GB2185161A - Controls for electrical heating elements of water heaters - Google Patents

Abstract

The control may be applied to a domestic hot water system or particularly to a water boiling vessel. An NTC thermistor is coupled in close heat transfer relationship with the heating element and a PTC thermistor is exposed to steam produced when the water boils. A first detector circuit IC1a responds to the rate of change of resistance of the NTC thermistor so as to turn off a triac in response to a rapid resistance change representative of the heating element being powered when there is no water or insufficient water in the vessel, and also is responsive to a predetermined low resistance condition indicative of excessive element temperature. A second detector circuit IC1b turns off the triac, possibly after a delay, in response to a predetermined PTC thermistor resistance representative of the PTC thermistor being heated by steam. In an alternative arrangement, (Fig 1), the NTC and PTC thermistor are series connected to form a potential divider, the junction point of the thermistor being connected to a single detector circuit responsive to rate of change of resistance of either thermistor. The divider may include a current sensor which operates a boil-dry indicator lamp. The triac may be cooled by close thermal contact with water in a pocket in the vessel, (Fig 4A), or the triac may have a chip secured to a large-area copper base plate mounted on the vessel body via an electrically insulating layer so that the triac is cooled by the water that is being heated, (Fig 4c). <IMAGE>

Description

SPEC'FICATION Controls for electrically powered heating elements Field of the invention This invention generally concerns improvements in and relating to controls for electrically powered heating elements. Whilstthe invention will be part icularly described herein with reference to controls for the electric heating elements of water boiling vessels, especially kettles but also including jugs, pots, pans, urns etc., the invention is not limited to such controls and has wider application for example to controls for immersion heaters of domestic hot water systems.

Backgroundofthe invention It is well known to provide two levels of control associated with the heating element of, for example, an electric kettle. Afirst control is known as a dry boil ordryswitch on control and hasthefunction of disconnecting the heating element from its power supply in the event that the kettle boils dry or is switched on without containing any water, and a second control is known as a steam sensor or auto matic switch-off control and has the function of disconnecting the heating element from its power supply when water boils in the kettle and steam is produced.It is common in so-called automatic kettles to provide a steam sensor control together with a dry switch on control, the latter serving to preserve the integrity ofthe element in the event that the kettle is accidentally switched on empty and, with the advent in recent years of water boiling vessels wherein the vessel body is formed of synthetic plastic material, serving additionallyto protect againstthefire hazard which otherwise might result if the element were grossly to overheat with consequent melting of the plastics material.

Many different forms of kettle controls have been developed through the years and Otter Controls Ltd.

of Buxton, Derbyshire, England have been at the forefront of such developments ever since the invention of the snap-acting bimetallic switch actuating element described in GB 600055 by E.H. Taylor, one of the founders of Otter Controls Ltd. Whilst attempts have been made to develop an electronic control, which is not in itself a difficulttask given the increasingly rapid developments in electronics technology in recent years, the problem has always been encountered that the existing controls based on bi metallictechnology were so sophisticated and could be manufactured so inexpensively that no equivalent electronic control could be found which was competitive with the bimetallic controls.It has hitherto been our experience that electronic controls capable of an equivalent element protection and automatic switch-off on boiling function have been more expensive than a conventional bimetallic control by several orders of magnitude, and in consequence of this electronic kettle controls have not yet won ac cesstothe marketplace.

Some prior art electronic kettle controls known to us are disclosed in GB 1356502 (Belling), GB 1426399 (Belling), GB 2135143(TI Russel Hobbs) and US 4278873 (General Electric Company), and other electronic heating element controls are described in GB 1147422 (General Electric Company) and US 3665159 (Whirlpool Corporation). In so far as we are aware, none of these prior art electronic kettle controls have ever been successfully marketed in competition with bimetallic controls.

Objects and summary of the invention Accordingly, a principal object of the present invention is to provide an electronic control for an electric kettle or other water heating appliance which can be made price competitive with at least the more sophisticated conventional bimetal controls.

Another object ofthe present invention is to provide an electronic control for an electric kettle or like water heating appliance which is precise in its operation and avoids the problems arising with temperature overshoot as experienced with conventional controls, namely the tendency for the element head portion to continue to rise in temperature as a result of heat transferfrom the element proper even afterswitch-off of the element, and ac- cordingly enables water heating vessels formed of synthetic plastics materialsto be usedwith greater security.

