EP0855131A1

EP0855131A1 - Electric heaters

Info

Publication number: EP0855131A1
Application number: EP96933551A
Authority: EP
Inventors: John Crawshaw Cronk My Chree Arbory Road TAYLOR; Keith Barrie Radnor House DOYLE
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1995-10-11
Filing date: 1996-10-11
Publication date: 1998-07-29
Also published as: CN1135060C; GB2321579B; GB2321579A; WO1997014269A1; CN1203731A; GB9807806D0

Abstract

The desired power rating of a thick film heater is obtained by making electrical connections to the heater track at positions which define the appropriate resistance therebetween.

Description

Electric Heaters

This invention relates to electric heaters, in particular but not exclusively to heaters of the plate element type for use in domestic appliances, such as water heating vessels, water heaters and irons.

A plate heater comprises an insulated substrate on which a resistive heater track is provided. Typically a glass, ceramic or glass ceramic insulating layer is provided on a metal base such has a metal plate, and the track laid down on the insulating layer. This may be achieved by bonding thin resistive material to the substrate or, more usually, by using printed circuit techniques in which a resistive ink is laid down on the substrate or by metal spraying for example. The result of either of these techniques is a heater in planar form which may conveniently form, be formed on, or bonded to the base of a heating vessel.

Resistive heating tracks may also be laid down on non-planar substrates such as tubular or other three dimensional objects.

EP-A-0574310 discloses one example of a water heating vessel having a plastics body with a printed- circuit type plate heater forming its base. Electrical connection is made at each end of the spiral track, by wires which are bonded to the track and which are connected to the electrical supply. It is, however, very difficult to produce printed circuit heaters with a consistent resistance and consequently a consistent power output . Variations in the printed track thickness, in the nominal resistivity of the ink and in the processing times and temperatures lead to variations in the final track resistance and thus the wattage of the heaters. In practice manufacturers do well to maintain wattages within ±15% of the nominal wattage. However, International Electrical Standards require the wattage to be within +5% to -10% of the nominal value. Clearly if out of tolerance parts are merely rejected, as is done by some manufacturers, the scrap rate will be high, possibly 25%, which iε wasteful and expensive.

Variouε attempts have been made to overcome this problem. Manufacturing tolerances may be improved by using expensive materials and clean room facilities. This however is expensive. Alternatively, a track may be 'trimmed' using a laser to increase its resistance. However, for a track of a size required for say a 2KW heating effect, this would be both difficult and expensive . The present invention seeks to overcome these problems, and from a first aspect provides a method of manufacturing an electrical heating assembly compriεing a heater of the type comprising a resistive heating track laid down on an insulating substrate, the heater having a predetermined resistance corresponding to a desired heater wattage, the method comprising: a) laying down on said substrate a resistive heating track having a resistance greater than the predetermined resistance,- b) determining locations on said track at which electrical connections may be made thereto to give the predetermined resistance therebetween; and c) making electrical connections to the track at said locations . Thus in accordance with the invention, the resistive heating track is intentionally made to have a higher resistance than the desired resistance (say 5% higher at the worst tolerance condition) , and electrical connections are then made to the track at selected positions to give the desired track resistance between the electrical connections .

With such a method, scrap can be substantially avoided and, at the same time, a desired track resistance obtained without the need for expensive manufacturing facilities or materials

The predetermined resistance value need not be the one needed to provide the "nominal" rating for the heater but may be chosen, for example, anywhere within the internationally required tolerances For example it may be desirably to choose a resistance which produces a heater power within the -10% section of the tolerance range, so as to reduce the power density of the heater, and thus prolong heater life

In practical terms, the track is made longer than a length required to provide the predetermined nominal track resistance and the electrical connections made inwardly from at least one end thereof to give the desired track resistance.

From a second broad aspect, therefore, the invention provides an electrical heater assembly comprising a heater of the type having a resistive heating track provided on an insulating substrate, the heater track having an electrical supply connection made thereto at a location spaced from an end thereof

The positions at which electrical connections are to be made to the track may be determined in a number of ways In a first method, the overall resistance of the track may be measured, and from that, knowing the resistivity of the track material, calculating the amount by which the track should be shortened to provide the desired resistance Alternatively, the required amount of shortening necessary can be pre-calculated for a range of measured resistances, and the figure taken from a table or the like In a further alternative method, the resistance between pairs of selected positions on the track may be measured directly until the desired resistance is measured between those positions

One of the connections of each pair may be in a fixed position, for example at one end of the track Furthermore one of the connections may be made to the track before determining the required position of the other. The track may be provided with a number of discrete connection locations, for example one or more contact pads, at which the electrical connection may be made to t.

