of potentiometer Rc causes a desired shift in the proportional control band with regard to the heat output of the heating element 3. In other words, a shift occurs in the temperature (of TH 1 ) at which a given control voltage, in the proportional band, is'applied to pin 8 of ICi. Thus adjustment of potentiometer Rc causes a change in the timing of the burst firing pulses applied to triac T2 which, in turn, causes a variation in the amount of power supplied to the heating element 3. With a given setting of potentiometer Rc, a variation in room temperature will cause a corresponding change in the resistance of thermistor TH 1 and this will cause a change in the control voltage applied to pin 8 of Cl.Thus, a change in room temperature will cause a proportional change in the current supplied by triac T2 to the heating element 3, in the proportional band. The parallel network R4, TH1 is also connected to a network including fixed resistor R6 in parallel with potentiometer RV2 and a series fixed resistor R7. The junction between R6, RV2 and R7 is connected to pin 8 of the integrated circuit IC2. The values of R6, RV2 and R7 are selected so as to supply a suitable control voltage to pin 8 of IC2, which voltage also varies with a change in resistance of thermistor TH 1 due to a change in room temperature. However, as the latter control voltage is derived resistively from the network connected to pin 8 of IC1, the control voltage applied to pin 8 of IC2 is always lower than that applied to pin 8 of IC1.Hence, the control voltage applied to pin 8 of IC2 will change the timing of the burst firing pulses applied to triac T1 at a different temperature and the amount of current supplied to heating element 2 will be varied accordingly.

Outside of the proportional control band, the triacs T1 and T2 either supply maximum current (i.e. continuous cycles of AC when the control voltage is lower than the minimum voltage in the control band), or they supply no current (i.e. when the control voltage exceeds the maximum voltage in the control band).

In general, the two zero voltage switches IC1, IC2 are connected to a common voltage dividing network which includes a common temperature sensing element TH 1 and respective voltage control means RV2, RV1 and Rc. The voltage control mean RV2 and RV1 are adjusted such that the duty cycles of the respective heating elements 2, 3 are controlled at different sensor temperatures. The voltage control means Rc is provided for adjustment, by the user, so as to adjust the voltages applied to both pins 8 of IC1 and IC2 for regulating the currents supplied to heating elements 2, 3 respectively in the proportional band. Outside the proportional band, the elements 2, 3 are either fully on or off.

Both of the integrated circuits IC1, IC2 are supplied with unidirectional current from a diode D2 having resistor R1 in parallel therewith. This current is supplied to respective resistors R2, R8 connected to respective pins 2 or IC1 and IC2.

Various capacitors Ci-C6 and respective resistors R3 are also connected to pins 6, 1, 7 and 3 of each integrated circuit IC1 and IC2 as shown in the drawing. These components are required in the normal circuit operation of integrated circuits IC2, IC2 and hence their function need not be described in detail.

The following example of a typical cycle of operation is given to assist in explaining the visual effect of the illustrated circuit on the heating element 1, 2 and 3. Element 1 is at the top, element 2 is in the middle and element 1 is at the bottom.

(a) Given a situation in which a room is being maintained at 200C and the middle element (2) is working on a 50% duty cycle, the top element (3) will be off.

(b) If the room temperature suddenly falls by (say) 1 0C due to an opening door, the middle element (2) will switch on for approx. 4- seconds, off for - second, and so on, thereby maintaining the room temperature at 200C, the top element (3) will still be off.

(c) If the room temperature falls by another 1 OC, the middle element (2) will be switched on completely and the top element (3) will now be working on a 50% duty cycle (approx.).

The lower element (1) is on all the time. The top (3) and middle (2) elements appear to function independently; only coming into operation when the room temperature demands it.

Regarding (b) above, the burst period is selected for optimum results. If the burst period is 5 seconds (and it could alternatively be 1 second, for example), the middle element switches through one complete cycle, from ON TO ON, in 5 seconds. It can, for example, be ON 4.5 seconds, OFF 0.5 seconds; ON 2.5 seconds; OFF 2.5 seconds; or ON 0.5 seconds, OFF 4.5 seconds.

Visible temperature fluctuations can just be seen but tend to be obscured by the adjacent element which is on full load. If the burst were reduced to one second, fluctuations should not be observed.

Of course during the respective duty cycle, whether part of a 5 second burst period at a 1 second burst period, there will be times when the element colour is black rather than red because the "ON" period is not long enough to raise the temperature into the visible spectrum.