The present invention resides in the realization that more precise temperature control can be achieved relatively simply and inexpensively by use of tem perature-sensitive thermistors orothersolid- state electronic devices in circuits where, ratherthan simply seeking to detect absolute temperature levels as in the priSr a rt proposals, instead or additionally the rate of change oftemperature js monitored. In a dry boil situation for example, narbelywhen the vessel is switched on when empty, the temperature of the heating element and associated vessel parts exhibits a relatively rapid and readily detectable increase as soon as the vessel is switched on.Likewise the generation of steam when water boils within a vessel causes a relatively rapid temperature rise abovethe liquid level inthevesselwhich again is readily detectable as a temperature rate of change. In a boil dry situation, the heating element and associated vessel parts will remain at a substantially constanttemperature until the vessel begins to boil dry, whereupon there will occur a temperature rise detectable by a combination of rate of change and absolute temperature, for example.

Thermistors are the preferred temperatureresponsive devices to be utilized in the practice ofthe present invention, on account inter alia oftheirsmall size, robust and reliable construction and relative low cost. As is well known, there are basicallytwo kinds of thermistors, namely NTC (negative temperature coefficient) and PTC (positive temperature coefficient) thermistors and the two types have dif ferentcharacteristics. An NTC thermistor exhibits a resistance which decreases as its temperature increases and a PTC thermistor exhibits a resistance which increases as its temperature increases, and furthermore NTC thermistors currently available have a relatively gradual resistance/temperature characteristic whereas PTCthermistors have a char acteristicwhich exhibits a much more abrupt resist ance change once a particulartemperature is reached. By virtue of their respective characteristics, NTCthermistors might be regarded as being intrinsically better suited to the sensing of an incipient dry boil situation whereas PTC therm istors might be regarded as being intrinsically better suited to steam sensing, but as will become apparent from the following this is not necessarily the case.

According to one exemplary aspect of the present invention, an electronic kettle control which we believe shows great promise and has the potential to replace at least some of the more complex and more expensive bimetallic controls utilises an NTC (nega tivetemperature coefficient) thermistorfor sensing the rate of temperature rise of a heating element in ordertodetectan abnormal dryswitch on orboildry condition, and utilises a PTC (positive temperature coefficient) thermistorto detect the generation of steam when the kettle boils.

The NTC and PTC thermistors can be connected in series with each other in a potential divider circuit with a detector coupled to the junction point ofthe two thermistors, in which case in order to enable certain user switching arrangements and status indicators to be included a current sensor arrangement may be provided for determining whether one orthe other or both thermistors is responsible for a detected change in the potential at the junction ofthetwo thermistors which could stem from eitherthermistor. Alternatively, and possibly preferably if user switching facilities and status indicators are to be included, the two thermistors can be separated into functionally different detecting circuits.

Thus in an exemplary embodiment ofthe invention as described in detail hereinafter separate dry boil and steam detection circuits are coupled to a triac drive for the heating element with each circuit arranged to inhibit the operation ofthe triac so as effectively to disconnect the element in response to a respective dry boil or steam detect condition. The dry boil detection circuit comprises an NTCthermistor coupled to a modified Schmitt trigger detector circuit and arranged to be responsive to a high rate oftem- perature rise in the element and also to be responsiveto a high temperature level in the element. If a kettle fitted with the embodiment in question were to be switched on when empty, the rate oftemperature rise in the element would be detected and the element switched off in consequence.Were the kettle to be allowed to boi I dry, th boil dry,throughfailureofthesteam sensorforexampleorasa result of the kettle lid being left offthus allowing steam to exit the kettle without actuating the steam sensor circuit, the circuit would detectthis condition by a combination of rate of rise and over-temperature. The steam sensor circuit comprises a PTC thermistor coupled to a modified Schmitttrigger detector circuit and arranged to be responsive to a sudden rise in the resistance of the PTC thermistorwhich occurs when it is heated by steam generated when water boils in the kettle.

