GB2354927A

GB2354927A - Water heating element with adjustable resistance track associated with bimetal control

Info

Publication number: GB2354927A
Application number: GB0013545A
Authority: GB
Inventors: Robert Andrew O'neill; Robin Keith Moore
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1999-07-21
Filing date: 2000-06-02
Publication date: 2001-04-04
Anticipated expiration: 2020-06-02
Also published as: GB2354927B; GB0013545D0

Abstract

A water boiling vessel having a thick film heating element 3 provided with a bimetallic over- temperature protection control includes a thick film auxiliary heater 11 provided on the heating element in the bimetal response region 7 and a resistive track 13 connected in series with the auxiliary heater the track being contactable selectively along its length so as to vary its effective resistance and thereby vary the current through the auxiliary heater. The auxiliary heater increases the heating of the bimetallic control so that it operates at a lower temperature thereby switching off the heater prematurely. An integrated 360{ cordless appliance inlet connector and dry boil control includes a slider 55 to connect selectively to the resistive track. The resistive tack may have discrete contact points 14' and may be arranged in parallel with a second resistive track 25 which control power to a keep warm element 21. A second bimetal response area 8 is also provided.

Description

2354927 IMPROVEMENTS RELATING TO WATER HEATING VESSELS

Field of the Invention:

This invention concerns improvements relating to electrically powered water heating vessels such as domestic kettles and hot water jugs for example, and, more particularly, concerns arrangements whereby such vessels may be switched off or their power supply reduced at a water temperature below boiling.

Background of the Invention:

Steam controls which operate to switch off or reduce the power supply to the electric heating element of a water boiling vessel when water boils in the vessel are well known, and commonly operate by responding to the generation of steam when water boils in the vessel, the stem venting from the vessel interior into a region of the vessel whereat a thermally-sensitive switch actuator, a bimetal or shape memory effect device for example, is located.

Controls are also well known which are intended to protect the heating element of a water boiling vessel from overheating, for example as a result of the vessel being switched on empty or being allowed to boil dry, and commonly comprise a bimetallic switch actuator which is held in close thermal contact with the heating element and is calibrated to operate at a certain overtemperature.

It has also been proposed to provide a water heating vessel with means whereby hot water can be provided at a temperature lower than boiling, for 2 example for use in making coffee. Electronic control arrangements have been proposed which enable the temperature of the hot water produced by a water heating appliance to be selected by the user, and bimetallic arrangements are known which operate similarly to an adjustable thermostat, in that the operating temperature of the bimetal is controllable by the user, or which operate by varying the heat transfer relationship between the heating element and the bimetal, for example by moving the bimetal relative to the heating element.

The bimetals that are employed in controls for water heating vessels are nowadays commonly of a stressed, snap-acting type in view of the fact that such bimetals have well defined characteristics which are stable over a period of time. So-called creep acting bimetals were previously employed but are not nowadays widely used, mainly on account of their relative lack of stable operating characteristics. Creep acting bimetals, which exhibit a progressive deflection or deformation as a function of increasing temperature, as opposed to the snap-acting type which exhibits a change from one stable state to another at a predetennined temperature, are however useful in applications where the operating temperature of a control is required to be adjustable. It is however, disadvantageous to use a creep-acting bimetal to operate switch contacts directly since the relatively slow movement of the bimetal (as compared to a snap-acting bimetal) will give rise to arcing at the switch contacts with disadvantageous consequences and for this reason, where 3 creep-acting bimetals are proposed to be used in present day controls, it would be good design practice to isolate the bimetal from the switch contacts of the control by means of a snap-acting mechanism, an overcentre lever for example, which translates the slow movement of the bimetal into a snap-action at the switch contacts. For obtaining temperature adjustability, the mechanical relationship between the creep bimetal and the snap-acting mechanism can be adjustable.

One such arrangement as abovedescribed is proposed in Strix Limited's GB-A-2 329 523. A standard U18 control as presently available from Strix has a creep bimetal riveted to the metal carrier plate of the control and the creep bimetal is arranged to interact with the trip lever of the control by way of a manually operable selector which provides for adjustability of the mechanical relationship between the creep bimetal and the trip lever. The arrangement of GB-A-2 329 523 provides the U18 control with the additional facility of an adjustable means for switching off the power supply to a heating element at a temperature below boiling which is selectable, in dependence upon the calibration and adjustment of the creep bimetal, by operation of the manually operable selector.

For the reasons abovementioned, creep bimetals are, in our considered opinion, an undesirable component for inclusion in a control for a water boiling appliance. It is in the nature of creep acting bimetals that some forin of initial temperature calibration is required during manufacture, and it is 4 notable that the embodiment described in GB-A-2 329 523 includes set screws which have to be ad usted to determine the action of the creep bimetal. This adds to the cost and renders the control susceptible to variation of its operating temperature as the calibration and adjustment changes as the control ages.

Objects and SummM of the Invention:

It is thus the principal object of the present invention to provide for adjustability of the water temperature at which a control will operate to switch off or reduce the power supply of the heating element of a water boiling vessel without evoking the disadvantages that are inherent in the arrangement that is described in GB-A-2 329 523 abovementioned.