Claims (8)

CLAIMS 1. Heating apparatus comprising at least two electrical heating elements, namely, a first and a second heating element, and a thermostatic power controller, said power controller having a temperature sensor and means capable of controlling the amount of power supplied to said elements, in accordance with a variation in the temperature of said sensor, such that; (a) the first element is periodically provided with said power, at a first duty cycle, when the temperature of said sensor is at a predetermined upper level; (b) the first element is periodically provided with said power, at a second duty cycle, when the temperature of said sensor is at a predetermined mid-range level, said second duty cycle having a mark space ratio which is larger than that of the first duty cycle; and (c) the first element is provided with a maximum amount of power, which is greater than that delivered in the second duty cycle, when the temperature of said sensor is at a predetermined lower level, the second element then being periodically provided with said power at a third duty cycle. 2. Heating apparatus according to claim 1 wherein each duty cycle is such that, at least for most of the time, any visible flicker is substantially avoided. 3. Heating apparatus according to claim 1 or 2 wherein said means capable of controlling the amount of power supplied to said elements comprises gate-controlled devices in serries with the first and second heating elements respectively, said devices being controlled by respective zero voltage switches. 4. Heating apparatus according to claim 3 wherein said zero voltage switches are connected to a common voltage dividing network which includes said temperature sensor for transducing a change in temperature to a change in resistance, first and second voltage control means commonly connected to said sensor for adjusting the duty cycles of said first and second heating elements respectively, the duty cycles of the first and second heating elements being controlled at different sensor temperatures, and third voltage control means for adjusting the response of said first and second zero voltage switches to the temperature of said sensor. 5. Heating apparatus according to claim 1 or 2 wherein first and second triacs are connected in series with the first and second heating elements respectively, and wherein said means capable of controlling the amount of power supplied to said elements comprises first and second zero voltage switches connected to said first and second triacs respectively, and a common voltage dividing network which includes said temperature sensor and first, second and third voltage control means, said first and third voltage control means being connected in series to said temperature sensor whereby a proportional voltage is applied to a control terminal of said first zero voltage switch, said second terminal of said first zero voltage said second voltage control means also being connected to said temperature sensor and to a control terminal of said second zero voltage switch to supply a lower proportional voltage thereto, said control terminals receiving the respective proportional voltages in order to control burst firing of said first and second triacs respectively, whereby said first and second duty cycles are controlled at different sensor temperatures within a proportional control band. 6. Heating apparatus according to any one of the preceding claims wherein said temperature sensor is a thermistor. 7. Heating apparatus substantially as herein described with reference to the accompanying drawing. New claims or amendments to claims filed on 23 November 1 983. Superseded claims 1 to 7 New or amended claims CLAIMS

1. Heating apparatus comprising at least two electrical heating elements namely, a first and a second heating element, and a thermostatic power controller, said power controller having a temperature sensor and first and second independent means capable of variably controlling the amount of power supplied respectively to said first and second elements, in accordance with a variation in the temperature of said sensor, such that; (a) the first element is periodically provided with said power, at a first duty cycle, when the temperature of said sensor is at a predetermined upper level;; (b) the first element is periodically provided with said power, at a second duty cycle, when the temperature of said sensor is at a predetermined mid-range level, said second duty cycle having a mark space ratio which is larger than that of the first duty cycle; and (c) the first element is provided with a maximum amount of power, which is greater than that delivered in the second duty cycle, when the temperature of said sensor is at a predetermined lower level, the second element then being periodically provided with said power at a variable duty cycle.

2. Heating apparatus according to claim 1 wherein respective voltage control means are provided for adjusting the duty cylces of the respective first and second heating elements at different sensor temperatures.

3. Heating apparatus according to claim 1 or 2, wherein each duty cycle is such that, at least for most of the time, any visible flicker is substantially avoided.

4. Heating apparatus according to any one of claims 1-3, wherein said means capable of controlling the amount of power supplied to said elements comprises gate-controlled devices in series with the first and second heating elements respectively, said devices being controlled by respective zero voltage switches.

5. Heating apparatus according to claim 4 wherein said zero voltage switches are connected to a common voltage dividing network which includes said temperature sensor for transducing a change in temperature to a change in resistance, first and second voltage control means commonly connected to said sensor for adjusting the duty cycles of said first and second heating elements respectively, the duty cycles of the first and second heating elements being controlled at different sensor temperatures, and third voltage control means for adjusting the response of said first and second zero voltage switches to the temperature of said sensor.

6. Heating apparatus according to claim 1 wherein first and second triacs are connected in series with the first and second heating elements respectively, and wherein said means capable of controlling the amount of power supplied to said elements comprises first and second zero voltage switches connected to said first and second triacs respectively, and a common voltage dividing network which includes said temperature sensor and first, second and third voltage control means, said first and third voltage control means being connected in series to said temperature sensor whereby a proportional voltage is applied to a control terminal of said first zero voltage switch.

said second voltage control means also being connected to said temperature sensor and to a control terminal of said second zero voltage switch to supply a lower proportional voltage thereto, said control terminals receiving the respective proportional voltages in order to control burst firing of said first and second triacs respectively, whereby said first and second duty cycles are controlled at different sensor temperatures within a proportional control band.

7. Heating apparatus according to any one of the preceding claims wherein said temperature sensor is a thermistor.

8. Heating apparatus substantially as herein described with reference to the accompanying drawing.