Once the desired connection positions are determined, electrical connections may be made to the track at these positions It may be the case that these electrical connections will not be made at the time of manufacture of the heater itself, but subsequently, for example during assembly of a heating vessel to which the heater is mounted. In such circumstances, the heater may be provided with means indicative of the positions at which electrical connections should subsequently be made. For example, markers may be provided at the relevant positions on the track. From a further broad aspect, therefore, the invention also provides a method of manufacturing an electrical heater of the type comprising a resistive heating track laid down on an insulating substrate, the heater having a predetermined resistance corresponding to a desired heater wattage, the method comprising- a) laying down on sa d substrate a resistive heating track having a resistance greater than the predetermined resistance,- b) determining locations on said track at which electrical connections may be made thereto to give the predetermined resistance therebetween; and c) marking said heater to indicate where said connections should subsequently be made to the track. The heater track may be of the type that requires a protective overglaze If so, this may be applied before or after the desired electrical connection positions are established. In a first method, the connection positions may first be established, and the overglaze then applied with a window or windows at the appropriate positions to allow access to the track therebelow. In a second method, an overglaze may be provided with a number of windows allowing access to the track therebelow, and the desired connection positions then determined by selecting those locations which give the desired resistance within a suitable tolerance. Preferably, however, the track is of a material that requires no protective overglaze. This reduces manufacture costs by obviating the need for an overglazing step, and allows easier access to the track. The track may also be such as to require contacts to be formed thereon to allow an electrical connection to be made thereto. If so, suitable contact pads may be provided at the desired positions on the track, for example provided in the windows of the protective glaze. Preferably, however, the track is such as to permit a direct electrical connection to be made thereto. This avoids the need for, and thus the cost of, providing separate contacts on the track.

Most preferably, therefore, the track is produced by spraying or printing from a printable or sprayable ink that does not require an overglaze comprising pure nickel or a high nickel alloy.

In the preferred embodiment, therefore, the track is substantially exposed, with no overglaze and is such that an electrical connection may be made to it without the need to provide a separate contact on the track.

The electrical connection made to the track may be a soldered or brazed connection, but preferably, a contact is urged against the connection location on the track to make an electrical connection. Preferably therefore the contact is a resilient contact. Such contacts may conveniently be mounted on a contact carrier which may form part of a control for the heater, as will be discussed further below. Most preferably the contact is in the form of a knife edge to provide a good line of contact with the track.

The shape of the track may be chosen to suit a particular application. Generally, however, printed or sprayed circuit heaters are sensitive to sharp bends, with current density increasing around the inside of a bend, leading to potential hot spots which could damage the heater. Sharp bends should, generally therefore be avoided, and a spiral track shape for example a generally circular spiral shape is therefore preferred. A single spiral has the advantage that the voltage drop between adjacent turns is relatively low, so that they may be arranged closely together without danger of arcing between the turns.

In applications, as mentioned above, where electrical connections are made to the heater track through carrier-mounted contacts, the positions of the contacts relative to one another will be fixed for ease of production. Preferably therefore, the track should be able to accommodate such contacts in a number of different positions with a constant spacing therebetween. This may most easily be effected by having parallel lengths of track to which the respective electrical connections can be made.

Thus, in the embodiments described above the track may be provided with inner and outer parallel arcuate portions, most preferably parallel circular portions, extending around at least a portion and preferably a substantial portion thereof. Circular portions are particularly preferred irrespective of the track configuration therebetween, since the heater and contacts may be positioned in a desired relative rotational orientation to give a desired heater resistance. It will be appreciated that the inner portion could equally be in the form of a disc or point on the axis of the outer track portion. When a heater is to be mounted, say in the base of a water heating vessel, the position of the connections to be made to the heater track may be fixed relative to the vessel . Accordingly the mounted position of the heater in the vessel will have to be such that when the assembly of the vessel is complete, the connections to the track will be in the correct position to give the desired wattage for the heater. This may be facilitated by, providing an indicium on the heater for alignment with a corresponding datum on the vessel to ensure the correct position of the heater relative to the contacts in mounting, rather than indicating on the heater the positions at which the electrical connections.

