GB2228396A - Electric hotplate - Google Patents

Description

1 IMPROVEMENTS IN ELECTRIC HOTPLATES This invention relates to electric

hotplates and applies primarily to hotplates for cooking hobs.

Conventionally electric hotplates have been provided with heating elements consisting of a coil of resistive wire, generally of nichrome, which has a very small temperature co-efficient of resistivity. As a result, the resistance of the heating element and therefore the power dissipated at any power setting is virtually independent of the temperature of the element.

If contrary to the conventional practice, the element is of a material having a positive tewperature co-efficient of resistivity, z:he current flow and therefore the power dissipated is greater when the element is cold and reduces as the element reaches its operating temperature. This has the advantage that the hotplate will warm up more rapidly and that it is less liable to damage through accidental overheating, for example in the case of a cooker hob, if the hotplate left switched on without a saucepan being placed on it One problem that arises in hotplates having heating elements with a positive temperature co-efficient of resistivity is that if the hotplate is cooled over part of its area, for example by a cold saucepan being placed part way thereon, the cooler portions have a lower resistance and therefore the current through the element is increased. Consequently those portions which are not 1. 1 i S cooled, will become overheated and they are more liable to be burnt out. This problem also manifests itself where the base of the saucepan is warped and therefore contacts the hot plate over a limited area thereof. An object of the invention is to reduce the probability of this happening.

A further problem with such hotplates is that because the resistance of the element is low on switching on, the initial current may be unacceptably large and may cause damage. One particular embodiment of the invention is directed to this problem.

The invention is defined in the claims appended hereto and the design of a hotplate according to the invention will now be described with reference to the accompanying drawings, in which Figure 1 shows the interconnection of the sections of an element employed in the hotplate according to the invention and Figures 3 and 4 are diagrammatic plan views of three different embodiments of the invention, showing how the elements are disposed on the hotplates.

Referring first to Figure 1, the element of the hotplate comprises not a single conductor, but sixteen conductive sections R all of a material having a substantial positive temperature co-efficient of resistivity, and all having substantially similar resistance when measured at the same temperature. The elements are connected in four groups of four. The elements in each group are connected in parallel and the N n groups are Figure 1, example to Z connect in paralle If now the hotplate saucepan are reduced.

3connected in series with one another. In represents an input terminal connected for an energy regulator. The conductors X,Y and the elements to connect the respective groups 1 and B represents an output terminal for connection to the neutral supply line. it will be appreciated that the overall resistance of this assembly is equal to the resistance of any one section.

Initially, when power is applied to the element, a large current flows because the resistance is low and the sections heat up rapidly. By the time the elements reach operating temperature, the resistance is much higher and the currenc is muci reduced, cherety minimizing the risk of an element burning out even If the element is left uncovered at maximum power. a saucepan is placed so as partly to cover those sections which lie under the cooled and their resistance is much However, because of the series parallel arrangement. current will tend to be diverted through the cooler sections, so reducing local overheating.

Figure 2 is a diagrammatic section in plan of a hotplate according to the invention. The hotplate is a laminated structure comprising a metal plate coated with a ceramic insulating layer on which are deposited resistive films forming the sections of the heating element and conductive films forming the interconnections. The electrical arrangement is as 4 described above with regard to Figure 1.

The resistive sections are composed of a metal/glass mixture, the proportion of metal to glass (typically from 50150 to 9515 percent metal/glass) being selected to give a suitable resistivity. Nickel is a particularly suitable metal for use In the resistive sections, although other metals, such as cobalt, and alloys are also used for this purpose. The conductors are mainly or entirely metallic in composition.

The metal and glass are finely powdered and thoroughly mixed in the required proportions with a screen printing medium to produce a viscous ink which can be readily screen printed in the required position on the hotplate. The conductors may be applied by similar means. After screen printing, the printed hotplate is dried at a moderate temperature and is then fired at a high temperature to bind the resistive sections and the conductors to the ceramic substrate. Connections are made to the supply conductors and a further protective layer may be applied.

The conductive and resistive pattern is shown in Figure 2. The outermost conductor A and the innermost conductor B are connected to a supply and between them are intervening conducting rings X, Y and Z between which the resistive elements are connected concentrically and in parallel groups. There are four parallel resistive sections between each consecutive pair of conductors. Because of the concentric arrangement. a saucepan partially covering the hotplate will reduce the temperature of some sections in each of the groups and the increased current will be diverted through these cooler sections. Conversely, if a single continuous element were employed, the whole of the increased current would pass through the hottest part of the element. increasing the risk of a burn out in that region.

Further embodiments of this invention are shown in Figures 3 and 4. In the embodiment shown in Figure 3, two annular conductors C, D are connected to the elements R by a plurality of radial conductors F, G. Rad4Lal conductors f connect each group of four sections R to the outer annular conductor C and radial conductors G connect each group of sections R to the inner annular conductor D.

The embodiment shown in Figure 4 is broadly similar to that of Figure 3 bur has three annular conductors: outer and inner annular conductors C, D are as before but there is an intermediate annular conductor E. In Figure 4, there are only two sections R in each group but there are twice as many groups and therefore the number of sections in Figure 4 is the same as that in Figure 3. The groups of sections R are split into inner and outer sets by the intermediate annular conductor E.

Each group of sections R in the outer set is connected to the outer annular conductor C by radial conductors F, and is also connected to the intermediate 6 annular conductor E by radial conductors H. Each group of sections R in the inner set is connected to the intermediate annular conductor E by radial conductors I, and is also connected to the inner annular conductor D by radial conductors G.

In the embodiment shown in Figures 3 and 4, the annular conductors C, D, E are connected to a suitable supply at terminals T. Each embodiment is suitable for use with an energy regulator and the Figure 4 embodiment is suitable for use with a switched power supply.

It will be appreciated that the annular conductors C,D,E need not be situated on the plate but could instead take the form of a wired ring main.

In a modification of the invention, the intermediate conductors X, Y and Z or the radial conductors F,G, H and I are made of a material which has a strong negative temperature co-efficient of resistivity, such that its resistance is comparable with that of the heating elements sections at room temperature but falls to a much smaller value, preferably negligibly low, but at most one-quarter of the resistance of a section at the operating temperature.

This modification increases the resistance of the elements appreciably at room temperature and therefore reduces the otherwise objectionably large initial current on first switching on.

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Claims (5)

7 CLAIMS

1. An electric hotplate having a heating element comprising a plurality of sections of a material having a positive temperature coefficient of resistivity the sections being connected in parallel in groups, and the groups being connected in series between supply terminals.

2. An electric hotplate according to claim 1, in which the heating element comprises four groups each of four sections, the sections having substantially the same resistance.

3. An electric hotplate according to claim 2 in which the groups are arranged concentrically.

claim 1 or

4. An electric hotplate according to any preceding claim, having a laminated structure and in which the heating element sections each comprise a film of resistive material applied to an Insulating substrate.

5. An electric hotplate according to any preceding claim in which the groups of element sections are interconnected by conductors having a negative temperature co-efficient of resistivity such that at operating temperatures, the interconnecting conductors have a resistance which is small in relation to that of the element sections.

Published 1990atThe Patent Office, State House. 86.71 High Holborn, London WC1R4TP.Purther copies maybe obtainedfrom. The Patent otrice. Bales Branch, St Mary Gray. Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Gray, Kent, Con. 1187 1. 1