TRANSIENT SUPPRESSOR

Abstract: A transient suppressor having at least tow electrical leads, at least one electrical lead having a plastic insulating sleeve allowing above a pre-determined temperature a spring attached to the suppressor to short out at least two leads of the sup­ pressor, should be improved. Heatshrink tubing, which tears at the point of stress, is placed on at least one lead. Above the pre-determined temperature, the ideal diameter of the heatshrink tubing is smaller than the diameter of the lead passing through it.

Description: This invention relates to a transient suppressor according to the preamble of claim 1. In certain applications it is important to ensure that failure of an electronic component renders it short circuit. This is particularly applicable to transient voltage suppression devices because any other type of failure will result in damage to circuitry that the device is installed to protect. In the past such devices as gas discharge tubes and transient absorption Zener diodes have been protected by various external systems which short the device terminals when the device reaches a certain temperature. Two methods of achieving this result have commonly been used. In the first method a mass of solder is melted when the device reaches a pre-determined temperature and it is this molten solder which shorts out the device. This failure mode can be unpredictable in its operation and can lead to the hazard of molten solder being ejected from the device. It has proved difficult to incorporate a reliable system based on this principle into a very small device package. The second method of ensuring a short circuit failure mode uses a change in the physical characteristics of plastic sleeving material to effect the shorting. This change allows a spring to short out the contacts of the device. The change of physical characteristics takes place at a pre-determined temperature. As a result of this change of physical characteristics at a predetermined temperature, melting of the plastic insulating sleeve allowing a short to occur has been used, as well as a longitudinal shrinking of the tubing which again allows shorting. Both methods have problems due to reproducibility and reliability. It is possible for the short to occur intermittently. These can be lead to degradation of the transient suppressor device and the circuitry it is protecting. The present invention provides an improved transient suppressor according to claim 1 in which the aforementioned disadvantages are overcome. A transient suppressor according to this invention using heatshrink tubing, which tears at the point of stress, works much better than a transient suppressor according to the prior art. At least one piece of heatshrink tubing is placed on at least one electrical contact of the transient suppressor. The inside diameter of this tubing is selected so that there is a loose fit of the tubing on the lead, but when taken above its heatshrink temperature, its diameter is reduced to a value lower than the diameter of the electrical contact passing through it. A spring is attached so that it is able to put additional stress on the heatshrink tubing of at least one electrical contact so that at least one heatshrink tubing tears at the point of contact allowing an irreversible short to occur. The transient suppressor can have two or more electrical contacts (diode, foldback diode, thyristor, triac, gas discharge tube, varistor). The spring can be attached to an electrical contact in order to tear the heatshrink tubing on at least one other electrical contact to short out the contacts of the device. The spring can also be attached to another point in order to tear the heatshrink tubing on at least two electrical contacts to short out the contacts of the device. The spring can be a spring clip in order to put stress on the pieces of several heatshrink tubings of several electrical contacts. The electrical contacts can be bent in order to concentrate the additional stress upon the heatshrink tubing which additional stress is caused by a spring. There can be used several springs. Those skilled in the art will readily be able to determine from the following explanations and by routine trial and experiment suitable modifications which cause at least one heatshrink tubing to tear at a pre-determined temperature. In order that the invention may be more fully understood an embodiment thereof and its operating characteristics will now be described by way of illustrative example, reference being had to the accompanying drawings wherein: FIG 1 shows schematically a longitudinal section of a transient suppressor according to the invention FIG 2 shows schematically the front sight of the transient suppressor of FIG 1. FIG 1 shows a semiconductor transient suppressor. It consists of a moulded plastic body 1, from which protrude three leads 2, 3, 4. Leads 2 and 4 are bent as shown. Placed on leads 2 and 4 are two pieces of heatshrink tubing 5, 6. The inside diameter of this tubing is selected so that it is a loose fit of the tubing on the lead, but when taken above its heatshrink temperature, its diameter is reduced to a value much lower than the diameter of lead passing through it. A spring clip 7 is attached to lead 3 and is insulated from leads 2 and 4 by the heatshrink tubing. When the temperature of lead 2 or lead 4 exceeds the heatshrink temperature of the sleeving, it shrinks rapidly. Its ideal diameter is smaller than the lead diameter so it is under tension. The points of contact 8 and 9 of the clip put additional stress on the heatshrink tubing, concentrated by the effect of the bend in the wire. In addition the heatshrink tubing is softened at elevated temperature. It tears at the point of contact, allowing an irreversible short to occur. A suitable type of heatshrink tubing has been found to be type CRN, manufactured by Raychem Limited, Swindon, England. This invention has proved particularly effective when applied to the Semitron L10-140 CD, manufactured by Semitron Cricklade Ltd., Cricklade Swindon, Wiltshire SN6 6HQ, GB. This device is of the "foldback" type as proposed in European Patent number EP-A-0088 179. The L10-140 CD is designed to protect telecommunications circuitry from lightning strikes. The device is particularly effective in this application and always exhibits the required short circuit condition when its rating is exceeded. If, however, accidental 240 volt AC mains contact occurs, the device can undergo a progressive degradation in its performance parameters, which is not acceptable. A device manufactured according to the teaching of this invention, however, does not suffer such a degradation. An assembly manufactured according to the above teaching goes short circuit within one minute when 240v AC mains currents between 200mA and 5A are passed through it. The circuitry the L10-140 CD is installed to protect is completely undamaged by the mains contact. Although the invention has been applied by this embodiment to semiconductor devices, it could be used for other products such as gas discharge tubes and varistors.

Claims: Transient suppressor having at least two electrical leads (2,3,4), at least one electrical lead (2,4) having a plastic insulating sleeve (5,6) allowing above a pre-determined temperature a spring (7) attached to the suppressor to short out at least two leads (2,3;3,4) of the suppressor, characterized in that at least one piece of heatshrink tubing (5,6) placed on at least one lead (2,4), that the inside diameter of this tubing is selected so that there is a loose fit of the tubing on the lead, but when taken above its heatshrink temperature, its diameter is reduced to a value lower than the diameter of lead passing through it, that the attached spring (7) is able to put additional stress on the heatshrink tubing of at least one lead so that at least one heatshrink tubing is able to tear at the point of contact, allowing an irreversible short to occur. Transient suppressor according to claim 1, characterized in that pieces of heatshrink tubing (5,6) are placed at all leads (2,3,4). Transient suppressor according to claim 1, characterized in that only one piece of heatshrink tubing (5,6) is placed at just one lead. (2,4). Transient suppressor according to one of the claims 1 to 3, characterized in that at least one lead (2,4) is bent at the point of contact (8) of the spring (7) in order to concentrate the stress exerted upon the heatshrink tubing (6) by the spring. Transient suppressor according to one of the claims 1 to 4, characterized by a spring clip (7). Transient suppressor according to one of the claims 1 to 5, characterized by a semiconductor suppressor device. Transient suppressor according to one of the claims 1 to 5, characterized by a gas discharge suppressor device. Transient suppressor according to one of the claims 1 to 5, characterized by a varistor suppressor device.

Inventors: BELL, MALCOLM SANDERSON
Applicants: SEMITRON INDUSTRIES LIMITED
Application year: 1987
Main classification: H02H9
IPC codes: H01H37, H01T1, H02H9