The thus arranged kettle control circuit is controlled by two user-operab!e control buttons, start and stop. In normal use, operating the start button will cause the kettle to boil and turn off. By holding the start button, the kettle will continue to boil unless a dry boil condition exists. The kettle can be turned offat anytime by means ofthe stop button. While power is being supplied to the element a green indicator is lit, but if the temperature of the element either rises very quickly or exceeds a fixed tem- perature then the element will be turned off and the condition indicated by lighting a red indicator.No power can then be supplied to the element until it has cooled and the red indicator is rxtinguished by either pressing the stop button or unplugging the kettle and then pressing the start button. The existence of a mains supply is indicated by means of an orange indicator which is lit so long as the mains supply is connected.

Whilst our currently preferred prototype control utilises an NTC thermistor to sense the rate of rise of the temperature at the element head, that is the part of the element whereby it is secured to the vessel body, and a PTC therm istor to sense the steam tem- perature when water boils in the vessel, it is within the ambit ofthe present invention in its broadest aspects to employ an NTCthermistor based rate of temperature rise sensorforthe steam sensing function. In the hereinafter described preferred embodiment, the steam sensor part of the circuit could thus be repiaced bya circuitverysimilarto (and possibly even identical to) that utilised for the dry boil protection.An even more simple circuit could be obtained, assuming that user controls and status indicators were not required, if the circuit hereinbefore mentioned and hereinafter described where the NTC and PTC thermistors were series connected in a potential divider circuit and a detector monitored the potential at the junction of the two thermistors was to be modified so as to place two NTCthermistors (preferably connected in parallel for the greatest sensitivity) in one arm ofthe potential divider circuit and a fixed resistance in the other arm, one of the two NTC thermistors being designated the dry boil protector thermistor and in use being coupled in direct heat transfer relationship with the element and the other being designated the steam sensor thermistor and in use being located to besubjectto heating by steam generated when water boils in the vessel .

It is furthermore to be appreciated thatwhilsta preferred electronic control in accordance with this invention for a water boiling vessel would comprise both dry boil protection and steam sensing functions so that the vessel, an electric kettle for example, was not only fully element protected but also was auto matic in operation, the invention is not limited to the provision of both of these controls and could be embodied in the provision of one only of the element protection and steam sensing controls.

In using a triac for controlling the heating element of an electric kettle or the like, a problem arises in that cooling of the triac must be accommodated.

Triac cooling is conventionally accomplished by use of a heatsinkintheform of an aluminium extrusion which is expensive and physically large, making kettle styling difficult. To enable the heatsinkto be omitted, we have successfully tested two proposals.

Afirst proposal has been to modify the vessel body to contain a water pocket specifically for cooling the triacwhich is fitted to be in close thermal contact with water in the pocket. When the kettle is filled, the triac-cooling water pocket fills with water and when the kettle is switched on the heat from the triac is absorbed by the water in the pocket. The water pocket can be made sufficiently large to contain sufficientwaterto allow for repeated boiling, but yet so small in relation to the volume of water heated in the kettle that when the relatively cool triac-heated water is emptied with water heated by the kettle element, no significant cooling of the main volume of water occurs.An alternative proposal isto make use ofthe water being heated to cool the triac, that is without providing a special water pocket within the kettle body, and the difficulty in this regard is to find a triac having a sufficiently low thermal resistance between chip and case to avoid the chip going overtemperature when the water in the kettle is at or near boiling point.Commonly available triacs having el ectrical insulation between chip and case, as is considered a requirement for application to kettle controls, do nothave a sufficiently iowohip-to-case thermal resistance to be coolable sufficiently by means of water at 1 OO"C, but we have constructed and successfully tested low chip-to-case thermal resistance triacs wherein the triac chip is mounted upon a relatively large area copper plate which in turn is electrically insulated from the kettle body. The copper plate serves to distribute the heat ofthetriac over a wider area thereby enabling it to be satisfactorily cooled by water at 1 OO"C.

Thus, in accordance with another aspect ofthe present invention there is provided an electrically hea ted water boiling vessel incorporating a triac arran- ged to control the power supply to the heating element of the vessel and wherein the triac is cooled in operation by means of water contained within a water pocket within the vessel, the water pocket being arranged so as to fill with water as the vessel is filled and to empty as the vessel is emptied.

