GB2358530A

GB2358530A - Temperature responsive control circuit for an electric kettle

Info

Publication number: GB2358530A
Application number: GB0001593A
Authority: GB
Inventors: Robert Andrew O'neill
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2000-01-24
Filing date: 2000-01-24
Publication date: 2001-07-25
Anticipated expiration: 2020-01-24
Also published as: GB2358530B; GB0001593D0

Abstract

A kettle or similar water heater has a control circuit with NTC and PTC resistors 2,3 formed on a thick film heating element. The junction 5 of the resistors is connected through a relay coil 6 to a capacitor 7. As the water temperature rises, the values of the resistors 2,3 change so that the voltage at junction 5 increases and current flows through the coil 6 to charge the capacitor 7. When the kettle boils, the voltage stabilises and charging current ceases, so that the coil 6 allows the relay contacts to open and switch off the kettle The circuit also responds to an excessive rate of temperature rise caused by a dry condition.

Description

2358530 IMPROVEMENTS RELATING TO CONTROLS FOR LIQUID HEATING APPLIANCES

Field of the Invention:

This invention concerns improvements relating to controls for liquid heating appliances and, more particularly concerns boiling sensors for use, for example, in domestic electric water boiling appliances such as kettles and hot water jugs though the invention has wider application.

Backn_ound of the Invention:

It is well known to provide electrically heated water boiling appliances with thermomechanical steam sensors arranged to switch off or reduce the power supply to the heating element in response to the generation of steam when water is boiled in the appliance. Steam sensors have commonly comprised a bimetallic or other thermomechanical temperature sensitive element arranged at a location of the appliance at which steam can impinge upon the thermally sensitive element when water boils in the appliance. An exemplary steam sensor is the J-type steam sensor manufactured and sold by Otter Controls Limited which is described in GB-A-2 212 664.

In some conventional arrangements, the steam sensor has been accommodated at an upper location of the vessel, commonly in a handle structure, adjacent to an aperture in the vessel wall permitting steam generated 2 within the vessel to access the thermal sensor of the steam control. In otlipi known arrangements, the steam sensor has been mounted in the base of the vessel and a steam pipe is provided in the vessel for conveying steam from tle upper part of the vessel to the location of the steam sensor in the base of the vessel.

Electronic controls for liquid heating vessels, kettles for example, also known which provide automatic switch-off on boiling. However,l electronic controls are typically more expensive than conventional bimetallic controls by several orders of magnitude. Otter Controls Limited ha.!e proposed a number of electronic controls for use with liquid heating vessel S and these are described in GB 2 185 161 A and GB 2 228 634 A. However, 1 despite the extensive range of facilities that such controls provide, including heating element overtemperature protection, the circuitry of such controls is rather complicated.

Objects and Summary of the Invention:

The object of the present invention is to provide a new and improveo boiling sensor for an electrically powered liquid heating appliance which lials more simple circuitry than is described in GB 2 185 161 A ang GB 2 228 634 A for detecting when the heating element stops rising r.'ti 1 temperature as the liquid reaches boiling point.

3 In broad ternis, the present invention resides in the appreciation that by connecting the coil of an electromechanical relay in the charging circuit to a capacitor and arranging that charge current flows in the relay coil only so long as the liquid temperature is rising, a boil sensor can be achieved since the relay will drop out when the current in its coil stops as a result of the liquid temperature stabilizing when the liquid boils.

For providing charge current through the relay coil only so long as the liquid temperature is rising, a sensor arrangement in the form of a potential divider circuit comprising an NTC-resistance and a PTC-resistance in series can be utilized, the potential divider circuit providing a varying potential at its junction so long as the temperature is rising and the relative values of the NTC and PTC resistances are changing and this varying potential determining the charging of the capacitor through the relay coil. The potential divider circuit could possibly utilize only a single variable resistance, but a greater potential variation is achieved if two resistances are employed which vary in opposite senses, namely one reducing and the other increasing in resistance as the temperature changes.

Described in detail hereinafter is an exemplary embodiment of the invention which further includes a diode coupled across the relay coil to enable the capacitor to discharge current back through the potential divider in the event that, liquid having been boiled and the relay deactivated, the system can be reset to enable a further volume of liquid to be heated, for example by 4 temporarily connecting the junction of the potential divider to ground 1)) depression of a spring loaded reset button so that the capacitor is discharged.

