Capacitor School Varistors

The varistor is a resistive device with angle-modulated volt-ampere characteristics. It is mainly used for voltage clamping when the electronic circuit is subjected to overvoltage and absorbs excess current to protect sensitive devices. The abbreviation VDR is a nonlinear overvoltage protection electronic device with high voltage sensitivity.

Varistors have two tests:

1. Differential mode transient over-voltage test: When selecting a varistor, the voltage should be selected to be slightly larger than 300V. This voltage level varistor clamp voltage is about 1KV. Then on the differential-mode circuit, an ideal voltage source with an internal resistance of 2Ω and a square-wave pulse of 6 kV is connected in series with the varistor, and the instantaneous value current is about 5-1 kV/2Ω=2500A. Finally, 681KD14 was used as a varistor. Its peak current is 4.5KA, active work voltage 385VAC, clamp voltage 1.1KV. However, after the common-mode inductor, the circuit has been protected. After verification, the rectifier diode selects the familiar 1N4007.

2. Common mode surge test: When testing ACL-PE or ACN-PE 6KV surge, common mode surge test, the common mode path is equivalent to a voltage source with internal resistance of about 12Ω and pulse voltage of 4KV. In series with the common mode choke and Y capacitor. Therefore, the Y capacitor adopts a Y1 rating capacitor, and its compressive strength is high, and the energy of the 4KV common mode surge is not enough to damage it. Therefore, it is only necessary to ensure that the fixed wiring and other wiring remain certain indirect and can easily pass through the common mode. Surge test.

However, due to the sudden surge test, common mode inductors will form high voltage on both sides, causing arcing. If it is close to the nearby components, it is also possible to damage the surrounding components. Therefore, a discharge tube or a varistor can be connected in parallel to limit the voltage, so that the arc extinguishing effect is achieved.