From a further broad aspect, therefore, the invention provides a method of manufacturing a liquid heating vessel incorporating an electrical heater of the - type comprising a resistive heating track laid down on an insulating substrate, the heater having a predetermined resistance corresponding to a desired heater wattage, and a set of electrical contacts for making electrical connection to said track, said contacts being arranged in a predetermined position with respect to said vessel, the method comprising: a) laying down on said substrate a resistive heating track having a resistance greater than the predetermined resistance,- b) determining the position relative to the vessel in which the heater should be mounted so that, when mounted, the contacts of the vessel will form electrical connections with the track to provide the predetermined resistance therebetween,- c) providing an indicium on said heater indicative of the said position, for alignment with a datum on the vessel such that on mounting said heater and said contacts in said vessel, electrical connections will be made to the track at the desired positions,- d) mounting said heater in said vessel with said indicium aligned with said datum,- and e) mounting said electrical contacts to said heater. The determination of the alignment position could be effected in a number of ways. For example, the overall resistance of the track could be measured, and the position for the indicium taken from tables showing say the required angular or linear displacement from the end of the track for different measured resistances of the track. These figures can be calculated in advance, knowing the nominal resistivity per unit length of the track material, or more preferably from actual measurements made .

Preferably, as stated above, the contacts are provided on a control unit of the vessel which is mounted in the base of the vessel. This control may, for example, provide an overheat protection function for the element, and could, for example, comprise a bimetallic actuator which, when the control is mounted in position, is arranged in thermal contact with the heater. However, the control may be an electronic control, which leads to a further preferred feature of the invention.

In known electronic controls for liquid heating vessels, the electronic components require a drive voltage lower than that of the mains voltage, typically 50V on a 240V supply. Up to now, this has been provided from a suitable take off across the mains supply. However, in a preferred feature of the present invention, the voltage may be provided by making an electrical connection to the heating track itself. Preferably therefore, the heating track has at least one connection position intermediate the respective supply connection positions, to allow an intermediate voltage to be 'picked off¹ the track.

The concept of providing a drive voltage for the control of a plate-type heater is novel in itself and from a yet further broad aspect, therefore the invention provides, in combination, an electric heater of the type comprising a resistive heating track laid down on an insulating substrate, and an electronic control therefor, with an electrical connection made between said track and said control at position chosen to provide a desired drive voltage for the control .

From another aspect the invention provides an electrical heater comprising a resistive heating track laid down on an insulating substrate, the track having an electrical connection location provided intermediate its ends to permit a voltage lower than that applied to the track to be picked off the track.

The position at which this connection is made may be determined empirically. For example if a voltage of 50V is required from a 240V supply, the connection -should be made approximately 1/5 along the length of the heater track.

In the heater track arrangements described above with parallel track portions for making connection to a power supply, if the intermediate connection position on the track is fixed, the voltage picked off at that point may vary from heater to another heater if the power connection points change, since the relative track lengths on either side of the intermediate point will vary. Accordingly, the track may have a further connection portion parallel to its power connection portions. For example a spiral track may have an additional circular arcuate portion intermediate circular arcuate end positions. Such parallel track arrangements are particularly advantageous in applications where the power supply connections and the intermediate voltage connections are in a predetermined spatial relationship to one another, for example, by being mounted on a common carrier such as a control housing. In such arrangements, as the relative positions of the track and power supply connections are changed to give a desired track resistance therebetween, the position of the intermediate connection will also change, to some extent compensating for the change in the ratio of the track lengths on either side thereof. It will be apparent that the invention also extends in broad terms to heaters suitable for use in the present invention. From a yet further broad aspect, therefore, the invention provides a heater of the type comprising a resistive heating track laid down on an insulating substrate, said track having electrical contact means spaced from at least one end thereof to which an electrical power connection may be made.

The contact means is preferably a portion of the track itself although it could equally be a separate electrical contact provided thereon or connected thereto.

Preferably the track has a first exposed portion for connection of a first electrical power contact thereto and a second exposed portion, parallel to said first portion for connection to second electrical power contact thereto.

