Varistor

A varistor is an electronic component designed to protect circuits against excessive voltage. Construction is quite similar to that of a capacitor - just like a capacitor it consists of two metal plates separated by an insulator. When the voltage between the two plates reaches a certain value, the insulator breaks down and admits the flow of current.

Varistors have a capacitance and could be called capacitors; likewise, all capacitors have a breakdown voltage. The difference is that in most capacitors, breakdown is highly undesirable, and usually results in the destruction of the device. Varistors on the other hand are designed to repeatedly withstand breakdown.

Important parameters for varistors are response time (how long it takes the varistor to break down), maximum current and a well-defined breakdown voltage. Capacitance is undesirable since high capacitance allows high frequency signals to leak away.

See also transient voltage suppression diode[?]