Varistors how to choose

Under normal circumstances, in the protection circuit, the minimum value of the varistor voltage should meet the requirement of formula (1). In addition, the large value of the varistor voltage should also meet the requirements of formula (2) according to the withstand voltage level of the protection circuit:

V1 ≈2. 2Vac or V1≈2. 0Vdc (1)

V1 ≤ 0. 9Vz/ Kp (2)

In the formula: V1 --- represents the varistor voltage value under 1mA direct current;

Vac --- represents the effective value of the AC voltage;

Vp - p --- represents the peak-to-peak value of the AC voltage;

Vdc --- DC voltage value.

In the formula: Vz ——— indicates the impulse insulation withstand voltage value of the protected equipment (or component);

Kp — Limit voltage ratio is a material-related constant.

For example: an electronic ballast's AC voltage is 220Vac, then the minimum voltage of the varistor should be V1 = 115 & 2; & 220V = 467V. According to the relevant IEC regulations, the protection level of electronic equipment is Class D protection, and its insulation voltage value is generally specified as 115kV.

In fact, for a specific electronic ballast product, if the pulse resistance voltage of the rectifier circuit is 1000V-1200V, a common varistor with a current capacity of 0-10 K, if the Kp value is 117-118, it will be calculated and pressed. The large pressure sensitive value of the sensitive resistor should be V1 ? 019Vz/K?019 <RTIgt;1200/117</RTI> 643 V. Therefore, for the ballast products of 220Vac power systems with relatively stable supply voltage, MYG3/300 (varistor voltage 470 V) type varistors are generally used.

For ballasts with a supply voltage of 120Vac, MYG3/150 (varistor voltage 250V) varistors are usually used. Of course, under the condition that the circuit design has been determined, the protection level of the actual circuit can also be determined through a large number of tests, and more appropriate varistors can be selected.