EP0058336B1

EP0058336B1 - Zinc oxide varistor with reduced fringe current effects

Info

Publication number: EP0058336B1
Application number: EP19820100710
Authority: EP
Inventors: Herbert Reynold Philipp
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1981-02-13
Filing date: 1982-02-02
Publication date: 1987-08-19
Also published as: EP0058336A3; DE58336T1; EP0058336A2; DE3277042D1; JPS57152106A; MX157914A

Description

Background of the Invention

The application relates to the technical field of zinc oxide varistors.
US-Patent 3 913 056 discloses a method for preventing currents from channeling through preferred current paths within zinc oxide varistors.
Zinc oxide varistors, as currently manufactured, comprise a sintered disc of zinc oxide material having a pair of electrodes on opposing surfaces of the disc. The perimeter of the disc is covered with an insulating layer of glass or ceramic material to prevent the transport of current across the outside surface of the disc perimeter. When the electrodes are applied to the flat faces of the disc, a portion of the disc material in the vicinity of the disc rim is not covered with the electrode material because the physical transition from the flat disc surface to the peripheral rim may not be well defined. During varistor operation, current transports directly through the zinc oxide material covered by the electrode material. However, in actuality some current also transports through the zinc oxide material in the vicinity of the rim not covered by electrode material. This current is referred to as "fringing current" and can be a source of failure when the varistor is operated during conditions of overload. The transport of fringing current in the rim portion of the disc not covered by electrode material causes a concentration of current at the edges of the disc electrodes. The current concentration at the electrode edges causes an excess electrical stress in this area which leads to premature disc failure during overload conditions.
The object of this invention is to provide a zinc oxide varistor having substantially reduced fringing current effects and consequently a greater overload capability.

Summary of the Invention

According to the invention there is provided a zinc oxide varistor comprising a disc-shaped sintered body having opposed faces and a circumferential wall between said opposed faces, and an electrode applied to each said face, the edges of said electrodes being spaced from the edges of said body to leave circumferential marginal portions of said faces devoid of electrode material, said zinc oxide varistor having the features of the characterizing portion of claim 1 or of claim 5.

Brief Description of the Drawings

Figure 1 is an end view of a zinc oxide varistor according to the prior art in partial section showing the transport of varistor current and fringing cu rrent;
Figure 2 is a side view in partial section of a zinc oxide varistor having a reduced cross sectional area in the vicinity of the varistor rim; and
Figure 3 is a side view in partial section of a zinc oxide varistor having a pair of circumferential grooves extending around the disc perimeter.

