SURGE ABSORBING DEVICE

Abstract: External terminals (8a,8b) made by the slits (9) extend from the centre of the electrode plate (7a), such that the electrode plates (7a),(7b) come into uniform contact with the electrode (6) and good soldering can be obtained. At the same time, a current density inside the varistor becomes uniform so that satisfactory high resistance to a surge current can be obtained.

Description: The present invention relates to a surge absorber for protecting electronic devices from abnormal over voltages such as a switching surge voltage, a lighting surge voltage, and so forth. Current diversification of functions of electronic devices have put electronics using semiconductors into practical use in various fields including household devices, data processing devices, communication systems and industrial devices. Semiconductors such as ICs, LSIs, thyristors and so forth used in these electronics have excellent performance but are very sensitive to abnormal voltages which may be applied by electrostatic discharges, lightning and so forth, so that problems such as malfunction or breakdown of the electronic devices tends to be caused when abnormal voltages are applied thereto. It is therefore a matter of great significance to be able to take suitable measures to protect against surge voltages from the point of view of security and improvement of the electronic devices. Figs. 4(a), 4(b) and 4(c) show an example of a known surge absorber. Referring to these Figures, a reference numeral 1 denotes a tabular varistor element usually made of semiconductor ceramics mainly composed of zinc oxide, strontium titanate or the like, while 2 denotes electrodes which are provided on the corresponding portions of the front and rear sides of the varistor element 1 by, for example, firing of silver paste. The electrode 2 on the rear side of the varistor element 1 is now shown in these Figures. Numerals 3a and 3b show metallic electrode plates made of materials which have a high degree of electrical conductivity such as copper or brass. The electrode plates 3a and 3b are connected to the electrodes 2 by solder paste soldering. A reference numeral 4a designates an external terminal which extends from a portion of the electrode plate 3a. Usually, the external terminal 4a is made of the same material as the electrode plate 3a. This applies also to another external terminal 4b provided on the rear side. The ends of the external terminals 4a and 4b are connected to an electric circuit by soldering or by means of bolts. Practically, the surge absorber is resin-coated or resin-molded such that only the ends of the external terminals are exposed (not shown in Figs. 4(a) to 4(c)). The operation of this known surge absorber will be described hereinunder. The external terminals 4a and 4b are connected between the power supply lines, signal lines or grounding lines of the device to be protected, so as to absorb any abnormal voltage which has been introduced into these lines due to, for example, electrostatic discharge, lightning surge, and so forth. The surge current produced by the abnormal voltage flows from the external terminal 4a on the front side of the surge absorber to the external terminal 4b on the rear side thereof, through electrode plate 3a, electrode 2, varistor element 1, and the corresponding portions on the rear side of the surge absorber so that the voltage, suppressed to a safe level, is applied to the device to be protected. This known arrangement, however, suffers from a disadvantage in connection with the production of the devices. Namely, in producing the surge absorber, external terminals 4a and 4b are held by means of jigs so as to support the varistor element 1 therebetween by spring action of the external terminals 4a, 4b during soldering of the electrodes 2 and the electrode plates 3a, 3b together. In such a case, the varistor element 1 is supported solely by the external terminals 4a and 4b only at the free ends thereof so that a slight variation in the thickness of the varistor element 1 prevents uniform contact between the electrode plates 3a, 3b and the electrodes 2 on the element 1, resulting in non-uniform soldering. In order to solve the abovementioned problem, Japanese Utility Model Publication No. 44-17568 discloses a resistor terminal comprising a terminal plate and an external connection terminal which, in essence, extends from the centre part of the terminal plate by virtue of narrow cut-outs formed in the terminal plate on either side of the external connection terminal. The cut-outs provide the external connection terminal with a flexibility which allows the terminal plate 17 to make uniform contact with a varistor electrode before soldering them together. However, even with the use of such a terminal, it is likely that fluxes and air bubbles will remain between the terminal plate and the varistor electrode, resulting in a poorly soldered connection. These