GB2033708A

GB2033708A - Method and apparatus for controlling temperature

Info

Publication number: GB2033708A
Application number: GB7932219A
Authority: GB
Original assignee: Joseph Ltd N C
Current assignee: Joseph Ltd N C
Priority date: 1978-09-16
Filing date: 1979-09-17
Publication date: 1980-05-21

Abstract

A method of and apparatus for controlling the temperature of a substance in an appliance having an electrical heating element, (1) comprises the steps of supplying electricity to the heating element (1) to generate heat therein which is transmitted to the substance, detecting the temperature of a selected part of the element with an electronic temperature sensor (4) e.g. a thermistor, a diode or a resistor which is thermally coupled with said element (1), feeding an output signal from the sensor (4) to a control circuit and cutting off the electricity supply to the element (1) when the temperature of said selected part of the element rises above a value corresponding to a predetermined temperature of the substance being heated. <IMAGE>

Description

SPECIFICATION Method and apparatus for controlling temperature This invention relates to a method and apparatus for controlling the temperature of a substance in an electrical appliance which has a heating element.

In known heating processes, the temperature of a substance being heated has been detected by a first bi-metal strip or electronic temperature sensor, and the temperature of the element has been detected by a second bi-metal strip or electronic temperature sensor in thermal contact with said element. When the substance being heated reached a desired temperature, the electricity supply to the element was cut off and also, should the element over heat the supply would also be cut off.

This method has the disadvantage that two temperature sensors are necessary.

Another method used has been to detect both the temperature of the element and the temperature of the vapour of the substance being heated by a single electronic sensor.

This method has the disadvantage that to enable the sensor to effectively detect the temperature of the vapour of the substance being heated, the sensor is at a distance from the element. Thus its response to changes in the temperature of the element is slow and the element is not effectively protected from overheating.

It is an object of the present invention to provide a method whereby the above disadvantages are overcome or reduced.

According to a first aspect of the invention we provide a method of controlling the temperature of a substance in an appliance having an electrically energisable heating element, comprising the steps of supplying electricity to the heating element to generate heat therein which is transmitted to the substance, detecting the temperature of a selected part of the element with an electronictemperature sensor which is in thermal contact with said element, feeding an output signal from the sensor to a control circuit and cutting off the electricity supply to the element when the temperature of said selected part of the element rises above a value corresponding to a predetermined temperature of the substance being heated.

In this specification, by "electronic temperature sensor" we mean a component of an electronic circuit of which an electrical parameter changes with temperature with a sufficiently high temperature coefficientto enable the electronic circuitto detect the parameter change and react thereto. Usually the parameter is resistance.

The positioning of the sensor in thermal contact with the heating element provides the additional advantage that in the event of the heating element being energised without the presence of the substance to absorb heattherefrom,the sensor will detect a rise in the temperature of the element above said value and hence the electricity supply to the element will be cut off in this case also.

This method is particularly applicable for controlling the temperature of a liquid being heated in a vessel, to cut off the electricity supply to the element when the temperature of the selected part of the element rises above a predetermined value due to the liquid reaching a desired temperature. The electricity supply may be cut off when the liquid reaches its boiling point or at any desired temperature.

Different parts of an element have different temperatures when the element is energised. When the sensor is placed in thermal association with the selected part of an element it does not necessarily only detect the temperature of that part as it is also subjected to the temperatures of other parts of the element.

Thus when the temperature sensor is referred to as detecting the temperature of a selected part of the element, it should be understood that it is also subjected to some degree to the temperatures of the other parts of the heating element.

According to another aspect of the invention, we provide a heating appliance having a heating element which in use heats a substance, and a control circuit comprising an electronic temperature sensor positioned in thermal contact with a selected part of the heating element, the sensor being operative to provide a signal to the control circuit which receives said signal and cuts off the electricity supply to the heating element when the temperature of said selected part of the element rises above a value corresponding to the predetermined temperature of the substance being heated.

Preferably, a single sensor only is provided to provide an output signal to the control circuit to cut off the electricity supply to the heating element both in response to the substance being heated reaching a predetermined temperature, and the element overheating as a result of it being energised without the presence of substance to absorb heat therefrom.