Most preferably the first portion and second portions are concentric circular arcs . The inner portion may for example be provided near the centre of the heater track. However in another embodiment, rather than having a track in the form of say a single spiral, the track may be formed to double back on itself, so that the inner track is arranged adjacent the outer track portion, and runs in the opposite direction thereto. Such an arrangement has the advantage that a much greater range of reεistance variation can be achieved, if necessary, since both connection portions have relatively large radii. A possible disadvantage however is that the turns of the tracks must be kept further apart since a greater voltage potential exists between adjacent turns.

If necessary, a plurality of parallel track portions may be provided at one end of the track so as to give an even greater scope for resistance variation, one of the electrical connections being made selectively with one or other of the track portions. Further, the track may comprise a third exposed portion, parallel to and intermediate said first and second portions to which may be made a further electrical connection to pick off from the track a lower voltage than the supply voltage. In the most preferred embodiment the track may therefore comprise three concentric circular arcuate portions .

Preferably the heater substrate is in the form of a plate, most preferably a metal plate having an insulating layer provided thereon. The resistive track and the various connection positions thereof may be formed of the same resistive - material and/or have the same resistivity. However, it would be possible to produce the connection locations or region of the track from a different material and/or resistivity and/or width from the rest of the track by, for example, a multiple printing process in which different resistive materials or thicknesses of resistive material are laid down successively. In particular the material of the connection location or region may be of a material to which electrical connection may more easily be made, for example one which is less prone to oxidation.

Furthermore, whilst it is preferred to have the heater track of the invention arranged substantially in the same plane, it would be possible to provide it in a plurality of planes. For example, it would be possible to have a first portion of the track overlaid with a glaze or the like and a second portion, containing connection locations provided on top of that glaze, with electrical connection between the two portions of the track.

It will be appreciated that the concept of providing a heater track with an intermediate contact location will have wider application than in just, say, electronic controls. The intermediate location may be used not just, therefore, to 'pick off' a voltage from the track.

For example, controls may be applied across one or both portions of the heating track which may thereby be selectively energised. The different sections of the track may, therefore have different resistances or heating effects and could be used, for example, in an airpot or the like to provide a liquid boiling or simmering effect.

In very broad terms, therefore, the invention also extends to an electrical heater comprising a resistive heating track laid down on an insulating substrate, the track having an electrical contact location provided intermediate its ends.

The invention also extends to a water heating vessel incorporating a heater or heater assembly according to the invention, and a method of making such a vessel incorporating the step of producing a heater assembly according to the method of the invention

It was mentioned above that regions of the track in the vicinity of the connection locations may be made of a material having a different resistance to the rest of the resistive heating track. In another aspect of the invention, it has been found that it may be advantageous to form a section of track between an electrical location portion and the heating track proper of a low resistance material, such as silver. This has the advantage that that portion of the track will not produce a substantial amount of heat which is advantageous, in that it reduces the amount of heat flowing into the connection location and thus into a contact made with that region. This is particularly so in overheat conditions, where repeated heating of contacts may lead to annealing of say leaf springs, leading to loss of contact pressure, which may in turn result in further resistance heating at the contact locations .

From a yet further aspect, therefore, the invention provides an electrical heater of the type comprising a resistive heating track provided on an insulating substrate, said heater having at least one electrical connection location which is spaced from the heating track proper by a length of material having a lower resistance, preferably a substantially lower resistance than the heating track material.

Preferably the low resistance material is silver or has a high silver content. This will provide excellent electrical connection to the heating track proper, but will also produce very little heating effect.

The connection location is preferably formed as a contact pad, for example of silver, which again provides for an excellent electrical connection to be made to the heater. The low resistance material may partially or wholly overlie or underlie the heating track. Preferably, however, it overlaps with an end portion of the heating track.

The length of low resistance material will be determined on a case by case basis to provide sufficient heat insulation for a contact. However, preferably the length of said lower resistance material is at least 8mm.

In the preferred embodiment, the low resistance material is applied using conventional track application techniques such as silk screen printing. Accordingly, the material may be applied in the form of a silver or high silver content ink.