unfavourable states undesirably impair the performance with regard to withstanding the surge current capacity, which is one of the most critical requirements for the surge absorber, resulting in a lowered reliability of the surge absorber. The soldering of the electrode plates 3a, 3b to the electrodes 2 is usually conducted by printing paste solder on the surfaces of the electrodes 2, drying the paste solder and pressing the electrodes plates 3a, 3b onto the electrodes 2 under application of heat. Such a method, however, is costly. In order to overcome the above-described problems of the prior art, the present invention provides a surge absorber in which the electrode plates to be soldered to the electrodes on both sides of a tabular varistor have a specific structure. Each electrode plate to be soldered to the electrode of the varistor has a plurality of line electrodes which extend from a single point or a line. With this arrangement of the invention, since each electrode plate has the form of a plurality of line-like electrodes, soldering can be achieved uniformly without allowing flux and air voids to remain between the electrode plates and the electrodes. Further this arrangement also reduces the production cost remarkably because the soldering can be effected by a solder dip method without the aid of solder paste. Figs. 1 to 3 are illustrations of different embodiments of a surge absorber in accordance with the present invention in which sections (a), (b) and (c) are side elevational views, front elevational views and perspective views, respectively. Figs. 4(a), 4(b) and 4(c) are a side elevational view, a front elevational view and a perspective view of a conventional surge absorber respectively. Fig. 1 shows an embodiment of a surge absorber in accordance with the present invention. Sections (a), (b) and (c) of this figure are a side elevational view, a front elevational view and a perspective view of this embodiment. Referring to Fig. 1, a reference numeral 5 denotes a varistor element made of a material mainly composed of, for example, zinc oxide, 6 denotes electrodes formed on the front and rear sides of the varistor element 5, and 8a and 8b denote external terminals. These portions correspond to the varistor element 1, electrodes 2 and external terminals 4a, 4b of the conventional surge absorber. Numerals 10a and 10b denote, respectively, one of the line electrodes which extend radially outwardly from a point. These line electrodes are soldered to a corresponding electrode 6 by dip-soldering. The line electrodes 10a and 10b are made of a material which have high electrical conductivity, e.g., copper, a copper alloy or iron. Each line electrode 10a, 10b has a width of 0.5 to 2.5 mm, and the external electrodes 8a and 8b are connected to the central regions from which the line electrodes 10a and 10b radially extend. Further, according to the present invention, the surge absorber in its entirety is covered or molded with an insulation resin coat except perhaps for the free end parts of the external terminals 8a, 8b. However, the insulation resin coat is not shown in Fig. 1, for the sake of brevity. Incidentally, in the case of forming an insulation layer made of glass, resin or the like, at the periphery of the side surfaces of the varistor 5, the abovementioned resin coat is not always indispensable. Meanwhile, the external terminals according to the present invention, can have a role of support means for supporting the body of the surge absorber in addition to their intrinsic electric terminal role. The operation of the surge absorber having the described construction will be explained hereinunder. As in convention case, a surge current flows through the varistor element 5 when a surge voltage is applied thereto and the surge is absorbed substantially in the same manner as in the conventional case. In this embodiment, since the connection to the electrodes 6 is achieved through a plurality of radial line electrodes 10a, 10b, flux and air voids generated in the course of soldering can easily escape through the gap between adjacent line electrodes without being trapped between the line electrodes and the electrodes 6, whereby the line electrodes 10a, 10b are uniformly soldered to the electrodes 6, thus preventing any degradation in the performance with regard to the surge current capability. The use of the radial line electrodes 10a, 10b enables the use of the solder-dip method which is inexpensive to carry out. Namely, the external terminals 8a and 8b are suitably held such that the varistor element 5 is pinched between the line electrodes 10a, 10b and these parts are dipped in a solder bath whereby the soldering is effected without requiring printing of solder, drying and heating which have to be employed in ordinary paste soldering methods. Further, since the external terminals 8a, 8b are bent toward each other, less space is required to connect the electric terminals, and