JP2017054868A - Varistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compact and flow-adaptable varistor that is set to an open mode even when the varistor is broken because of application of an overvoltage thereto.SOLUTION: In a varistor in which a varistor element 11, an external electrode 12 disposed on the surface of the varistor element 11, and a pair of frame terminals 13 joined to the external electrode 12 are covered with an insulating outer body 14, the pair of frame terminals 13 include joining portions 13a joined to the external electrode 12, protruding portions 13b protruding from an exterior body 14, and connecting portions 13c between the joining portions 13a and the protruding portions 13b. Heating portions 13d having the maximum current density are formed in the connecting portions 13c.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a varistor used in various electronic devices.

  Electronic parts used in automobiles, industrial equipment, etc. are remarkably changing in the usage environment, and revisions to standards that have not been required for this kind of electronic parts and their uses are becoming increasingly diverse. As a result, there is an increasing demand for electronic components that protect vulnerable electronic circuits from unexpected noise and pulses. Generally, varistors are used for these demands. However, when an overvoltage is applied to the varistor, the varistor breaks and enters a short mode. If the current continues to flow in the short mode, it may generate heat and damage other parts. On the other hand, heat generation is prevented by combining a varistor and a thermal fuse.

  As prior art document information related to the invention of this application, for example, Patent Document 1 is known.

International Publication No. 2011-162181

  However, a combination of a varistor and a thermal fuse has a problem that the shape becomes large or reflow soldering cannot be performed.

  In order to solve the above-mentioned problems, the present invention comprises a varistor element, electrodes disposed on both sides of the varistor element, and a pair of frame terminals joined to these electrodes, which are covered with an insulating exterior body. It is a varistor, and a pair of frame terminals is composed of a joint part joined to an electrode, a projecting part projecting from an exterior body, and a connection part between the joint part and the projecting part, and the current density in the connection part The largest heat generating part is formed.

  According to the configuration of the present invention, when an overvoltage is applied to the varistor and the current continues to flow in a state of being broken and in the short mode, heat is generated at the connection portion having a high resistance value, the connection portion is melted, and the current is Stop flowing. In addition, since a complicated mechanism such as when combined with a thermal fuse is not required, it can be miniaturized and can be adapted to reflow soldering.

Sectional drawing of the varistor in one embodiment of this invention The top view before the assembly of the frame terminal used for the varistor in one embodiment of the present invention

  Hereinafter, a varistor according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a step view of a varistor according to an embodiment of the present invention. In FIG. 1, a varistor element 11 includes a plurality of varistor layers 15 made of a ceramic material containing zinc oxide as a main component and internal electrodes 16 facing each other with the varistor layer 15 interposed therebetween. A pair of external electrodes 12 that are electrically connected to the internal electrode 16 are formed on both end faces of the element 11. Further, the frame terminal 13 is electrically and mechanically joined to the external electrode 12, and the entire varistor element 11 is molded with an exterior body 14 made of epoxy resin.

  The frame terminal 13 protrudes from the side surface of the exterior body 14 and serves as an electrode for connection to an external circuit. A portion of the varistor element 11 that is joined to the external electrode 12 is a joint portion 13a, a portion of the frame terminal 13 that is outward from the exterior body 14 is a projection 13b, and a portion between the joint 13a and the projection 13b. The frame terminal 13 is referred to as a connection portion 13c.

  FIG. 2 is a top view of the frame terminal 13 before assembly. The frame terminal 13 is made of an alloy mainly composed of tin-plated copper having a thickness of about 0.1 mm and a width of about 2.5 mm. Assemble as a varistor. Here, a through hole 17 having a diameter of about 1.0 mm is formed at a position to be the connection portion 13c. When current flows through the frame terminal 13, the current density is maximized at the portion where the cross-sectional area is the smallest. Therefore, the portions on both sides of the through hole 17 become the heat generating portion 13d having the highest current density.