This aspect of the invention also extends to the combination of a heater with a resilient contact in contact with said connection location, most preferably a contact formed by or provided on a leaf spring. As stated above, the possible problems of loss of contact pressure, and indeed transmission of heat to other parts of an appliance can be mitigated by the invention.

Some embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:-

Figure 1 is a perspective view of a heater according to the invention;

Figure 2 shows the track profile of the heater of Fig. 1;

Figure 3 shows the heater of Fig. 1 positioned on the base of a liquid heating vessel ,-

Figure 4 shows a control for the heater positioned thereover; Figure 5 shows the control of Fig. 4 mounted to the heater;

Figure 6 is a plan view of a further form of heater track suitable for use in the invention,-

Figure 7 is a further embodiment of the invention,- and

Figure 8 shows a yet further embodiment of the invention.

With reference to Figures 1 and 2, a thick film plate heater 2 for mounting in or on, or forming the base of, a water heating vessel is illustrated. It comprises a stainless steel plate 4 of diameter 81 mm and thickness about 0.5 mm, having an electrically insulating ceramic layer 6 on a surface thereof, on top of which layer is printed, in a known manner, a thick film resistive heater track 8 of pure nickel or a high nickel alloy which is exposed over its length, having no protective overglaze.

The track 8 is about 2.4 mm wide and comprises an outer circular end portion 10 of a constant mean radius R_x of about 34.5 mm extending substantially completely around the periphery of the plate 4 and an inner circular end portion 12 of a constant mean radius R₂ 5.25 mm extending approximately 300° around the axis of the plate and concentric to the outer end portion 10. The track 8 also has an intermediate concentric circular portion 14 of constant mean radius R₃ of about 16.75 mm. The circular portions 10, 12, 14 are joined by respective inner and outer spiral portions 16, 18. The transitions between the respective spiral and circular portions are smooth curves, so as to prevent current concentrations, and thus hot spots, at areas of tight curvature. The spacing between adjacent turns of the spiral track portions is about 0.8 to 1 0 mm. A mounting stud 20 is mounted centrally of the plate 4.

The plate 4 is intended to be mounted on, in or as, the base 24 of a liquid heating vessel 26, as shown schematically m Fig. 3. Furthermore, electrical connections are made to the track 8 at three locations, 9, 11, 13 one in each of the respective circular portions 10, 12, 14 of the track, by respective resiliently mounted electrical contact blades 30, 32, 34 projecting from the body 36 of an electronic control unit for the heater, (as shown schematically in Fig. 3) . As shown in Fig 4, the control body is mounted 36 is mounted over the stud 20.

The control body 36 s mounted m a predetermined angular orientation in the vessel base relative to a datum 38 shown schematically in Fig. 3. Contacts 30 and 32, are connected to line and neutral power supply terminals mounted in the control body 36 for providing a power supply to the track 8, while contact 34 is connected to electronic components of the control to provide a drive voltage therefor from the track 8.

The track 8 is manufactured with an overall resistance, measured between its ends, greater than the resistance required to give the desired wattage of the heater Accordingly the electrical supply connections must be made to the inner and outer track portions 12, 10 by the contacts 32, 30 of the control body 36 in positions so as to define a length of track therebetween with a predetermined resistance corresponding to a desired wattage of the heater

Since the control body 36 is mounted in a fixed position relative to the vessel base, when the vessel is being assembled, the heater 2 must be mounted in a particular angular orientation in the vessel base to ensure that the contacts 30,32 engage the track 8 in the correct positions when the control body 38 is mounted in the vessel base 24 To facilitate this, the heater is marked with an indicium 42 intended to be aligned with a datum 38 in the vessel

One way of establishing the position for the indicium would be to measure the overall resistance of the track 8, and calculating the position from this by knowing the length of the track, the resistivity of the track material and the position relative to the datum 42 that electrical connections will be made to the track in the vessel For example, it might be calculated that the track would have to be shortened by a given figure to obtain its desired resistance, and the position of the indicium then displaced from an end connection point by a predetermined amount to allow for the relative position of the connection position relative to the vessel when the heater is installed These figures could all be precalculated and tabulated so that the appropriate position could merely be read off the table or could be calculated for each element during final manufacture and testing An alternative way of positioning the indicium on the heater would be to measure the resistance between a pa r of probes having the same relative positions as the contacts 30, 32 and moving the probes together around the axis of the track 8 until the desired resistance is measured Once this position has been reached, the indicium 42 may be made on the heater 2 with the same orientation relative to the determined connection positions as the orientation of the contacts 30,32 relative to the vessel datum 38. For example if the contact 30 is arranged 10° offset from the datum 38, then the marking 42 on the heater would be provided 10° offset from the corresponding connection position on the outer track 10.