further, they have a self-supporting ability. Moreover, if the surge absorber is covered or molded with a resin coat or the like in its entirety, except for the free ends of the external terminals 8a, 8b, it is possible to enhance the weather resistance and insulation of the surge absorber. A second embodiment of the present invention will be described with reference to Fig. 2. Sections (a), (b) and (c) of this Figure are a side elevational view, a front elevational view and a perspective view respectively of the second embodiment. This second embodiment is discriminated from the first embodiment only in the form of the line electrodes. Namely, in contrast to the line electrodes 10a, 10b which radially extend from a point, the line electrodes 11a, 11b in this embodiment extend from both sides of a single linear line-like branch part. This surge absorber operates substantially in the same manner as the embodiment shown in Fig. 1. A third embodiment of the present invention will be described with reference to Fig. 3. Sections (a), (b) and (c) of this Figure are a side elevational view, a front elevational view and a perspective view respectively of the third embodiment. In this embodiment, the line electrodes are wholly or partially connected at their outer ends. Namely, line electrodes 12a and 12b which are similar to those of Fig. 1 are connected together at their outer ends by means of a common line electrode 13. Although in the embodiment shown in Fig. 3 all the line electrodes 12a or 12b are connected together at their outer ends, this is only illustrative and some of these line electrodes may not be connected. This surge absorber operates in the same manner as that shown in Fig. 1. However, entanglement of the ends of the line electrodes 12a and 12b is avoided by the provision of the line electrodes 13 so that the efficiency of the assembly work is improved and the force with which the varistor element is held is also increased advantageously. As has been described, according to the present invention, each electrode plate is constructed in the form of a plurality of line electrodes which extend from a single point or a single line, and an external terminal is led from one of these line electrodes. Thus, according to the invention, solder between the electrode plates and the electrodes on both sides of the varistor element, can be effected uniformly without allowing flux and air voids to remain between the electrode plates and the electrodes, whereby a surge absorber having an improved surge current capability and high degree of reliability can be obtained. In the embodiment in which the electrode plate has the form of a plurality of line electrodes, soldering can be effected by a solder dipping method without making use of solder paste, so that the cost can be reduced remarkably. In this embodiment, entanglement of the line electrodes at their free ends can be avoided by providing a line electrode which connect at least some of the line electrodes. This arrangement also contributes to improvement in the efficiency of the assembly work and ensures that the varistor element is held with a higher stability. In addition, in the arrangement in which the ends of the external terminal are bent inwardly as viewed in the direction of the thickness of the varistor element, the space for connection of the electrical terminals is advantageously saved. Furthermore, a higher degree of weather resistance and insulation power can be obtained by coating or molding the surge absorber such that only the ends of the external terminals are exposed.

Claims: A surge absorber comprising a tabular varistor element (5), an electrode (6) provided on each side of said varistor element, and electrode plates (10, 11, 12) soldered to the respective electrodes, characterised in that each of said electrode plates has a plurality of line electrodes (10a,b, 11a,b, 12a,b) which extend from a point or a line, and an external terminal (8a, 8b) which is led from one of said line electrodes. A surge absorber according to claim 1, characterised in that the ends of said line electrodes (10a,b, 11a,b, 12a,b) are wholly or partially connected through a line electrode. A surge absorber according to claim 1, characterised in that said external terminals (8a, 8b) extend substantially in the same direction and the outer ends of said external terminals are bent inwardly as viewed in the direction of the thickness of said varistor element (5). A surge absorber according to claim 1, characterised in that said surge absorber is coated or molded with an insulating resin such that only the ends of said external terminal (8a,b) are exposed.

Inventors: HASHIZUME TAIZOU - -CHOME, IGARASHI HOMERU SUMIYOSHI-CHO, SUMIYOSHI MIKIO - NAGAONISHIMA, HASHIZUME, TAIZOU 1206-386, 8-CHOME, IGARASHI, HOMERU 375, SUMIYOSHI-CHO, SUMIYOSHI, MIKIO 17-23, NAGAONISHIMACHI 1-CHOME
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H01C7
IPC codes: H01C7, H01C1