Electrical components incorporating a temperature responsive device

Abstract

 An electrical component has at least two electrodes (4,5) which become short-circuited when a temperature responsive device associated with the component and formed by a resilient wire (1) covered by a sleeve (2) of heat shrinkable material responds to an increase in temperature. The heat shrinkable sleeve (2) is made of a plastics material having a heat responsive memory and is formed so as to shrink longitudinally on the application of heat to thereby initiate the short-circuiting function.

Description

[0001] The present invention relates to electrical components incorporating a temperature responsive device and more particularly to such components in which the temperature responsive device is provided to perform a protective function on the component or on an associated component.

[0002] Electrical components incorporating temperature responsive devices of various forms are known. Thus, the temperature responsive device may take the form of a bimetallic member which, when heated, is subjected to distortion which in turn can be employed to perform an open-circuiting or short-circuiting function. However such devices are relatively expensive and require careful adjustment.

[0003] Another form of temperature responsive device comprises a resilient electrically conductive member which is normally maintained in a stressed condition by means including a heat softenable material, the arrangement being such that when the heat softenable material becomes soft at an elevated temperature, the stress in the resilient electrically conductive member is released to enable it to perform a short-circuiting or open-circuiting function. In such devices the heat softenable material may be a metallic fusible material, for examle a relatively low melting point alloy, or a fusible insulating material such as a thermoplastics material which becomes soft at a desired temperature.

[0004] Also, where the device employs a fusible insulating material which normally holds the stressed resilient electrically conductive member out of engagement with a cooperating contact, although the heat softening of the insulating material allows the resilient electrically conductive member to engage the contact, there is always the possibility that a film of the softened insulating material remains interposed between the engaging parts thereby preventing the protective short-circuiting function from occuring.

[0005] It is an object of the present invention to provide an improved form of temperature responsive device for electrical components.

[0006] From one aspect the invention provides an electrical component comprising a temperature responsive device including a member of plastics material having a heat responsive memory and which is formed so as to shrink longitudinally on the application of heat.

[0007] From another aspect the invention provides an electrical component having at least two electrodes or regions which either become short-circuited or open-circuited when a temperature responsive device associated with the component responds to an increase in temperature and wherein the short-circuiting or open-circuiting function is controlled by a member of plastics material having a heat responsive memory and which is formed so as to shrink longitudinally on the application of heat.

[0008] The invention may particularly be employed as an overheating protection device attached to a component such as an excess voltage arrestor which may take the form of a pair of spaced electrodes defining a spark gap and arranged in a gas filled enclosure. Such arrestors may also take the form of three electrode devices in which case the third electrode is generally located in the region of the spark gap.

[0009] In one preferred form of the invention, the temperature responsive device comprises a length of a stiff resilient metal wire which is surrounded over at least part of its length by a sleeve of an insulating plastics material having a heat responsive memory characteristic such that on the application of heat it will shrink in the lengthwise direction. This shrinkage of the sleeve in the lengthwise direction may either result in one or more regions of the wire being exposed to perform a short-circuiting function, or alternatively result in one or more exposed regions of the wire being covered by the insulating plastics sleeve to perform an open-circuiting function.

[0010] In an alternative embodiment of the invention, a member of insulating plastics material having a heat responsive memory may be arranged as a separator between two parts, at least one of which is resiliently urged towards the other and the application of heat causes the member of insulating plastics material to shrink lengthwise sufficiently to allow the two parts to come into contact and thereby perform a short-circuiting function.

[0011] Various plastics materials are available which have a heat responsive memory. Preferred materials for use in the present invention include irradiated polyolefins and modified polyvinylidene fluorides.

[0012] The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 shows one form of temperature responsive device according to the invention,

Figure 2 shows a plan view of an electrical component incorporating the device of Figure 1,

Figure 3 is a side view of the component,

Figure 4 is a plan view similar to Figure 2 when the component has been subjected to overheating,

Figure 5 is a side view of a further embodiment,

Figure 6 is a side view of yet a further embodiment, and

Figure 7 is a plan view of another embodiment.

[0013] Figure 1 shows a simple form of temperature responsive device in which a wire 1 of spring temper, for example a plated spring steel wire or a beryllium copper wire, is located within a sleeve 2 of heat shrinkable material, that is to say a plastics material having a heat responsive memory. The sleeve 2 has a cut-out or notch 3 at the mid point of its length so that the wire 1 is exposed and may make contact with a conductor such as the centre terminal pin of a three electrode excess voltage arrestor.

[0014] Figure 2 is a plan view of the casing of such a three electrode excess voltage arrestor comprising two electrodes defining a spark gap and having terminal pins 5 and an intermediate electrode having a terminal pin 4. Figure 3 is an elevation of the same device. The arrestor is fitted with a temperature responsive device as shown in Figure 1, and as can be seen in Figure 2, the cut-out or notch 3 permits electrical contact between the centre pin 4 and the spring temper wire 1 whilst the sleeve 2 insulates the end pins 5 from the spring temper wire. The exposed wire may be attached to the pin 4 or merely rest against it.