Preferably, the circuitry of the invention also includes a resistiVe component arranged to allow a limited amount of drain current to flow, through the relay, in use, when the water boiling condition is nearly reachefl.

By appropriately selecting the drain resistance, a small current just below [tie hold current threshold of the relay can flow when the potential divider is in 1 ts hot condition and the relay will release only when the charging current cease rather than when it falls to a low value which might occur before boil OCCILTS as the rate of temperature rise slows.

Advantageously, the circuit of the invention as generally described in the foregoing can not only detect boiling, but can also be utilised to detect a n I excessive rate of temperature rise indicative of an abnormal dry switch on condition. As the temperature rises above normal boiling temperature, the value of the NTC resistor will become almost constant, while that of the PTC resistor will continue to rise. This will give a point at which the volta output of the potential divider stops rising, allowing the relay to release.

The invention can be embodied as a stand-alone component adapted to be secured in good heat transfer relationship with a heating element, but mO';t conveniently can be formed integrally with a thick film heating element wit 11 the NTC and PTC resistances printed onto the heating element and officir components of the circuit, except possibly the capacitor, constituted by surface mounted components.

The invention further extends to a liquid heating vessel including a heating element assembly as described above.

The above and further features of the invention are set forth with particularity in the appended claims and will be described hereinafter by reference to an exemplary embodiment which is illustrated in the accompanying drawing.

Brief Description of the Drawin

Figure 1 is a diagram of the electronic circuitry of the invention proposed for use with an electrically heated water boiling appliance.

Detailed Description of the Embodiment

Figure 1 shows electronic circuitry 1 embodying the invention which is intended for use with an electrically heated domestic water boiling appliance. As shown, the electronic circuitry 1 comprises a potential divider in the form of an NTC-resistance 2 and a series connected PTC-resistance 3, and a charging circuit 4 connected at the junction point 5 of the potential divider. The charging circuit 4, as shown, comprises a relay coil 6 and a capacitor 7 connected in series, with a drain resistor 8 connected across the capacitor 7 and a diode 10 connected across the relay coil 6. Preferably, the 6 NTC-resistance is of a D-NTC 2115 or 2116 type and the PTC-resistance is ol a D-PTC 2611 or 2650 type. The circuit is adapted to be powered by a 240 volts AC mains supply rectified by diode 11.

In use, when the water to be heated is initially in cold condition, the NTC-resistance 2 is high (typically 200 K Q) and the PTC-resistance 3 is low (typically 6 K Q) and a correspondingly low output voltage (typically 7.5 V) is delivered at junction point 5 in the circuit 1. This voltage is applied to tte input side of the charging circuit 4 and serves to initiate the charging of the capacitor 7 via the relay coil 6, the relay being urged into closed condition hly the charging current which passes through the coil 6 and the heating elemeig consequently being powered. As the water heats up, so the NTC-resistance decreases and the PTC-resistance 3 increases and the voltage developed -, t junction point 5 rises. The voltage applied to the charging circuit 4 therefolte increases with rising water temperature, and the capacitor 7 corresponding! y continues to charge with the charging current through relay coil 6 holding the relay contacts in closed condition. This situation only changes when the wator temperature reaches boiling point and stops rising so that the voltage output It junction point 5 becomes static (typically 50 V at boil). This causes via cessation of current flow in the circuit 4 as the capacitor 7 stops charging and the cessation of current in the relay coil 6 causes the relay contacts to open sp as to break the circuit to the heating element and switch it off.

7 In the illustrated embodiment, additional components 8, 10 are incorporated into the charging circuitry 4 in order to facilitate circuit reset and adjustment of the circuit response time before switch off of the appliance in response to a sensed boil condition.

The component 8 is a drain resistor which, as shown, is coupled to a junction point 9 of the charging circuit 4 between the relay coil 6 and the capacitor 7. The function of the drain resistor 8 is to pass a small amount of current which is just below the hold-current threshold of the relay. The value of this current will increase as the water temperature rises, so the value of drain resistor 8 should be selected on the basis of the hot value of the potential divider output so as to ensure that the relay only releases when the charging current ceases, rather than when it falls to a low value, which might occur before boil as the rate of temperature rise slows.

