Varistors — Resistance That Changes With Voltage
A varistor is a voltage-dependent resistor. Unlike a potentiometer, thermistor, or LDR, its resistance changes mainly because of the voltage applied across it. The most familiar type is the metal-oxide varistor, usually shortened to MOV.
Its main job is protection
At ordinary circuit voltage, an MOV ideally draws very little current. When a transient voltage rises high enough, the MOV conducts much more strongly and helps clamp the surge by diverting current.
large voltage surge → much lower resistance
A simple surge-protection arrangement
An MOV is therefore normally connected across the circuit being protected, not in series with the load. During a surge it may conduct a very large current for a short time.
Varistors are strongly non-linear
Calling a varistor a “variable resistor” can be misleading if it suggests a smooth knob-like adjustment. Its voltage-current relationship is intentionally non-linear. Below its operating region it conducts little; as voltage rises into the clamping region, current can increase dramatically.
Important MOV specifications
| Specification | Why it matters |
|---|---|
| Continuous working voltage | The normal voltage that may be applied continuously. |
| Varistor voltage | A specified voltage measured at a stated test current. |
| Clamping voltage | The voltage that appears across the MOV at a specified surge current. |
| Peak surge current | The short-duration current the part can withstand under specified conditions. |
| Energy rating | How much transient energy the MOV can absorb under stated test conditions. |
Not the same as a zener diode
Both devices can be used for overvoltage protection, but they are different components with different voltage-current characteristics, speed, power and energy capabilities, polarity behavior, and applications. An MOV is commonly used for relatively high-energy AC or mains transients; zener and TVS diodes are common in lower-voltage electronic protection.