Whilst the arrangement of GB-A-2 329 523 is not restricted in its application to any particular kind of electric heating element, its greatest application at the present time is to electric kettles and hot water jugs utilizing so-called thick film heating elements. A thick film heating element comprises a generally planar substrate, commonly but not essentially formed of metal, such as stainless steel for example, which carries a printed or otherwise formed heating track or layer on at least one side thereof, electrically insulating layers being provided as necessary between the heating track or layer and the substrate and overlying the heating track or layer. In accordance with the present invention, in one of its aspects, we propose to make use of the properties inherent in thick film heating elements to achieve adjustability of the operating temperature of a thermally sensitive control associated with a thick film heating element. More particularly, we propose to enable the thermal output of a predetermined part of a thick film heating element which is to have a thermal sensor associated therewith to be selectively adjusted, whereby the performance characteristics of the sensor in relation to the volume of water being heated will change correspondingly.

According to the present invention, in one of its aspects, therefore, an electrically powered water heating vessel has a thick film heating element, a thermally-responsive control is located in thermal contact with a predetermined part of the thick film heating element, and means are provided for enabling the thermal output of that part of the thick film heating element to be selectively adjusted.

Thick film heating elements are commonly manufactured by printing with conductive ink onto the surface of a substrate formed of stainless steel and provided with one or more insulating layers, commonly of glass. On such a printed element the track temperature is related to the water temperature, normally exceeding it by an amount which depends on the power density of the track and on the thermal conductivity of the substrate and dielectric. In the case of our X4 control described in our GB-A-2 339 088 on a 2.5 to 3kW element the track temperature will be about 1600C, that is 60'C above the boiling water temperature. In order to avoid nuisance tripping of the control during boiling the control bimetal is normally set at about 180T. If, on the track portion which heats the bimetal, the heat output is increased, then the 6 bimetal can be made to nuisance trip at a water temperature below boiling point. If the amount of additional power applied to that part of the track were arranged to be controlled, then the temperature at which the control "nuisance tripped" could be controlled to give water at a controlled temperature below boiling. In application to a cordless vessel, because of the latch out function of the X44 (latched) variant, the control would remain off until the associated cordless vessel was lifted off its base and reset. On the X46 (auto cycling) or X48 (auto cycling with thermal fuse) variants, the control would cycle to maintain the water at the preselected temperature. Of course, the present invention is not limited in its application to our X4 series of controls.

The means by which the heat output of the track beneath the bimetal may be adjusted could take a number of forms, but the preferred means is to add an auxiliary track beside the main heater track at the location where the bimetal senses the heater temperature. The current through the auxiliary track could be controlled by connecting it via a variable resistance to the power supply. It is anticipated that a power of around 20W would be necessary to give a range of about 200C depression of switch off point. This could be done by provision of a variable resistor separate from the heating element and having a control knob accessible to the user of the appliance, but such a resistor would be costly and would dissipate a considerable amount of heat, making it inefficient. The control could be electronic, since the control of a power of 20W is well within the capability of low cost components. However 7 the preferred option is to form the variable resistor on the thick film heating element in the form of a resistive track printed on the element, and to provide an arm carrying a brush or sliding contact arranged to make electrical connection to the resistive track at any selected position along the track or to any selected one of a plurality of contacts connected to predetermined track locations which preferably are spaced apart from each other unequally to provide a linear change of power to the auxiliary track as the brush or sliding contact is moved, the arrangement thus increasing or decreasing the amount of resistive track included in circuit with the auxiliary heater. There could also be a "boil" position where the sliding contact made no contact with the resistive track, providing a setting where the auxiliary heater track has no effect and the vessel boils water normally. Such an arrangement would however mean that with the sliding contact in its "boil" position, no heat would be generated by the auxiliary track and thus no contribution to the overall element power would be available from that area of the heater. This is inefficient use of the available space on the heating element and would lead to an unnecessarily large and costly heater, and to avoid this it is preferred in accordance with the present invention to arrange that the "boil" position places the entire resistive track in circuit and that the power density in the auxiliary track is the same as in the adjacent main power track so that both operate at the same temperature. The techniques of printing tracks for variable resistors with sliding contacts are well known and would be directly 8 applicable to this construction. The resistive track could be an arc preferably concentric with the element substrate disc (and/or the appliance), and the adjustment arm could project from a slot in the wall of the vessel body and be arranged to slide around the body. Alternatively the arc could be formed as a small diameter arc of more than 180', as would be normal in a conventional potentiometer, in which case the sliding arm could be pivoted from the centre of the arc and a lever could be connected to the sliding arm and project from the side of the appliance.

In the above embodiment which contemplates the use of an X4 control in the practice of the invention, the bimetal actuator of the control would also continue to function as a dry boll protector. However if a more flexible lower power solution is desired, then a thermal sensor separate from those provided on the X4 control could be provided. This sensor should have a low thermal capacity to reduce the power required from the auxiliary heater track. It could take the fon-n of a PTC sensor, with an electronic control system, or preferably it could be a small contactstat. Such a contactstat would require a smaller (in area) heater track to make it function and, since the temperature of the heater depends on its power density (not the absolute power), then a small track will only require a small amount of power. The temperature setting of the contactstat would be approximately 105'C to avoid nuisance tripping during boiling, and the auxiliary heater track could be arranged to be adjustable to provide between 0' and 65'C temperature rise to provide water selectively at 9 temperatures from 40'C for heating babies bottles up to boiling. A further advantage of this approach is that the auxiliary heater track would normally be run at very low or no power, which would limit the amount of scale build up.

Scale build up over the main bimetal heater area results in a rise in running temperature, which could affect the switch off point of a temperature control based on the main bimetal. This is, after all, the reason why the bimetal is set higher than the running temperature to avoid nuisance tripping.

