IMPROVEMENTS RELATING TO ELECTRIC HEATING ELEMENTS

Abstract: In order to accommodate different heating tracks with different power densities on a thick film heating element the thick film heating element is formed with a multi-layer structure in which different heating tracks are in different layers separated by electrical insulation.

Description: IMPROVEMENTS RELATING TO ELECTRIC HEATING ELEMENTS Field of the Invention: This invention concerns improvements relating to electric heating elements and more particularly concerns thick film heating elements which comprise a resistive heating track or layer formed on a substrate which commonly comprises stainless steel with an electrically insulating layer between the stainless steel and the resistive heating track or layer, though ceramics material substrates are also known. Such thick film heating elements are nowadays used in liquid heating appliances, such as kettles and hot water jugs for example, and also find wider application. Background of the Invention: It has previously been proposed to provide more than one heating track on a thick film heating element, see for example GB-A-2 343 352, GB-A-2 335 588 and our British Patent Application No. 0013545.9. In such previous applications for thick film elements with multiple tracks, the power density of the additional heater track(s) has always been sufficiently low to allow it to be located on the heating element surface and not occupy too much space. However, where the power required from the additional tracks(s) becomes significant, there may be insufficient space on an element disk to accommodate the required tracks. An example of an appliance where this is the case is one which is intended to heat a range of different liquids, where a high power (2-3kW) is needed for rapid boiling of water, and a lower power (6-700 W), and lower power density, is needed for heating milk, without the milk becoming burnt on the base of the vessel. The low power density of the lower power track means that it cannot be tightly laid into a small area, but may occupy as much space as the high powered track. A large element diameter to give sufficient area is not desirable, since this leads to appliance styling difficulties and high costs. Objects and Summary of the Invention: It is thus the principal object of the present invention to overcome or at least substantially reduce the abovementioned problem. According to the present invention this object can be achieved by a thick film heating element having a multi-layered construction with different heating tracks on different layers which are separated by electrically insulating material. More particularly, in order to overcome the abovementioned problem, it is proposed in accordance with the present invention to print two or more tracks one on top of the other, separated by a thin dielectric layer. With this proposal, which is based on multi-layer thick film electronics technology, the withstand voltage for the intermediate layer does not have to be as great as the basic insulation required between the mean heating element and the steel substrate which has to withstand a high voltage test of 1250N. The intermediate layer would only have to withstand 250N, perhaps with a high voltage test of only 500N. thus the intermediate insulating layer can be thinner than the main insulation, say 60-80 microns compared to 150 microns for example. The increased total insulating thickness between the additional track(s) and the substrate will raise the running temperature of that track, but since we are aiming for a low power density, the resulting temperature rise will be acceptance. The reduced thickness of the intermediate layer will also assist in keeping temperature rises down. The manufacturing process for such a thick film heating element might for example comprise the following steps: 1. Onto a prepared steel substrate print the main dielectric layer and fire; 2. Print the main heater track and fire; 3. Print the intermediate insulating layer and fire; 4. Print the additional heater track and fire; 5. Print silver connection pads and fire; and 6. Print a protective cover coat and fire. The various firing operations may be combined by firing multiple layers at once, as is usual in the manufacture of thick film elements, and operations 5 and 6 may be reversed or combined. The additional track(s) may include other features, such as an adjustable resistance control track as described in our aforementioned British Patent Application No. for example, and tappings from either track may be provided to give access for controls or indicators, similar to that described in our GB-A-2 329 759 or GB-A-2 325 396. Connections to the underlying track could be made through holes in the intermediate insulating layer, and access to the tracks for bimetal or other sensors can also be similarly provided. The above and further features of the present invention are set forth in the appended claims and will be clearly understood from consideration of the following description which is given with reference to the accompanying drawings. Description of the Drawings: Figures 1A to IE illustrate various steps in the manufacture of an exemplary thick film heating element embodying the present invention; and Figure 2 is a perspective cross-sectional view of a thick film heating element manufactured by the steps of Figures 1 A to IE. Detailed Description of the Embodiment: In the following, since the manufacture of thick film heating elements is well known, only brief details of the various manufacturing steps will be described. Referring to the drawings, Figure 1A illustrates the provision of a dielectric layer 1 on one surface of a stainless steel substrate 2. The dielectric material can for example consist of a glass, ceramics or glass ceramics material which is applied onto the surface of the substrate in powder form and is then fired, as is well known. Figure IB shows the provision of a main heater track 3 on top of the dielectric layer 1, for example by means of a printing step employing electrically conductive ink followed by a firing step. Note that the form of the track 3 that is shown in Figure IB is exemplary only. Figure 1C illustrates the provision of an intermediate electrically insulating layer 4 over the track 3, but with connector terminal parts 5 and 6 of the track 3 left exposed in a window opening 7 provided in the layer 4. A second window opening 8 exposes a portion of the main heater track 3 for access by a thermally-responsive control (not shown). Figure ID shows the provision of a further heating element track 9 on top of the intermediate electrically insulating layer 4 and Figure IE shows the provision of a further, protective electrically insulating layer 10 overlying the further heating element track 9, the layer 10 being formed with window openings 11, 12 and 13 which, respectively, register with the window opening 7 in the intermediate insulating layer 4, provide access to the terminal parts 14 and 15 of the heating element track 9, and expose a portion of the additional heating element track 9 for access by a further thermally-responsive control (not shown). At appropriate stages in the manufacturing process as abovedescribed the terminal parts 5, 6, 14 and 15 are provided with low resistance silver coatings which may also be provided, as is well known, at the locations where the heating element tracks 3, 9 turn back upon themselves so as to avoid the phenomenon of current crowding. Figure 2 is a perspective cross-sectional view of a thick film heating element manufactured as described above. It is to be noted that the thicknesses of the various layers are greatly exaggerated for the sake of clarity. Having described the invention in the foregoing in relation to a particular embodiment, it is to be well appreciated that the embodiment is in all respects exemplary and that modifications and variations thereto are possible without departure from the spirit and scope of the invention as set forth in the appended claims.

