GB2320614A

GB2320614A - Self-limiting heaters

Info

Publication number: GB2320614A
Application number: GB9726015A
Authority: GB
Inventors: Brian G Sellers
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1996-12-20
Filing date: 1997-12-10
Publication date: 1998-06-24
Also published as: CA2224022A1; GB9626517D0; GB9726015D0

Abstract

A self-limiting electric heater comprises a PTC element 1 in the form of a body having first and second faces 2,3 opposite one another and first and second ends 4,5 opposite one another, and a series resistance element 6 in form of a layer (preferably a coating) on the first face 2 so as to be in intimate thermal contact with it. This layer 6 is carried by an insulating layer 8 so that it is in effective electrical contact with the PTC material 1 only in the neighbourhood of the end 5. External connections 12, 13 are made to the PTC material and to the layer, both in the neighbourhood of the other end 4. A further series resistance element 7 may be applied to the other face 3 of the PTC element 1. Insulating the series resistance layer(s) electrically from the PTC element in this way (but maintaining thermal contact) stops the PTC material from tending to short out the series resistance layer(s) when it is cold and has low resistance, minimising inrush current.

Description

Self-Limiting Heaters This invention relates to self-limiting electric heaters utilising a resistive material having, over a relevant temperature range, a positive and substantial temperature coefficient of electrical resistance - for brevity "PTC" materials - and more especially (but not exclusively) utilising PTC ceramics.

As is well-known, most electrical conductors have a fairly small coefficient of resistance over the whole range of temperatures at which they might be used, with the result that heating elements based on them need thermostats or other control devices to avoid local or general overheating. Even with the use of such control devices there remains a risk of local overheating as, even if the control device is in intimate thermal contact with all of the heating element, it will ordinarily only respond to its average temperature.

Thermal runaway is possible when using materials for which the temperature coefficient of resistance is negative, with overheated areas drawing disproportionate current and so becoming even more overheated. PTC materials can, in principle, be used to make electric heaters in which, over an appropriate temperature range, resistance rises so rapidly with temperature that for a given applied voltage there is a more or less sharply defined "switching" temperature that will not be exceeded, irrespective of ordinary variations in ambient temperature and the length of time for which the heater has been energised, because the operating power rapidly reduces as the temperature rises, and in which any hot spot that might arise carries reduced current and thus tends to cool.

However, the resistance of PTC materials at temperatures low compared with the switching temperature may be so low that the "inrush" current on turning on the heater when it is cold is excessive in comparison with the steady load after its temperature has stabilised, and the sharper the switching temperature the more likely this is to present difficulties.

Also, for some PTC materials, the temperature coefficient of resistance at these low temperature may in fact be negative.

Series resistances can be used to limit inrush current, but they will then generate a significant part of the heat at all temperatures, and the effect may be that the PTC material functions merely as a thermostat.

It has been proposed (US5057674) that series resistances should be constituted by layers (coatings) on the surface of the PTC element of the heater and that electrodes for external connections should be placed at opposite ends of the heater so that the current-flow is generally along the length of the layer structure rather than directly across the layers. The concept is that this ensures that the series resistances are in intimate thermal contact with the PTC element (to avoid the risk that hot spots in the series resistances do not influence the temperature of the PTC element) and that current flows through relatively large volumes of PTC material, minimising the effect of any inhomogeneity.

The benefits of this proposal are limited, however, because the lower the temperature (and the greater the amount of heat being generated) the more the PTC element tends to short out the series resistances, lessening their effect in controlling inrush current and concentrating heat generation at the ends of the elements.

In accordance with the present invention, a selflimiting electric heater comprises a PTC element in the form of a body having first and second faces opposite one another and first and second ends opposite one another and at least one series resistance element in the form of a layer on the first face and is characterised in that the said layer is in effective electrical contact with the PTC material only in the neighbourhood of the first end and that means are provided for making external connections to the PTC material and to the said coating, both in the neighbourhood of the second end.

