The working principle, advantages and disadvantages of SPD varistor and its application

SPD varistoris one of the most widely used voltage limiting devices, including silicon carbide varistor and zinc oxide varistor. The commonly used zinc oxide varistor is an overvoltage protection device. It is made of zinc oxide as raw material, a variety of trace metal oxides, mixed and sintered, and its outer surface is sealed with epoxy resin

The working principle of SPD varistor: The varistor is equivalent to a variable resistor, which is connected in parallel in the circuit. When the circuit works normally, the impedance is very large, the leakage current is very small, which is equivalent to an open circuit, and has little impact on the circuit. However, when the high surge voltage comes, the resistance value of the rheostat drops instantaneously, so that the large current can flow through and the voltage can be clamped at a certain value. Because the surge voltage value of the rheostat depends on its physical size Therefore, different surge current values can be obtained

Selection and application of SPD varistor: The rheostat should work within its rated parameters, otherwise it may cause thermal deterioration or even breakdown of the rheostat. The failure mode of the varistor is mainly short circuit. If the short circuit lasts too long, it will cause explosion and fire and damage the surrounding components. Therefore, the following selection items must be observed:

1. The selection of SPD varistor. It is controlled according to the power supply voltage. Considering the fluctuation range of the working voltage of the power grid, sufficient margin should be left when selecting the voltage value of the varistor. Generally, the domestic fluctuation range is 30%. It should also be considered that the power supply voltage continuously applied to the rheostat should not exceed the "continuous working voltage" value listed in the specification table. For the lightning arrester of 220 V~240 V AC power supply, it is appropriate to select the voltage of 470 V~620 V varistor. Selecting the voltage of higher voltage varistor can reduce the failure rate and prolong the service life, but the residual voltage is slightly increased

2. Selection of flux. The nominal discharge current of the varistor shall be greater than the required surge current or the surge current that may occur every year. The nominal discharge current shall be calculated according to the value of more than 10 shocks in the surge life rating curve of the varistor, which is about 30% of the surge current

3. Parallel connection of varistor. When aSPD varistorcannot meet the requirements of the nominal discharge current, multiple varistors should be used in parallel. Sometimes in order to reduce the limit voltage, even if the nominal discharge current meets the requirements, multiple varistors need to be connected in parallel. It should be noted that when rheostats are used in parallel, they must strictly match the same parameters to ensure the uniform distribution of current

4. Varistor fault protection. The failure mode of the varistor is usually short circuit. In order to prevent the power supply short circuit caused by the failure of the varistor from causing fire, a temperature fuse or thermal circuit breaker can be connected in series on each varistor. The temperature fuse should have good thermal coupling with the rheostat. When the rheostat fails, the heat generated by it will fuse the temperature fuse, thus isolating the failed rheostat from the circuit and ensuring the safety of the equipment