GB2230900A

GB2230900A - Electrical components

Info

Publication number: GB2230900A
Application number: GB8909319A
Authority: GB
Inventors: Kelvin Loader
Original assignee: Semitron Cricklade Ltd; Semitron Industries Ltd
Current assignee: Semitron Cricklade Ltd; Semitron Industries Ltd
Priority date: 1989-04-24
Filing date: 1989-04-24
Publication date: 1990-10-31
Also published as: CA2015216A1; GB8909319D0

Abstract

An electrical component such as an excess voltage arrester includes an electrode (3, 4) and electrically conductive member (5) resiliently biased for contacting the electrode (3, 4), and a spacer (13) normally holding the member out of electrical contact with the electrode (3, 4). The spacer (13) is heat softenable such that on softening due to overheating of the component a part of the electrically conductive member (5) not having the spacer (13) directly between it and the electrode (3, 4) is permitted to move into direct electrical contact with the electrode (3, 4). The arrangement allows a small gap between the electrode (3, 4) and conductive member (5) to be provided. <IMAGE>

Description

ELECTRICAL COMPONENTS This invention relates to electrical components.

The invention is particularly concerned with electrical components having at least two electrodes which it is required should be shorted together if the component overheats.

An example of such a component is an excess voltage arrester which acts to protect an electrical circuit from transient voltage surges but which should not be allowed to reach too high a temperature.

An excess voltage arrester can be a semi-conductor device in which a PN junction in a semi-conductor body is connected across a voltage supply to a circuit so as to be reverse biased with the breakdown voltage of the reverse biased junction selected to be somewhat greater than the voltage normally developed by the supply. In normal use the junction is substantially non-conductive but in the event of a transient voltage surge such that the supply voltage exceeds the breakdown voltage, the reverse biased junction breaks down and conducts the current surge associated with the voltage surge so as to protect the circuit from the surge.

Another type of excess voltage arrester comprises a gas-filled enclosure and a pair of electrodes housed within the enclosure and which define between them a discharge gap. Such an arrester is connected across equipment it is desired to protect against excess voltage, the discharge gap breaking down on the occurrence of excess voltage.

For both the above types of voltage arrester, should the arrester be operative to protect a circuit over a period sufficiently long for the arrester to be in danger of overheating, it is required that the arrester should fail in a safe manner and so as not to leave the circuit unprotected.

Arresters are known in which a blob of solder normally maintains two electrodes in non-shorted condition. At a temperature approaching too high a temperature the solder melts to permit resilient bias to assert itself to bring about shorting of the electrodes. As melting of the solder tends to occur relatively slowly it is difficult to ensure that shorting occurs positively, and the arrester tends to be bulky overall due to the inclusion of the solder blob. GB-2078025-B discloses an excess voltage arrester having an overheating protection device which is formed by a length of resilient electrically conductive wire on which there is a sleeve of heat softenable electrically insulating material. A portion of the sleeve is trapped between the wire and a rigid member electrically connected to an electrode.On overheating the arrester the sleeve softens and the resilient wire forces it way through the sleeve and into electrical contact with the rigid member to establish an electrically conductive path through itself between the arrester electrodes. However, due to the necessarily small area of cut through, and movement of the material of the sleeve during cooling, permanent shorting of the two electrodes may not occur.

Instead intermitant shorting tends to take place which is undesirable.

According to the present invention there is provided an electric component having an electrode, an electrically conductive member resiliently biased for contacting the electrode, and a spacer normally holding the member out of electrical contact with the electrode, the spacer being heat softenable such that on softening thereof due to overheating of the component a part of the electrical conductive member not having the spacer directly between it and the electrode is permitted to move into direct electrical contact with the electrode.As, in this component the deformation of the spacer permits direct contact between the member and the electrode, it being parts of the member and the electrode not having any part of the spacer directly interposed between them that come into contact, it is possible to provide a very small gap of accurate size between the electrode and the member and hence a fast response to excess voltage is achievable, this being enhanced by having the spacer in direct contact with the electrode.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing in which: Figure 1 is a side view of an electrical component; Figure 2 is a plan view from above of the component; and Figure 3 is a plan view from below of the component.

The electrical component shown in the Figures is an excess voltage arrester which includes a semiconducter device having a body 1 of cylindrical form with a central electrode 2 and end electrodes 3 and 4, the body 1 being held in a clip 5.

The clip 5 is a U-channel of electrically conductive resilient material, for example phosphor bronze, berefillium copper, or spring steel, the upstanding ends 6 and 7 of which are biased inwardly so that they grip therebetween the body 1. The web 8 of the grip 5 has a central aperture at 9 in which a central pin 2A extending from the central electrode 2 is a push fit so that there is electrical connection between the electrode 2 and the clip 5. This connection is enhanced by lancing the clip to the central pin after fitting the body 1 in the clip, such lancing also resisting disassembling of the body and clip.