A further aspect of the invention, useful to maintain water at a preset temperature, for example simmering, or for maintaining a water bath at a set low temperature, makes use of the principle disclosed in our GB-A-2 248 144 which describes how a temperature may be controlled by the reset temperature of an automatically resetting bimetal control by ensuring that the bimetal differential is less than the self heating caused by the current flow through the bimetal. In the case of the present invention, the heating is proposed to be supplied not by through current but by the auxiliary heater track, which raises the bimetal through its differential until it switches off. The track then rapidly falls to the water temperature and, if this is below the remake temperature of the bimetal, then the control switches the heater back on and repeats the cycle.

This cycling action continues, with the water being heated intermittently, until the water reaches the bimetal remake temperature, whereafter the bimetal will cycle slowly to maintain the heater and the water at its remake temperature just as in GB-A-2 248 144. To take best advantage of this effect it would be desirable to have a variable remake bimetal, that is to say an adjustable bimetal, with a fixed auxiliary track power, but a variable (adjustable) remake bimetal could be used with an adjustable auxiliary track power, in which case one of these adjustments, possibly the power of the auxiliary heating power, might be factory set and the other enabled to be determined by the user of the appliance by provision of an appropriate control on the appliance.

Adjustability of the remake temperature of a bimetal, particularly a dished, snap-acting bimetal as best contemplated for use in the practice of the present invention, is readily provided for example by provision of an ad ustable j back-stop for the bimetal and/or by provision of adjustability of the stressing of the bimetal.

In the practice of this aspect of the invention, the controlled temperature would be unaffected by any scale build up, since at the controlled temperature there is no temperature drop across the dielectric/substrate/scale sandwich. It should be noted in connection with the previously mentioned example of the cycling X46, that if the normal remake temperature (which is above I OOOQ of such a control is used in the practice of this further aspect of the invention, then the water temperature would tend to rise steadily until it boiled. For this reason the alternative contactstat version with its lower setting and remake below 1000C, is to be preferred where a controlled maintained water temperature is desired.

In accordance with this ftirther aspect of the invention, therefore, the thick film heating element of an electrically powered water heating vessel has a predetermined part thereof provided with an auxiliary heater and a snap-acting bimetallic actuator is arranged in thermal contact with such part of the thick film heating element so that, in use, its operation is dependent upon the heat output of said auxiliary heater, said snap-acting bimetallic actuator being adjustable as regards it remake temperature.

Switching off the appliance on boil can be done by any of the known ways, such as by use of our Jl or Z5 controls described in GB-A-2 212 664 and GB-A-2 331 848 respectively, or by means of the concept described in GB-A-2 265 070, or by sensing the rate of rise of the water temperature as in our electronic kettle control described in GB-A-2 228 634.

According to yet another optional feature of the invention a thick film heating element provided with an auxiliary heating track at a location whereat a thermal control, for example a bimetallic control, is to be juxtaposed with the heating element and with means enabling the heat output of the auxiliary track to be adjusted, all as aforesaid, additionally has a keep warm track and means enabling the heat output of the keep warm track to be adjusted. The means enabling the heat outputs of the auxiliary and keep warm tracks are preferably commoned so that one and the same manual operation can perform both functions and preferably comprise respective variable resistance elements juxtaposed with each other and arranged to be adjusted by means of one and 12 the same slider control, and the slider control preferably has an off position where no contact is made with either variable resistance element so that neither the auxiliary heater nor the keep warm heater is operative.

A water heating vessel provided with such a heating element and with an X4 element protector control monitoring the condition of the heating element, with one of its bimetallic actuators located to be responsive to the heat developed by the main heating element track and by the auxiliary track and the other located to be responsive to the heat developed by the main heating element track and by the keep wann track, can have an on/off control constituted by the reset switch of a boil sensing control such as our aforementioned JI or Z5 stearri controls and a further control constituted by the abovementioned slider or, more preferably, can have the two such controls commoned together. Such a water heating vessel can, with the tracks and bimetals appropriately configured, demonstrate different modes of operation, namely a boil mode wherein the slider control is in its off position so that neither the auxiliary heater track nor the keep warm track is operative and the vessel is controlled by operation of the boil sensing control, a boil and keep warm mode wherein the slider control is positioned so that the keep warm track provides its maximum power output and the auxiliary track its minimum whereby the vessel contents are initially heated to boiling by operation of the main heater track which is then switched off by the Z5 steam control whereafter the keep warm track keeps the vessel contacts close to boiling, and 13 a heat and keep warm mode wherein the slider control is positioned at an intermediate position so that the auxiliary heater operates to switch off the power supply to the main heating element track when the vessel contents are at some selected sub-boiling temperature and the keep warm track then maintains the vessel contents at that selected temperature. By selection of the appropriate mode, the user of such a water heating vessel can obtain boiling water, or water that has boiled and which has been maintained close to boiling, or water that has just been heated to some selected intermediate temperature, and the mode selection is simple and uncomplicated.

The above and further features of the present invention are set forth in the appended claims and will be ftirther explained by reference to the following detailed description of exemplary embodiments which are illustrated in the accompanying drawings.

Description of the Drawings:

Figure 1 illustrates in plan view an exemplary track layout of a thick film heating element for an electrically powered kettle or hot water jug embodying the present invention.

