A varistor is a voltage-limiting protection device. Utilizing its non-linear characteristics, when an overvoltage occurs across its terminals, the varistor clamps the voltage to a relatively fixed value, thus protecting downstream circuits. Another important function of varistors is transient overvoltage protection in circuits. Although they have a large current-carrying capacity, their energy capacity is relatively small. Furthermore, because the maximum pulse width of their inrush current is much smaller than the actual pulse current width of high- and medium-power semiconductor systems, short circuits, burnout, and failures frequently occur. Currently, the most commonly used varistors on the market are made of zinc oxide (ZnO). The main reasons for their failure are as follows: a. Insufficient withstand voltage. This is easy to understand. If a product's operating voltage is 220V, and you use a varistor with a withstand voltage of 180V or less, it will definitely break down and be damaged. b. Excessive Current and Surge MYG05K specifies a current of 0.1mA. The nominal voltage of MYG07K, MYG10K, MYG14K, and MYG20K refers to the voltage across the varistor when a DC current of 1mA passes through it. In products, especially those requiring plugging and unplugging, this will accelerate the damage to the varistor because the surge during plugging and unplugging is relatively large (the devices at both ends are not grounded). This weakens the varistor's withstand voltage and TVS protection capabilities, resulting in a higher damage rate. Recommended varistor overheat protection technologies: (1) Using a spring to hold low-melting-point solder This technology is currently used by the vast majority of manufacturers. A low-melting-point solder joint is added to the pin of the varistor, and a spring holds this solder joint. When the leakage current of the varistor is too large and the temperature rises to a certain level, the solder at the solder joint melts. Under the pulling force of the spring, the solder joint quickly separates, thus disconnecting the varistor from the circuit. At the same time, the alarm contact is activated, sending an alarm signal. (2) Encapsulation Technology. To prevent varistors from emitting smoke, catching fire, or exploding when they fail, some manufacturers use this technology to encapsulate them. However, when a varistor fails, internal arcing occurs, causing the sealing material to fail and producing carbon. This carbon then sustains the arc, often leading to internal short circuits and blackening of the equipment. (3) Isolation Technology. This technology encapsulates the varistor in a sealed enclosure, isolating it from other circuits to prevent the spread of smoke and flames. Isolation technology is a simple and effective method when all backup protections fail, but it requires significant equipment space and also requires preventing smoke and flames from escaping through openings in the enclosure's leads.