Preferably the layer constituting the (or each) heating element is a coating formed in situ on the face of the PTC element, but in some cases a preformed layer may be used provided a sufficiently intimate thermal contact is established. In most cases the layer (or each of them) will need to extend substantially from end to end of the PTC element; it does not necessarily need to cover the whole area of the face of the PTC element to which it is applied, but this will in most cases be preferred.

Preferably the means for making an external connection to the PTC material in the neighbourhood of the second end comprises a further series resistive element in the form of a resistive coating or other layer on the second face of the PTC element and in effective electrical contact with it only in that neighbourhood and means for making an external electrical connection to this further series element in the neighbourhood of the said first end; the structure thus becomes at least in general terms rotationally symmetrical.

The PTC element will often be a rectangular slab of uniform thickness, but many other structures are possible and may be useful in particular cases: for example, a fluid heater might use an element in the form of a tube in which the opposite faces are the inner and outer peripheral surfaces.

Preferably the PTC element is of a PTC ceramic, such as one of those based on barium titanate which are readily available on the market, but the invention is also applicable to polymer-based PTC materials.

The series resistance may be selected from a wide variety of readily available materials, depending on the properties desired; it may have a negative or negligible temperature coefficient of resistance, or even a positive one provided it is small compared with that of the PTC element over the working temperature range. Thus it may be of a resistive metal, carbon-loaded or other conductive polymer, conductive ceramic, or p- or n-type semiconductor, as appropriate to the purpose and overall design of the heater.

Preferably each of the areas in the neighbourhood of the respective ends of the PTC element to which effective electrical connections are made are areas including adjacent parts of both the face and the end of the element, though an area confined either to the face or to the end could be used.

In each case, the area preferably extends to all or nearly all of the width of the resistive element.

To establish the effective electrical contact between the PTC element and the (or each) series resistance layer at the appropriate end, and prevent it over the remaining area of the element, it will be necessary to prepare appropriately one or both of the areas. Especially when the (or each) series resistance layer is a coating, the area of the PTC element in the neighbourhood of the end where effective electrical connection is required may need to be pre-coated with an electrode material or at least sensitised to promote formation of a conductive connection when the series resistance material is applied. Alternatively, or in addition, the remaining area may need to be coated (or otherwise supplied) with an electrically insulating layer of adequate thermal conductivity in relation to its thickness.

At least when the PTC material is a ceramic, we prefer to do both of these, whether or not it is strictly necessary.

Preferably the electrode material is a metal of relatively high conductivity (or there are plural layers of electrode material, at least one of which layers is of such a metal).

Composite electrode materials, such as a mixture of metal and glass powders may also be used. Preferably (in the case of a ceramic PTC element) the insulating coating material is ceramic, glass or a vitreous enamel. Suitable techniques are well-known in the art of making PTC heaters, and will not be described here.

The heaters in accordance with the invention may be used singly or grouped into any appropriate array: for example, large numbers of heaters based on PTC chips with dimensions of a few millimetres can be connected in parallel by arraying them in linear form with a pair of longitudinally-extending wire busbars to which the external electrodes of the chips are respectively connected and enclosed in suitable polymeric insulating material (or inorganic insulating material, with a metal jacket if required) to make a heating cable for pipetrace or other well-known applications. In such a context, it is an advantage of the invention that it enables the physical size of the chips to be reduced, for a given supply voltage, compared with a chip in which the current is not constrained to flow predominantly in the direction of the plane of the chip; in addition to the direct effect of the small dimensions of the chip itself, the better control of inrush current means that conductors of large cross-section are not needed for current-carrying capacity and the generation of relatively low power well distributed over the areas of the chips at least reduces the need for the individual lead wires of the chips and the wire busbars to serve as a means of conducting heat away from the chips. Thus by using large numbers of small chips with relatively low individual heat outputs, the cable can be made of small cross-section to enhance flexibility (and save cost of conductors, and of insulating and jacketing materials) . Furthermore, a cable with large numbers of closely-spaced low-power chips is more tolerant of chip defects or damage, since their effect will be more closely localised.

The invention will be further described, by way of example, with reference to the accompanying drawing, which is a diagrammatic cross-section through one form of selflimiting heater in accordance with the invention; it may be a flat rectangular ceramic chip or a tube seen in longitudinal section through one wall. The thicknesses of the component layers are exaggerated and the length foreshortened for clarity, and no proportions are to be inferred from the diagram.

