The principle of varistor use is that, after connecting to the protected equipment, it should not affect the normal operation of the equipment while effectively providing transient overvoltage protection. To this end, in addition to the varistor's technical parameters, the following issues should also be considered when selecting a varistor: ⑴ Varistor Voltage Selection Taking into account the deviation between the varistor's actual varistor voltage and the rated voltage (should be 1.1-1.2 times the rated voltage), the potential fluctuation range of the AC power supply voltage (should be 1.4-1.5 times the rated voltage), and the relationship between the AC voltage peak and effective value (should be 1.4 times). Therefore, a varistor with a varistor voltage of 2.2-2.5 times the rated voltage should be selected. In DC circuits, a varistor with a varistor voltage of 1.8-2 times the rated DC voltage is often selected. ⑵ Current Capacity Selection In principle, the varistor should be selected based on the maximum transient surge current it may encounter, but this is difficult to achieve. In practice, the varistor is selected based on the application or the test level specified in the product test standard. According to the former, a 1kA (8/20μs current wave) varistor can be used for thyristor rectifier protection; a 3kA varistor is used for surge absorption in electrical equipment; a 5kA varistor is used for lightning protection and overvoltage absorption in electronic equipment; and a 10kA varistor is used for lightning protection. According to the latter, a composite wave (generator generates a 1.2/50μs voltage wave when the output is open-circuited; an 8/20μs current wave when the output is short-circuited; the internal resistance of the generator is 2Ω) is often used to online assess the device's ability to withstand lightning surge interference. During a 4kV test, the maximum current absorbed by the protector can reach 2kA; for a 6kV test, the maximum current absorbed is 3kA. However, when selecting a varistor, the current carrying capacity should be appropriately increased. This is because a varistor with a higher current carrying capacity should have a relatively small residual voltage drop when absorbing the same surge current. This also provides a larger protection margin for the selected varistor. (3) Intrinsic Parasitic Capacitance Varistors have inherent capacitance, ranging from hundreds to thousands of pF, depending on their dimensions and nominal voltage. This inherent capacitance makes them unsuitable for use in high-frequency applications, as it can affect system operation. They are suitable for use in power-frequency systems, such as for power line protection and thyristor rectifier protection. Varistors have relatively high instantaneous power but very low average continuous power, so they cannot operate in the on state for extended periods.
