A varistor, also known as a voltage-dependent resistor, is an electronic component with a nonlinear varying resistance, dependent on the voltage applied. The impedance is high under nominal load conditions, but will sharply decrease to a low value when a voltage threshold, the breakdown voltage, is exceeded. They are often used to protect circuits against excessive transient voltages.
When the circuit is exposed to a high 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 partially absorbed, protecting the circuit. This behavior makes them suitable to protect circuits during voltage surges.
Metal oxide varistors (MOVs) are one of the most common types of varistors. They are voltage dependent, nonlinear devices that provide transient suppression. MOVs are composed primarily of zinc oxide with small additions of other metal oxides. The structure of the body consists of a matrix of conductive zinc oxide grains separated by grain boundaries. These boundaries are responsible for blocking conduction at low voltages and are the source of the nonlinear electrical conduction at higher voltages.
The use of a metallic oxide within their construction means that MOV's are extremely effective in absorbing short term voltage transients and have higher energy handling capabilities. As with the normal varistor, the metal oxide varistor starts conduction at a specific voltage and stops conduction when the voltage falls below a threshold voltage.
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