The component 10 is a reset diode to enable discharge of the capacitor 7 in the event of the junction point 5 of the potential divider being temporarily coupled to ground, for example by momentary depression of a push-button reset switch (not shown) connected across PTC resistor 3. With the circuit in its hot, boiled condition and the relay contacts open circuit, such discharge of capacitor 7 will enable the heating element to be powered again as the capacitor 7 recharges from junction point 5 of the potential divider and charge current again flows through the relay coil 6.

8 The circuit values should be carefully selected to ensure that ar excessive current will not flow into the relay coil 6 and capacitor 7 in the event that the circuit is reset whilst water in the appliance is still close [c 1 boiling and the output voltage of the potential divider circuit is still high.

The circuit of Figure 1 advantageously provides an effective and ch(>,,p electronic sensor which is especially convenient for use with a thick film-type heating element. In such an arrangement, it is envisaged that the thick fi.ltn heating element would be secured to or constitute a surface (a base surface, for example) of the appliance and the NTC and PTC-resistance componen is would be printed directly on the heating element dielectric. The remainiog circuit components, except possibly the capacitor 7, could be surface moll it components mounted on the surface of the thick film element.

Having thus described the present invention by reference to a prefen-od embodiment, it is to be appreciated that the embodiment is in all respects exemplary and that modifications and variations are possible without departure from the spirit and scope of the invention. For example, the filtering action of the relay coil 6 and the capacitor 7 in the embodiment could possibly be improved, if desired, by provision of additional smoothing capacitors in the charging circuit. Additionally, while the potential divider s described has utilised an NTC-resistance and a PTC-resistance in series, as is preferred, either of these could be replaced by a fixed value (ZTC) resistance as mentioned hereinbefore.

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Claims

CLAIMS:

1. A temperature responsive circuit for an electrically heated water boiling appliance, said circuit comprising means for providing a temperature dependent output voltage and a charging circuit coupled thereto, the charging circuit including a relay coil connected in series with a capacitor such that capacitor charging current flows in the relay coil, the arrangement being such that the relay will open its contacts when said output voltage stabilizes and the capacitor stops charging.

2. A circuit as claimed in claim 1 wherein the means for providing a temperature dependent output voltage comprises a potential divider including at least one temperature dependent resistance.

3. A circuit as claimed in claim 2 wherein the potential divider comprises an NTC resistance and a PTC resistance connected in series.

4. A circuit as claimed in any preceding claim further comprising means coupled to the capacitor for enabling the capacitor to be selectively discharged.

5. A circuit as claimed in claim 4 wherein the means enabling tht capacitor to be selectively discharged includes a reset diode.

6. A circuit as claimed in any preceding claim further including means associated with the charging circuit for permitting a predeterTnined small amount of current to flow through the relay coil irrespective of the state of charge of the capacitor.

7. A circuit as claimed in claim 6 wherein a drain resistor is coupled across the capacitor to permit said small amount of current flow through tbe relay coil.

8. A circuit as claimed in claim 3 or in any of claims 4 to 7 as dependent on claim 3 wherein the NTC-resistance, preferably of the D-NTC 2115 cr 2116 type, is about 200 K ohms (K Q) when said water boiling appliance is i ki 1 cold condition, and the NTC-resistance is about 30 K ohms (K 2) when sai d water boiling appliance is in boiling condition.

9. A circuit as claimed in claim 3 or in any of claims 4 to 8 as dependert on claim 3 wherein the PTC-resistance, preferably of the D-PTC 2611 or 265P type, is about 6 K ohms (K 2) when said water boiling appliance is in cold 11 condition, and the PTC-resistance is about 9 K ohms (K Q) when said water boiling appliance is in boiling condition.

10. A circuit as claimed in claims 8 and 9 wherein the potential at the junction of the NTC and PTC resistances varies from about 7.5 V when said water boiling appliance is in cold condition to about 50 V when said water boiling appliance is in boiling condition.

11. A circuit as claimed in any preceding claim wherein actuating means are associated with the contacts of the relay for permitting the water boiling appliance in use to undergo a sustained boiling condition.

12. A temperature responsive circuit substantially as herein described with reference to the accompanying drawing.

13. A heating element assembly incorporating a circuit as claimed in any preceding claim.

14. A heating element assembly as claimed in claim 13 comprising a heating element of thick film construction having said temperature responsive circuit constructed thereon.

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15. A liquid heating vessel including a heating element assembly a., claimed in claim 13 or 14.