Figure 2 is a schematic circuit diagram showing a thick film heating element, for example as in Figure 1, in a water heating vessel provided with dual dry-boil protection and with a steam control; Figure 3 is a plan view of an alternative thick film heating element embodying the present invention; 14 Figures 4A, 4B, 4C and 4D show how a modified form of our X4 control interfaces with the heating element of Figure 3, Figure 4A illustrating the areas of the heating element that arc required to register with specific parts of the control, Figure 4B showing a plan view of the modified X4 control assembled with the heating element, Figure 4C showing an enlarged perspective view illustrating the modification of the X4 control and Figure 4D being a scrap cross-sectional view of the modified part of the control; Figures 5 and 6 are plan views of yet further alternative thick film heating elements embodying the present invention; Figure 7 is a plan view of yet another thick film heating elemcnt embodying the present invention which additionally includes a controllable keep warm track; and Figure 8 is a circuit diagram illustrating the use of a thick film heating clement according to Figure 7 with a control substantially as shown in Figures 4A, 413, 4C and 4D.

Detailed Description of the Embodiments:

Referring to Figure 1, a circular thick film heating element is shown in plan view as comprising a stainless steel disk I upon which there is forincd an electrically insulating layer 2 upon which a conductive ink printed heating element proper 3 is provided. The heating clement proper 3 is made up of heater track sections 4 which are joined by conductive bridges 5 at their extremities to avoid current crowding where the track changes direction. The heating element shown is adapted for use with an X4 control having two generally similar bimetallic switch actuators which, when the control is affixed to the heating element by spot welding of metal mounting feet of the control to the stainless steel disk I at the three locations 6, register with the heating element in good heat transfer relationship therewith generally in the regions 7 and 8 thereof. The main power supply terminals to the heating element comprise conductive pads 9 and 10 which are arranged to be contacted by spring terminals of the X4 control.

In accordance with the teachings of the present invention, an auxiliary heater track 11 is provided in the region 7 of the heating element which co-operates with one of the bimetals of the X4 control and, coupled thereto by means of a conductive section 12, is a resistive track section 13 which, in use, will serve to enable the thermal output of auxiliary heater track 11 to be adjusted. The auxiliary heater track I I is coupled at one end to main heater terininal 9 and resistive track section 13 has associated therewith a schematically illustrated wiper 14 which is connected to that pole of the power supply which supplies terminal 10 of the main heating element 3. In use, the series-connected resistive track section 13 and auxiliary heater track I I are connected in parallel with the main heating element track 3 and the position of the wiper 14 along the resistive track section determines the heat output of the auxiliary heater I I and thus, as hereinbefore described, determines the 16 operation of that bimetallic actuator of the X4 control that registers with region 7 of the thick film heating element.

The position of wiper 14 on the resistive track section 13 is made controllable by the user of the vessel and preferably, to enable the effect of the auxiliary heater I I to be switched off when the vessel is to be used for boiling water, has a control position whereat it makes no contact with the resistive track section 13. The mechanical arrangement of the wiper 14 and/or its slider knob is preferably such that contact between the resistive track section 14 and the wiper 14 is made and broken with a snap action so as to minimize arcing, and the respective part of the resistive track section, which will preferably be provided at the high resistance end thereof, may be configured to withstand such arcing as might occur.

Figure 2 shows the thick film heating element of Figure I in an electrically heated water boiling vessel provided with a dual-protection dry boil control such as our aforementioned X4 control and with a steam control such as our aforementioned J1 or Z5 controls. In the figure, the two dry boil controls are designated 21 and 22 and are designed to register with the regions 7 and 8 of the thick film heating element, and the steam control is designated 23. The control position of wiper 14 where it does not make contact with slider track 13 is designated 24, this position of the wiper serving to isolate auxiliary heater track 11 so that the vessel operates as a water boiling vessel.

17 Figure 3 shows an alternative thick film track layout designed specifically for use with a modified form of our X4 control which is described hereinafter. Employing the same reference numerals as were used in relation to Figure 1, the thick film heating element of Figure 3 has regions 7 and 8 designed to register with the bimetals of the X4 control and the region 7 has associated therewith an auxiliary heater track I I which is connected to a track section 13 which is designed to co-operate with a control slider 14 to constitute a variable resistance enabling the thermal output of the auxiliary track 11 to be controlled by the vessel user.

Figures 4A to 4D illustrate the attachment of a modified X4 control to a thick film heating element as shown in Figure 3. Figure 4A merely shows the regions of the heating element which are required to register with specific parts of the control, namely the regions 6 whereat the feet of the metal chassis of the control are affixed to the metal substrate I of the heating element, the regions 7 and 8 whereat the bimetals of the X4 control thermally contact the heating element track, the terminal regions 9 and 10 whereat the spring terminals of the X4 control make electrical contact with the heating element track, and the variable resistance track section 13 adapted to be contacted by a control wiper of the X4 control.

Referring to Figure 4B, this shows a modified X4 control 40 affixed to the heating element. As is known, and as described in GB-A-2 339 088 aforementioned, the X4 control comprises a cordless vessel inlet connector 41 18 of the 360' type pioneered by us which enables a cordless vessel to be set down upon its base in any relative rotational orientation and, integrated therewith, first and second heating element protector controls generally designated 42 and 43 which can be bimetallic or a combination of one bimetallic and one fusible material as described in our abovementioned application. Provision is made at location 44 of the control for the attachment of a Z5 stearn control to the X4 control.

Figures 4B, 4C and 4D show the modification to the X4 control. The metal chassis of the control has an extended part 51, which has a central pivot hole 52 at the centre of the arc of an are shaped section 53 with end stops 54.

