What is SPD varistor?

What is SPD varistor

"Varistor" is a resistance device with nonlinear voltage-ampere characteristics, mainly used to clamp the voltage when the circuit is over-voltage, absorb excessive current, and protect sensitive devices. So, what is SPD varistor

What is theSPD varistor

1. At present, 'zinc oxide' (ZnO) varistor is widely used. Its main material is composed of divalent element zinc (Zn) and hexavalent element oxygen (O)

2. The rheostat is a voltage limiting protection device. Using the nonlinear characteristics of the varistor, overvoltage is generated between the two poles of the varistor, which can clamp the voltage at a relatively fixed voltage value, thus realizing the protection of subsequent circuits. The main parameters of varistor include voltage, current capacity, junction capacitance, response time, etc

3.SPD varistorhas a response time of ns, which is faster than gas discharge tube and slightly slower than TVS tube. Generally, its response speed to overvoltage protection of electronic circuit can meet the requirements. Generally speaking, the junction capacitance of the varistor is in the order of hundreds of Pf to thousands of Pf. In many cases, it should not be directly applied to the protection of high-frequency signal lines. When applied to the protection of AC circuit, the junction capacitance will increase the leakage current, so it should be fully considered when designing the protection circuit. The current capacity of the varistor is large, but less than that of the gas discharge tube

How to select SPD (Surge Protection Device)

When selecting the surge protector, the characteristics of leakage current should be fully considered. Generally, the leakage current of surge protector is required to be within a certain range, and it is not allowed to rise automatically during operation. Once the leakage current is found to rise automatically, it should be eliminated immediately, because the instability of the leakage current is the direct reason for accelerating the aging and explosion of the surge protector. At present, from the perspective of the manufacturing process of the varistor, there are two different batching methods and manufacturing processes:

1. The initial leakage index of the varistor produced by one process is low, only 3~6 A, but once used, with the service time of the surge protector increasing year by year, it will be in an unstable state for a long time, resulting in overheating, aging, failure of the surge protector, and may cause fire

2. The leakage current of the varistor manufactured by another production process is 20~30 A, but it is stable for a long time, and will not gradually increase with the service time of the surge protector, and will not cause rapid deterioration and failure of the surge protector

Is leakage current detection of SPD surge protector important

In order to ensure whether the surge protector put into use is aging or invalid, the lightning protection unit will conduct detailed detection of the leakage current index of the surge protector in the routine lightning protection test every year. When I1ma is measured for the first time, the measured value of the leakage current Iie of the surge protector shall not exceed the nominal value of the manufacturer's Iie; When the number of valve plates cannot be determined, the measured value of surge protector shall not be greater than 20 μ A。 When I1ma is measured subsequently, the measured value of leakage current Iie of the surge protector composed of a single MOV and multiple movs shall not be greater than 1 time of the first measured value