Background of the Invention: Electric kettles and hot water jugs have conventionally been provided with an element protector control, for protecting the heating element against overheating for example as a result of the vessel having been switched on empty, and a separate steam sensor control, for automatically switching off or reducing the supply of electricity to the heating element when water boils in the vessel and steam impinges upon the control. An example of an element protector control is the X-series control, available from us, which is substantially as described in GB-A-2 194 099 with reference particularly to Figures 3A, 3B and 3C thereof and provides two levels of overtemperature protection, and an exemplary steam control is the J-series control, also available from us, which is substantially as described in GB-A-2 212 664 with reference particularly to Figures 3A to 3M thereof. Both of these controls, and other functionally similar controls, employ thermally-sensitive devices in the form of bimetallic actuators.

Proposals have been made to achieve the functions of heating element overtemperature protection and steam sensing by means of a single sensor. A single sensor electronic control is described in GB-A-2 228 634, and in GB-A-1 143 834 there is described a proposal to incorporate an enclosure within the water chamber of a water boiling vessel, the enclosure being arranged to fill with water when the chamber is filled and to be heated by the vessel heating element so that, when the water boils, the water in the enclosure is driven therefrom by steam, thereby simulating a dry boil situation within the enclosure which can be sensed by a bimetallic or other thermal sensor.

Despite its apparent simplicity and promised advantages, the arrangement of GB-A-1 143 834 was, so far as we are aware, never put into production. When we attempted to follow the teachings of GB-A-1 143 834 we found that we could not obtain a satisfactory single sensor control arrangement with a heating element as described in GB-A-1 143 834. We were, however, able to devise solutions to the problem of making the enclosure idea of GB-A-1 143 834 work and these are described in WO-A-9318631, WO-A-9318632 and WO-A-9511515. The first two of these involve the utilisation of heating elements of the conventional sheathed type in which a resistance heating wire is packed into a metal sheath with mineral insulating material between the wire and the sheath, and the latter makes use of so-called thick film heating elements which comprise an electrically

insulating substrate, commonly of stainless steel with an insulating layer of glass for example, upon a surface of which a resistance-heating track or layer is provided. Thick film heating elements are becoming increasingly popular on account of their clean appearance and potentially high watts density which gives reduced heating times.

In our British Patent Application No. 9816645.7 there is described a solution to the problem which can arise, particularly with the proposal of WO-A-9511515, that the operation of the control may be voltage sensitive.

This is a particularly European problem, since supply voltages in Europe can vary from one country to another within a range of from 210 volts to 250 volts. According to the solution described in GB 9816645.7 abovementioned, the resistance heating track of a thick film heating element has a portion designated to register with an enclosure as described in WO-A-9511515 and formed of a material having a negative temperature coefficient of resistance, the remainder of the heating element track being formed of a material having a positive temperature coefficient of resistance. As is explained in GB 9816645.7, this arrangement can compensate for differences in supply voltage insofar as the operation of the enclosure and its attendant bimetallic sensor is concerned.

The single bimetal control principle as described in the foregoing thus enables a single bimetallic sensor, responsive to the temperature of a particular part of the heating element underlying an enclosure, particularly a

thick film heating element, to function both to protect the heating element against overheating and also to switch off the electricity supply to the heating element when water boils in the vessel. The vessel still has to have an on/off switch and this can be arranged by use of a manually resettable control to which the appliance (vessel) manufacturer can affix his styled rocker knob.

However, this enforces design limitations as regards the location of the rocker knob (on/off switch) and requires an element temperature control having the facility to be switched both on and off.

Obiects and Summarv of the Invention: It is the principal object of the present invention to overcome or at least substantially reduce the abovementioned problem.

According to the present invention, a single sensor control in accordance with the teachings of any of our above-mentioned proposals further includes a relay connected in series with the heating element and the control, the relay operation being arranged to be dependent upon the flow of electric current to the heating element, for example by provision of a current transformer in the circuit, and the relay furthermore being manually operable.

A water boiling vessel in accordance with the teachings of the present invention including a single bimetallic control and an electromagnetic relay switch would thus operate as follows: (1) Start-the relay switch is de-energised, its contacts are open, and no current flows through the heating element.

(2) The contacts of the relay are manually closed (by an"On" switch or push button. Current flows, via the current transformer, to the heating element. The output of the transformer serves to energise the relay coil, holding the contacts closed, so the"On"switch may be released.

(3) When the water boils, the single bimetal control switches off the element. No current flows, so the output of the current transformer drops to zero and the relay is de-energised, opening its contacts.

(4) The water ceases boiling and the single bimetal control resets automatically. No current flows because the relay contacts are open.

(5) The cycle may be repeated by closing the relay contacts again.

The boil cycle may be interrupted at any time by manually breaking the circuit, for example by manually opening the relay contacts or by operating a separate"Off"switch.

(6) In the event of switching on dry or boiling dry, the normal functions of the element protector control serve to protect the element by switching it off. In this case the relay circuit will de-energise and will require the user to switch on to reset.