A plastics moulding 55 has a pivot peg 56 and an overlapping arm 57 and is designed to snap into place as shown, so that the pivot peg 56 locates in the pivot hole 52 of the chassis extension. The overlapping arm 57 fits under the chassis extension to prevent the moulding coming off its pivot and acts as a guide as the moulding is rotated. At the outer end of the moulding is a box section 58 with a snap fitting lid 59. A carbon brush assembly 60 fits into the box section, with the brush projecting towards the element surface. The braid pigtail of the brush assembly has a quick connect receptacle attached which is fitted to the tab 61 of the X4. This tab is connected to the neutral supply point of the element via the neutral side dry boil thermal control, as shown in the circuit diagram of Figure 2. 19 As is also seen from the circuit diagram of Figure 2, the boil control,

which may be either directly plugged into the X4 or remotely mounted (as is the case in the Figures, plugging into the receptacles marked 62 and 63 on the isometric view), will act to switch off the power to the element, independently of the connection of the brush assembly. This will ensure that in the event of a failure of the lower temperature feature, the boil control will always prevent the water boiling continuously.

When the X4 control is mounted to the heating element, the brush makes contact with the arc shaped slider track 13. The angle of rotation of the moulding is greater than the angle subtended by the slider track so that at one extreme of travel the brush is not in contact with the slider track. In this position the auxiliary heating track 12 is not energised and the appliance will behave as a normal automatic kettle. This is the "Boil Position" in Figure 2.

As the brush engages the slider track, current will flow in the auxiliary track, causing the live side dry boil blade to operate at a temperature depending on the power in the auxiliary track, as already described, and varying the position of the brush on its slider track will vary the power in the auxiliary track, and hence the water temperature at which the control will switch off.

Figures 5 and 6 show yet further alternative thick film track layouts designed for use with the aforementioned modified X4 control. Employing the same reference numerals as were used in relation to Figures I and 3, the thick film heating elements of Figures 5 and 6 each have regions 7 and 8 designed to register with the bimetals of the X4 control and the region 7 has associated therewith an auxiliary heater track I I which is connected to a track section 13 designed to cooperate with a control slider of the X4 control to constitute a variable resistance enabling the thermal output of the auxiliary track 11 to be controlled by the user of the vessel.

In both of the heating element layouts of Figures 5 and 6 the main power track 4 is shown reasonably wide, whereas the temperature control auxiliary track I I is somewhat narrower, since it is of lower power and thus higher resistance. The arc shaped control track 13 is moved away from the stroke of the sliding brush contact and has a series of silver links 14' printed over (or under) it. The silver links lead to a series of closely spaced silver pads 15 which lie in an arc beneath the path of the brush contact. The spacing of the pads ensures that the brush is always in contact and may bridge two pads with little loss of contact area.

The silver pads 15 have two purposes. Firstly, it is known that making contact to resistive areas of a thick film printed heater can cause problems from localised overheating, which may bum out the track or the contacting component. Although the current through the track in this case is relatively low, the material of the track has a much higher resistivity than the main track and may cause problems. Thus it has been our practice always to provide silver pads when making spring contact to a thick film heater. The problem may be worse with the proposed sliding contact when the point of contact is 21 moving and any arcing could damage the track surface. Secondly, it will be noticed that the segments of the arc shaped track 13 between the silver links 14' are not of equal length. This allows a linear change of power to the auxiliary track as the brush is rotated, rather than the hyperbolic relationship that would occur with equal length segments or with direct contact with the arc shaped track. This will allow a linear temperature scale on the appliance.

A further feature of the design of Figures 5 and 6 is that there is no "boil" position where the brush is isolated from the arc shaped track. Such a position would mean that no heat is generated in the auxiliary track, and thus no contribution to the overall element power would be available from that area of the heater. This is inefficient use of the available space and would lead to an unnecessarily large and costly heater. In the present layout, in the "boil" position, which is with the whole arc included, the power density in the auxiliary track is arranged to be the same as in the adjacent main power track.

Thus the auxiliary track will run at the same temperature as the main boil track, not causing nuisance tripping and contributing both to the normal power output of the element and to providing the necessary dry boil response from the bimetal blade at that position. As the brush is moved to the minimum included arc length, so the power density in the auxiliary heater track is increased, leading to the bimetal being actuated at progressively lower temperatures, according to the invention.

22 Referring now to Figure 7, shown therein is yet another embodiment of a thick film heating element according to the present invention which, in addition to incorporating an auxiliary track and associated arc shaped slider track enabling the heat output of the auxiliary track to be controlled, also incorporates a keep warm track and an associated control enabling the heat output of the keep warm track to be controlled. The control for the keep warm track is actually commoned with the control for the auxiliary track by virtue of comprising an arc shaped slider track parallel to and concentric with the arc shaped slider track that is associated with the auxiliary track and by virtue of employing the same control to determine the position of respective carbon brushes associated with the respective arcuate slider tracks. As will be explained more fully hereafter, this arrangement enables a kettle or hot water jug, for example, selectively to provide boiling water, or water which has been boiled and then has its temperature maintained close to boiling, or water which is heated to a selected temperature lower than boiling and then is maintained substantially at that temperature. Furthermore the arrangement hereinafter described enables this to be achieved by use of just one manual control.

The same reference numerals are used in Figure 7 as were used to denote like parts in previous embodiments. As shown, the thick film heating element I has arcuate main heater track sections 4 connected at their ends by conductors 5 so as to avoid current crowding, the main heater track extending 23 between neutral and live terminals 9 and 10 respectively. An auxiliary track section 11 is connected between a tapping point 20 of the main heater track, selected to apply the required sub-mains voltage to the auxiliary track section, and an adjustable resistance defined by the track section 13 with its associated contact points 14' and tapping points 15 to be contacted selectively by a sliding brush contact. As shown, an end tapping point 15' is provided upon which the brush contact can be parked out of electrical contact with the track section 13; when the brush contact is in this position, the auxiliary track I I is not powered.