Yet another aspect of the invention provides an electrically heated water boiling vessel incorporating a triac arranged to control the power supply to the heating element ofthe vessel and wherein the triac is arranged to be cooled in operation bythewater being heated in the vessel, the triac having a relatively low chip-to-case thermal resistance such as to enable the triac to be coolable in operation by boiling water by virtue of the triac chip being mounted on a relatively large area thermal diffuser.

Further features, aspects and advantages ofthe present invention will become apparentto those skilled in the art from consideration of the following detailed description given with reference to the accompanying drawings.

Description ofthe drawings Figure lisa schematic showing of a first electronic kettle control circuit embodying the present invention; Figure2is a circuit diagram of a second electronic kettle control circuit embodying the present invention in a presentiy preferred form; Figure 3shows graphs illustrative of the NTC thermistor operation in the circuit of Figure 2; and Figures 4A, 4B and 4Care sketches illustrative of other aspects of the present invention.

Specific description ofthe invention Referring to Figure 1, a circuit is illustrated which uses two thermistors to form a potential divider. One thermistor, a FTC, is used to detectthe presence of steam when the kettle boils, and the other, an NTC, is placed in good thermal contact with the element cup.

These are coupled via a diode D1 and capacitor Cto a trigger circuitTwhich can ultimately be used to control the element. If either the PTC or NTC are heated, the voltage at pointAwill fall. Provided that D1 is reverse biased, the voltage at point B will fall according to the rate of change ofthe voltage at pointA because of the capacitor coupling. The faster the voltage at point A changes the biggerwill be the voltage produced at point B. ThetriggerTcan therefore be set to operate when the rate of change of voltage at the pointA reaches a higherthan normal value, such as might happen when the kettle is turned on with no water. If thevoltage at pointAfalls belowthevoltage at point B, diode D1 will be forward biased and the triggerTwill be operated regardless of rate of rise.

This might happen during boil dry or steam detection.

The circuit of Figure 1 has been shown to work well. However, this particular single chain arrange- ment has one or two drawbacks, particularly that certain user switching arrangements and status indicators cannot be included without addition offurther components on account of the fact that with the circuitshown in Figure 1 no determination can be made as to whether the PTC or the NTC is responsible for a fall in the voltage at pointA. It alsoirestricts the values of NTC which can be used. Forthese reasons the two detecting circuits were separated forthe prototypecontrol circuit described hereinafter with referenceto Figure 2.

In orderto provide for determination of which of the PTC a nd NTC thermistors is responsible for a detected change in the voltage at pointA, in orderto provide for the provision of user control facilities and status indication for example, it would however be necessary onlyto provide a current sensing arrange ment responsive to changes in the currentflowing through the series-connected thermistors.In the quiescent condition of the circuit of Figure 1 a certain currentwill flow in thethermistorchain, andthis cur- rent will increase in response to heating of the NTC thermistor and reduce in responseto heating ofthe PTC therm isto r. It could thus be arranged that theresponse of the NTC thermistor for example to a dry boil condition, namely a reduction of the resistance ofthe NTCthermistorand a corresponding increase in the currentflowing in thethermistor chain, isdet- ected by a current-sensitive switch and gives rise to a corresponding signal which can be utilized to light a respective indicator lamp for example so as to provide an indication to a user of the fact that a dry boil condition has occurred. Such a current-sensitive switch mightfor example simply comprise a transistor arranged to be appropriately biassed into or out of conduction in response to the development of a set signal protential across a series resistor coupled to the thermistor chain when an appropriately sign ificantvariation occurs in the currentflowthrough the thermistors.

Referring now to Figure 2, the control circuit shown therein is fully electronic and provides both enhanced dry boil protection and automatic steam cutout. It is controlled by two buttons which are designated start and stop/reset. In normal use, operating the start button will causethe kettle to heat upand boil and to turn off as with conventional automatic kettle controls. By holding the start button the kettle will continueto boil unless or until a dry boil condition exists. The kettle can be turned off at anytime using the stop button. While power is being supplied to the element, a green LED is lit.Mains supply is indicated bya permanently lit orange LED. Ifthetem- perature ofthe element cup either rises very quickly or exceeds a fixed temperatu re, the element will be turned offandthe condition indicated bya red LED.

No power can be supplied to the element until it has cooled and the red LED is extinguished by either pressing the stop button or unplugging the kettle and then pressing 'start'.