  The operation of the varistor configured as described above will be described.

  The varistor is normally connected between the signal line or power supply line and ground, and hardly conducts current during normal operation, but when a high voltage such as a pulse is applied, it becomes low resistance and protects the device by letting it escape to ground. To do. If the voltage is higher than the predetermined voltage, the varistor returns to normal, but if an extremely large current flows, the varistor may break down and enter a short mode. When the varistor connected to the power supply line enters the short mode, current continues to flow, and the temperature rises further, possibly causing damage to peripheral components.

  On the other hand, in the present embodiment, when an excessive current flows through the varistor element 11 and the varistor element 11 enters the short mode, the temperature of the varistor element 11 becomes 400 ° C. or higher. In such a state, the heat is conducted to the frame terminal 13, and the frame terminal 13 is also in a high temperature state. Since the resistance value of the metal increases as the temperature rises, the frame terminal 13 also has a higher resistance value than that at room temperature. In such a state, the connecting portion 13c of the frame terminal 13 is provided with the heat generating portion 13d having the maximum current density. Can prevent further temperature rise.

  Here, it is important that the heat generating portion 13d is provided inside the exterior body 14. By being provided in the exterior body 14, it becomes easy to become a high temperature state by the heat from the varistor element 11, and it can prevent being cooled by outside air. The position of the heat generating portion 13d is more preferably formed at a position closer to the protruding portion 13b than the joint portion 13a. This is because the tin plating provided on the surface of the frame terminal 13 is melted to easily form a cavity, and the fusing at the heat generating portion 13d can be more easily performed.

  Further, when W is the width of the connecting portion 13c and A is the maximum width of the through hole 17, it is more preferable that 1/2 ≦ (W−A) / W ≦ 3/4. If (W−A) / W is smaller than ½, the strength of the frame terminal 13 becomes weak, which is not preferable. On the other hand, if (W−A) / W is larger than 3/4, fusing at the heat generating portion 13d becomes difficult, which is not preferable.

  The heat generating portion 13d is formed by providing the through hole 17 in the connecting portion 13c. However, the heat generating portion 13d may be formed by a method such as partially reducing the width or partially reducing the thickness. Absent. However, the provision of the through hole 17 makes it difficult for the frame terminal 13 to be removed from the exterior body 14, and therefore it is more preferable to form the heat generating portion 13d by the through hole.

  In order to set the varistor to the open mode, it is possible to provide the two frame terminals 13 with a heat generating portion 13d on one side. However, since the heat generating positions of the varistor elements are likely to vary, both the frame terminals 13 are used. It is desirable to provide the heat generating part 13d in

  The varistor according to the present invention can provide a varistor that can be in an open mode as a varistor even when an overvoltage is applied and the varistor element is in a short mode, and that can be reduced in size and can be used for reflow soldering. Can be industrially useful.

DESCRIPTION OF SYMBOLS 11 Varistor element 12 External electrode 13 Frame terminal 13a Joint part 13b Protrusion part 13c Connection part 13d Heat generating part 14 Exterior body 15 Varistor layer 16 Internal electrode 17 Through-hole

Claims (4)

A varistor comprising: a varistor element; an external electrode disposed on a surface of the varistor element; and a pair of frame terminals joined to the external electrode; and the pair of frame terminals Is composed of a joint part joined to the external electrode, a projecting part projecting from the exterior body, and a connection part between the joint part and the projecting part, and heat generation that maximizes the current density in the joint part. A varistor characterized by forming a part. The varistor according to claim 1, wherein the heat generating portion is formed in the connection portion by providing a through hole in the connection portion. The varistor according to claim 1, wherein the heat generating portion is formed at a position of the connecting portion that is closer to the protruding portion than the joint portion. The varistor according to claim 2, wherein when the width of the connecting portion is W and the maximum width of the through hole is A, 1 / 2≤ (W-A) / W≤3 / 4.

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