A varistor is a voltage-limiting protection device. Utilizing its nonlinear characteristics, when an overvoltage occurs between its terminals, the varistor clamps the voltage to a relatively fixed value, thereby protecting downstream circuits. Another important function of varistors is transient overvoltage protection in circuits. While they have a large current capacity, their energy capacity is limited. Furthermore, because their maximum impulse current pulse width is far smaller than the actual pulse current width of high- and medium-power semiconductor systems, they often short-circuit, burn out, and fail.

The varistor commonly used in the market today is made of zinc oxide (ZnO). The main reasons for its failure are as follows:
a. Insufficient voltage resistance
This is easy to understand. If a product has an operating voltage of 220V and you use a varistor rated at 180V or less, it will inevitably break down and fail.

b. Excessive current and surge
The specified current for MYG05K is 0.1mA. The nominal voltage for MYG07K, MYG10K, MYG14K, and MYG20K refers to the voltage across the varistor when a 1mA DC current is passed through it. Using this in products, especially devices that need to be plugged in and out, will cause the varistor to be damaged more quickly, because the surge when the product is plugged in and out is relatively large (the equipment at both ends is not grounded). At this time, the varistor's withstand voltage will cause the product itself and the TVS protection capability to be weakened, resulting in a higher damage rate.

Recommended several varistor overheating protection technologies:
(1) Using a spring to pull the low-melting-point solder technology
This technology is currently used by most manufacturers. A low-melting-point solder point is added to the pin of the varistor, and then a spring is used to pull this solder point. When the leakage current of the varistor is too large and the temperature rises to a certain level, the solder at the solder point melts. Under the action of the spring's pulling force, the solder point quickly separates, thereby cutting off the varistor from the circuit and simultaneously linking the alarm contact to send an alarm signal.

(2) Potting technology.
To prevent the varistor from smoking, catching fire or exploding when it fails, some manufacturers use this technology to pot the varistor. However, since arcing occurs inside the varistor when it fails, the sealing material fails and carbon is generated, which in turn maintains the arc. This often leads to short circuits and blackening inside the equipment.

(3) Isolation technology.
This technology places the varistor in a sealed box, isolating it from other circuits to prevent the spread of smoke and flames from the varistor. When all backup protections fail, isolation technology is a simple and effective method, but it requires a large amount of equipment space and also prevents smoke and flames from escaping from the lead openings in the box.