The use principle of the varistor is that it cannot affect the normal operation of the device after it is connected to the protected device and can effectively implement instantaneous overvoltage protection for the device. Therefore, in addition to the technical parameters of varistor, the following problems should be considered in the actual selection:
⑴ Selection of pressure-sensitive voltage
Considering the deviation between the actual pressure-sensitive voltage of the varistor and the nominal voltage (1.1 ~ 1.2 times of the nominal voltage should be considered), the possible fluctuation range of the power supply voltage in the AC circuit (1.4 ~ 1.5 times of the rated voltage should be considered), and the relationship between the peak AC voltage and effective value (1.4 times of the rated voltage should be considered), A varistor with a voltage sensitive of 2.2 to 2.5 times the rated voltage should be selected. In the direct current circuit, the voltage sensitive voltage is often selected for the DC voltage rating of 1.8 ~ 2 times the varistor.
⑵ Selection of flow capacity
In principle, the maximum transient surge current that can be experienced should be selected, but it is difficult to do so. In practice, it is nothing more than in accordance with the occasion of use, or in accordance with the test grade specified on the product test standard to select the varistor.
According to the former, 1kA (8/20μs current wave) varistor can be used for protection of thyristor rectifier; 3kA for electrical equipment surge absorption; 5kA is used to absorb overvoltage of lightning strikes and electronic equipment; 10kA for protection against lightning strikes. According to the latter, the common integrated wave (1.2/50μs voltage wave is generated when the generator is open output; The current wave of 8/20μs is generated when short circuit output. The internal resistance of the generator is 2Ω) to check the ability of the equipment against lightning surge interference online. At 4kV test, the maximum current absorbed by the protector can reach 2kA. For the 6kV test, the maximum absorption current is 3kA. But in the actual selection, it should be appropriate to increase the current capacity of the selected varistor. Because of the large flow capacity of the varistor, when absorbing the same amount of surge current, should have relatively small residual voltage drop; At the same time, for the selection of varistor, there is also a larger protection margin.
⑶ Inherent parasitic capacitance
Varistors have an inherent capacitance problem, ranging in value from hundreds to thousands of pF, depending on external size and nominal voltage. The inherent capacitance of varistor determines that it is not suitable for use in high frequency situations, otherwise it will affect the normal operation of the system. Suitable for use in power frequency system, such as the protection of power supply line, silicon controlled rectifier protection, etc.
The instantaneous power of varistor is relatively large, but the average sustained power is small, so it can not work for a long time