Varistor protection

Varistor is one of the common electronic components for entry, and the figure of varistor can often be seen in LED. The varistor provides protection for the circuit system, which is very important.

Varistor is a resistance element with zinc oxide as the main component. The surge current tolerance and nonlinear coefficient of this element are very large. Below the threshold voltage, the resistance is very high and almost no current flows. If the threshold voltage is exceeded , The resistance is drastically reduced, and the amplified current can be leaked. Due to this characteristic, as a protection component of electronic and electrical equipment, it plays a great role in the absorption of abnormal voltage and the absorption of lightning surges.

Operating overvoltage is an overvoltage generated when the electromagnetic energy is rapidly converted and quickly released when the working state of the circuit changes suddenly. Varistors can be used to protect various power equipment, motors, etc., to prevent this overvoltage.

Atmospheric overvoltages are caused by lightning strikes, and most of them are inductive overvoltages. Lightning strikes produce overvoltages on transmission lines. Such overvoltages have a high voltage value and cause great harm. Therefore, measures must be taken for electrical equipment to prevent atmospheric overvoltages. It is generally used in parallel with the equipment. If electrical equipment requires low residual pressure, multi-level protection can be used.

In order to prevent semiconductor devices from being burnt out during operation, varistors are commonly used to protect them, and a varistor is connected to the transformer once, which can effectively protect the transistor from damage caused by overvoltage. In the normal state, the varistor is in a high resistance state with only a small leakage current. When subjected to an overvoltage, the varistor quickly becomes a low resistance state, and the overvoltage energy is absorbed by the varistor in the form of discharge current After the surge voltage disappears, when the circuit or component bears the normal voltage, the varistor returns to a high-resistance body.

 When cutting off a circuit containing inductive loads such as contactors and relays, the overvoltage can exceed several times the power supply voltage. The overvoltage can cause arcing and spark discharge between the contacts, burning the contacts, and shortening the life of the equipment. Due to the shunt effect of the varistor at the high potential, the contacts are protected. When the varistor and the coil are connected in parallel, the overvoltage between the contacts is equal to the sum of the power supply voltage and the residual voltage of the varistor. The energy absorbed by the varistor is the energy stored in the coil. When the varistor is connected in series with the contact, the The overvoltage is equal to the residual pressure of the varistor, and the energy absorbed by the varistor is 1.2 times the energy stored in the coil.