PACKAGING FOR PTC-BASED CIRCUIT PROTECTION DEVICES

Abstract: Circuit protection devices which comprise a PTC conduc­ tive polymer element and an enclosure, and which are less likely to cause damage to other electrical components through release of carbonaceous dust. On the interior surface of the enclosure, there is a projection or other disconformity which is spaced apart from the electrodes and which provides a site for arc initiation. In this way, the danger of erosion creating a hole in the enclosure is reduced. Preferably each of the electrodes is surrounded by a projection. Figure 2 illustrates a preferred form of projection.

Description: BACKGROUND OF THE INVENTION This invention relates to circuit protection devices comprising PTC conductive polymers. Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them, are well known. Reference may be made, for example, to U.S. Patent Nos. 2,952,761; 2,978,665; 3,243,753; 3,351,882; 3,571,777; 3,757,086; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,950,604; 4,017,715; 4,072,848; 4,085,286; 4,117,312; 4,177,376; 4,177,446; 4,188,276; 4,237,441; 4,242,573; 4,246,468; 4,250,400; 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,361,799, 4,388,607, 4,398,084, 4,413,301, 4,425,397, 4,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216, 4,442,139, 4,459,473, 4,473,450, 4,481,498, 4,502,929, 4,514,620, 4,517,449, 4,529,866, 4,534,889, and 4,560,498; J. Applied Polymer Science 19 , 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18 , 649-653 (1978), Narkis et al; and European Application Nos. 38,713, 38,714, 38,718; 74,281, 92,406, 119,807, 134,145, 84,304,502.2, 84,307,984.9, 85,300,415.8, 85,306,476.4 and 85,306,477.2. Particularly useful devices comprising PTC conductive polymers are circuit protection devices. Such devices have a relatively low resistance under the normal operating conditions of the circuit, but are "tripped", i.e. converted into a high resistance state, when a fault condition, e.g, excessive current or temperature, occurs. When the device is tripped by excessive current, the current passing through the PTC element causes it to self-heat to an elevated temperature at which it is in a high resistance state. The increase in resistance is accompanied by an expansion of the PTC element along an expansion axis. Such devices, and PTC conductive polymer compositions for use in them, are described for example in U.S. Patents Nos. 4,237,411, 4,238,812; 4,255,698; 4,315,237; 4,317,027; 4,329,726; 4,352,083; 4,413,301; 4,450,496; 4,475,138; and 4,481,498; in European Patent Publication Nos. 38,713, 134,145, 158,410, 201171, 201993, 201172 and 198598. The wall can be associated with a plurality of protection devices whose electrodes pass through the wall. As noted in U.S. Patent No. 4,481,498, failure of protection devices based on PTC elements comprising carbon black dispersed in a polymer can result from the formation of a permanent conductive path between the electrodes, as a result of the deposition of carbonaceous dust, evolved from the PTC element when it is tripped, onto a surface which joins the electrodes, typically the interior of an enclosure which surrounds, but is spaced apart from the PTC element. It has been found that improved results are obtained by reducing the likelihood that an arc is struck between the electrodes, via carbonaceous dust deposited on the interior of the enclosure, and by ensuring so far as possible, that if such an arc is struck, the result of the arc is not a permanent conductive path (or "track"). Typically the measures taken to avoid "tracking" result in erosion of the polymeric enclosure, and we have found that under some circumstances this can result in unforeseen disadvantages. In particular, we have found that erosion of the enclosure can result in the electrodes no longer being a tight fit in the exit ports of the enclosure and/or can result in the creation of holes in the enclosure through which carboneaceous dust can escape and cause undesirable electrical effects, for example by causing short circuits in a printed circuit board onto which the protection device is mounted. In some cases, the likelihood of creating holes in the enclosure can be reduced by increasing the wall thickness of the enclosure or by increasing the distance between the exit ports. However, in many cases these expedients are insufficient or cannot be employed because of the end use or other requirements of the device, e.g. the need for the electrodes to be connected to a printed circuit board with a fixed separation between the connections and/or the need to keep the dimensions of the device below fixed limits. DE-C-893829 discloses a cam-operated switch having channels in its plastics insulating body to prevent the formation of creepage current