Conductive mechanism and rules of use of varistor

The varistor is a voltage-limiting protection device that utilizes the nonlinear characteristic of the varistor. When an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value. Realize the protection of the latter circuit.

The junction capacitance of a varistor is generally in the order of several hundred to several thousand Pf. In many cases, it should not be directly applied to the protection of high-frequency signal lines. When it is used in the protection of an AC circuit, it will increase leakage due to its large junction capacitance. The current needs to be fully considered when designing the protection circuit. The varistor has a large flow capacity but is smaller than a gas discharge tube.

Conductive Mechanism of Varistor: The I-U characteristic curve of a typical varistor can be generally divided into a pre-breakdown zone, a breakdown zone and a recovery zone. In the pre-breakdown region, when the applied voltage is lower than the varistor voltage, the higher the grain boundary barrier of the material is, the higher the varistor is, and the high resistance is mainly derived from the grain boundary.

When the applied voltage reaches the breakdown voltage, the resistance will decrease sharply with the increase of voltage, the grain boundary barrier will be broken down gradually, the resistance value is mainly determined by the grain resistance, and the breakdown region is small in voltage variation. The current changes by 6 to 8 orders of magnitude, the varistor maintains a fairly high value of a, and the recovery region is a characteristic region in which the nonlinearity drops again and disappears in the large current region. The characteristics of the varistor in the pick-up zone are controlled by the grain characteristics in the ceramic.

Usage rules:

1. Regardless of how the voltage changes, its large fluctuation value cannot exceed the large allowable value of the varistor. If the continuous operation voltage exceeds the large allowable value, the use time of the varistor is reduced.

2. When the varistor is connected between the power line and the ground, the voltage between the power line and the ground may increase due to some reasons such as lack of good grounding. The varistor equipment we use must use a higher power supply. The voltage between the line and the earth is higher.

3. The current absorbed by the varistor during operation is as small as possible.