How to select SPD varistor?

SPD varistorHow to select the type

When the electrical resistance breaks down or reverses, the voltage at both ends of the resistance will be limited to a fixed level, and the current through the resistance will increase rapidly. Therefore, the varistor can be used to release current and clamp voltage. So, how to select SPD varistor

SPD varistorHow to select the type?

Although the varistor can absorb a large amount of surge energy, it cannot withstand excessive current (generally milliampere level) 。 Therefore, when selecting the varistor as overvoltage protection, the leakage current it can withstand must be considered. When selecting a rheostat, the following parameters are usually considered:

1. Varistor voltage: the breakdown voltage of the resistance electrode, which is actually measured by the manufacturer

2. Current capacity: pulse current at 25 ℃. It reflects the tolerance of the rheostat. When the current capacity is insufficient, the varistor will be irreversibly broken down. The greater the current capacity, the stronger the resistance of the rheostat

3. Nominal voltage: Because rheostats are often used in parallel with devices, the voltage of rheostats must be consistent with the rated voltage of parallel devices or devices

4. Nonlinear index: the relationship between voltage and current of the varistor is constant if I=(U/K) is greater than 1. K. This parameter reflects the sensitivity of the rheostat

What are the basic performances ofSPD varistor

1. Protection characteristics. When the impact strength (or impact current Isp=Usp/Zs) of the impact source does not exceed the specified value, the limit voltage of the varistor is not allowed to exceed the surge withstand voltage (Urp) that the protected object can withstand

2. Impact resistance, that is, the varistor itself should be able to withstand the specified impact current, impact energy, and the average power when multiple impacts occur successively

3. There are two vital characteristics. One is the continuous working voltage life, that is, the varistor should be able to work reliably within the specified time (hours) under the specified ambient temperature and system voltage conditions. The second is impact life, that is, the number of times that can reliably withstand the specified impact

4. After the varistor is inserted into the system, in addition to the protection function of 'safety valve', it will also bring some additional effects, called "secondary effect", which should not reduce the normal working performance of the system. At this time, three factors are mainly considered, one is the capacitance of the varistor itself (tens to tens of thousands of PF), the other is the leakage current under the system voltage, and the third is the influence of the nonlinear current of the varistor on other circuits through the coupling of the source impedance

The resistance material of the varistor is semiconductor, so it is a kind of semiconductor resistance. At present, "zinc oxide" (ZnO) varistor is widely used. Its main material is composed of divalent element zinc (Zn) and hexavalent element oxygen (O). Therefore, in terms of material, zinc oxide varistor is a "II-VI oxide semiconductor". In Taiwan, China Province of China, varistors are referred to as "surge absorbers" and sometimes also as "surge suppressors (absorbers)"