Adjacent the ends 6, 7 of the clip 5 there are apertures 10 and 11 through which pass end pins 3A and 4A extending from the end electrodes 3 and 4. There is clearance between each pin 3A and 4A and the clip 5 in the region of the aperture 10 or 11 so there is no electrical connection at either of the pins 3A or 4A with the clip 5.

In the normal operational condition illustrated there is also no electrical contact between either end 6 or 7 of the clip and the adjacent electrode 3 or 4 since each clip end includes a recess 12 in which there is housed a heat sensitive spacer 13 which projects from the recess to hold the parts of the clip on either side of the spacer clear of the electrode. Each spacer 13 consists of a substantially rigid carrier 13A, for example a metal wire, and heat sensitive insulating material, for example a sleeve 13B of plastics material. The dimension D of the gap maintained between each clip end and the adjacent electrode can be for example, 0.08 mm. Alternatively the spacer can consist solely of heat sensitive insulating material.

In use the arrester is mounted with the pins 3A and 4A connected with an equipment which it is desired to protect against excess voltage and the pin 2A connected to ground. It will be appreciated that as the spacers 13 hold the clip ends clear of the electrodes 3 and 4 there is no connection through the clip between the electrodes 2, 3 and 4.

On occurance of an excess voltage between the end electrodes 3 and 4, or between either of these electrodes and ground, the arrester operates to protect the equipment. If the arrester continues so to operate for too long a period, the resultant heat generated deforms the heat sensitive insulating material of the spacers 13 so that the resiliance of the clip asserts itself and the gaps which the spacers 13 have been maintaining between the end electrodes and the clip ends closed to short the end electrodes together and to earth through the clip. The arrester ceases to operate, so that dangerously high temperatures are avoided, but the equipment remains protected as the end electrodes are now earthed.

As the deformation of the spacers permits direct contact between the clip and the arrester electrodes, it being parts of the clip and the electrodes not having any part of the spacers directly interposed between them that come into contact, it is possible to provide a very small gap of accurate size between each electrode and adjacent clip end and hence a fast response to excess voltage is achievable, this being enhanced by having the spacers in direct contact with the arrester ends, that is in direct contact with the excess heat source. The clip, which itself carries the short circuiting current, can have very low short circuit current resistence and hence can conduct very high currents.

Although the voltage arrester described is a semiconductor device it could be a gas-filled arrester device, or the component could be some other electrical component having an electrode and an electrically conductive member such as either of the clip ends described above resilient biased for contacting the electrode but normally held spaced therefrom by a spacer such as the spacer 13 described above, the spacer deforming as described above, on occurence of excess heat in the component, to permit the member directly to contact the electrode.

Claims

1. An electric component having an electrode, an electrically conductive member resiliently biased for contacting the electrode, and a spacer normally holding the member out of electrical contact with the electrode, the spacer being heat softenable such that on softening thereof due to overheating of the component a part of the electrical conductive member not having the spacer directly between it and the electrode is permitted to move into direct electrical contact with the electrode.

2. An electrical component as claimed in claim 1, wherein the component is elongated and has an electrode at each end, the member is a U-channel having its upstanding ends inwardly biased to grip the component at the end electrodes, and there is a spacer between each end of the member and the adjacent electrode.

3. An electrical component as claimed in claim 2, wherein each spacer is received in a recess in its end of the member, the spacer normally projecting from the recess to hold the parts of the end that are on each side of the spacer clear of the electrode.

4. An electrical component as claimed in claim 1, 2 or 3, wherein the or each spacer comprises a sleeve of heat sensitive insulating material mounted on a carrier.

5. An electrical component as claimed in claim 1, 2 or 3, wherein the or each spacer consists solely of heat sensitive insulating material.

6. An electrical component substantially as herein before described with reference to the accompanying drawing.

6. An electrical component substantially as herein before described with reference to the accompanying drawing Amendments to the claims have been filed as follows 1. An electric component having an electrode, an electrically conductive member resiliently biased for contacting the electrode, and an electrically insulating spacer normally holding the member out of electrical contact with the electrode, the spacer being heat softenable such that on softening thereof due to overheating of the component a part of the electrical conductive member not having the spacer directly between it and the electrode is permitted to move into direct electrical contact with the electrode.

4. An electrical component as claimed in claim 1, 2 or 3, wherein the or each spacer comprises a sleeve of heat sensitive insulating material that is mounted on a carrier.