PTC CONDUCTIVE POLYMER DEVICES

Abstract: A PTC device which comprises a housing (1); two metal members (2, 3), each of which comprises an elastically deformed retaining element (22, 32) inside the housing and a terminal element (21, 31) outside the housing; and (3) a PTC component (4) which is removably retained within the housing by elastic recovery forces of the retaining elements. The PTC component includes a laminar resistive element (43) composed of a PTC conductive polymer, and two electrodes (41, 42), each in contact with one of the retaining elements. The PTC device is particularly useful for providing a connection, accompanied by overcurrent protection, between two wires, e.g. in loudspeaker assemblies.

Description: PTC CONDUCTIVE POLYMER DEVICES This invention relates to PTC conductive polymer devices. Many electrical devices comprise two laminar electrodes and, sandwiched between them, a laminar resistive element which exhibits PTC (positive temperature coefficient) behavior, in particular circuit protection devices. The PTC resistive element may be composed of conductive polymer (this term being used to denote a composition comprising a polymer and, dispersed or otherwise distributed therein, a particulate conductive filler) or a ceramic, e.g. a doped barium titanate. When a conductive polymer is used, such devices are generally prepared by stamping (or otherwise cutting) a plurality ofthe devices out of a laminate of a sheet ofthe conductive polymer between two metal foils. When a ceramic is used, such devices are usually prepared by applying liquid electrode material to the major surfaces of a preformed laminar resistive element, and solidifying the liquid electrode material. Devices of this kind are sometimes installed between two separate resilient retaining members which bear on opposite faces ofthe device. Typically, there are two metal retaining members which are directly opposite to each other and which bear on the outer faces ofthe electrodes. More specialized installations are described for example in U.S. Patent Nos. 4,924,204 (Uchida) and 5,451,921 (Crawford et al.). Care is needed to ensure that an appropriate pressure is generated and maintained between the retaining members and the PTC device. One disadvantage of some ofthe known installations is that the retaining members are secured to a larger structure, e.g. a motor housing, in a way that is difficult to accomplish and/or permits the retaining members to move relative to each other, thus changing the pressure that they exert on the PTC device. In other installations, the retaining- members extend from the PTC element in the form of terminals (which may be male or female) and these terminals are connected to complementary connectors which are fixed to a circuit board or other rigid substrate. We have now discovered improved PTC devices which can be remarkably simple, and which are, therefore, cheap and easy to make, but which nonetheless offer other important advantages. The devices ofthe invention are particularly suitable for providing a connection, accompanied by overcurrent protection, between two wires whose ends are free (either because a length of wire has been cut to enable the PTC device to be inserted, or because two wires from different sources are to be joined together). We have found the devices to be particularly useful for protecting loudspeakers from excessive currents. In a first preferred aspect, this invention provides a PTC device which comprises (1) a housing which is composed of an electrically insulating material; (2) a first metal member which (a) is secured to the housing, and (b) comprises (i) a first terminal element which lies outside the housing, and (ii) a first retaining element which is elastically deformed and at least a part of which lies within and is spaced apart from the housing; (3) a second metal member which (a) is secured to the housing, and (b) comprises (i) a second terminal element which lies outside the housing, and (ii) a second retaining element which is elastically deformed and at least a part of which lies within and is spaced apart from the housing; and (4) a PTC component which (a) comprises (i) a first laminar metal electrode, (ii) a second laminar metal electrode which is parallel to the first electrode, and (iii) a laminar resistive element which is composed of a PTC conductive polymer and which is sandwiched between the first and second electrodes, (b) is positioned between the first and second retaining elements, with the first electrode being in physical and electrical contact with the first retaining element and the second electrode being in physical and electrical contact with the second retaining element, and (c) is removably maintained in position between the first and second retaining elements by elastic recovery forces exerted on the first and second electrodes by the first and second retaining elements. In a second preferred aspect, this invention provides an electrical assembly which comprises first and second wires each having a female connector secured thereto, and a PTC device according to the first aspect ofthe invention in which the terminal elements are male terminal elements respectively secured to the female connectors. The housing can be of any appropriate shape but is preferably an open box, so that the PTC element can be easily inserted and removed. Preferably the housing comprises (i) a base having first and second spaced-apart apertures therethrough, and (ii) side walls extending away from the base; and the first metal member passes through the first aperture and the second metal member passes through the second aperture. Preferably the base is rectangular, and the side walls are at right angles to the base. The side walls provide physical protection to the retaining members and the PTC element. Preferably, therefore, the walls are higher than both the PTC element and the retaining elements. The apertures in the base ofthe housing can pass through the body thereof, but preferably each is in the form of a slot having one edge defined by a side wall. The metal member and the side wall and/or the base can have