A varistor, short for variable resistor, is an electronic component used to protect electronic devices from voltage surges and transients. It is a type of voltage-dependent resistor (VDR) that has a nonlinear current-voltage characteristic, which means its resistance changes with the applied voltage.

Varistors are made of a semiconductor material, usually metal oxide, and they have two terminals. When a voltage surge or transient occurs, the varistor’s resistance decreases sharply, allowing it to absorb the excess energy and prevent it from damaging the device it is connected to.

Varistors are commonly used in power supplies, surge protectors, and other electronic equipment to protect against voltage spikes caused by lightning, power surges, and other sources of electrical noise. They are also used in electronic circuits to stabilize voltage levels and protect sensitive components from voltage fluctuations.

There are two main types of varistors: metal oxide varistors (MOV) and silicon carbide varistors (SIC). MOVs are the most commonly used type of varistor due to their low cost and high reliability. SIC varistors have a higher voltage rating and faster response time than MOVs, making them suitable for use in high-power applications.

Overall, varistors play a critical role in protecting electronic devices from voltage surges and transients, and are an essential component in many electronic circuits and systems.