Referring to Figure 2, the dry boil sensor is comprised of the NTCthermistorand a modified Schmitt trigger configured on IC1 a which is one half of an 082 dual operational amplifier and senses lackofwater either by a high rate oftemperature rise orjust a high temperature in the element cup. An NTC thermistor is used here because its resistance changes over a wide range oftemperatures as is desirable for rate of rise sensing.

The NTC and R1 form a potential divider which is connected via D1 and C2 to 1C1 a. A referencevoltage is generated by ZD1 and a voltage just less than this is applied to the non-inverting input of IC1 a by means ofthe potential divider formed by R3 and R4.

This state is ensured on power up by the low imped ance of C1 and C2 as the power supply is turned on making sure that the inverting input is higherthan the non-inverting input.

If the NTC is heated rapidly in a dry boil situation, the impedance ofC2 appears small to the falling voltage applied which reduces the voltage on the inverting input of ICla. If this falls belowthe voltage on the non-inverting input, the circuit will change state. D3 will now be forward biased ensuring the output of the amplifier stays nearto Ov until SW1 is operated.

R1 is so chosen that if the temperature of the NTC reaches about 110"C, Dl will bye forward biased, triggering the circuit regardless of rate of rise.

The steam sensor is comprised of the PTCthermistor and a modified Schmitttrigger configured on IC1 b which is the other half ofthe dual operational amplifier and is driven by a potential divider formed by the PTC and R8. A PTC is used because it has a very non-linear characteristicsuited to steam sen sing,that isto say itsuddenly exhibits a substantial resistance change upon exposure to steam.

On power up, ensuresthatthe output ofthe trigger IClb is near Ov. The circuit can only be chan ged to its otherstate by operating SW2.Thissetsthe non-inverting input to about -6.5V. When the PTC is heated by steam, its resistance rises quickly so that when thevoltage on the inverting input falls below -6.5V,the circuit reverts to its off state. By holding SW2, the kettle can be kept boiling providing the dry boil sensor has not operated.

The kettle element E is series connected with a triac T between the neutral and liveterminals of the mains supply. Triac drive is provided by IC2 which is a PBL 3708 trigger circuit available from RIFAAB of Stockholm,Sweden. The trial is on when the IC2 input voltage is below about -4.5V. Diodes D4 and D5 form an OR gate so that if eitherthe dry boil orthe steam sensor has a Ov output, thetriac is off.

In Figure 3,the typical variation with time of the voltage and temperature at the NTC are shown.

Graph A shows what happens when the kettle is turned on from cold with no water. The current is stopped about7.5 seconds later. An elementcup overshoottemperature of only about 1050C is reached. The dry boil detector has to be insensitive to the rate of rise shown in graph Bwhich represents a normal operating condition ofthe kettle with water in it. It is steepest here since the voltage output per"C is highest in this temperature range in order to pro vide maximum sensitivity to rate of rise at tempera turesaround boiling point Having thus described a preferred form of electronic kettle control in accordance with the present invention, reference may now be had to Figures4A, 4B and4Cwhich show sketches relevant to the mounting of the triac of such a control to the kettle.

As previously mentioned herein, a fundamental pro blem with using a triac to control a kettle element is the need to use a heatsinkfortriac cooling. Con ventionallythis heatsinktakes the form of an aluminium extrusion which is expensive and physically large, making the incorporation ofsuch a heatsink into a kettle styling difficult. In attempting to omitthe heatsinkwe have successfully tested two possibilities as described below.

Referring to Figure 4A, the kettle body can be modified to include a water pocket which fills with water when the kettle is filled and empties when the kettle is emptied. Thetriac is fixed to be in close thermal contact with the water in the pccketso that the heat from the triac is absorbed bythe water. There can be sufficient water in the pocketto allow for repeated boiling, and whilstthewater in the pocketwill be heated by the triac and also indirectly by the kettle element, nonetheless effective cooling ofthetriac can be achieved.

