How to choose varistor?

 

 

 

       Before selecting a varistor, you should first understand the following technical parameters: The nominal voltage refers to the voltage across the varistor at the specified temperature and DC current [3]. Leakage current is the value of the current flowing through the varistor when a maximum continuous DC voltage is applied at 25 °C. The gradation voltage refers to the voltage peak presented at both ends of the varistor when passing 8/20 gradation current pulses [3]. The flow rate is the peak current [3] when a predetermined pulse current (8/20 μs) waveform is applied. Surge environmental parameters include maximum inrush current Ipm (or maximum surge voltage Vpm and surge source impedance Zo), surge pulse width Tt, minimum time interval Tm between adjacent surges, and reservation at the varistor The total number of surge pulses is N during the working life.

Generally speaking, varistors are often used in parallel with protected devices or devices. Under normal conditions, the DC or AC voltage across the varistor should be lower than the nominal voltage, even when the power supply fluctuates worst. It should not be higher than the maximum continuous working voltage selected in the rated value. The nominal voltage value corresponding to the maximum continuous working voltage value is the selected value. For overvoltage protection applications, the varistor voltage should be greater than the actual circuit voltage. Generally, the following equation should be used: VmA=av/bc [4]:

a is the circuit voltage fluctuation coefficient; v is the circuit DC working voltage (effective value when AC); b is the varistor voltage error; c is the aging coefficient of the component; the calculated VmA actual value is 1.5 times the DC working voltage [4], the peak value should also be considered in the AC state, so the calculation result should be expanded by 1.414 times [5].

 

What should I pay attention to when choosing?

 

       (1) It must be ensured that when the voltage fluctuation is maximum, the continuous working voltage will not exceed the maximum allowable value, otherwise the service life of the varistor will be shortened;

       (2) When a varistor is used between the power supply line and the ground, the voltage between the line and the ground may rise due to poor grounding. Therefore, a varistor with a higher nominal voltage than the line-to-line use is usually used. 6].

The surge current absorbed by the varistor should be less than the maximum flux of the product.