paths between the contacts of the switch. A feature of the invention is to provide, on the interior surface of the enclosure, a projection such that there is an increased probability that, under at least some of the fault conditions likely to be encountered, when an arc is struck between the electrodes, the arc includes the projection, the projection being so constructed and arranged that when such an arc is struck, the resultant erosion does not lead to the disadvantages noted above. Thus the projection can have a shape and/or a chemical composition which is different from the main part of the enclosure. The projection has a shape such that the thickness of the carbon black which gathers thereon is relatively small, so that when the arc is struck, the carbon dust can be relatively easily blown away, thus preventing the formation of a "track". A particularly effective projection is one which crosses, preferably substantially at right angles, at least the shortest, and preferably all, of the paths on the interior of the enclosure between the electrodes. The projection is preferably so shaped and located that it provides an area which, relative to the remainder of the interior of the enclosure, is thermally well insulated and on which the thickness of carbon black is relatively small, with a consequently relatively large resistance per unit of path length. It is theorized that as a result, when an arc is struck within the enclosure, the arc includes at least a part of the projection, eg. its end, on which there is relatively thin layer of carbonaceous dust, so that the arc causes the dust to be blown off the projection, thus preventing the formation of a track. We have obtained particularly good results by providing, around the base of at least one of the electrodes, a projection which projects towards the PTC element from the wall defining the exit port and which is spaced apart from the electrode. As discussed below, the dimensions of the projection which will give the optimum results depend upon the dimensions of the other parts of the device and the conditions of operation of the device. However, those skilled in the art will have no difficulty, having regard to their own knowledge and the disclosure of this specification, in determining dimensions which will result in a useful improvement. Thus, in accordance with the present invention, there is provided (A) a circuit protection device which comprises (1) a PTC element composed of a conductive polymer composition which exhibits PTC behavior and which comprises a polymeric component and, dispersed in the polymeric component, carbon black; and (2) two electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element; and (B) an enclosure which (1) encloses and is spaced apart from the PTC element; and (2) comprises a first wall which contains a first exit port through which passes one of the electrodes, and a second wall portion which contains a second exit port through which passes the other electrode; at least one of said wall portions comprising (i) a base which defines an exit port; and (ii) a projection which projects from the base towards the PTC element, and which is spaced apart from and substantially surrounds the electrode passing through the exit port, the projection being such that its presence substantially reduces erosion of the base when the device is repeatedly converted to a high resistance state under conditions which cause the PTC element to evolve carbonaceous dust, and having a shape such that the thickness of carbon black which settles thereon if the device is subjected to successive conversions to a high resistance state, is relatively small, and is such that if an arc is struck between the electrodes, the arc includes at least the end of the projection. BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated in the accompanying drawing, in which Figure 1 is an exploded perspective view of an apparatus comprising a plurality of circuit protection devices, and Figure 2 is a cross-sectional view of one of the electrodes and the corresponding wall portion of Figure 1. DETAILED DESCRIPTION OF THE INVENTION In the preferred embodiment of the invention the gap between the electrode and the projection should not be too small, since it will then become bridged by the carbonaceous dust. On the other hand, it should preferably not be too big, since the area at the base of the electrode will then also be a likely site for arcing, thus defeating the objectives of the invention. I have found that when at least one of the electrodes is surrounded by a projection, the separation between the electrode and the projection surrounding it is preferably 0.2 to 2.5 mm (0.008 to 0.1 inch), particularly 0.25 to 1.5 mm (0.010 to 0.060 inch), especially 0.25 to 1.0 mm (0.01 to 0.04 