Schematic symbol of varistor

The varistor is a voltage limiting type protection device. Using the non-linear characteristics of the varistor, when an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby protecting the latter circuit. The main parameters of the varistor are: varistor voltage, current capacity, junction capacitance, response time and so on.

The response time of the varistor is ns, which is faster than the air discharge tube and slightly slower than the TVS tube. Under normal circumstances, the response speed of the overvoltage protection for the electronic circuit can meet the requirements. The junction capacitance of varistor is generally in the order of several hundred to several thousand pF. In many cases, it should not be directly applied to the protection of high-frequency signal lines. When it is used in the protection of AC circuits, it will increase leakage due to its large junction capacitance. The current needs to be fully considered when designing the protection circuit. The varistor has a large flow capacity but is smaller than a gas discharge tube.

The varistor voltage (min (U1mA)) and the through-current capacity of the varistor should be considered in the circuit design. In the DC loop, there should be: min (U1mA) ≥ (1.8 ~ 2) Udc, where Udc is the DC rated operating voltage in the loop. In the AC loop, there should be: min (U1mA) ≥ (2.2 ~ 2.5) Uac, where Uac is the effective value of the AC operating voltage in the loop.

The above value principle is mainly to ensure that the varistor has an appropriate safety margin when it is applied in the power supply circuit. In the signal loop, there should be: min (U1mA) ≥ (1.2 ~ 1.5) Umax, where Umax is the peak voltage of the signal loop. The flow capacity of the varistor should be determined according to the design specifications of the lightning protection circuit. In general, the varistor has a flow capacity greater than or equal to the flow capacity of the lightning protection circuit design. The varistor can be mainly used for DC power supply, AC power supply, low frequency signal line, and antenna feeder with feed.