An additional track section 21 is provided in the centre of the heating element and serves as a keep warm track. As shown, the keep warm track 21 is connected at one end to an additional power supply terminal 22 provided on the heating element by means of a conductor 23 and, at its other end, is connected by means of a conductor 24 to a further adjustable resistance 25 configured similarly to and physically in parallel with the adjustable resistance that is associated with the auxiliary track section 11. The provision of the two adjustable resistances physically in parallel and closely proximate to each other enables both to be operated by means of one and the same control slider.

The control that is used with the heating element of Figure 7 is a variant of the X4 control described hereinbefore with reference to Figures 4A to 4D. According to this variant, the sliding brush mechanism of the Figure 4 proposal is provided with two separate brushes (or a single broader one) 24 attached to a flexible lead or braid, which in turn is connected to one terminal of the boil control Z5 by means of the standard X4 tabs and receptacles provided for the purpose. The variant X4 has a third spring contact 22, in addition to the standard live and neutral element contacts, which is located beside the auxiliary track 11. Figure 8 shows how the control connects to and interacts with the heating element.

The control is fitted with a high temperature bimetal blade (I 80/1900C) on its neutral side to protect the main and auxiliary tracks. A low temperature blade (I 10/120"C) is fitted to the live side to act as a keep warm thermostat and provide secondary protection. The potentiometer slider has either one or two contacts (depending on the design, to ensure that both resistance tracks have good contact pressure), which contact the two variable tracks, so that as one resistance increases, the other is reduced. At one end of the travel there is an isolated position, to act as an "Off'. The auxiliary track 11 is shown being fed from a tapping, part of the way along the main track, to give a lower voltage which is better suited to the available resistive materials used to print its heater track. This is simply for convenience, and the auxiliary track could be fed from the full supply voltage if desired.

The heating element of Figure 7 when combined with the abovedescribed control enables the following selectable modes of operation, namely:

1. Boil: With the slider in the Off position, the kettle may be switched on by means of the Z5 control, which fimctions as usual, and switches the kettle off when the water boils. Since the slider contacts are not connected to anything, they have no effect and the auxiliary and keep warm tracks are inoperative.

2. Boil and Keep Warm: With the slider position in the max position, when the Z5 is switched on the water will come to the boil and switch off the Z5. The slider contact of the keep warm track 21 provides the maximum power to that track which is designed to keep the water close to boiling. The slider contact of the auxiliary track I I gives a very low heat to its associated heater track, which is insufficient to cause the bimetal (thermostat) associated with it to operate. The water may be re-boiled by operating the Z5 at any time.

3. Heat and Keep Warm: With the slider positioned between max and min, and when the Z5 is switched on, the main track will heat the water until the thermostat bimetal is tripped by the auxiliary track 11. After this the keep warm track 21 will maintain the water temperature. The two variable resistors are arranged so that the "keep warm" setting follows the "Vari-Temp" setting determined by auxiliary track I I as the slider is moved, so that the maintained temperature is close to 26 the Vari-Temp temperature. There will clearly be some variation in the temperature given by the keep warm track, depending on ambient conditions and the amount of water in the kettle, so the track is preferably targeted always to give a slightly lower temperature than the Vari-Temp track. This means that the thennostat bimetal will cycle occasionally, as necessary, to switch on the main heater and return the water temperature to the required value. This avoids the possibility of the water temperature slowly rising in an uncontrolled manner, and at the same time limits the cycling rate of the thermostat to increase its lifespan.

Dry boil protection of the system is provided by the dry boil bimetal of the X4 which locates in the region 8 of the heating element. This will switch off the main tracks, but leave the keep warm track 21 energised if the slider is not at the off position. The keep warm track 21 is positioned centrally to ensure that its heat does not cause damage to seals or the vessel wall. In the event of failure of the dry boil bimetal, the other bimetal associated with auxiliary track I I will operate, albeit somewhat more slowly because of the lower track power density. Even if the slider is set to off, there will be sufficient thermal transfer from the main tracks to ensure that this low set bimetal will operate. Should the keep warm track remain energised, it is probable that it will act to maintain the Vari-Temp bimetal in an operated 27 condition, giving a form of manual latch, to be reset by disconnecting the kettle from the supply. However this action is dependent on the slider position and is not relied on in the design of the system.

The design of the "user interface" is a matter of choice. Two separate controls could be provided, one for the boil control, and one for the slider.

However this would still leave the dilemma of having two "Off' positions, and a simple user may not realise that both controls must be off to completely switch off the appliance. A better scheme,is to have a single, sliding, control, which has three states: Off, where both the Z5 and the sliders are in their "Off' positions; a Boil position, where the Z5 is switched on, and can return the control of the "Off ' state, the sliders remaining in their "Off ' position; and a Simmer range of positions, where the Z5 is on, and can have no effect on the sliding control, and the sliders are between Max and Min. The status of the appliance would be indicated by indicating lamps. Such a mechanism would be within the capability of a reasonably skilled mechanism designer, and might make use of lost motion and Geneva wheel type functions.

