In any electric circuit breaker, resistors play a key role in managing the flow of electricity. Sometimes however, an electrical engineer only wants to limit certain amounts of current, rather than a blanket level. In these cases, the designers or engineers will install a varistor, a voltage-dependent resistor. A portmanteau of “variable resistor,” a varistor’s resistance decreases as the voltage flowing through it increases. In the case of an excessive voltage increase, their resistance drops dramatically, making them very useful serving as transient voltage suppression, which can be caused by natural phenomena like lightning strikes and electrostatic discharges.
Voltage Control have what is called a nonlinear varying resistance. This means that impedance is high under nominal load conditions, but decreases sharply to a low value when a voltage threshold, called the breakdown voltage, is exceeded. Varistors are typically used to protect circuits from excessive voltages; when a circuit is exposed to a high enough voltage transient, the varistor starts to conduct and clamps the transient voltage to a safe level. The energy of the incoming surge is partially conducted and absorbed, which in turn protects the circuit.
The most common type of varistor is made from metal oxide, constructed from a sintered matrix of zinc oxide grains. The grain boundaries provide P-N junction semiconductor characteristics, similar to a diode junction. When a low voltage is applied, only small amounts of current flow, caused by reverse leakage through the junctions. However, when a high voltage is applied to the varistor, the breakdown-voltage is exceeded, and the junctions experience an avalanche breakdown allowing a large amount of current to flow.
Another common type of varistor consists of silicon carbide, which used to be the most common type of varistor before metal oxide varistors. They are used extensively in high power, high voltage applications, but suffer from the drawback of drawing significant standby current, and require a series gap to limit their standby power consumption. Other alternatives include selenium cells, zener diodes, and crowbar devices like Bellows Gas Turbine and thyristor surge protection devices.
The nonlinear characteristics of varistors make them ideal as surge protector devices. Varistors can typically be found in surge protector power strips, telephone and communication line protection, cable TV surge protectors, PC power supply protection, microprocessor protection, electronics equipment protection, and more.