Alternatively, the water being heated in the kettle can be utilised to cool thetriac. This is theoretically possible even as the water is heated towards boiling because the maximum permitted chip temperature formosttriacs is 125"C, butasa practical matterthe problem arises that available triacs do not have a sufficiently low thermal resistance between theirchip and case to enable a chip temperature below 1 25"C with a case temperature of 1 OO"C. This is partly due to the need to use a triac having its case electrically in sulated from its chip as shown in Figure 4B. For any given thickness of electrical insulation the thermal resistance can be reduced by increasing the area of insulation as shown in Figure 4C. It is thus possible to purchasetriac chips and manufacture a special low thermal resistance triac capable of safely carry- ing thefull electrical load of a kettle with the "heatsink" represented by the water in the kettle operating at 100 C.

The electronic kettle control aforedescribed could, if desired, omit the steam sensor if only element protection against boil dry or dry switch on situations is required. The dry boil protection circuit is based on an NTCthermistorand has the unusualfeaturethat when the kettle is switched on with no water in it the rapid rate of temperature rise of the kettle element is quickly detected and the element switched off. The main advantage of this system is that excessive over shoottemperatures in the element can be avoided, which is particularly useful in plastics bodied water boiling vessels such as kettles and jugs.In the case of a kettle which is allowed to boil dry, the element protector circuit detects this condition by a combination of rate of rise and overtemperature again achieving low overshoot elementtemperatures. The detection of an abnormally high rate of rise ofthe elementtemperature by means of the electronic control of the present invention can be effected earlierthan would the detection of an overtemperature by means of a conventional bimetallic sensor and thus excessive overshoot temperatures can more readily be avoi- ded by means of the electronic control of the invention.

As an illustration, a control in accordance with the present invention as herein described is capable of responding earlyto a dry switch-on situation ata temperature of about 45"C at the element head and yetwill still allow boiling at 100"C; no other known bimetallic or other control is capable of doing this.

Because of the rapid response obtained to a dry switch-on situation, overshoot temperatures (caused by heattransferfrom the element proper to the element head after switch-off ofthe element) even forafurred up (limed or scaled) kettlewould not exceed about 130"C, whereas for conventional bimetallic controls the overshoot temperatures ex perienced in normal operation can typically be as high as 230"C. Such high overshoot temperatures as have been conventionally experienced give rise to requirements for suitable materials capable of with standing such temperaturesto be used for element sealing rings and forthe vessel body itself, having regard to the currenttrend towards water heating or boiling vessels formed of synthetic plastics mat erials. Polypropylenewould be an ideal material for moulded kettle bodies, but it has a relatively low melting point; the control of the present invention would adequately protect vessels made from poly propylene. The ability to use such lowtemperature materials in the construction of kettles and theircon- trols would provide considerable cost savings.

It has been proposed to utilize a thermal fuse as a safety backup in case of dry-boil cut-outfailure, part icularlyforvessels formed of synthetic plastics mat erials which can melt down and catch fire in the event of such a failure. However, the rupturetem- perature of such a thermal fuse has to be set quite high (about230 C) in orderto prevent premature rupture by temperature overshoot due to normal dry-boil cutout operation. As a consequence, the subsequent overshoot dueto dry-boil cut-outfailure makes this form of backup protection impractical.

With the control of the present invention, howeve, much lower overshoot temperatures of about 1 200C to 130"Coccurdunng a normal dry-boil operation and a much lowertemperaturethermal fuse can be used, resulting in quite acceptable overshoottemperatures in the event offailure of the dry-boil protection circuit. Therefore, a thermal fuse is, with the present invention, a suitable safety backup device.

This provides a considerable cost saving as compared to the use of a secondary bimetallic device for backup protection.

Other significant advantages provided by the pre- sent invention stem from the fact that the reset time required after response to a dry-boil condition is very short. When a conventional bimetallic dry-boil cutout operates, the bimetal blade moves away from the element head and is relatively insensitive to cool ing of the element head byfilling the kettle with cold water, but in the practice of the present invention the dry-boil sensing thermistor maintains contact with the element head and responds quickly to cooling of the head. Similarly, a short resenttime after boiling can be obtained duetothe low thermal capacity of the steam sensor. Typicallythe steam sense circuit can be reset some 6 seconds after sustained boiling.

The invention furthermore does not give rise to radiofrequency interference such as can arise with conventional controls by arcing at switching contacts, and suffers no contact wear problems, and is capable of sustained boiling without stressing or otherwise damaging the control in anyway. The electronics ofthe control also provides scope for additional features such as simmering or heating to any selected temperature.

