The working nature of varistor and the cause of aging of varistor

1. Working properties of varistor

In order to meet the space size requirements of equipment miniaturization and meet the needs of functional upgrades, the working principle of varistor is high impedance when there is no transient overvoltage. However, as the surge current and voltage increase, its impedance will continue to decrease, and its current-voltage characteristics are strongly nonlinear. When a varistor is used as a filtering application, one of its protection functions is degraded, which may be the result of repeated surges of voltage surges.

The main factor that reduces the reaction time of some early pressure sensitive components is the parasitic inductance caused by the finished package. A 25 to 50 mm lead is used in the structure, which exhibits an inductance of 0.6 nH/mm, while a high self-inductance forms the characteristics of a radial wire varistor.

2. The cause of aging of varistor

The aging of the varistor is that in most cases, the P-N junction will cause a short circuit when it is overloaded and cannot be rotated to a normal state. The diode element in the varistor is broken down under repeated action of the electric shock. The low-resistance linearization of the resistor body is gradually intensified, the varistor voltage is getting lower and lower, and the leakage current is getting larger and larger. As the MOV body temperature increases, the leakage current is larger, forming a vicious circle, and the temperature of the MOV rises. Reach the ignition point of the outer packaging material.

This condition is called a high-impedance short circuit (about 1kΩ), and the Joule heat causes the MOV heat to increase and concentrates into the weak point, and the weak point material melts. After forming a short-circuit hole of about 1kΩ, the power supply continues to push a large current into the short-circuit point, forming a high heat and causing a fire. The research results show that if there is a manufacturing problem in the varistor, it is prone to early failure, and the repeated effects of the electric shock with little strength will accelerate the aging process and make the aging failure appear earlier.