What causes the varistor to be damaged?

The rheostat is a voltage limiting protection device. By using the nonlinear characteristics ofvaristor, when overvoltage occurs at its two poles, the voltage can be clamped at a relatively fixed voltage value, thus realizing the protection of subsequent circuits. Varistors also play a very important role in the transient overvoltage protection of circuits. Although its flow is large, its energy capacity is not large. In addition, because the maximum pulse width of its impact current is far less than the actual pulse current width of large and medium-sized power semiconductor systems, short circuit or burning fault often occurs

Varistor is a kind of protective element and a variety of semiconductor resistor. It has the characteristics of small size, wide working range, fast response, strong impact current resistance and small leakage current. Now, the commonly used varistor in the market is a kind of (ZnO) Varistor made of zinc oxide. The main reasons for its damage are as follows:

1. Insufficient withstand voltage

This is easy to understand. If the working voltage of a product is 220V, but the varistor you use is below 180V, it will definitely be broken down

2. Excessive current and surge

MYG05K specifies that the current passing through is 0.1mA. The nominal voltage of MYG07K, my10k, MYG14K, MYG20K refers to the voltage value at both ends of the varistor when the 1mA DC current passes through. The use of devices in the product, especially those that need to be plugged in and out, will lead to faster damage of the varistor, because the surge when the product is plugged in and out is relatively large (the equipment at both ends is not supplied). At this time, the withstand voltage of the varistor will weaken the protection ability of the product itself and TVS, resulting in a higher damage rate

Severalvaristorsoverheat protection technologies:

1. Use spring pull low melting point solder

At present, most manufacturers adopt this technology. Add a low melting point solder joint at the pin of the rheostat, and then use the spring to pull the solder joint. When the leakage current of the rheostat is large and the temperature rises to a certain degree, the solder joint will melt, and the solder joint will rapidly separate under the pull of the spring, so that the rheostat will be separated from the circuit, and the linkage alarm contact will send an alarm signal at the same time

2. Packaging

In order to prevent smoke, fire or explosion when the rheostat fails, some manufacturers will use this technology to package the rheostat. However, when thevaristorfails, an internal arc will be generated, resulting in the failure of the sealing material, the generation of carbon, and the maintenance of the arc, which often leads to the internal short circuit and blackening of the equipment

3. Isolation

In this technology, the rheostat is installed in a closed box and isolated from other circuits to prevent the smoke and flame spread of the rheostat. When various backup protections fail, isolation can be regarded as a simple and effective method, but it needs to occupy a large amount of equipment space, and at the same time, smoke and flame shall be prevented at the opening of the box lead