Aswill I be a ppreciated by those skilled in the re- levant arts, many variations and modifications of the described arrangements could be made without departure from the invention. For example, the PBL 3708 integrated circuit employed in the circuit of Figure 2 could, with appropriate modification of the remainder ofthe circuit, be replaced byan SL441C zero voltage switch as available from Plessey Semiconductors. Furthermore, while the use of an NTC thermistorfordryboil sensing and a PTCthermistor for steam sensing is preferred, the invention could be made to operate with two NTC thermistors ortwo PTC thermistors or with a PTC thermistor for dry boil sensing and an NTCthermistorforsteam sensing in dependence upon the particular characteristics of the specific devices utilized, the essence of the invention being to sense the rate of change of temperature by use of an appropriate temperature-responsive device rather than to require the use of any specific form oftemperature responsive device. Additionally, the invention could embody otherfeatures such as for example a facility whereby the sensed boiling of water in a vessel does not immediately result in the vessel element being switched off but rather delays the switch off for several seconds to ensure that the entire contents of the vessel are brought to boiling temperature and there are no relatively cold spots within the vessel.

Claims (31)

1. An overtemperature control for an electrically heated water heater, the said control being respon sive to the rate oftemperature rise within the heater.

2. An overtemperature control for a water heater which is responsive to the rate oftemperature rise in the heater for determining the supply of heating energy.

3. An overtemperature control for protecting a water heating vessel against the disadvantageous effects of being switched on without there being suf ficientwater in the vessel, the said control compris- ing a temperature sensor responsivetothe rate of rise of the temperature of the vessel heating element irrespective of the absolute temperature level for switching off the supply of powerto the heating element.

4. An overtemperature control according to claim 3 which is further responsive to absolutetem- peratures above a predetermined level forswitching offthe supply of power to the heating element.

5. Asteam sensorforawater heaterwhich com- prises a temperature sensor and means responsive to the rate of change ofthe sensor outputfor detecting the generation of steam.

6. In combination in or for an electrically powered water heating vessel, an overtemperature control responsive to the rate of change ofthe temperature of the vessel heating elementfor determining the supply of power thereto, and a steam sensor for determining the supply of power to the vessel heating element in response to the gener- ation of steam in the vessel.

7. A combination according to claim 6 wherein the steam sensor is responsive to the rate of change ofthetemperatureabovetheliquid level in the vessel for detecting the generation of steam.

8. An electronic, thermally-responsive control for an electric kettle or otherwater heating vessel for switching offthe supply of power to the heating el ementofthevessel in repose to a sensed pred etermined temperature condition,the control com prising a temperature-responsive thermistor and circuitry responsive to the rate of variation of the resistance of said thermistor for detecting said pred etermined temperature condition and, in response thereto, switching off the supply of power to the el ement.

9. An electronic control according to claim 8 for use as a dry switch on protector and wherein the cir cuitryresponsivetothe rate of variation of the resist ance of the thermistor is arranged to respond to an excessive rate of variation indicative of the vessel el ement being powered without there being sufficient water in the vessel.

10. An electronic control according to claim 8 for use as a steam sensor and wherein the circuitry re sponsive to the rate of variation ofthe resistance of the thermistor is arranged to respond to a rate of var iation indicative ofthe thermistor being subjected to heating by steam generated in the vessel.

11. An electronic control according to claims 9 and 10 wherein the control comprises afirstthermis- torto be mounted in close heattransfer relationship with the vessel element and a second thermistor to be mounted to be subjected to heating by steam generated in the vessel, and said circuitry is arranged to be responsive to respective thermistors to switch off the supply of powerto the vessel element when the element is powered without there being sufficient water in the vessel and also when steam is generated in the vessel.

12. An electronic control according to claim 11 wherein a first detector circuit is associated with the first thermistor for detecting an excessive rate of temperature rise indicative of a dry switch on condition, and a second detector circuit is associated with the second thermistor for detecting a rate of temperature rise indicative of steam generation in the vessel.

13. An electronic control according to claim 11 wherein the two thermistors are connected in one arm of a potential divider circuit and the circuitry responsiveto the two thermistors for switching offthe supply of powerto the vessel element is coupled to a junction of said potential divider circuit to be respon sive to the voltage variationsthereatwhich occur in consequence of the resistance variations ofthe thermistors.

