A varistor is a voltage-limiting type of protector that uses the non-linear characteristics of the varistor. When an overvoltage appears at its poles, the varistor can clamp the voltage to a relatively fixed voltage value, thus protecting the later circuit. Another very important role of the varistor is to protect against transient overvoltage in the circuit. Although it has a large current capacity, its energy capacity is not large. In addition, because its impact current maximum pulse width is far smaller than the actual pulse current width of large and medium power semiconductor systems, it often causes short circuit or burnout and failure phenomena.

The commonly used varistor on the market today is a zinc oxide material varistor, which will be damaged for the following reasons.

a. Insufficient pressure resistance

This is easy to understand if a product has a working voltage of 220V and you use a varistor that is 180V or smaller, then it will definitely break down and be damaged.

b. Current and surge are too large

MYG05K specifies that the current passing through is 0.1mA, MYG07K, MYG10K, MYG14K, MYG20K nominal voltage refers to the voltage value at both ends of the varistor when 1mA DC current passes through. In use in products, especially devices that need to be plugged and unplugged, this will more quickly promote the damage of the varistor, because the surge at the time of plugging and unplugging of the product is relatively large (both ends of the device are not grounded), then the pressure resistance of the varistor will cause the product itself and TVS protection ability to weaken and thus produce a higher damage rate.

Recommend several varistor overheating protection technologies:

(1) Using spring to hold low melting point solder technology

This technology is currently used by most manufacturers, adding a low melting point solder joint at the lead of the varistor, and then using a spring to hold this solder joint. When the varistor leaks too much current and the temperature rises to a certain degree, the solder at the solder joint melts and breaks. Under the pull of the spring, the solder joint quickly separates, thus cutting off the varistor from the circuit. At the same time, it links up with an alarm contact and sends out an alarm signal.

(2) Potting technology.

In order to prevent smoke, fire or explosion when the varistor fails, some manufacturers will use this technology to potting the varistor. However, due to arcing inside the varistor when it fails, it causes the sealing material to fail and produce carbon. Carbon will also maintain the arc, which often leads to internal short circuit and blackening of equipment.

(3) Isolation technology.

This technology installs the varistor in a sealed box body and isolates it from other circuits to prevent smoke and flame spread from the varistor. In case all backup protection fails, isolation technology is also a simple and effective method. However, it requires occupying a large space in equipment and also preventing smoke and flame from escaping from holes where box body leads are located.