cooperating members which help to provide a secure fitting between the metal members and the housing. The first and second metal members are preferably each a single piece of appropriately shaped metal. Each metal member must be secured to the housing, preferably by a secure bond between the member and the base at a single location e.g. in the region of an aperture in a base through which the member passes. Preferably the members are not otherwise secured to the housing. However, there may be slidable contact between the members and the housing so as to modify the elastic deformation ofthe member, and in consequence the forces exerted on the PTC component. The retaining elements preferably each have a generally planar portion which is parallel to the electrodes ofthe PTC component, and which preferably carry one or more convex portions which contact the electrodes. The convex portions are preferably offset from each other. There may be one convex portion on each planar portion, or two or more convex portions on one or both ofthe planar portions, preferably arranged so that the sum ofthe forces exerted on the PTC component is at right angles to the plane ofthe electrodes. The planar portion preferably lies between an elastically deformed base portion which slopes towards the other retaining element, and an entry portion which slopes away from the other retaining element. The entry portions serve to guide a PTC element into place as it is inserted between the retaining elements, and the ends thereof may bear against the side walls ofthe housing to modify the elastic deformation ofthe retaining elements. The retaining elements are preferably mirror images of each other except for the convex portions. The terminal elements are preferably male terminals. Especially when they are, the housing preferably includes an insulating separating member which extends from a housing base between the terminal elements, and which ensures that there can be no electrical contact between terminated wires which are secured to the terminal elements. The housing may also comprise means for securing the device to a structure, e.g. a loudspeaker box. The PTC conductive polymers used in the present invention preferably comprise a crystalline polymer component and, dispersed in the polymer component, a particulate filler component which comprises a conductive filler, e.g. carbon black or a metal. The filler component may also contain a non-conductive filler, which changes not only the electrical properties ofthe conductive polymer but also its physical properties. The composition can also contain one or more other components, e.g. an antioxidant, crosslinking agent, coupling agent or elastomer. For use in circuit protection devices, the PTC composition preferably has a resistivity at 23 °C of less than 50 ohm-cm, particularly less than 10 ohm-cm, especially less than 5 ohm-cm. Suitable conductive polymers for use in this invention are disclosed for example in U.S. Patent Nos. 4,237,441 (van Konynenburg et al), 4,304,987 (van Konynenburg), 4,388,607 (Toy et al), 4,514,620 (Cheng et al), 4,534,889 (van Konynenburg et al), 4,545,926 (Fouts et al), 4,560,498 (Horsma et al), 4,591,700 (Sopory), 4,724,417 (Au et al), 4,774,024 (Deep et al), 4,935,156 (van Konynenburg), and 5,049,850 (Evans et al). When a PTC device is tripped, most ofthe voltage dropped over the device is normally dropped over a relatively small part ofthe device which is referred to as the hot line, hot plane or hot zone. In the devices ofthe invention, the PTC element can have one or more features which help the hot line to form at a desired location, usually spaced apart from both electrodes. Suitable features of this kind for use in the present invention are disclosed for example in U.S. Patents Nos. 4,317,027 and 4,352,083 (Middleman et al), 4,907,340 and 4,924,072 (Fang et al). Particularly useful PTC elements for use in this invention comprise two metal foil electrodes, and a PTC conductive polymer element sandwiched between them, especially such elements which have low resistance at 23 °C, generally less than 50 ohm, preferably less than 10 ohm, more preferably less than 5 ohm, particularly less than 3 ohm, e.g. less than 0.1 ohm. Particularly suitable foil electrodes are electrodeposited nickel foils and nickel-plated electrodeposited copper foil electrodes, in particular as disclosed in U.S. Patents Nos. 4,689,475 (Matthiesen) and 4,800,253 (Kleiner et al). A variety of laminar devices which can be used in the present invention are disclosed in U.S. Patent Nos. 4,238,812 (Middleman et al), 4,255,698 (Simon), 4,272,471 (Walker), 4,315,237 (Middleman et al), 4,317,027 (Middleman et al), 4,330,703 (Horsma et al), 4,426,633 (Taylor), 4,475,138 (Middleman et al), 4,724,417 (Au et al), 4,780,598 (Fahey et al), 4,845,838 (Jacobs et al), 4,907,340 (Fang et al), and 4,924,074 (Fang et al). The electrodes can be modified so as to produce desired thermal effects. Referring now to the drawings, Figures 1 and 2 show side views, at right angles to each other, of a PTC device ofthe invention, Figure 1 being taken on line I-I of Figure 2, and Figure 2 being taken on line II-II of Figure 1. In the Figures, there is an insulating housing 1, to which are attached first and second metal members 2 and 3, each of which comprises a terminal element 21,31 which lies outside the housing 1 and a retaining element 22,32 which lies within the housing 1. The retaining members 22,32 are elastically deformed and removably retain between them a PTC component 4 which comprises laminar metal electrodes 41,42 and a PTC conductive polymer resistive element 43 sandwiched between the electrodes. Each ofthe retaining elements 22,32 comprises convex portions 221,321 which contact respective electrodes 41,42; the convex portions are shown directly opposed to each other but can be offset. The housing includes insulating separating member 11 which extends from the base ofthe housing between the first and second terminal elements 21,31. On one side of the housing there is a layer 5 of an adhesive which can be used to secure the housing to a loudspeaker assembly.

