Introduction to the working principle of varistor

Varistoris a resistance device with nonlinear volt-ampere characteristics, which is mainly used to clamp the voltage when the circuit is subjected to overvoltage, absorb excessive current, and protect sensitive devices. The resistance material of rheostat is semiconductor, which is a voltage limiting protection device. Using nonlinear characteristics, overvoltage occurs between the two poles, which can clamp the voltage at a relatively fixed voltage value, thus realizing the protection of the subsequent circuit

First, the working principle of the varistor

When the voltage applied to the varistor is lower than its threshold value, the current flowing through it is very small, which is equivalent to a resistance with infinite resistance. That is to say, when the voltage applied to it is lower than its threshold value, it is equivalent to a switch in the disconnected state.

Second, precautions for the varistor

(1) It must be ensured that the continuous working voltage does not exceed the maximum allowable value when the voltage fluctuation is large, otherwise the service life of the varistor will be shortened

(2) When using the varistor between the power line and the ground, sometimes the voltage between the line and the ground increases due to poor grounding, so the nominal voltage is usually higher than that used between the lines

III. The function of the varistor

is characterized by that when the voltage applied on it is lower than its threshold value 'UN', the current flowing through it is very small, equivalent to a closed valve. When the voltage exceeds UN, its resistance decreases, so that the impact of current surges flowing through it on other circuits does not change much, thus reducing the impact of overvoltage on subsequent sensitive circuits. With this function, the abnormal overvoltage that often occurs in the circuit can be suppressed and the circuit can be protected from overvoltage

IV. The application type ofvaristor

In different applications, the purpose of applying the varistor and the voltage/current acting on the varistor should be different, so the requirements are different. Distinguishing this difference is very important for correct use

1. Surge suppression: refers to the varistor used to suppress lightning overvoltage, switching overvoltage and other transient overvoltage. It is random and non-periodic, and the peak value of current and voltage may be large. Most varistor devices belong to this category

2. High power: refers to the varistor used to absorb the periodic continuous pulse group, such as the one parallel to the switching power converter. Generally, the energy value can be calculated, and the voltage peak is small, but the average power is quite large due to the high frequency

3. High-energy: refers to the pressure-sensitive device used to absorb the magnetic energy in the generator excitation coil, lifting electromagnet coil and other large inductance coils. For this application, the main technical index is the energy absorption capacity

The protection function of the varistor can be repeated many times in most applications, but sometimes it is also made into a "disposable" protection device like a current fuse. For example, a rheostat with short circuit contacts in parallel with some converter loads