The relay and switches may be situated on the vessel, but preferably the vessel is cordless and the relay and switches are situated within the cordless base. This gives a very simple appliance, in which the vessel proper has only the element and its protector, and as with the above single bimetal

control arrangements, no steam duct or separate steam control is required. In addition, however, no rocker or manual control of any type is required on the vessel proper, giving complete freedom of design. This is particularly appropriate for metal vessels, where the provision of plastics mouldings and rocker knobs spoils an otherwise clean design. By situating the controls within the cordless base a further advantage arises in that as soon as the vessel is lifted from the base, the circuit is broken and the relay is de-energised.

Thus the connector part in the base is not connected to the supply and additional safety is achieved. In addition, even if the vessel is replaced on the base, it will have to be manually switched on. The control system additionally means that the vessel cannot be left switched on but unplugged (or plugged into a switched off socket outlet) so that accidental dry switch on is less likely.

Of course, any type of cordless connection system could be utilized, such as for example the 360° cordless connection system made and sold by us as the CS4/CP7 system which is substantially as described in WO-A-9406185. We believe that such a construction is inventive in itself, in that it provides a cordless appliance connector which cannot be switched on without the appliance proper being in place.

The invention is not restricted to utilization of bimetallic temperature sensitive controls and any kind of thermally responsive switch could be employed. Additionally, sensors could be utilized comprising PTC (positive temperature coefficient of resistance) materials as disclosed for example in

our British Patent Application No. 9807385.1 which describes and claims a thick film heating element having associated therewith a PTC material for determining the current supply to the coil of an electromagnetic relay switch.

It is thus not necessary to break the circuit to cause operation. The current sensing component may also use a reduction in the load current below a set value to de-energise the relay. When applied to a PTC protected element, the reduction of current caused as the PTC resistance rises can be used. This has an advantage in that it is not necessary to maintain the PTC at a high temperature (using energy) to hold the element off, which means that the whole element will be cold and hence safer. In one implementation on a thick film printed element, the PTC resistor would be located under the enclosure to sense both boil and dry boil. The rise in resistance, giving rise to a fall in element current, would de-energise the relay. In this application a PTC having a knee characteristic (giving a rapid rise in resistance above a set temperature) would be preferred. Since PTC thick film printing materials are not readily available, the PTC sensor could be a discrete surface mount component. A normal linear characteristic could also be made to work and would lead to an element with no additional control components assembled to it, which would be a cheap solution. NTC and PTC tracks provided on the heating element to give a sort of thermal runaway may be used, where the power of the element as a whole is transferred from the NTC to the PTC

portions as the PTC resistance rises on sensing a rise in temperature, leading to an overall drop in current.

The teachings of the present invention could also be applied to a conventionally constructed vessel with a steam tube and a steam switch, allowing a retrofit to an existing design. In this case the steam switch need only be a simple self-resetting thermal cut-out, with no manual actuator. A sealed contactstat could be used. The lack of a manual rocker and linkage would have safety and constructional advantages, and would simply the design of switches of a suitably sealed design to resist condensation.

A further advantage arises because the invention can be implemented using standard, readily available, components. No additional contacts are needed on the cordless system, as were required by the electronic kettle (GB 2 228 634) so the standard CS4/CP7 or CS2 described in GB-A-2 241 390 may be used. The element protector control can be of any auto-reset type, such as our X2 or X3 controls. The latching relay is readily available, and together with the current transformer, costs around £1. This will replace around 60p of components, together with the necessary cover mouldings normally found on a conventional kettle. It has the constructional advantages claimed for the single bimetal control, including lack of steam sealing and spillage problems.

The teachings of the present invention are applicable to vessels incorporating any kind of electric heating element, though in the case of die

cast elements or elements of the mechanical Blitzkocher construction, modification of the heating element to provide a location which is normally cooled by the presence of water, and which has a heater between it and the control actuator may be necessary. This can be achieved by providing a sump in the water-side surface of a die cast element, situated very close to the inner side of part of the sheathed heating element which is enclosed in the casting, and providing an apertured cover over the sump. A similar modification of a Blitzkocher type heating element could be effected.

The above and further features of the present invention are set forth with particularity in the appended claims and, together with the advantages thereof, will be well understood from consideration of the following description given with reference to the accompanying drawings.

Description of the Drawings: Figure 1 is a schematic circuit diagram of a heating element control circuit embodying the teachings of the present invention; and Figures 2A, 2B and 2C show, respectively, a perspective view of the underside of an exemplary die cast heating element, a view similar to Figure 2A but with a bimetallic control shown affixed to the heating element, and a perspective view of the upper (water) side of the heating element of Figures 2A and 2B showing a sump-defining pocket formed in the surface thereof.

