What is varistor how it's worked and their application (MOV/VDR)
What is a varistor? Well, consider the following. You probably have a prototype board on your desk that contains the simple circuit shown below. It is a basic circuit: a power supply and load. There is no protection for the load, or the supply, from voltage transients. With no circuit protection device, what could happen? A voltage transient might damage a sensitive component, or worse, the entire circuit.

You remember the old joke: the most expensive devices get destroyed trying to protect the cheapest parts. Your design does not have to suffer this fate. When it comes to voltage protection, consider the relatively unknown device: the varistor.
What is a Varistor?
A varistor is sometimes called a “voltage dependent resistor.” The construction of a varistor is comparable to a ceramic capacitor, but its function is similar to a diode. (Why aren’t they called Capodes?) When enough voltage activates a varistor, it provides a low impedance path. This action redirects the energy away from the circuit, providing protection.

Metal Oxide Varistors (MOVs) are ceramic structures made with zinc oxide grains. These grains create diode junctions which are interconnected. Individual junctions have a breakdown voltage of only 3.5 volts. Imagine diodes randomly assembled. Those in series would increase the voltage capability. Any reversed diodes would reduce leakage current flow. The interconnected junctions in the ceramic structure mean low leakage current with a high voltage rating.
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