Then, having mounted the heater to the vessel base 26 with the indicium 42 aligned with the datum 38 and the control 36 subsequently mounted to the vessel base, the contacts 30, 32 should make electrical connection with the track 8 in the correct positions to give the predetermined track resistance.

An electrical connection is also made to the intermediate track portion 16 by the contact 34. This connection allows a voltage lower than the supply voltage to be picked off the track to drive electronic _^components of the control.

A satisfactory connection to the intermediate track portion 14 will be made substantially irrespective of the relative angular positions of the control body 36 and the heater track 8 because of the concentricity of the intermediate and end track portions.

The intermediate contact 34 is arranged such that in a reference position, the voltage picked off is a desired value.

In the embodiment shown, the voltage picked off at the intermediate connection is intended to be about 1/5 the supply voltage i.e. around 50V for a 240V supply voltage. Accordingly, the intermediate connection to the track should be made approximately 1/5 along the track length. The precise voltage picked off by the contact 34 will vary from slightly, depending on the relative angular positions of the control body 36 and the track 8, as the relative lengths of track between the intermediate connection and the end connections will vary. However the variation should not be sufficiently large to adversely affect the control components . In the embodiment described above, the amount of resistance variation achievable is approximately the resistance of the outside circular track portion 10 Indeed the approximate maximum and minimum track areas (and thus resistances) which can be defined between the contacts are about 2500 and 2100 mm² respectively, giving a total possible variation in areas of about 400 mm², or about 19% of the minimum area of resistance A greater variation could be achieved using a heater track configuration as shown in Fig 6 In this embodiment, the track 50 is in the form of a double spiral and has an outermost, substantially circular track end portion 52 and a further substantially circular track end portion 54 arranged inwardly adjacent thereto Respective electrical connections (not shown) are made to these respective end portions 52,54, as in the earlier embodiment The advantage of this arrangement is that a greater range of adjustment can be achieved, since the inner circular track end portion 54 is of substantially greater length than in the earlier embodiment, and also since the respective end portions 51,54 run in opposite directions relative to one another Thus the variation in resistance possible with such a track shape is equal to the sum of the lengths of the track portions 52,54

If the track is to be used with an electronic control as described above, an intermediate circular track portion may be provided in the appropriate position In this embodiment, the spacing between adjacent turns of the track must be kept sufficiently large to prevent electrical breakdown, since, for example the tracks 52 and 54 will be at a full mains voltage potential relative to one another A spacing of 2mm may therefore be typical

A further embodiment of the invention is described in Fig 7 In this embodiment, a heating track 60 comprises an inner portion 62 of relatively low resistance, about 14Ω and an outer portion 64 of relatively high resistance, about 190Ω. This track is laid down e.g. printed on an insulating substrate, which is then overlaid e.g. overprinted with an outer contact ring 66 connected to the outer portion of the track, an inner contact ring 68 connected to the inner portion of the track and a number of contact pads 70 arranged on an intermediate portion 72 of the track 60. These contact rings and pads are of a material which will allow a good electrical contact to be made to the track. They do not need to provide a heating effec . A protective overglaze is provided over the heater track 60, but this is provided with windows in registry with the contact pads 70, and does not extend inwardly to the inner contact ring 68 or outwardly to the outer contact ring 66. Thus electrical contact, for whatever purpose can be made to the track via the exposed inner and outer rings 66,68 and the contact pads 70. An application of such a heater would be in an airpot or the like where different heating effects are required first to boil and then to simmer liquid in a vessel .