[0015] Figure 4 is a plan view of the same component after a sustained excess voltage has been applied to the arrestor resulting in overheating. It can be seen that the sleeve 2 has retracted or shrunk longitudinally under the effect of heat and electrical contact has now been made between the spring temper wire 1 and the end pins 5. The amount of shrinkage may be up to about 50% of the length of the sleeve. Since the end and centre pins are now electrically connected one to the other, the discharge through the arrestor is stopped and no further heating can occur. Also because the terminals are so connected together, an item of equipment fitted with the arrestor is still protected against damage due to overvoltage or excess voltage surges.

[0016] Figure 5 illustrates an embodiment of surge arrestor which is not fitted with pins and the centre of the spring temper wire is welded at 6 direct to the centre electrode 7 of the arrestor. The extremities of the wire 1 are performed at la such that they are biassed into contact with the end electrodes 8 but are normally electrically isolated from them by the sleeve 2. Here again when the sleeve 2 shrinks longitudinally under heat, the wire contacts the electrodes 8 and shorts them together with the electrode 7.

[0017] The principle of the invention, namely employing a heat shrinkable sleeve which shrinks longitudinally on the application of heat may be extended to other types of excess voltage arrestors, as for example is illustrated in Figure 6. In this embodiment a gas discharge tube 10 is welded between two terminal plates 11, and ceramic spacers 12 are also provided between the plates. One of the spacers encloses a compression spring 9 which carries a heat shrinkable sleeve 2a normally shrouding the ends of the spring. When the sleeve 2a shrinks longitudinally with heat, the ends of the spring are exposed to contact the electrodes and perform a shorting function. The spring may be reverse wound over part of its length to anchor the sleeve at an intermediate point.

[0018] Figure 7 is a plan view of a component 13 having a temperature responsive device arranged so that a normal connection is broken or open-circuited in response to overheating. The temperature responsive device again comprises a spring tension wire 14 enclosed in a sleeve 2 of heat shrinkable plastics material which in this case has three cut-outs or notches 15 arranged opposite contact pins 16, by virtue of which all three pins are normally interconnected. On application of heat, the sleeve will retract longitudinally causing the two outer cut-outs or notches to move from the vicinity of the pins, and leave a portion of the insulating sleeving interposed between the wire and the pins so that the circuit is opened.

[0019] Whilst the invention has been specifically described, in the main, as applied to excess voltage arrestors, it may obviously be applied to any other electrical component in order to protect it from damage due to overheating. For example the component, e.g. of Figure 7 could be an integrated circuit or microchip. Moreover the temperature responsive device could itself be formed as a separate component which is mounted in the vicinity of and appropriately connected to a component or components which it is to protect from overheating.

Claims

1. An electrical component comprising a temperature responsive device characterised in that it includes a member of plastics material having a heat responsive memory and which is formed so as to shrink longitudinally on the application of heat.

2. An electrical component having at least two electrodes or regions which either become short-circuited or open-circuited when a temperature responsive device associated with the component responds to an increase in temperature, characterised in that the-short-circuiting or open-circuiting function is controlled by a member (2) of plastics material having a heat responsive memory and which is formed so as to shrink longitudinally on the application of heat.

3. An electrical component as claimed in claim 1 or 2, characterised in that the temperature responsive device comprises a length of a stiff resilient metal wire (1) which is surrounded over at least part of its length by a sleeve (2) of an insulating plastics material having a heat responsive memory characteristic such that on the application of heat it will shrink in the lengthwise direction.

4. An electrical component as claimed in claim 3, characterised in that the shrinkage of the sleeve (2) in the lengthwise direction results in one or more regions of the wire (1) being exposed to perform a short-circuiting function.

5. An electrical component as claimed in claim 3, characterised in that the shrinkage of the sleeve (2) in the lengthwise direction results in one or more exposed regions of the wire (1) being covered by the insulating plastics sleeve to perform an open-circuiting function.

6. An electrical component as claimed in claim 1 or 2, characterised in that the temperature responsive device comprises a member (2) of insulating plastics material having a heat responsive memory arranged as a separator between two parts, at least one (la) of which is resiliently urged towards the other (8) and the application of heat causes the member (2) of insulating plastics material to shrink lengthwise sufficiently to allow the two parts (la and 8) to come into contact and thereby perform a short-circuiting function.

7. An electrical component as claimed in claim 3 or 4, characterised in that the temperature responsive device is in the form of a coil spring (9).

8. An electrical component as claimed in any preceding claim, characterised in that the plastics material having a heat responsive memory is selected from irradiated polyolefins and modified polyvinylidene fluorides.

9. An electrical component as claimed in any preceding claim, characterised in that it comprises an excess voltage arrestor in the form of a pair of spaced electrodes defining a spark gap and arranged in a gas filled enclosure.

Drawing