Where the substance to be heated is a liquid which is contained in a vessel, the sensor may be separated from or arranged to be shielded from any vapour being generated by the liquid, and from the liquid itself. Alternatively, the sensor may be in the vessel in thermal contact with the substance being heated.

The control circuit may be arranged to cut off the electricity supply to the element when the liquid boils or at any other desired temperature.

The electronic temperature sensor and the element may be attached to a plate which is adapted to be secured to the appliance or may be positioned adjacent a tube through which an electrical resistor of the element passes. Alternatively the sensor may be positioned in a housing on the element or in any other position provided that it can detect the temperature of the selected part of the element.

The sensor may be a thermistor or a diode or any other electronic device which has a parameter such as resistance which changes with temperature and is compatible with a control circuit. For example, a resistance wire which may be made of nickel.

The resistance wire may comprise a voltage dropping resistor which derives, a low voltage used to operate the control circuit from a high voltage used to energise the heating element.

The control circuit may have a triac to control the electricity supply to the element on and off, and the triac may be controlled by a zero voltage switch to reduce radio frequency interference to a minimum.

Optionally, the circuit may include an electromechanical relay to control the electricity supply to the element.

In either case, the circuit may be arranged so that when the current to the element is switched off it is not switched on again when the temperature of the selected part of the element falls below the predetermined value.

At least some of the components comprising the control circuit may be provided in the form of an integrated circuit.

Two embodiments of the invention will now be described with the aid of the accompanying drawings in which: FIGURE 1 shows a perspective view of a heating element and thermistor mounted on a plate, FIGURE 2 shows a diagrammatic representation of a control circuit for use in conjunction with the element of Figure 1, FIGURE 3 shows a perspective view of a heating element and conductor of high temperature coefficient, mounted on a plate and, FIGURE 4 shows a diagrammatic representation of a control arrangement for use in conjunction with the element of Figure 3.

In use, the heat produced by a heating element is continuously absorbed by the substance being heated.

In a kettle for example, the liquid absorbs heat from the element and maintains its temperature below a particular value.

As the liquid reaches a predetermined temperature however, the temperature of the element will rise above the particular value.

The present invention provides a method including using a sensor to sense when the temperature of the element rises above the value corresponding to the predetermined temperature of the substance being heated and apparatus for carrying out the method.

Referring first to Figure 1 there is shown an immersion heating element 1 such as is commonly used in electrical appliances such as kettles. Two electric contact pins 2, connected to the electrical resistor of the element which is contained within an insulated tube 7 extend through a mounting plate 3 which, in use, is bolted or otherwise attached to and in sealing engagement with a wall of the appliance.

Athermistor temperature sensor 4 is positioned in thermal communication with the element in a position between the contact pins 2. If required, thermal communication between the sensor and the plates 3 may be improved by attaching the sensor to the plate by thermal conducting paste.

Insulated leads 5 extend from the thermistor temperature sensor 4 to an electronic circuit such as that shown in Figure 2. The circuit is arranged to switch off the electricity supply to the element 1 when the thermistor 4 detects that the temperature of the selected part of the element 1 has reached a value corresponding to a predetermined temperature of the substance being heated.

The control circuit will now be described with reference to Figure 2.

The terminals 2 are connected to the mains electricity supply by a lead (not shown) from which it is distributed in two ways.

Firstly,the mains supply is fed to the heating element 1 to energise the element and thus heat the substance in the appliance. The electricity supply to the element is controlled by a triac 14 which acts as an on/off switch. When the triac is in a conducting state, electricity is supplied to the element 1, and when it is in a non-conducting state no current is supplied to the element 1. A neon light 15 or alternatively a light emitting diode indicates when the triac 14 is in a non-conducting state.

Secondly, the mains supply is rectified by a diode 10 and its voltage reduced by a resistor 11 to provide a substantially 12 volt D.C. supply which is stabilised by a reservoir capacitor 13, a resistor 9 and a Zener diode 12.

The triac 14 will conduct when a trigger current is supplied to it Triacs often produce radio frequency interference, but in this circuit a zero voltage switch arrangement comprising two transistors 18 and 19 is used to reduce this interference to a minimum. This ensures that trigger current is supplied at the beginning of the mains cycle only.

A resistor 27 acts as a collector load resistor to the transistors 18 and 19 and two further resistors 23 and 24 are provided to form a potential divider to provide the drive current to the transistors 18 and 19.

