Several important parameters of varistor and varistor selection are: large allowable voltage, large clamping voltage, and surge current that can be withstood.
Firstly, it should be ensured that the large allowable voltage of the varistor is greater than the large value of the power supply output voltage; secondly, it should be ensured that the large clamping voltage does not exceed the large surge voltage allowed by the subsequent circuit; afterwards, the inrush current flowing through the varistor should be ensured It will not exceed the surge current it can withstand.
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Other parameters, such as rated power and high-energy pulse that can withstand, can be determined by simple check calculation or experiment. It should be noted that the varistor has a problem of performance degradation.
Gas discharge tubes, gas discharge tubes are switching devices. Compared with varistor, it has some different characteristics, such as long turn-on delay, continuous current after turning on, small inter-electrode capacitance, high insulation resistance, and small leakage current Etc., so it is often used in parallel with the varistor. For example, when connected in series, it can solve the problems of large leakage current of the varistor, long-term performance degradation or failure; when connected in parallel, the response time of the protection circuit is accelerated, and most of the current is divided after the gas discharge tube breaks down.