varistor

A varistor is an electrical component whose resistance changes depending on the voltage applied to it. This property makes it useful for protecting electronic circuits from voltage surges.12

Semiconductors, like those used in varistors, have electrical properties between those of conductors and insulators. In a varistor, this variable resistance is achieved by using materials such as zinc oxide ceramics. When the voltage across a zinc oxide varistor is low, it has very high resistance, allowing little current to flow. However, if the voltage increases beyond a certain threshold, the resistance drops dramatically, and a large current can flow, effectively diverting the excess voltage away from the sensitive circuit.2