Description of the Preferred Embodiment

A zinc oxide varistor 10 having the composition disclosed within U.S. Patent 3,928,245 is shown in Figure 1. Varistor 10 comprises a sintered zinc oxide disc 11 having a pair of top and bottom metallic electrodes 12A, 12B on opposing surfaces. Electrodes 12A, 12B extend across disc 11 leaving a top portion D_i between edge 13 of top electrode 12A and edge 14 of disc 11. A portion D₂ on the bottom of disc 11, between edge 15 of bottom electrode 12B and edge 14 of disc 11, is also not covered by electrode material. The height of disc 11 is defined by D₃. Dotted path 16 extending between top electrode 12A and bottom electrode 12B through disc 11 defines "fringe" area S₁ through which fringe current B₁ transports in the direction indicated by arrows. The direction of ordinary current transport between electrodes 12A, 12B is depicted by arrow A. As described earlier, the transport of current through fringe area S₁ causes damage to the disc material near the edges 13 and 15 of top and bottom electrodes 12A and 12B respectively.
Dotted paths A and B₁ of Figure 1 represent filaments of current flowing from top electrode 12A to bottom electrode 12B due to a difference of potential between the two electrodes. The relationship between current density and electric field E in a zinc oxide disc is approximately given by the equation J = KE" where K is a constant of proportionality and n is an exponent which ranges between approximately 15 and 60.
When dotted paths A and B₁ of Figure 1 represent paths of constant current density, the field along each path is constant and inversely proportional to the path length.
For example, when path B₁ is 2 percent longer than path A, the field along path B₁ is 0.98 times as great as path A. For a value of exponent n equal to 15, the current density along path B₁ is (0.98)¹⁵, which equals 0.74 times path A. For a value of exponent n equal to 40, the current density along path B₁ is (0.98)⁴°, which equals 0.45 times path A. When the field is reduced to 0.96 times path A, by increasing the path length by 4 percent, the current density along path B₁ is (0.96)⁴°, which equals 0.20 times path A for a value of exponent n equal to 40. This shows, therefore, that the higher the value of exponent n, or the longer the average effective fringing current path B_i, the lower the fringing current density.
Figure 2 shows a zinc oxide varistor 10 having a top electrode 12A and a bottom electrode 12B with areas D₁ and D₂ between electrode edges 13 and 15 and disc periphery 14. Perimeter 14 in this embodiment is concave such that the minimum disc diameter approximates that for the electrodes 12A, 12B. Fringing current path B₂, as shown, is greater than fringing current path B₁ of Figure 1. Consequently, the fringing current density is greatly reduced for the embodiment depicted in Figure 2.
A disadvantage with the embodiment of Figure 2 is that the disc corners 17 are sharp and become subject to damage by chipping.
A further embodiment having reduced fringing current is shown in Figure 3 to consist of a zinc oxide disc 11 similar to that of Figure 1, with at least one circumferential groove 18 formed by abrading, machining or mold-forming techniques. For the embodiment shown in Figure 3 two grooves 18 are shown, one proximate the top and the other proximate the bottom of disc 11 and extending within disc 11 to a distance approximately in line with electrode edges 13 and 15. The fringing current path B₃ is seen to be greater than B₁ of Figure 1, such that the fringing current density is proportionately reduced. It is to be clearly understood that the arrangement of grooves 18 can be modified depending upon the desired results. One or more grooves 18 can be employed, and the shape and depth of the grooves can be designed to accommodate various manufacturing techniques.
A specific example containing two grooves 18, such as shown in Figure 3, and ranging from about 1.5 to 2.5 mm (0.06 to 0.10 inches) in radius extending entirely around the perimeter of disc 11 was manufactured having the following dimensions. Height D₃ measures about 34 mm (1.375 inches) and separation distances D₁ and D2 for top and bottom electrodes 12A and 12B measured about 2 mm (approximately 0.086 inches). The maximum electric strength measured higher for this example than any previous varistors having the configuration shown in Figure 1.
Varistor 10 also contained an insulating coating of glass or ceramic material 9 around the perimeter of the disc and covering the zinc oxide material within grooves 18. In some instances, where the voltage demand across varistor 10 is not too high, a plurality of grooves 18 extending along the disc perimeter can increase the voltage "creep" to the extent that no glass or ceramic coating 9 need be employed. The addition of a glass or ceramic coating 9 over discs having a plurality of grooves 18 further increases the breakdown voltage capability of varistor 10.

Claims

1. A zinc oxide varistor (10) comprising: a disc-shaped sintered body (11) having opposed faces and a circumferential wall between said opposed faces, and an electrode (12A, 12B) applied to each said face, the edges (13, 15) of said electrodes being spaced from the edges of said body to leave circumferential marginal portions (D₁, D₂) of said faces devoid of electrode material, characterized by atleast one circumferential groove (18) created in said circumferential wall, the depth of said groove being chosen to limit the overload current densities in the positions of said body located adjacent the electrode edges and the bottom of said groove.

2. The zinc oxide varistor defined in claim 6, wherein the depth of said groove (18) is approximately equal to the width of said circumferential marginal portions (D_i, D₂).

3. The zinc oxide varistor defined in claim 2, wherein said circumferential wall is provided with a pair of circumferential grooves (18).

4. The zinc oxide varistor defined in claim 3, wherein the spacing between said grooves (18) is greater than the spacing between each said groove and the face of said body adjacent thereto.

5. A zinc oxide varistor (10) comprising: a disc-shaped sintered body (11) having opposed faces and a circumferential wall between said opposed faces and an electrode (12A, 12B) applied to each said face, the edges (13, 15) of said electrodes being spaced from the edges of said body to leave circumferential marginal portions (D_i, D₂) of said faces devoid of electrode material, characterized by said disc wall having a concave configuration (14).