Failure mode analysis and precautions of varistor

Date：2022-03-15 18:34:48Views：1245

Varistor is a voltage limiting protective device. Using the nonlinear characteristics of varistor, when overvoltage occurs between the two poles of varistor, varistor can clamp the voltage to a relatively fixed voltage value, so as to realize the protection of subsequent circuit.The failure mode of varistor is mainly short circuit. However, short circuit will not cause damage to varistor, because the resistance is at the positive and negative inlet of power supply; The insurance is good, which proves that it is not caused by short circuit or overcurrent. It may be that the surge energy is too large and exceeds the absorbed power to burn the varistor; When the overcurrent is too large, it may also cause the valve plate to be cracked and open circuit.

In the application process of varistor, when its performance degrades, there are two common degradation modes: the first is open circuit mode and the second is short circuit mode. The open circuit mode mainly occurs when the MOV flows through the surge current far beyond its own withstand, which is usually manifested as the explosion of the body of the zinc oxide varistor, but this mode will not cause combustion. Short circuit modes can be divided into aging failure and transient overvoltage failure.

First, let's look at the aging failure of zinc oxide varistors. This problem mainly refers to the gradual aggravation of the low resistance linearity of the resistor. At this time, the leakage current will increase maliciously and be injected into the weak point, resulting in the melting of the material at the weak point and the formation of a short-circuit hole of about 1000 Ω. After that, the power supply continues to push a large current into the short-circuit point to form a high heat and fire. The results show that if the varistor has manufacturing defects, it is prone to early failure, and the multiple effects of electric shock with low strength will also accelerate the aging process and make the aging failure appear earlier.

The transient overvoltage damage of varistor is a situation that causes device damage in a short time. The so-called transient overvoltage damage refers to that a strong transient overvoltage occurs in a short time, which makes the resistor perforated, resulting in greater current and high heat fire. The whole process occurs in a short time.

According to the performance of varistor after failure, it can be divided into three common failure states, namely deterioration, cracking and perforation. In case of deterioration, the real object shows that the leakage current increases when testing the varistor with a multimeter, and the varistor voltage decreases significantly until it is zero. When it is in the burst condition, the ceramic burst phenomenon will occur when the varistor suppresses the overvoltage, which is very obvious. When the zinc oxide varistor is perforated, the ceramic outer layer of the resistor will be electrically punctured and perforated.

Precautions for use of varistor:

1. It must be ensured that when the voltage fluctuates greatly, the continuous working voltage will not exceed the large allowable value, otherwise the service time of varistor will be shortened.

2. When the varistor is used between the power line and the earth, sometimes the voltage between the line and the earth rises due to poor grounding, so the varistor with higher nominal voltage than that between the lines is usually used. The surge current absorbed by the varistor shall be less than the flow of the product.

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