A varistor, short for "variable resistor," is an electronic component commonly used to protect electrical circuits from transient voltage spikes or surges. It is also known as a voltage-dependent resistor (VDR) or metal oxide varistor (MOV).
The varistor's main function is to regulate and limit voltage fluctuations in a circuit. When operating under normal conditions, it exhibits a relatively high electrical resistance, allowing only a small current to pass through it. However, when subjected to a sudden surge in voltage, such as from a lightning strike or power surge, the varistor rapidly changes its resistance to a much lower value, effectively diverting the excessive current away from the sensitive components in the circuit.
Varistors are constructed using a semiconductor material, often a metal oxide, sandwiched between two metal plates or electrodes. The behavior of the varistor is determined by the properties of this semiconductor material, which has a non-linear voltage-current characteristic. This non-linearity is crucial for its protective function.
Varistors are commonly used in electronic devices, power distribution systems, surge protectors, and other electrical systems where protection against voltage spikes is necessary. By absorbing and dissipating excess energy, varistors help prevent damage to components and ensure the overall stability and safety of the electrical system.