How to use varistors correctly?

The resistance material of varistors is semiconductor, which is also a type of semiconductor resistor and one of the essential electronic components for engineers. Varistors can often be seen in LED switching power supplies or high-power power equipment. In addition, because their maximum pulse width of the impulse current is much smaller than the actual pulse current width of large and medium power semiconductor systems, short circuits, burns, and failures often occur. So how to use varistors correctly to avoid damage? The detailed introduction is as follows:

Reasons for the burnout of varistors:

1. Aging failure, manifested as an increase in leakage current and a significant decrease in varistor voltage until it reaches zero;
Aging failure refers to the gradual increase in low resistance linearization of the resistor, leading to a malignant increase in leakage current and concentrated flow into weak points. The weak point material melts, forming a short circuit hole of about 1k Ω. The power supply continues to push a larger current into the short circuit point, resulting in high heat and fire. This type of accident can usually be avoided by a hot melt joint connected in series with a varistor. Transient overvoltage damage refers to the phenomenon where a strong transient overvoltage causes perforation of the resistor, resulting in a larger current and causing high heat and fire. The entire process occurred in a relatively short period of time, to the point where the hot melt contacts on the resistor did not have time to melt.
2. Transient overvoltage damage.
3. The frequency of overvoltage protection;
4. Surrounding working temperature;
5. Is the varistor subjected to compression;
6. Whether it has passed quality certification;
7. The surge energy is too large, exceeding the absorbed power;
8. Insufficient voltage resistance;
9. Excessive current and surge, etc.

How to use varistors correctly? Xiaoyi will give you a detailed explanation.

1. Varistor voltage UN (U1mA): The voltage at which a 1mA DC current passes through a varistor is usually used to indicate whether it is conducting. This voltage is called the varistor voltage UN. Varistor voltage is also commonly represented by the symbol U1mA. The error range of pressure-sensitive voltage is generally ± 10%. In experiments and practical use, a 10% decrease in varistor voltage from normal values is usually used as a criterion for varistor failure

2. Maximum continuous operating voltage UC: refers to the maximum AC voltage (effective value) Uac or maximum DC voltage Udc that the pressure-sensitive resistor 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 present on the varistor when a specified 8/20 μ s wave surge current IX (A) is applied to the varistor.

4. Leakage current Il: The current flowing through the varistor when the maximum DC voltage Udc is applied. When measuring leakage current, a voltage of Udc=0.83U1mA is usually applied to the varistor (sometimes 0.75U1mA is also used). Generally, the static leakage current Il is required to be ≤ 20 μ A (there are also requirements for ≤ 10 μ A). In practical use, what is more concerned is not the magnitude of the static leakage current value itself, but its stability, that is, the rate of change after impact testing or under high temperature conditions. After the impact test or under high temperature conditions, if the rate of change does not exceed twice, it is considered stable.

For a long time, the safety of using varistors has been a concern that needs to be taken seriously. As varistors are widely used in household appliances and other electronic products, they play a role in overvoltage protection, lightning protection, suppression of surge currents, absorption of peak pulses, amplitude limiting, high-voltage arc extinguishing, noise reduction, and protection of semiconductor components. Therefore, it is recommended that everyone use high-quality varistors.