The invention having been described herein by reference to specific arrangements and an exemplary embodiment, it is to be understood that modifications and variations thereto would be possible without departure from the scope of the invention. For example, in accordance with the teachings of GB-A-2 340 367, the main track portions 3 of the thick film heating element I could be formed to exhibit a PTC (positive temperature coefficient of 28 resistance) characteristic whereas the auxiliary heater 11 could be formed with an NTC characteristic for the purpose of compensating for supply voltage variations. Furthermore, whereas in the foregoing the auxiliary heater is formed as an integral part of a thick fihn heating element, the invention is applicable to the control of other kinds of heating elements by means of bimetallic actuators. For example, an underfloor heating element of the metal sheathed, mineral insulated, resistance wire type could be controlled in accordance with the teachings of the present invention by means of a bimetallic control by provision of a separate auxiliary heater, for example a small thick film heater, in close thermal contact with the bimetal and by provision of a variable resistance to control the auxiliary heater output. The metal mounting plate of an X4 control (item 27 in Fig ID of GB-A-2 339 088 for example) could even be provided with a small thick film heater on a part thereof extending under one of the bimetals for this purpose.

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Claims

Claims:

1. In or for an electrically powered water heating vessel, a thick film heating element having a predetermined part thereof adapted to be juxtaposed with the bimetallic actuator of a thermally-responsive control, and means enabling the thermal output of that part of the thick film heating element to be selectively adjusted.

2. A heating element as claimed in claim I wherein said part of the heating element has an auxiliary heater track formed thereon, and said adjustment means is such as to enable the current through the auxiliary heater track to be selectively adjusted.

3. A heating element as claimed in claim 2 wherein said ad ustment i means comprises an adjustable resistance including a resistive track section formed on the thick film heater.

4. A heating element as claimed in claim 3 wherein said resistive track section has an associated plurality of contact portions coupled thereto at spaced apart locations.

5. A heating element as claimed in claim 4 wherein said locations are unequally spaced apart so as to enable the output of the auxiliary heater track to be linearly adjusted.

6. An electrically powered water heating vessel including a thick film heating element, a thermally responsive control for protecting the heating element against overheating, said control including a bimetallic actuator juxtaposed with the thick film heating element, an auxiliary heater associated with said bimetallic actuator, and means enabling the heat output of said auxiliary heater to be adjusted.

7. A water heating vessel as claimed in claim 6 wherein the bimetallic actuator is juxtaposed with a predetermined part of the thick film heating element which is provided with an auxiliary heater track constituting said auxiliary heater.

8. A water heating vessel as claimed in claim 7 wherein said adjustment means comprises an adjustable resistor connected in series with said auxiliary heater track, said adjustable resistor comprising a resistive track section formed on the thick film heating element and means enabling the operative length of said resistive track section to be adjusted.

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9. A water heating vessel as claimed in claim 8 wherein a movable wiper is associated with the resistive track section for enabling the operative length thereof to be adjusted.

10. A water heating vessel as claimed in claim 9 wherein the wiper cooperates with a plurality of contacts coupled to respective locations of said resistive track section.

11. A water heating vessel as claimed in claim 10 wherein said locations are unequally spaced apart whereby adjustment of the wiper linearly adjusts the output of the auxiliary heater track.

12. A water heating vessel as claimed in claim 9 or 10 or I I wherein the wiper is additionally selectively movable to a position whereat it makes no contact with the resistive track thereby disconnecting said auxiliary heater.

13. A water heating vessel as claimed in claim 9 or 10 or 11 wherein the power output of the auxiliary heater track matches that of the main heater track when the maximum length of resistive track section is selected.

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14. An electrically powered water heating vessel including a heating element, a thermally-responsive control associated with the heating element for protecting the same against overheating, an auxiliary heater associated with said control, and means enabling the heat output of said auxiliary heater to be selectively adjusted thereby to adjust the vessel temperature at which said control will operate.

15. A water heating vessel as claimed in claim 14 wherein said control comprises a snap-acting bimetallic element and means are provided enabling the operating characteristics of said bimetallic element to be adjusted.

16. A water heating vessel as claimed in claim 15 wherein the heating element is a thick film heating element, the auxiliary heater comprises a track formed on said thick film heating element, and for adjusting the heat output of said auxiliary heater, a resistive track is provided on the thick film heating element in series with the auxiliary heater and a wiper is associated with said resistive track for adjusting the resistance thereof by adjusting the effective length thereof.

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17. A water heating vessel as claimed in claim 16 wherein the wiper cooperates with a plurality of contacts connected to respective locations of said resistive track section.

18.A water heating vessel as claimed in claim 17 wherein said locations are unequally spaced apart whereby linear adjustment of the wiper linearly adjusts the output of the auxiliary heater.

19. A water heating vessel as claimed in claim 16 or 17 or 18 wherein the power output of the auxiliary heater track matches that of the main heater track when the maximum length of resistive track section is selected.

20. An electrically powered water heating vessel including a thick film heating element having associated therewith a temperature-sensitive control comprising an automatically resetting snap-acting bimetallic actuator in thermal contact with the heating element, an auxiliary heater being associated with the bimetallic actuator for causing the same to snap from its cold to its hot configuration in a relatively short time period, and the remake temperature setting of the bimetallic actuator being adjustable, the arrangement being such as to enable 34 water to be heated in the vessel to the remake temperature of the bimetallic actuator and maintained at that temperature.

21. A water heating vessel as claimed in claim 20 wherein the auxiliary heater comprises a track formed on the thick film heating element.

22. A water heating vessel as claimed in claim 20 or 21 wherein the heat output of the auxiliary heater is arranged to be adjustable.

