GB2101773A - Improvements in and relating to heaters - Google Patents

Abstract

The problem of the matching of the output of a space heater RL to the rise in ambient temperature resulting from the use of the heater is dealt with by controlling the supply of energy to the heater by means of an energy regulator fitted with a manually operable control VR to preset the maximum rate at which the regulator can feed energy to the heater. The energy regulator also incorporates a temperature sensing device TH1 that monitors changes in ambient (room) temperature and adjusts the energy regulator accordingly, reducing the mean power fed to the heater by the "burst-fire" control of triac T1 as the sensed temperature rises. The circuit shown operates by comparing the voltage at pin 5 of an integrated circuit with that at pin 4. The latter voltage depends on a divider circuit preferably incorporating a second temperature sensitive device TH2 and is a threshold voltage with which the ramped voltage on pin 5 is compared. The heater may be a radiant or convector heater, or a cooker element. <IMAGE>

Description

SPECIFICATION Improvements in and relating to heaters This invention relates to heaters and has particular but not exclusive reference to space heaters that heat by the emission of radiant heat from electrically powered radiant elements.

Conventionally, heaters incorporate some form of heat control, usually a thermostat, which simply turns off the supply of energy to the heater when a temperature determined by the setting of the thermostat is reached.

In the case of thermostatically-controlled spaced heaters, the user, in an attempt to reach a comfortable room temperature quickly, frequently sets the thermostat to its highest setting and then finds that a comfortable room temperature is exceeded before the thermostat turns off the supply.

If, however, the thermostat is set to the comfortable room temperature, the user finds that when the thermostat turns off the supply, the room space itself has not reached the comfortable temperature.

The disadvantages just described can be overcome to some extent by the use of a "proportional" control providing a gradual reduction in supply from full rate at the beginning of a heating period to a minimum sufficient to maintain the comfort temperature.

However, this form of control is fully effective only when the rate of fall in heat output is fully balanced by the rate of increase in room temperature. Such full balancing is difficult to achieve in practice, particularly with radiant heaters because the emission of radiant heat is not directly proportional to the temperature of the radiant heating element and the temperature depends upon the means supply of power to the heater.

Accordingly, it is an object of the present invention to provide a heater construction in which the problem just referred to is largely overcome.

According to the present invention, a control system for a heater comprises an energy regulator for regulating at least part of the supply of energy to the heater, a temperature sensitive device for controlling the operation of the regulator in accordance with changes in temperature sensed by the device, and a manually-operable control for setting the energy regulator to allow energy to be supplied at a predetermined rate.

In one embodiment of the invention suitable for a space heater, the temperature sensitive device is exposed to the ambient air temperature.

In another embodiment also suitable for a space heater, the latter includes an element emitting radiant heat, the temperature sensitive device is so arranged that it responds substantially exclusively to radiant heat emitted by the element.

By way of example only, embodiments of the invention will now be described in greater detail with reference to the accompanying drawings of which: Figure 1 is a block schematic illustrating the principles of the invention, Figure 2 is a circuit diagram partly in block schematic form of a first embodiment, Figures 3 and 4 are explanatory graphs, Figure 5 shows the disposition of control elements in a radiant electric fire, and Figure 6 shows diagrammatically part of a radiant electric fire incorporating the second embodiment.

Fig. 1 shows in block schematic form only, the principal elements of the invention. The invention is embodied in an electric heater having a radiant heating element E the supply of A.C. power to which is controlled by a semi-conductor switch S1, for example a silicon controlled rectifier or a triac. Switch S1 is triggered into conduction by a control signal supplied over leads L1 from an energy regulator shown as block ER. Operation of the regulator is determined by the setting of a manually-operable control VR and a temperature sensitive device TSD, for for example a thermistor.

Preferably, the energy regulator incorporates an integrated circuit providing an output that enables the switch to operate on the wellknown burst firing principle, and which incorporates zero crossing detection means ensuring that a burst is initiated only at zero supply voltage. This minimises surges on the supply line, prolongs the life of the heating element and also reduces radio interference.

The manually-operable control is adapted to increase and decrease the mean power input to the heater according to the setting of the control and may operate from a predetermined minimum input up to a "fully-on" maximum at which the heater is continually supplied with power.

