In applying a power supply, a peripheral protection circuit is often created to guarantee the power supply’s reliability and security and to make certain that the power supply will not be destroyed while it is being used. For example, the surge protection circuit, the anti-static protection circuit, the EMI filter circuit, and the pulse swarm rejection circuit are all included in the peripheral circuit. In this study, the use and technique of selection of the MOV varistor that is used in the surge impact protection circuit will be discussed.
What is a varistor?
MOV is a protective device that limits the voltage and has nonlinear properties of voltage and current. Its body is a matrix structure made up of zinc oxide particles, and the grain boundaries that separate the particles have electrical properties comparable to those of a bidirectional PN junction.
When the pressure sensitivity is in normal usage, it is in the region of leakage current, but when the surge hits it, it enters the nonlinear area to discharge the surge current. In most cases, it is unable to reach the saturation area. When the pressure-sensor voltage is low, the pressure sensitive works in the current leakage area, showing a large resistance, and the leakage current is very small; When the voltage increases to the nonlinear region, the voltage changes little, the current changes in a considerable range and the voltage limiting characteristic is improved. When the voltage is raised once again, the pressure-sensitive element enters the saturation zone, exhibiting a low linear resistance; nevertheless, the current is quite high, and if it is allowed to remain in this state for an extended period, it would overheat, burn out, and maybe even explode. Because of this, a current of 1 mA cannot be sustained by the varistor for an extended period.
Varistor has the following qualities:
– Wide working voltage range
– A prompt reaction to the overvoltage impulse
– A very low leakage current
– A temperature resistance coefficient that is low.
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