Figure 8 shows a portion of a heater assembly according to a further embodiment of the invention. A control 74 of the heater assembly is electrically connected to an electric heater 76 via silver-plated beryllium-copper leaf spring contacts 78, having a section of 4.0 mm by 0.2 mm. In operation of the heater assembly, the control supplies electrical power to a palladium-silver heating track 80 of the electric heater 76. Each end of the heating track 80 terminates at a junction 82 with a silver "tail" 84. The silver tails 84 are thin layers of silver formed by screen printing using a silver or high silver content ink. The silver tails 84 may be applied to the heater substrate during manufacture either before or after the resistive heating track 80 is applied thereto In either case the heating track 80 and the silver tails 84 overlap at the junction 82 such that good electrical contact is made between them. The silver tail is overglazed with a layer of glass, glass ceramic, or ceramic, but at one end of the tail furthest from the junction 82 a window in the overglaze exposes a silver contact pad 86 having a diameter of 8mm The leaf springs 78 contact the contact pads 86 with a contact pressure of approximately 150gm (and not less than lOOgτn) and thereby make electrical contact with the heating track 80 via the silver tails 84

It will be seen that the resistive heating track 80 is spaced from the leaf spring contact 78 by the silver tail 84, which has a length of 10 mm from the edge of the contact pad 86 to the junction 82 Thus, there is no direct heating of the contact 78 by the heating track 80 The low resistance of the silver tail prevents resistive heating in the region of the contact pad, thereby reducing the temperature in that region, particularly in an overheat condition

With the arrangement shown in Figure 8, the maximum temperature attained by the leaf spring 78 during a 2 2 kW dry switch on cycle time of 7 seconds was 150°C After 50 hours of such cycling, as required by mandatory tests, there will be an approximately 15% reduction in the contact pressure of the leaf spring 78 due to heat annealing The contact resistance between the leaf spring 78 and the contact pad 86 will be increased thereby to a value not greater than lOmΩ, an acceptable rise above the initial value of 7mΩ In this case, the power dissipated at the contact will be less than IW for a 2 2kW, 9A heater

Variations of the above embodiment can be made within the scope of the invention For example, the track material 80 may extend under or over the silver tail to the contact pad 86, but effectively be short circuited by it.

Whilst in the embodiments above, the substrate has been shown as a flat plate, it may be provided with a circumferential groove for mounting the plate in or to the base of the vessel. In such arrangements, a plastics vessel side wall or base may have a downwardly depending section for receiving and mounting the channel of the heater. Such arrangements are described in our PCT application WO 96/18331. The invention is not limited to plate heaters, but may equally apply to heaters in which the substrate is tubular, polygonal in section, or three dimensional in configuration.

Furthermore, whilst the particular embodiments describe generally circular spirals, other track shapes can also be used for example triangular or rectangular or other polygonal spirals having a series of spaced turns of decreasing dimensions. For example if a track is being applied to a rectangular plate, the track turns may be generally rectangular to fill the edges of the plate.

The term "spiral" is thus meant to be interpreted broadly and not limited to just curved spirals.

Moreover, the invention in its broadest terms is not limited to the use of curved heating tracks, but any track shape, even a straight track. What is important is being able to make electrical connections to the track at desired locations in order to obtain a predetermined track resistance. It will thus be seen that in very broad terms, the invention provides a method of providing an electric heater of the type comprising a resistive heating track provided on an insulating substrate with a desired power output, comprising making electrical power connections to the track in positionε which define an appropriate electrical resistance therebetween.

Finally, whilst the invention has been disclosed in its preferred embodiment as being applied to a water heating vessel, it may also be applied for example to other water heaters, for example in dishwashers or immersion heaters, or other appliances such as irons.

Claims

l ims

1 A method of manufacturing an electrical heating assembly comprising a heater of the type comprising a resistive heating track laid down on an insulating substrate, the heater having a desired predetermined resistance corresponding to a desired heater wattage, the method comprising a) laying down on said substrate a resistive heating track having a resistance greater than the predetermined resistance, b) determining locations on said track at which electrical connections may be made thereto to give the predetermined resistance therebetween, and c) making electrical connections to the track at said locations

2 A method as claimed in claim 1 wherein the heater is provided with means indicative of the positions at which electrical connections should subsequently be made

3 A method of manufacturing an electrical heater of the type comprising a resistive heating track laid down on an insulating substrate, the heater having a predetermined resiεtance corresponding to a desired heater wattage, the method comprising a) laying down on said subεtrate a resistive heating track having a resistance greater than the predetermined resistance, b) determining locations on said track at which electrical connections may be made thereto to give the predetermined resistance therebetween, and c) marking said heater to indicate where said connections should subsequently be made to the track

4. A method as claimed in claim 2 or 3 modified by marking said heater with means for alignment with an indicium on a liquid heating vessel, the vessel indicium being arranged such that when the marking on the heater is aligned therewith, electrical connections within the vessel will be made to the heater in the desired position.