The temperature sensitive device operates, as the temperature sensed increases to reduce the means power input until the mean power input level is just sufficient to maintain a required temperature. The temperature sensitive device may be calibrated to come into operation to control the energy regulator at some preset temperature and to reduce the mean power input to the minimum when the required temperature, also predetermined, is reached.

Fig. 2 is a circuit diagram partly in block schematic form of an implementation of the energy regulator ER of Fig. 1.

The regulator comprises an integrated circuit type TDA 1024 incorporating a zero crossing detector responsive to main supply via resistor R7 on pin No. 6. The integrated circuit is also mains powered on pin No. 7 via resistor R8 and capacitor C5.

The integrated circuit also supplies d.c. low voltage power from pin No. 8 to a saw-tooth waveform generator comprising transistors TR1 and TR2 and associated components R1, R2, R3, R4 and C1. The saw-tooth waveform output appears at point X and is controllable in amplitude only by the setting of potentiometer VR which is the manually operable control referred to above.

The controlled saw-tooth output is applied across series connected resistor R5 and thermistor TH 1 with the result that, at junction Y between resistor R5 and thermistor TH1, appears a saw-tooth waveform whose amplitude depends both on the effective resistance of VR and on that of the thermistor TH 1. The output at point Y forms the control input to the integrated circuit on pin No. 5.

A reference input on pin No. 4 is obtained from the junction of a second thermistor TH2 and resistor R6 series-connected across the low voltage d.c. power supply as shown.

The output of the integrated circuit at terminal No. 2 is applied via resistor R9 to the trigger electrode of a triac T1 in series with the heater element E across the mains power supply.

For protecting purposes, a non-linear resistor R10 is parallel connected as shown with the triac T1 and the heater element E.

Fig. 3 shows the saw-tooth waveform generated by the combination TR1, TR2 and associated circuitry. The waveform appears on point Y and its amplitude can be changed by operation of VR and by changes in the resistance of thermistor TH 1 which has a negative temperature coefficient.

The integrated circuit generates a train of pulses timed to occur at the instants of zero supply frequency and the appearance of these pulses at pin No. 2 is determined by the potential at pin No. 5 relative to that at pin No. 4. When the potential at pin No. 5 exceeds that at pin No. 4 pulses appear on pin No. 2 but are inhibited otherwise. Each pulse turns on the triac at the instants of zero supply frequency and the time during which the triac remains turned on is determined by the number of pulses appearing on pin No. 2 during the time that the potential at pin No. 5 exceeds that at pin No. 4.

Waveform A, Fig. 3, is that of the saw-tooth generated under "cold" conditions and with VR at its highest setting and the resistance of thermistor TH1 at maximum, i.e. that at which the amplitude of the waveform is greatest. Horizontal line CT represents the potential at pin No. 4 with the resistance of thermistor TH2 at maximum. Under these conditions, pulses appearing at pin No. 2 as soon as the saw-tooth waveform rises upwardly across line CT and ceasing as the saw-tooth waveform falls below line CT.

Fig. 4 shows schematically that under the condition just described, the mean power supplied to the radiant element of the fire is at a maximum and that there is a short time period during which the element is not energised.

As the temperatures sensed by thermistors TH 1 and TH2 rise, the amplitude of the sawtooth waveform as applied to pin No. 5 falls, and, the potential applied to pin No. 4 rises.

During this time, the mean power applied to the radiant element falls because the time periods during which the triac is turned on shorten.

In due course, equilibrium is reached when the mean power supplied to the radiant element is just sufficient to maintain a preset temperature in the room. This is indicated by waveform B in dotted lines in Fig. 3, the new threshold HT being in dotted lines. As is indicated in Fig. 4, the mean power supplied to the radiant element has dropped considerably.

Fig. 5 indicates schematically a radiant electric fire 1 with bar elements 2, 3. A control knob 4 mounted at the side of the fire controls the setting of potentiometer VR. Beneath the fire is a housing 5 apertured to permit free passage of air therethrough and which contains the thermistors TH 1 and TH2. The control circuitry for regulating the supply of power to the elements is contained in a control box 6 at the rear of the fire 1.

