Varistors short circuit when an overage in potential is applied to it. These components are made of a material that’s insulating until a predetermined voltage appears across the part. When this voltage occurs, avalanche breakdown causes the varistor to act as a current shunt (Fig. 1); this prevents damage to the load that may result if failure of the protection circuit is undetected. When using varistors, ensure to select one with good durability against repeated surges.
Fig. 1: Typical Normalised Voltage Versus Current Characteristics Curve for Standard Varistors
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