The heater comprises a PTC ceramic layer 1 having upper and lower faces, respectively 2 and 3, and right and left ends, respectively 4 and 5. Over the whole area of each of the faces is applied a series resistance element in the form of a coating 6, 7 but in accordance with the invention this is not in electrical contact with the PTC material over its whole area but is separated from it by a respective electrically insulating sintered glass layer 8, 9 except that the resistive element 6 is connected to the PTC element by a deposited metal electrode 10 in the neighbourhood of its right-hand (first) end 5 and the resistive element 7 is similarly connected to it by an electrode 13 in the neighbourhood of its left-hand (second) end 4. In this example, as is preferred, these electrodes each extend both over a portion of the appropriate face (respectively 2 and 3) and an adjacent portion of the appropriate end (respectively 4 and 5) of the PTC element. The self-limiting heater is completed by electrodes 11 and 12 for making external connections to the series-resistance layers 7 and 6 respectively, in each case at the opposite end from that at which the layer is connected to the PTC element. The insulating layers 8 and 9 thus ensure that current can only pass between the external electrodes 11 and 12 by passing through nearly the whole length of each of the layers 6, 1 and 7 regardless of the temperature and consequent changes in relative resistivities of the layers.

Claims

1 A self-limiting electric heater comprising a PTC element in the form of a body having first and second faces opposite one another and first and second ends opposite one another and at least one series resistance element in the form of a layer on the first face characterised in that the said layer is in effective electrical contact with the PTC element only in the neighbourhood of the first end and that means are provided for making external connections to the PTC material and to the said coating, both in the neighbourhood of the second end.

2 A heater as claimed in claim 1 in which the means for making an external connection to the PTC material in the neighbourhood of the second end comprises a further series element in the form of a resistive layer on the second face of the PTC element and in effective electrical contact with it only in that neighbourhood and means for making an external electrical connection to this further series resistance element in the neighbourhood of the said first end.

3 A heater as claimed in claim 1 or claim 2 in which the or each said resistive layer is a coating on the PTC element.

4 A heater as claimed in any one of claims 1-3 in which the PTC element is of a PTC ceramic.

5 A heater as claimed in any one of claims 1-4 in which the series resistance has a negative or negligible temperature coefficient of resistance.

6 A heater as claimed in any one of claims 1-5 in which each of the areas in the neighbourhood of the respective ends of the PTC element to which effective electrical connections are made are areas including adjacent parts of both the face and the end of the element.

7 A self-limiting heater substantially as described with reference to the drawing.

8 An array of self-limiting heaters each as claimed in any one of claims 1-7.

9 An array in accordance with claim 8 in linear form with a pair of longitudinally-extending wire busbars to which the external electrodes of the individual self-limiting heaters are respectively connected, enclosed in insulating material to make a heating cable.

10 A self-limiting electric heater comprising a PTC element in the form of a body having first and second faces opposite one another and first and second ends opposite one another and two series resistance elements each in the form of a layer, the first layer being on the first face of the PTC element and the second layer on the second face of the PTC element wherein the said first layer is in effective electrical contact with the PTC element only in the neighbourhood of its first end, the said second layer is in effective electrical contact with the PCT material only in the neighbourhood of its second end and means are provided for making external connections to the said first layer in the neighbourhood of the second end and to the said second layer in the neighbourhood of the first end.

11 A heater as claimed in claim 10 in which each said resistive layer is a coating on the PTC element.

12 A heater as claimed in claim 10 or claim 11 in which the PTC element is of a PTC ceramic.

13 A heater as claimed in any one of claims 10-12 in which the series resistance element has a negative temperature coefficient of resistance.

14 A heater as claimed in any one of claims 10-13 in which the series resistance has a negligible temperature coefficient of resistance.

15 A heater as claimed in any one of claims 10-14 in which each of the areas in the neighbourhood of the respective ends of the PTC element to which effective electrical connections are made are areas including adjacent parts of both the face and the end of the element.