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Posted on 10 April 2019

Introduction to Varistors

 

 

 

 

 

 

 

Varistors are voltage-dependent resistors. They consist of a semiconductor material which is pressed in pulverised form and then sintered to create a solid disc. Their effect is based on the existence of numerous random pn-junctions at the contact points between the little fragments or grains. Most varistors are made of zinc oxide ZnO, which is why they are also known as metal-oxide varistors (MOV). The resistance of a varistor decreases as the voltage increases. In combination with a constant series resistance, this functions as a voltage divider. For brief voltage peaks, the series resistor can be replaced by an inductor. In converters featuring transformers, this is the stray inductance of the relevant transformer winding; in cases of direct connection to the mains, this is the inductance of the series reactor which is necessary here as well. Varistors can be used to attenuate overvoltage on the AC side or on the DC side, or even as single-switch snubbers.

A typical characteristic of a ZnO varistor, as given by the manufacturer, can be seen in Figure 1. This can be used to derive the corresponding peak voltage for a given peak current. The manufacturer specifies a maximum permissible peak current, which must not be exceeded even for a very short pulse duration. In ZnO varistors, the fundamental frequency power losses are normally negligibly low. One shortcoming of varistors is that they do not attenuate the voltage dv/dt. Thus, in thyristors with low dv/dt values, an additional RC snubber is needed.

ZnO varistor I-V characteristic
Figure 1. Current/voltage characteristic of a ZnO varistor (pulsed)

 

For more information, please read:

Design Notes - Varistors

Terms and Descriptions - Varistors

Selection Guide - Varistors

V-I Characteristics - Varistors

PSpice Simulation Model

 

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