A varistor, also known as a Voltage Dependent Resistor (VDR), is an electronic component that is used to protect electrical and electronic circuits from voltage surges and transient overvoltages. It is designed to have a nonlinear resistance characteristic, which means that its resistance decreases as the voltage across it increases. This property allows varistors to act like voltage-controlled switches.
Varistors are made from semiconductor materials, usually based on zinc oxide (ZnO), and they have two main terminals. When the voltage across a varistor is within its normal operating range, it exhibits a very high resistance and has little impact on the circuit. However, when the voltage exceeds a certain threshold (called the "clamping voltage" or "breakdown voltage"), the varistor conducts heavily, effectively providing a low-resistance path for excessive voltage to be dissipated safely.
The primary purpose of a varistor is to protect sensitive electronic components and devices from damage caused by voltage spikes, surges, or transient overvoltages, which can result from lightning strikes, power switching events, or other sudden changes in the electrical system. By shunting the excess voltage away from sensitive components, varistors help prevent equipment damage and enhance the overall reliability of electronic systems.
Varistors are commonly used in various applications, including power distribution systems, surge protectors, voltage regulators, consumer electronics, industrial equipment, and telecommunications devices. They come in different sizes, voltage ratings, and current-handling capabilities to suit various circuit protection needs.