Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
1999-03-26
2001-02-06
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S020000, C338S328000
Reexamination Certificate
active
06184769
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a monolithic varistor, and particularly to a monolithic varistor used for protecting electronic equipment from surge (abnormally high voltage).
2. Description of the Related Art
In order to cope with recent miniaturization of electronic equipment and increased signal-processing speed, electronic parts have been surface-mounted more frequently, and their operation frequencies have been increased. A non-linear resistor serving as a noise absorber is not an exception to this trend; a surface-mount-type varistor formed mainly of zinc oxide (ZnO) or strontium titanate (SrTiO
3
) has been put into practical use.
As a measure for reducing the size, especially the height, of a varistor, there has been proposed a method in which a plurality of varistor material layers and a plurality of internal electrodes are layered in order to form a monolithic varistor. However, in the case of a varistor that must have a varistor voltage of 100 V or greater, the distance between adjacent internal electrodes (hereinafter referred to as an “inter-internal electrode distance”) in the direction perpendicular to the layered varistor material layers and internal electrodes must be increased, so that employment of a layered structure is difficult.
However, thanks to recent improvements on varistor materials, the varistor voltage per unit inter-internal electrode distance has been increased, making employment of a layered structure possible in terms of varistor voltage. However, there has arisen a new problem that an increased varistor voltage causes a drastic decrease in maximum surge current that can be withstood by the varistor. Thus, the size of layered varistors cannot be decreased, and only varistors having a size similar to that of a single-layer-type varistor can be produced.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a compact monolithic varistor having an increased maximum surge current.
To achieve the above object, according to a first aspect of the present invention, there is provided a monolithic varistor comprising a sintered layered body and a pair of external electrodes disposed on opposite ends of the layered body. The sintered layered body comprises a plurality of varistor material layers and a plurality of internal electrodes, which are layered on one another. When T is defined as an inter-inner electrode distance in the direction perpendicular to the layered varistor material layers and the internal electrodes and Tx is defined as the distance between the external electrode provided on either end of the sintered layered body and the corresponding edges of the internal electrodes in a direction parallel to the layered layers, Tx is 1.5 to 3.0 times T.
Since the distance Tx between each external electrode and the corresponding edges of the internal electrodes is set to 1.5 to 3.0 times the inter-inner electrode distance, a high maximum surge current can obtained while a high varistor voltage is maintained, so that the size of a monolithic varistor can be decreased as compared with conventional single-layer type varistors.
According to a second aspect of the present invention, when Ty is defined as the distance between an outermost inner electrode and the surface of the sintered layered body, Ty is equal to or greater than T.
In this case, since the distance Ty between an outermost inner electrode and the surface of the sintered layered body is made equal or greater than the inter-internal electrode distance T, the maximum surge current is maintained constant, so that stable monolithic varistors having a reduced variation in maximum surge current can be obtained.
According to a third aspect of the present invention, Tx is 1.5 to 3.0 times T, and Ty is equal to or greater than T.
In this case, monolithic varistors having a stable and increased maximum surge current can be obtained.
In the varistors of the present invention preferably have a varistor voltage of 100 V or greater. In this case, the above-described advantageous effects become more remarkable.
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Hadano Kenjiro
Kaneko Kazuhiro
Kawada Tsuyoshi
Nakamura Kazutaka
Easthom Karl D.
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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