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Analysis of the causes of damage to the varistor How to use the varistor correctly?

163.com | 07/04/2021

The resistor body material of a varistor is a semiconductor, which is also a kind of semiconductor resistor, and it is also one of the necessary electronic components for engineers. Varistors can often be seen in LED switching power supplies or high-power power supplies. In addition, because its maximum impulse current pulse width is much smaller than the actual pulse current width of high and medium power semiconductor systems, short circuits or Burnout and failure phenomenon. So how to use varistor correctly to avoid damage? The details are as follows:

Causes of varistor burnout:

1. Aging failure, manifested by an increase in leakage current and a significant decrease in varistor voltage until it is zero;
Aging failure means that the low-resistance linearization of the resistor gradually increases, the leakage current increases viciously and concentrates on the weak point. After the weak point material melts and forms a short-circuit hole of about 1kΩ, the power supply continues to push a larger current into the short-circuit point. Formed a high fever and caught fire. This kind of accident can usually be avoided by a thermal fusion point in series with the varistor. The transient overvoltage damage refers to the fact that the stronger transient overvoltage makes the resistor body perforate, which leads to a larger current and high heat and fire. The whole process takes place in a relatively short time, so that the thermal fusion point set on the resistor body is too late to fuse.

2. Destroyed by transient overvoltage.

3, the number of overvoltage protection;

4. Ambient working temperature;

5. Whether the varistor is squeezed;

6. Whether it has passed the quality certification;

7. The surge energy is too large, exceeding the absorbed power;

8. The withstand voltage is not enough;

9. Excessive current and surge, etc.

How to use varistor correctly? Here is a detailed explanation for everyone.

1. Varistor voltage UN (U1mA): usually the voltage when a 1mA DC current passes through the varistor to indicate whether it is on or not. This voltage is called the varistor voltage UN. Varistor voltage is also commonly represented by the symbol U1mA. The error range of the varistor voltage is generally ±10%. In the test and actual use, the varistor voltage is usually reduced by 10% from the normal value as the criterion for the failure of the varistor

2. Maximum continuous working voltage UC: refers to the maximum AC voltage (effective value) Uac or the maximum DC voltage Udc that the varistor can withstand for a long time. Generally Uac≈0.64U1mA, Udc≈0.83U1mA

3. Maximum clamping voltage (limiting voltage) VC: The maximum clamping voltage value refers to the voltage appearing on the varistor when the specified 8/20μs wave impulse current IX (A) is applied to the varistor.

4. Leakage current Il: the current that flows when the maximum DC voltage Udc is applied to the varistor. When measuring the leakage current, usually add Udc=0.83U1mA voltage to the varistor (sometimes 0.75U1mA is also used). Generally, the static leakage current Il is required to be less than or equal to 20μA (there are also requirements for less than or equal to 10μA). In actual use, what is more concerned about is not the size of the static leakage current itself, but its stability, that is, the rate of change after the impact test or under high temperature conditions. After the impact test or under high temperature conditions, the rate of change does not exceed one time, which is considered stable.

For a long time, the safety of varistors has been an issue that needs attention. Because varistors are widely used in household appliances and other electronic products, they provide overvoltage protection, lightning protection, surge current suppression, and spike absorption. For limiting amplitude, high-voltage arc extinguishing, noise elimination, and protection of semiconductor components, it is recommended that you use high-quality varistors.

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