Varistors

The varistor is two terminal device. The varistor, like the diode, is sensitive to the voltage across it. However, the varistor is designed to conduct at a specified forward and reverse biased voltage.

Ideally, it acts like an open if the voltage across the varistor is less than the specification voltage. If the varistor voltage is exceeded with either polarity, it turns on and holds its rated voltage across itself and any connected component.

The schematic symbol of Figure (a) reflects the diode nature of the device. The two opposing diodes reflect that the device is designed to turn on in both directions as shown by its schematic symbol and by its characteristic curve of Figure (b). This figure shows this varistor turning on when its voltage drop exceeds 100 V in either direction. If its voltage drop is less than 100 V, the varistor is off and acts like an open.

Typically a varistor is placed across another electrical component to protect it from excessive voltage drops. If the voltage drop across the varistor is less than the specified voltage, the varistor acts like an open and plays no part in the circuit. If the voltage drop across the varistor exceeds its specification, then the varistor turns on and maintains a fixed voltage drop. In the example of Figure (b), if the varistor turns on, the voltage across it and its companion component is limited to 100 V. Thus the voltage drop across the component placed across the varistor could not exceed 100 V.