1. Varistor is one of the most used voltage limiting devices. Commonly used is zinc oxide (ZnO) varistor, which is mainly based on zinc oxide as raw material, adding a variety of trace metal oxides, by mixing molding, sintering assembly from a kind of overvoltage protection device, its outside encapsulated epoxy resin (can add pigment).
2. It is equivalent to a variable resistor, which is connected in parallel in the circuit. When the circuit is working normally, the impedance of varistor is very high and the leakage current is very small, which can be regarded as open circuit and has almost no effect on the circuit. But when a very high abnormal voltage comes, the resistance value of varistor drops instantly, that is, from high resistance to low resistance, the excessive inrush current is released to the earth, while the overvoltage is clamped within a certain safe voltage range, thus protecting the back-end line; when the abnormal surge disappears, the resistance value of varistor returns to the original high resistance state, and the line can work normally.
3. Characteristic curve: As can be seen from the figure, when the voltage is low, the varistor works in the leakage current zone, showing a large resistance, leakage current is very small; when the voltage rises into the nonlinear zone, the current changes in a fairly large range, the voltage does not change much, showing good voltage limiting characteristics; voltage rises again, the varistor enters the saturation zone, showing a very small linear resistance, due to the current is very large, a long time will make the varistor overheat and burn or even blow up.

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