Varistor

A varistor, short for voltage-dependent resistor, is a two-terminal nonlinear electronic device used primarily for surge suppression. It exhibits a highly non-linear current–voltage relationship: at normal operating voltages it presents a high resistance, but when the voltage exceeds a certain threshold, its resistance collapses rapidly and it conducts strongly to clamp the voltage on the protected circuit.

Most varistors are ceramic zinc oxide (ZnO) devices with dopants at the grain boundaries. The active material is a polycrystalline zinc oxide main phase with small amounts of metal oxides that create a network of grain-boundary barriers. Electrodes are placed on opposite faces, forming a two-terminal component. The result is a symmetric I–V response with a sharp turn-on, making varistors suitable for both line-to-line and line-to-ground protection.

Ratings describe the voltage at which clamping begins and the energy the device can absorb. Varistors have a nominal varistor voltage (V_VR or V_nom), often chosen so the device begins to conduct around the normal peak voltage of the system (for example, many line-voltage MOVs are rated around 275 VAC, corresponding to a varistor voltage in the hundreds of volts). Clamping voltage is specified at a defined surge current, and energy or surge ratings quantify how much energy the device can absorb. Temperature can shift the behavior, and leakage increases with temperature; repeated surges can cause gradual degradation and eventual failure (often to a short or open circuit).

Common forms include disc-type radial-leaded MOVs for through-hole mounting and surface-mount MOVs for automated assemblies. They are widely used across power supplies, consumer electronics, and telecom equipment to protect sensitive circuits from voltage transients. Standards such as IEC 61000-4-5 govern surge environments in which MOVs are employed.