Selecting Method of Varistor in Switching Power Supply

Although varistors can absorb large surge energy, they cannot withstand sustained currents above milliamperes, which must be taken into account when being used as overvoltage protection. In the selection of varistor, two parameters, nominal voltage-sensitive voltage V1mA and current-current capacity, are generally selected.

1. A is the voltage fluctuation coefficient of the circuit, and the general value is 1.2.

2. Vrms is the effective value of AC input voltage.

3, b is the error of varistor, the general value is 0.85.

4, C is the aging coefficient of the element, the general value is 0.9.

5. The peak value should be considered when √2 is in the AC state.

6. V1mA is the approximate value of voltage of varistor.

7. The peak value of current passing capacity, that is, the maximum pulse current is the maximum pulse current value when the ambient temperature is 25 ℃, for the specified impulse current waveform and the specified impulse current times, the variances of the voltage-sensitive voltage do not exceed ±10.

In the light of the foregoing, take a look at the relevant parameters corresponding to the type of varistor in this circuit.

The 07D471K AC300V DC385 varistor power supply 470V 423-517) limits the power supply 775Vp(v) 10lp(A) flow capacity 1200for one time (A) 600 for two times (A) the energy of 30 0.25 power (W)150 capacitance 1KN Z(pF).