WO2000065877A1

WO2000065877A1 - Safe thin film heater

Info

Publication number: WO2000065877A1
Application number: PCT/SE2000/000766
Authority: WO
Inventors: Antoine Dubedout
Original assignee: Aktiebolaget Electrolux
Priority date: 1999-04-21
Filing date: 2000-04-20
Publication date: 2000-11-02
Also published as: AU4445900A; DK199900542A

Abstract

Thin film heaters on a vitroceramic substrate are a desirable solution to provide an electric cooktop. However, the substrate becomes conductive at the high temperatures in use and even the upper surface, which is separated from the thin film heater by the substrate, may in use carry dangerous voltage. According to the invention the heater is subdivided into smaller areas which are supplied with lower and non-dangerous voltages.

Description

Safe thin film heater

The present invention relates to a thin film heater for cooking purposes comprising a conductive deposit on an insulating substrate.

Such heaters are well known in the art, ranging from windows and windscreens being heated in order to prevent condensation to heaters for cooking purposes. The latter have a power loading which is four times that of the power used for windows. This means that the substrate becomes very hot and some low-expansion glass ceramics loose their ability to insulate electrically. Although the heater is disposed on the lower side of the substrate, the high voltage terminal and the part of the resistive element, which has a voltage which is above the safety limit, will provide a voltage also on the top surface which supports cooking utensils. While in operation the top surface is so hot that touching it is most likely to cause burns, such touching will be avoided but metal cooking utensils will be touched and they may carry dangereous voltages. This means that heaters for use with a high voltage, such as the mains voltage which is in the range 110 V to 230 V, have to be provided with electrically insulating separating layers between the conductive deposit and the glass ceramic support for heaters on the lower side and cooking utensils on the upper side. This is a feature which is both complex during manufacture and which puts certain limits on the appearance of the cooktop.

It is a purpose of the invention to provide a cooktop which uses a high power density thin film as the heating element on a substrate of the kind commonly used for cooktops which does not display the above disadvantages and which is hence inherently safe. This is obtained according to the invention in that the area to be heated is subdivided into smaller areas, each supplied individually with a voltage below a pre-defined safety limit.

This construction has the further advantage that it becomes possible to control individual small areas in dependence of the thermal load constituted by a cooking utensil.

In an advantageous embodiment sets of neighbouring smaller areas share a busbar at a relatively low potential on one side and a busbar at a relatively higher potential on the other side. This enables a surface to be subdivided into stripes across the width of each there is only a relatively low voltage which is below any voltage which might provide a risk of an electric shock.

In a further advantageous embodiment all the busbars at a relatively low potential are connected to one and the same first main busbar, and all the busbars at the relatively higher potential are connected to one and the same second main busbar, said main busbars being supplied with a voltage which is below the predefined safety limit. In this manner there is obtained a subdivision of the area to be heated which needs only low voltage power supply and which will never exceed the safety limit, even when the vitroceramic material becomes conductive.

The invention will now be described more in detail with reference to the enclosed drawing, in which Fig. 1 shows an embodiment of the invention seen from below, and

Fig. 2 shows the same embodiment in a side view. In Fig. 1 a substrate 1 is shown provided with a thin film conducting deposit 2 on the lower side 3. On top of the deposit 2, i.e. in the downwards direction, stripes 3a, 3b, 3c, 3d, 3e of higher conductive material has been deposited, e.g. in the form of thick film. The stripes 3a-3e constitute busbars which are alternatingly connected to main busbar 4a and main busbar 4b. This is shown in Fig. 2. Main busbar 4a is connected to ground potential and main busbar 4b is connected to a voltage V which is sufficiently low to prevent any risk of electrical shock, but obviously of a magnitude sufficient to create the required current density in the thin film layer 2. It will be noted that in this embodiment it is not required that each of the stripes a, b, c, d is deposited as an individual smaller area; the deposit of the busbars provides both the power supply and the potential delimitation.

In fact, the invention may be realised as a retrofit to cooking surfaces which have been determined to be unsafe with high drive voltages.

Claims

C l a i m s

1. A thin film heater for cooking purposes comprising a conductive deposit (2) on an insulating substrate (1) , characterized in that the area to be heated is subdivided into smaller areas (a,b,c,d), each supplied individually with a voltage below a predefined safety limit.

2. A heater according to claim 1, characterized in that sets of neighbouring smaller areas (a,b,c,d) share a busbar (3a,3c,3e) at a relatively low potential (ground) on one side and a busbar (3b, 3d) at a relatively higher potential (V) on the other side.

3. A heater according to claim 2, characterized in that all the busbars (3a,3c,3e) at the relatively low potential are connected to one and the same first main busbar (4a) and all the busbars (3b, 3d) at a relatively higher potential are connected to one and the same second main busbars (4b) , said main busbars (4a, 4b) being supplied with a voltage which is below the predefined safety limit.