inch). The end of the projection should have a relatively narrow surface, preferably at most 1.25 mm (0.050 inch), particularly at most 0.75 mm (0.030 inch), especially at most 0.5 mm (0.020 inch), eg. 0.05 to 0.5 mm (0.002 to 0.020 inch), wide. Preferably the projection provides a surface which surrounds the electrode in a place substantially at right angles to the axis of the electrode. When, as in the preferred embodiment of the invention, at least one and preferably each of the electrodes is surrounded by a projection, the projection preferably has a substantially annular cross-section in all planes at right angles to the axis of the electrode. The projection can be cylindrical, but for ease of manufacture preferably has steeply sloping sides. Referring now to the drawing, Figure 1 shows a container which comprises a wall member 1 having pairs of exit ports 11,12 passing therethrough and a cover portion 2 which can be fitted to the wall member. The apparatus also includes five identical circuit protection devices 3, each comprising a pair of electrodes 31 and 32 which are embedded in a PTC conductive polymer element 33 and extend therefrom and fit through the exit ports 11 and 12 in the wall member 1. Figure 2 is a cross-section through a portion of the wall member 1 which contains an exit port 12. The wall portion comprises a base 121 which defines the exit port 12, and a projection 122 which has a thickness y at its extremity and a height z. When the electrode 32 is fitted into the exit port 12, it is spaced apart from the projection by a distance x which is slightly greater at the top than at the base.

Claims: Apparatus which comprises (A) a circuit protection device which comprises (1) a PTC element composed of a conductive polymer composition which exhibits PTC behaviour and which comprises a polymeric component and, dispersed in the polymeric component, carbon black; and (2) two electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element; and (B) an enclosure which (1) encloses and is spaced apart from the PTC element; (2) comprises a first wall portion which contains a first exit port through which passes one of the electrodes, and a second wall portion which contains a second exit port through which passes the other electrode; at least one of said wall portions comprising (1) a base which defines an exit port; and (2) a projection which projects from the base towards the PTC element, and which is spaced apart from and substantially surrounds the electrode passing through the exit port, the projection being such that its presence substantially reduces erosion of the base when the device is repeatedly converted to a high resistance state under conditions which cause the PTC element to evolve carbonaceous dust, and having a shape such that the thickness of carbon black which settles thereon if the device is subjected to successive conversions to a high resistance state, is relatively small, and is such that if an arc is struck between the electrodes, the arc includes at least the end of the projection. Apparatus according to Claim 1 or wherein the projection is spaced apart from the electrode by a distance of 0.2 to 2.5 mm (0.008 to 0.1 inch), preferably 0.25 to 1.5 mm (0.010 to 0.06 inch), particularly 0.25 to 1.0 mm (0.010 to 0.04 inch). Apparatus according to Claim 1 or Claim 2 wherein the end of the projection remote from the base provides a surface which surrounds the electrode, which lies in a plane substantially at right angles to the axis of the electrode, and which is at most 1.25 mm (0.050 inch) wide, preferably at most 0.75 mm (0.030 inch), particularly 0.2 to 0.5 mm (0.008 to 0.020 inch). Apparatus according to any preceding Claim wherein the projection has a substantially annular cross-section in all planes at right angles to the axis of the electrode and is spaced apart from the electrode by a distance of 0.25 to 1.25 mm (0.010 to 0.05 inch). Apparatus according to any preceding claim wherein the height of the projection above the base is at least 0.75 mm (0.030 inch), preferably 1.5 to 2.5 mm (0.060 to 0.1 inch). Apparatus according to any preceding claim wherein the enclosure comprises a rigid wall comprising substantially identical first and second wall portions. Apparatus according to any preceding claim wherein the enclosure encloses a plurality of substantially identical protection devices, all of the electrodes of the devices passing through substantially identical wall portions in a rigid wall of the enclosure. Apparatus according to any one of the preceding claims wherein substantially the whole of the enclosure is composed of the same material.

Inventors: RATELL, JOSEPH M.
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1986
Main classification: H01C7
IPC codes: H01H37, H01H69, H01C1, H01C7