Claims: CLAIMS 1. A PTC device which comprises (1 ) a housing which is composed of an electrically insulating material; (2) a first metal member which (a) is secured to the housing, and (b) comprises (i) a first terminal element which lies outside the housing, and (ii) a first retaining element which is elastically deformed and at least a part of which lies within and is spaced apart from the housing; (3) a second metal member which (a) is secured to the housing, and (b) comprises (i) a second terminal element which lies outside the housing, and (ii) a second retaining element which is elastically deformed and at least a part of which lies within and is spaced apart from the housing; and (4) a PTC component which (a) comprises (i) a first laminar metal electrode, (ii) a second laminar metal electrode which is parallel to the first electrode, and (iii) a laminar resistive element which is composed of a PTC conductive polymer and which is sandwiched between the first and second electrodes, (b) is positioned between the first and second retaining elements, with the first electrode being in physical and electrical contact with the first retaining element and the second electrode being in physical and electrical contact with the second retaining element, and (c) is removably maintained in position between the first and second retaining elements by elastic recovery forces exerted on the first and second electrodes by the first and second retaining elements. 2. A device according to claim 1 wherein the first retaining element has a first generally planar portion which is adjacent and parallel to the first electrode, and the second retaining element has a second generally planar portion which is adjacent to and parallel to the second electrode. 3. A device according to claim 2 wherein the first generally planar portion comprises at least one first convex portion which contacts the first electrode, and the second generally planar portion comprises at least one second convex portion which contacts the second electrode. 4. A device according to claim 3 wherein there are at least two first convex portions, and the sum ofthe forces exerted on the PTC component by the first and second convex portions is at right angles to the plane ofthe electrodes. 5. A device according to any of claims 2 to 4 wherein each ofthe retaining elements comprises (a) an elastically deformed base portion which slopes towards the base portion of the other retaining element, and (b) an entry portion which slopes away from the remote portion of the other retaining element, the planar portion of each retaining element lying between the base portion and the entry portion ofthe retaining element. 6. A PTC device according to any ofthe preceding claims wherein (a) the housing comprises (i) a base having first and second spaced-apart apertures therethrough, and (ii) side walls extending away from the base; and (b) the first metal member passes through the first aperture and the second metal member passes through the second aperture. 7. A device according to claim 7 wherein the housing includes an insulating separating member which extends from the base between the first and second terminal elements. 8. A device according to claim 8 wherein each ofthe first and second terminal elements is a male connector. 9. An electrical assembly which comprises (A) a first wire having a first female connector secured thereto; (B) a second wire having a second female connector secured thereto; and (C) a PTC device as claimed in any of claims 1 to 8, the first terminal element being a male terminal element which is connected to the first female connector, and the second terminal element being a male terminal element which is connected to the second female connector. 10. An assembly according to claim 9 which is mounted in a loudspeaker apparatus, the first and second wires conveying an audio signal to a loudspeaker which is also mounted in the loudspeaker assembly.

Inventors: FUKUYAMA SYOZO, HASANUMA TAKASHI, MIYASAKA NAOFUMI, FUKUYAMA, SYOZO, HASANUMA, TAKASHI, MIYASAKA, NAOFUMI
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1997
Main classification: H01C7
IPC codes: H01C1, H01C7