5. A method of manufacturing a liquid heating vessel incorporating an electrical heater of the type comprising a resistive heating track laid down on an insulating substrate, the heater having a predetermined resistance corresponding to a desired heater wattage, and a set of electrical contacts for making electrical connection to said track, said contacts being arranged in a predetermined position with respect to said vessel, the method comprising: a) laying down on said substrate a resistive heating track having a resistance greater than the predetermined resistance,- b) determining the position relative to the vessel in which the heater should be mounted so that, when mounted, the contacts of the vessel will form electrical connections with the track to provide the predetermined resistance therebetween; c) providing an indicium on said heater indicative of the said position, for alignment with a datum on the vessel such that on mounting said heater and said contacts in said vessel, electrical connections will be made to the track at the desired positions,- d) mounting said heater in said vessel with said indicium aligned with said datum,- and e) mounting said electrical contacts to said heater.

6. A method as claimed in any preceding claim wherein the step of determining the connection locations comprises measuring the overall resistance of the track.

7. A method as claimed in any of claims 1 to 5 wherein the step of determining the connection locations comprises measuring the resistance between pairs of selected track positions.

8. An electrical heater of the type comprising a resistive heating track provided on an insulating substrate, the heater track having a location spaced from an end thereof to which an electrical supply connection may be or is made.

9. A method or heater as claimed in any preceding claim wherein the track is provided with a number of discrete connection locations, at which the electrical connection may be made to it.

10. A method or heater as claimed in claim 9 wherein contact pads are provided at said discrete locations.

11. A method or heater as claimed in claim 9 or 10 wherein said track is provided with a protective overglaze, and windows are provided in the overglaze at the connection locations.

12. A method or heater as claimed in any of claims 1 to 8 wherein said track is unglazed.

13. A method or heater as claimed in any preceding claim wherein the track is configured such as to allow electrical contacts having a constant predetermined spacing therebetween to make electrical connection to the track in a number of different positions .

14. A method or heater as claimed in claim 13 wherein the track has parallel lengths of track to which the respective electrical connections can be made

15 A method or heater as claimed in claim 14 wherein said parallel portions are substantially circular.

16 A method or heater as claimed in any preceding claim wherein a further electrical connection location is provided intermediate the ends of the track

17 An electrical heater comprising a resistive heating track laid down on an insulating substrate, the track having an intermediate electrical connection location provided between its ends

18 In combination, an electric heater of the type comprising a resistive heating track laid down on an insulating substrate, and an electronic control therefor, with an electrical connection made between said track and said control at an intermediate position chosen to provide a desired drive voltage for the control

19 A method or apparatus as claimed m any of claims 16 to 18 wherein said track comprises a pair of parallel portions to which electrical connection can be made and a further portion parallel to said pair of parallel portions at which said intermediate connection may be made

20 A water heating vessel comprising a heater as claimed m any of claims 8 to 19

21 A method of providing an electric heater of the type comprising a resistive heating track provided on an insulating substrate with a deεired power output, comprising making electrical power connections to the track m positions which define an appropriate electrical resistance therebetween.

22. An electrical heater of the type comprising a resistive heating track provided on an insulating substrate, said heater having at least one electrical connection location which is spaced from the heating track proper by a length of material having a substantially lower resistance than the heating track material .

23. A heater as claimed in claim 22 wherein said lower resistance material is silver or has a high silver content .

24. A heater as claimed in claim 22 or 23 wherein the length of said lower resistance material is at least 8mm.

25. A heater assembly comprising a heater as claimed in any of claims 22 to 24 and a resilient contact in contact with said connection location.

26. A heater assembly as claimed in any of claims 25 wherein said contact is formed by or provided on a leaf spring.

27. A heater assembly as claimed in claims 25 or 26 wherein said contact is a power supply contact .

28. A heater or assembly as claimed in any of claims 22 to 27 wherein said lower resistance material is applied as an electrically conductive ink.