Both thermistors are exposed to a flow of air through the housing 5 induced by the heating effect of the radiant elements 1, 2 and the temperature of that air provides a measure of the heating effect of the elements.

It will be understood that a fixed resistor could be used in place of thermistor TH2 thus providing a fixed threshold. However, it is preferred to use the thermistor as this increases the sensitivity of the regulator because the threshold increases as the temperature sensed increases.

Some degree of control over the starting threshold value can be provided by inserting a variable resistor in series with thermistor TH2.

The frequency of the saw-tooth waveform is chosen so as to be high enough to give a reasonably steady heat output from the fire but not so high as to create excessive disturbance in the mains supply.

Alternative positions are possible for either or both thermistors. One or both of them could be exposed directly to heat from one or other of the elements 1, 2. This would enable a user to control the radiant heat output of the fire. If necessary, such a control may be associated with other sensors responsive to ambient temperature and these other sensors may be arranged to override the control in extremes of room temperature.

Fig. 6 shows schematically a possible location for a thermistor intended to respond to the radiant heat output of the fire.

The fire has a radiant bar element 7 located approximately at the focus of a reflector 8 apertured as at 9 to allow radiant heat from the element 7 to fall upon a thermistor TH3 located in a heat resistant housing 10 behind the reflector 8. Housing 10 ensures that thermistor TH3 is little affected by convected heat from the fire and by ambient temperature changes.

Thermistor TH3 is in the control circuit shown in Fig. 2 taking the place of thermistor TH1. In addition, thermistor TH2 would either be replaced by a variable resistor adjustable by a user to give the variable threshold referred to above, or be used to compensate for room temperature in a manner such that, in conditions of low room temperature, a higher radiant output is achieved thus preserving comfort conditions which, as is well known, are produced by a combination of the effects of radiant and convected heat.

It will be understood that the energy regulator described above may be used to control part only of the output of the heater. The remainder of the output may be controlled on an "on/off" basis only by a user, Also, the energy regulator can be used to control space heaters other than those of the radiant type, for example, it can be used with convention heaters and gas fires both radiant and convection.

The regulator can also be used to control other devices involving heaters, for example electric cookers.

Claims (11)

1. A control system for a heater comprising an energy regulator for regulating part at least of the supply of energy to the heater, a temperature sensitive device operatively connectd to the regulator for controlling the operation of the regulator in such manner that the supply of energy is continuously varied in accordance with changes in temperature sensed by the device, and a manually operable control for setting the energy regulator to a predetermined maximum rate of energy supply.

2. A control system as claimed in claim 1 in which the temperature sensitive device is operable to produce variations in the supply of energy only when the temperature sensed by the device exceeds a predetermined minimum temperature.

3. A control system as claimed in claim 2 in which means are provided for changing the predetermined minimum temperature.

4. A control system as claimed in claim 1, 2 or 3 in which the temperature sensitive device is operable to maintain the supply of energy at a minimum preset level when a preset temperature is sensed by the device.

5. A control system as claimed in any one of the preceding claims in which the temperature sensitive device comprises first and second thermally responsive elements, and in which the control system is such that the second element is used to provide a threshold temperature against which the temperature sensed by the first element is compared, the difference between the sensed temperature and the threshold temperature controlling the operation of the temperature regulator.

6. A control system as claimed in claim 5 in which the first element is responsive to radiant heat only emitted by the heater.

7. A control system as claimed in any one of the preceding claims in which the energy regulator includes means for deriving a quantity whose value determines the rate of supply of energy, the value being determined in accordance with the temperature sensed by the sensing device and by the setting of the manually operable control.

8. A control system as claimed in claim 7 in which the energy regulator comprises a saw tooth waveform generator and means for deriving therefrom control signals for regulating the supply of energy.

9. A control system as claimed in claim 8 in which the energy regulator is adapted to control energy in the form of an alternating current supply and in which the system includes a zero crossing detector and means for timing the appearance of a control signal to coincide with a zero amplitude point in the A.C. supply.

10. A control system for an electric fire substantially as herein described with reference to and as illustrated by Figs. 1, 2, 3 and 4 or Figs. 1, 2, 3, 4 and 5 of the accompanying drawings.

11. An electric fire including a control system as claimed in in any one of claims 1-10.