14. An electronic control according to any of claims 8to 13 wherein the or each said thermistor comprises an NTCthermistor.

15. An electronic dry boil protectorfor an electric kettle or other water boiling vessel comprising an NTC thermistor adapted to be coupled in close heat transfer relationship with the heating element ofthe vessel and circuitry responsive to an excessive rate of variation of the resistance of said NTC therm;stor, indicative of the element being powered without there being sufficient water in the vessel, for switching off the supply of powerto the element.

16. An electronic dry boil protector according to claim 15wherein said circuitry is arranged further moreto be responsive to an excessive temperature atthe NTC thermistorto disruptthe supply of power- to the element.

17. An electronic dry boil protector according to claim 15 or 16 in combination with a steam sensor adapted to disrupt the supply of powerto the vessel element when water boils in the vessel and steam impinges upon said steam sensor.

18. Acombination accordingtoclaim 17wherein the steam sensor includes a PTC thermistor and circuitry responsive to a predetermined change in the resistance of the PTC thermistor indicative of the impingement of steam thereon for disrupting the supply of power to thevessel element.

19. Acombination according to claim 18wherein the NTC thermistor of the dry boil protector and the PTC thermistor of the steam sensor are connected in a potential divider such that variation ofthe resist ance of eitherthermistor causesthe potential of a junction of the potential divider to vary, and circuitry responsive to the potential of said junction is respon sibleforswitching offthevessel element.

20. A combination according to claim 19 wherein current sensing means are associated with the pot ential dividerforsensing currentvariationstherein to enable it to be identified as to which ofthe NTC thermistors are responsible for achange in the pot ential at said junction.

21. Acombination according to claim 17 wherein separate detector circuits are associated with the NTC and PTC thermistors for deriving therefrom respective signals for switching off the vessel element.

22. A combination according to claim 21 wherein the outputs of the two detector circuits are OR gated to derive a vessel element switch off command.

23. A combination according to claim 21 or23 wherein said separate detector circuits each com prisea Schmitttriggercircuit.

24. A combination according to any ofthe pre- ceding claims wherein a triac is provided for controlling the supply of power to the kettle element and a triac drive circuit controls the triac operation in dependence upon the condition ofthe respective thermistor(s).

25. A combination according to claim 24 wherein the triac is so constructed as to be coolable in operation by water boiling in the vessel, the chip-to-case thermal resistance of the triac being relatively low by virtue ofthe triac chip being mounted on a relatively large area thermal diffuser.

26. An electrically heated water boiling vessel, such as a kettle for example, incorporating an electronic control as claimed in any of the preceding claims.

27. An electrically heated water boiling vessel according to claim 26 and wherein the control ofthe power supply to the kettle element is determined by a triac which is cooled by water contained within a water pocket formed in the vessel body.

28. An electrically heated water boiling vessel according to claim 27 wherein the water pocket is arranged to be filled when the vessel is filled and to empty when the vessel is emptied.

29. An electrically heated water boiling vessel incorporating a triac arranged to control the power supply to the heating element of the vessel and wherein the triac is cooled in operation by means of water contained within a water pocket within the vessel, the water pocket being arranged so as to fill with water as the vessel is filled and to empty as the vessel is emptied.

30. An electrically heated water boiling vessel incorporating a triac arranged to control the power supply to the heating element of the vessel and wherein the triac is arranged to be cooled in operation by the water being heated in the vessel, the triac having a relatively low chip-to-case thermal resistance such asto enablethetriacto be coolable in operation by boiling water by virtue of the triac chip being mounted on a relatively large area thermal diffuser.

31. An electrically heated water boiling vessel provided with a steam sensorforswitching offthe supply of power to the vessel element when the vessel boils and wherein the steam sensor comprises an electronic circuit including a PTCthermistor arranged to be exposed to steam generated when water boils in the vessel and to exhibit a corresponding predetermined resistance change, a detector responsive to the condition of the PTC thermistor, a triac drive circuit responsive to the output ofthe detector, and a triac arranged for determining the supply of power to the kettle element, the arrangement being such that in response to the generation of steam when water boils in the kettle the thermistor exhibits a resistance change which is detected by the detector so asto provide a triac inhibit output.