Claims: Claims: 1. A thick film heating element having a multi-layered construction in which different heating tracks are provided at different layers which are separated by electrically insulating material. 2. A thick film heating element as claimed in claim 1 comprising a metal substrate provided with an electrically insulating layer upon which at lest one heating track is provided, and wherein the thickness of said electrically insulating layer is greater than that of an electrically insulating layer overlying said at least one heating track. 3. A thick film heating element as claimed in claim 1 or 2, wherein window openings are provided in the one or more electrically insulating layers which cover the heating tracks so as to provide access to terminal portions of the tracks. 4. A thick film heating element as claimed in claim 1 or 2 or 3 wherein at least one window opening is provided in one or more of the electrically insulating layers of the element for enabling access to at least one of the heating element tracks by a thermally-responsive control. 5. A thick film heating element as claimed in any preceding claim wherein a main heating track is provided at one layer and an additional heating track is provided at another layer together with a means for adjusting the power output of such additional track. 6. A thick film heating element as claimed in claim 5 wherein said power adjustment means comprises a resistive track coupled to said additional heating track, said resistive track being configured to enable its resistance to be adjusted by operation of a control associated therewith. 7. A thick film heating element substantially as herein described with reference to the accompanying drawings. 8. A liquid heating appliance incorporating a thick film heating element as claimed in any of the preceding claims. 9. A liquid heating appliance as claimed in claim 8 having a high power (eg. 2 to 3kW) heating track for rapid boiling of water and a low power (eg. 6 to 700W) heating element for the heating of milk without the milk becoming burnt on the heating element. 10. A method of manufacturing a thick film heating element comprising: (i) preparing a metal (eg. stainless steel) substrate; (ii) providing an electrically insulating layer on a surface of said substrate; (iii) providing a first heating track on said electrically insulating layer; (iv) providing a second electrically insulating layer overlying said first heating track; (v) providing a second heating track on said second electrically insulating layer; and (vi) providing a third, protective electrically insulating layer overlying said second heating track; wherein access openings are provided in said second and third electrically insulating layers which enable terminal portions of said first and second heating tracks to be accessed for supplying electricity to the tracks.

Inventors: MOORE ROBIN KEITH, SMITH DAVID ANDREW, MOORE, ROBIN, KEITH, SMITH, DAVID, ANDREW
Applicants: MOORE ROBIN KEITH, OTTER CONTROLS LTD, SMITH DAVID ANDREW, MOORE, ROBIN, KEITH, OTTER CONTROLS LIMITED, SMITH, DAVID, ANDREW
Application year: 2001
Main classification: H05B3
IPC codes: H05B3