23. A water heating vessel as claimed in claim 22 as dependent upon claim 22 wherein an adjustable resistance is provided on the thick film heating element in series with the auxiliary heater.

24. A water heating vessel as claimed in claim 23 wherein said adjustable resistance is arranged so that adjustment thereof enables the output of the auxiliary heater to be controlled linearly.

25. A water heating vessel as claimed in claim 23 or 24 wherein the heat output of the auxiliary heater at one adjustment of the adjustable resistance matches that of the main heater track of the thick film heating element.

26. A thick film heating element as claimed in any of claims 2 to 5, or a water heating vessel incorporating such a thick film. heating element, or a water heating vessel as claimed in any of claims 6 to 13, 16 to 19 and 20 to 25, wherein the major part of the thick film. heating element has a PTC characteristic and the auxiliary heater has an NTC characteristic.

27. A thick film heating element as claimed in any of claims 2 to 5 and 26, or a water heating vessel incorporating such a thick film heating element, or a water heating vessel as claimed in any of claims 6 to 13, 16 to 19 and 20 to 26, wherein the heating element fin-ther comprises a "keep warm" track portion.

28. A thick film heating element as claimed in claim 27 wherein means are provided for enabling the power output of said "keep warm" track portion to be adjusted.

29. A thick film heating element as claimed in claim 28 wherein said ad. ustment means comprises an adjustable resistance including a J resistive track section formed on the thick film heater.

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30. A thick film heating element as claimed in claim 29 wherein said resistive track section has an associated plurality of contact portions coupled thereto at spaced apart locations.

31.A thick film heating element as claimed in claim 28 or 29 or 30 wherein the adjustment means of the "keep warm" track portion is adjacent to a corresponding adjustment means for the auxiliary heater track whereby both adjustment means may be adjusted together.

32. A thick film heating element as claimed in claim 31 wherein adjustment of the power output of the auxiliary heater track such as to increase the same is effected commonly with adjustment of the power output of the "keep wann" track portion, and vice versa.

33. A thick film heating element comprising a main heater track portion, an auxiliary heater track portion located at a position whereat a thermal control is to be juxtaposed with the heating element, a "keep warm" track portion, a first adjustable track portion connected to the auxiliary heater track and configured to enable its resistance to be selectively adjusted for selectively adjusting the power output of the auxiliary heater track, and a second adjustable track portion connected to the "keep warm" track portion and configured to enable its 37 resistance to be selectively adjusted for selectively adjusting the power output of the "keep warm" track.

34. A thick film heating element as claimed in claim 33 wherein said first and second adjustable track portions extend alongside one another so as to be adjustable simultaneously by one and the same manual control.

35. A thick film heating element as claimed in claim 34 wherein the first and second track sections are simultaneously adjustable in opposite senses so that as the resistance of one increases, so the resistance of the other reduces.

36. A thick film heating element as claimed in any of claims 33 to 35 in combination with a thermally-responsive control comprising a first bimetallic actuator juxtaposed with the heating element at said position thereof, a second bimetallic actuator juxtaposed with the heating element so as to be responsive to the temperature of said "keep warm" track portion, and adjustment means enabling said first and second adjustable track portions to be manually adjusted.

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37. A liquid heating vessel incorporating a heating element and thennally-responsive control as claimed in claim 36.

38. A liquid heating vessel as claimed in claim 37 farther comprising a boil control responsive to the boiling of liquid in the vessel to switch-off the power supply to the main heater track portion.

39. A thermally responsive control for a water heating vessel, said control including a bimetallic actuator arranged so as in use of the control to be responsive to the temperature of a heating element so as to protect the same against overheating, and wherein the control further comprises a selectively operable member enabling the heating of said bimetallic actuator to be controlled for adjusting the heating element temperature at which the control will operate.

40. A thermally responsive control as claimed in claim 39 which is adapted for use with a thick film heating element having an auxiliary heater located to interact with said bimetallic actuator and wherein said selectively operable member comprises part at least of an adjustable resistor for determining the current in said auxiliary heater.

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41. A thermally responsive control as claimed in claim 40 which is adapted for use with a thick film heating element further having a resistive track coupled in series with the auxiliary heater and wherein said selectively operable member comprises a slider mechanism for co-operation with said resistive track to vary the effective resistance thereof

42. A thermally responsive control as claimed in claim 39 or 40 or 41 wherein the control comprises first and second bimetallic actuators and only one of said actuators is arranged to be effected by said selectively operable member.

43. A thermally responsive control as claimed in claim 39 or 40 or 41 further comprising a fusible component actuator.

44. A thermally responsive control as claimed in any of claims 39 to 43 which is integrated with an appliance inlet connector.

45. A thermally responsive control as claimed in claim 44 wherein the appliance inlet connector is for a cordless appliance and enables the appliance proper to be set onto its base with any relative rotational orientation.

46. A thick film heating element substantially as herein described with reference to any of Figures 1, 3, 5, 6 and 7 of the accompanying drawings.

47. A thermally responsive control substantially as herein described with reference to Figures 4B, 4C and 4D of the accompanying drawings.

48. A thermally responsive control as claimed in claim 47 in combination with a thick film heating element as claimed in claim 46, in or for a water heating vessel.

49. An electrically powered water heating vessel wherein the electrical circuit of the vessel heating element and its associated controls is substantially as herein described with reference to Figure 2 of the accompanying drawings.

50. An electrically powered water heating vessel wherein the electrical circuit of the vessel heating element and its associated controls is substantially as herein described with reference to Figure 8 of the accompanying drawings.