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A "varistor" is a resistance device with non-linear volt-ampere characteristics. It is mainly used to clamp the voltage when the circuit is subjected to overvoltage and absorb excess current to protect sensitive devices. The resistor body material of the varistor is a semiconductor, which is a voltage-limiting 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 realizing the protection of the subsequent circuit. The main parameters of the varistor are varistor voltage, current capacity, junction capacitance, response time, etc.

1.Working principle of the varistor

When the voltage applied to the varistor is lower than its threshold, the current flowing through it is extremely small, which is equivalent to a resistor with infinite resistance. In other words, when the voltage applied to it is lower than its threshold, it is equivalent to an off-state switch.

When the voltage applied to the varistor exceeds its threshold, the current flowing through it increases sharply, which is equivalent to a resistor with infinite resistance. In other words, when the voltage applied to it is higher than its threshold, it is equivalent to a closed switch

2.Precautions for varistor

(1) It must be ensured that the continuous working voltage will not exceed the maximum allowable value when the voltage fluctuation is the largest, otherwise the service life of the varistor will be shortened;

(2) When a varistor is used between the power line and the ground, sometimes the voltage between the line and the ground rises due to poor grounding. Therefore, a varistor with a higher nominal voltage than the line-to-line use is usually used.

The failure mode of the varistor is usually a short circuit. In order to prevent the failure of the varistor from causing a short circuit of the power supply and catching fire, a temperature fuse or thermal release mechanism can be connected in series with each varistor. The temperature fuse should have a good thermal coupling with the varistor. When the varistor fails (high impedance short circuit), the heat generated by it will fuse the temperature fuse, thereby separating the failed varistor from the circuit and ensuring the equipment Security. When a higher power frequency temporary overvoltage acts on the varistor, it may cause the varistor to break down and short-circuit (low impedance short circuit) instantaneously, and the temperature fuse is too late to fuse, or it may catch fire. In order to avoid this phenomenon, an impact-resistant power frequency fuse can be connected in series with each varistor (single-use power frequency fuse may not blow when aging failure). The varistor can also be used in series with the ceramic gas discharge tube. The ceramic gas discharge tube does not conduct during normal operation, and the varistor has no leakage current, which can greatly extend the service life; when it is impacted by a surge, the ceramic gas discharge tube is the first to hit Then, the surge voltage is limited by the varistor. The total residual voltage is the sum of the two, which increases slightly (tens of volts); after the impact, the discharge tube cannot maintain conduction due to the varistor limiting the current The arc is extinguished and the normal working state is restored; when the varistor fails in a short circuit, the ceramic gas discharge tube will quickly fail due to the large power frequency current flowing through the ceramic gas discharge tube, but most of its failure modes are open circuits, so it is not easy to cause a fire.

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