A D.C. amplifier comprising transistors 20 and 21 and resistor 26, amplifies the signal from the zero voltage switch and the resultant signal is fed to the triac 14 via a triac gate limiting resistor 25, as a trigger.

The zero voltage switch arrangement is controlled by a transistor 22 which when conducting, shortcircuits the zero voltage switch and hence no trigger current is supplied to the triac 14. The transistor 22 is made to conduct as follows.

The resistance of the negative temperature coefficient thermistor 4 falls as the temperature of the element 1 increases as they are in thermal communication with each other as described above with reference to Figure 1. When the element 1 reaches a temperature predetermined by the parameters of the circuit and adjustable over a small range by the variable resistor 16, the resistance of the thermistor 4 falls sufficiently for a trigger signal to be supplied to a thyristor 28 thus causing itto conduct. This in turn causes the base of transistor 22 to become sufficiently negatively biased to allow current to flow through it Thus the zero voltage switch is short-circuited, the trigger current to the triac is cut off and hence the supply to element 1 is cut off.

The neon light 15 will light up when the supply to the element is cut off thus giving an indication that the desired element temperature has been reached.

The anode current to the thyristor 28 is fed by way of a normally closed push button switch 31 and a resistor 32. The switch 31 may be temporarily opened to break the anode current to the thyristor 28 and this will render it non-conducting once again.

A resistor 30 limits the gate currentto the thyristor 28.

This circuit also gives protection for the element if an appliance is switched on when there is no substance to heat as the temperature of the heating element 1 will rise very quickly above the predetermined value and the supply to the element will be cut off.

In place of the thermistor 4, any other temperature sensor such as a diode may be used, provided that the control circuit is made compatible.

The control circuit described with reference to Figure 2 is only an example. Any other circuit that can switch off the supply to the element in response to a change in an electrical parameter of a sensor, may be used.

Alternatively a circuit may be provided that will merely indicate when the substance being heated has reached the desired temperature and the supply may then be cut off manually or caused the cycle to maintain water temperature at a simmering temperature.

At least some of the components comprising the electronic circuit as may be used in the method described, may be provided in the form of an integrated circuit.

Although in the method described the sensor4 is described as being positioned in thermal contact with a mounting plate 3 of the element 1, it will be appreciated that it may be positioned in any other position provided that it can detect the temperature of a selected part of the element For example, it may be placed inside the tube through which the electrical resistance element 1 passes, or in a housing of the element 1.

Referring now to Figures 3 and 4 of the drawings, there is shown a second apparatus for carrying out the method the subject of the present invention.

Referring first to Figure 3, there is shown an immersion heating element 40 such as is commonly used in electrical appliances such as kettles. The element 40 is substantially similar to that shown in Figure 1 except that instead of a thermistor sensor in contact with the mounting plate 42 of the heating element, a sensor 41 comprising a nickel wire is provided within a tube mounted on the plate 42 substantially above the element 40 and thereby subjected to the heat produced by the element 40.

A first terminal 44 of the heating element 40 and a first terminal 45 of the sensor 41 are electrically connected together by a bridge connector 43 at the rear of the mounting plate 42.

Referring also to Figure 4, the arrangement of the element 40 and sensor 41 in relation to the control circuit 48 is shown.

The neutral N of the mains supply is connected to a second terminal 46 of the heating element 40 and the live L is connected via the contacts of a relay R to a second terminal 47 of the sensor 41.

The control circuit 48 is arranged so that when the contacts of the relay R are manually closed, the relay R is energised and the contacts are thus retained closed.

Thus current will be supplied to the heating element 40 and the substance in the appliance will be heated.

As the substance in the appliance is heated by the heating element 40 the temperature of the element 40 and the sensor 41 will rise correspondingly. The resistance of the sensor 41 is chosen so that there is a substantial voltage drop in the mains supply across the terminals of the sensor 41.

As the temperature of the sensor 41 rises, its resistance will also rise and thus a larger voltage drop will be produced across the terminals 45 and 47.

As the sensor41 is positioned above a selected part of the element 40, its temperature will rise by a greater amount than it would if subjected to only the increased temperature of the substance being heated, and also it will detect the temperature of the selected part of the element.

When the voltage across the terminals 45,47 reaches a predetermined value, the circuit 48 is arranged to de-energise the relay R and thus disconnectthe main supply from the element 40.