Detailed Description of the Embodiments:

Referring to Figure 1, the circuit shown is for a cordless electric water heating appliance, such as a kettle or hot water jug, comprising a vessel part and a base part and wherein a heating element in the vessel part is arranged to be powered via the base through the intermediacy of co-operating connectors provided on the two parts which operatively engage when the vessel part is set down on the base part. In Figure 1, the co-operating connectors are shown at 1 and 2, the vessel part is to the right hand side of the connectors 1 and 2, and the base part is to the left hand side of the connectors 1 and 2.

The vessel part has a heating element 3 and an element protector control 4. The heating element can for example be a thick film heating element provided with an enclosure as described in WO-A-9511515. The element protector control 4 can be an X-series control as described in GB-A-2 194 099 for example, the X-series controls providing dual protection for the heating element as is well known. The connector set 1,2 can for example be of the 360° kind described in WO-A-9406185 which enables the vessel part to be seated upon the base part irrespective of the relative rotational orientation of the two parts.

The base part has an electromagnetic relay 5 comprising contacts set 6 and operating coil 7, a current transformer 8 coupled to the relay coil 7 and arranged so that the relay coil 7 is energised so long as current is flowing in the heating element 3 of the vessel.

An"on"switch 9 is connected across the relay contacts 6 and an"off' switch 10 is connected in series with the relay contacts 6.

The contacts of the element protector control 4 are normally closed, but will open when water is boiled in the vessel, by virtue of the provision of the enclosure aforementioned, and also if the heating element 3 is powered without the vessel having been filled. The relay coil 7 is energized via current transformer 8 when the heating element 3 is powered. Otherwise, because the "off'switch 10 has been operated or the element protector control 4 has operated to open its contacts or the vessel part has been lifted off its base part, no current flows in the heating element 3 and the relay coil 7 is de-energised.

When relay coil 7 is de-energised the relay contacts 6 are open. The"on" switch 9 enables the open relay contacts 6 to be bypassed so as to cause the heating element 3 to be powered, so long as the vessel is seated on its base and the element protector 4 has not operated, and operation of the"on"switch 9 will, subject to these conditions, cause the relay coil 7 to be energised thereby closing relay contacts 6. The"off'switch 10, which can be integrated with the"on"switch 9 in an appropriate toggle switch, enables the circuit through the base and to the vessel to be broken. Otherwise, the operation and advantages of the circuit will be clear from the explanations provided herebefore.

Whilst the teachings of the present invention are, in principle, applicable to vessels incorporating heating elements of all the commonly used

kinds, some modification of die cast elements or elements of the mechanical Blitzkocher type construction, comprising a sheathed heating element clamped or clenched to the underside of a metal plate, may be necessary. This is because there is no location on such elements which is normally cooled by the presence of water, and which has a heater between it and the control actuator. To overcome this we propose to provide a sump in the water-side surface of the die cast element, situated very close to the inner side of part of the sheathed heating element which is enclosed in the casting. On a test sample this was done by machining a pocket in the upper surface of the heating element. The particular element that we used has a platform bailt (cast) up on its lower surface close to the heating element proper and an X2 control is mounted on this platform. Heat from the heating element proper is conducted to the platform sideways and lost to the water upwards, which gives a suitable running temperature for normal use and gives a rapid temperature rise when no water is present. The pocket abovementioned was machined from the upper surface of the heating element, effectively hollowing out the platform from above. A cover, formed of moulded plastics material for example, was secured over the pocket, and one or more suitably sized holes left to allow water to enter and steam to leave. The size of the hole (s) is determined by trial and error and depends on the heating element configuration and power density. Since the pocket is sunk below the top surface of the heating element, the cover may be made flush, giving a neat

appearance. A similar construction may be applied to heating elements of mechanical Blitzkocher type. Where such elements are used with metal bodied vessels, a hole might be cut in the vessel bottom and a sump welded in place underneath. The sump may be provided with a thermal link to the heating element, for example a copper loop pressed onto the outside of the element sheath or brazed to it, and the thermal control may be placed in thermal contact with the sump to create an equivalent construction to the die cast element described above. It is conceivable that a sump could be designed which has a restricted opening such that a separate cover is not required.

Figures 2A, 2B and 2C of the accompanying drawings show an exemplary die cast heating element modified as above described. Figure 2A shows the heating element as comprising a die cast metal body 20 having a sheathed, spiral heating element 21 incorporated into the underside thereof, the upper surface of the heating element, as shown in Figure 2C, being planar.

The body 20 of the heating element has integral mounting posts 22 and the heating element cold tails 23 and 24 can be seen. Also shown are integral cast metal pads 25,26 and 27 which serve for the mounting to the heating element of an X2 control 30 in the manner shown in Figure 2B. As shown in Figure 2C, the metal pad 26 is hollow thus defining a sump 35 as aforementioned which can be provided with an apertured cover to define a single sensor enclosure as previously explained herein.

Having thus described the invention by reference to specific embodiments, it is to be well understood that the embodiments are exemplary only and that modifications and variations thereto are possible without departure from the spirit and scope of the invention. For example, whereas the arrangement of Figure 1 employed an electromechanical relay, the same function could be performed electronically, for example by means of a triac circuit.