varistor

A varistor is a semiconductor component whose resistance drops sharply when voltage across it exceeds a threshold. The name combines "variable" and "resistor". Unlike a fixed resistor, a varistor exhibits highly nonlinear behavior: at normal operating voltage it conducts almost nothing, but the moment overvoltage arrives, it clamps down hard and shunts current away from sensitive downstream equipment. This makes it the workhorse of surge protection in industrial control panels, power supplies, and motor drive circuits.

The most common type is the metal oxide varistor (MOV), typically made from zinc oxide granules bonded with ceramic material, pressed into a disc and metalized on both faces. MOVs are rated by their clamping voltage (the voltage at which they conduct heavily) and energy absorption capacity, measured in joules. Typical industrial MOVs range from 14 mm to 32 mm diameter and clamp voltages span from 20 V to 1000 V or more. A 230 V three-phase supply, for instance, might use MOVs rated at 275 V or 320 V clamping voltage on the AC mains.

How they work in practice

Varistors are wired in parallel with the circuit they protect. During normal operation they draw negligible current. When a surge arrives, the MOV's resistance collapses to a few ohms within nanoseconds, creating a low-impedance path that diverts energy to ground or neutral. This happens before the surge voltage can reach downstream ICs, contactors, or solenoids. After the surge passes, the varistor returns to high resistance. The catch is that each surge degrades the material slightly; repeated large surges eventually cause the varistor to fail shorted or open, requiring replacement.

Failure modes matter: an MOV that fails shorted will draw continuous current and heat up dangerously; one that fails open leaves the circuit unprotected. Designers often pair a varistor with a thermal cutoff or fuse to prevent catastrophic failure. In critical applications, redundant varistors or hybrid protection schemes (combining varistors with transient suppression diodes) provide better reliability. The choice of clamping voltage is critical: too low and the varistor clamps the normal operating voltage too hard, causing waste heat; too high and it won't clamp the surge adequately.

Varistors degrade in humid environments and can cause electromagnetic interference due to their fast switching action. Industrial installations often use surge arresters instead of or alongside MOVs on main power lines, while varistors handle point-of-use protection for individual circuits. Replacement is routine maintenance: a failed MOV costs dollars but prevents damage worth thousands to a PLC or servo drive.