The contacts of the relay will remain open until manually re-set.

The sensor 41, of the second embodiment derives a low voltage for operation of the electronic circuit from the mains voltage and hence the need to provide a separate voltage dropper is avoided.

Further, heat dissipated in the sensor 41 in deriving the voltage is absorbed by the substance in the appliance and hence there is no need to provide any separate cooling means such as ventilation means for the voltage dropper and the heat produced contributes to the heating of the water.

Although the sensor has been described as being a nickel wire, it will be appreciated that any other sensor which has sufficiently high temperature coefficient of resistance to produce a substantial change in voltage drop to enable the control circuit to react thereto, may be used.

Further, any suitable control circuit 48 may be used.

It will be appreciated, that the method of controlling temperature described with reference to an electric appliance having an heating element, is intended to include any appliance or any industrial application where it is desired to control the temperature of a substance being heated by an electrical element.

Claims

1. A method of controlling the temperature of a substance in an appliance having an electrically energisable heating element, comprising the steps of supplying electricity to the heating elementto generate heat therein which is transmitted to the substance, detecting the temperature of a selected part of the element with an electronic temperature sensor which is in thermal contact with said element, feeding an output signal from the sensor to a control circuit and cutting off the electricity supply to the element when the temperature of said selected part of the element rises above a value corresponding to a predetermined temperature of the substance being heated.

2. A heating appliance having a heating element which in use heats a substance, and a control circuit comprising an electronic temperature sensor positioned in thermal contact being operative to provide a signal to the control circuit which receives said signal and cuts off the electricity supply to the heating element when the temperature of said selected part of the element rises above a value cor responding to a predetermined temperature of the substance being heated.

3. A heating appliance according to Claim 2 wherein a single sensor only is provided to provide an output signal to the control circuit to cutoff the electricity supply to the heating element both in response to the substance being heated reaching a predetermined temperature and the element overheating as a result of it being energised without the presence of substance to absorb heat therefrom.

4. A heating appliance according to Claim 2 or Claim 3 which is a vessel containing liquid, wherein the sensor is separated from or arranged to be shielded from any vapour being generated by the liquid and from the liquid itself.

5. A heating appliance according to Claim 2 or Claim 3 which is a vessel containing liquid wherein the sensor is in the vessel in thermal contact with the substance being heated.

6. A heating appliance according to Claim 4 or Claim 5 wherein the control circuit is arranged to cut off the electricity supply to the element when the liquid boils.

7. A heating appliance according to any one of the Claims 2 to 5 wherein the electronic temperature sensor and the element are attached to a plate which is adapted to be secured to the appliance.

8. A heating appliance according to any one of Claims 2 to 7 wherein the electronic temperature sensor is positioned adjacent a tube which an electric resistor of the element passes.

9. A heating appliance according to any one of Claims 2 to 6 wherein the sensor is a thermistor.

10. A heating appliance according to any one of Claims 2 to 8 wherein the temperature sensor is a resistance wire.

11. A heating appliance according to Claim 10 wherein the resistance wire is made of nickel.

12. A heating appliance according to Claim 10 or Claim 11 wherein the resistance wire comprises a voltage dropping resistor which derives a low voltage used to operative the control circuit from a high voltage used to energise the heating element.

13. A heating appliance according to any one of Claims 2 to 12 wherein the control circuit has a triac to control the electricity supply to the element.

14. A heating appliance according to Claim 13 wherein the triac is controlled by a zero voltage switch to reduce radio frequency interference to a minimum.

15. A heating appliance according to any one of Claims 2 to 12 wherein the circuit includes an electro-mechanical relay to control the electricity supply to the element.

16. A heating appliance according to any one of the Claims 2 to 15 wherein the circuit is arranged so that when the current to the element is switched off it is not switched on again when the temperature of the selected part of the element falls below the pre determined value.

17. A heating appliance according to any one of Claims 2 to 16 wherein at least some of the components comprising the control circuit are provided in the form of an integrated circuit.

18. A heating appliance substantially as described herein before with reference to and as shown in Figures 1 and 2 or 3 and 4 of the accompanying drawings.

19. A method substantially as described herein before with reference to and as shown in Figures 1 and 2 or 3 and 4 ofthe accompanying drawings.