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Selection and Precautions for Varistors

2020-04-13

Before selecting a varistor, it is important to understand the following relevant technical parameters: The rated voltage refers to the voltage value across the varistor under specified temperature and direct current conditions. The leakage current refers to the current value flowing through the varistor when the maximum continuous direct current voltage is applied at 25°C. The level voltage refers to the peak voltage presented across the varistor when an 8/20 level current pulse passes through it. The flow capacity indicates the peak current when a specified pulse current (8/20μs) waveform is applied. Surge environment parameters include the maximum surge current Ipm (or maximum surge voltage Vpm and surge source impedance Zo), surge pulse width Tt, the minimum time interval Tm between two adjacent surges, and the total number of surge pulses N during the varistor's predetermined service life.

Generally speaking, varistors are often used in parallel with the protected devices or systems. Under normal circumstances, the direct current or alternating current voltage across the varistor should be lower than the rated voltage. Even in the worst case of power supply fluctuations, it should not exceed the maximum continuous operating voltage selected from the rated values, and the rated voltage corresponding to this maximum continuous operating voltage is the selected value. For overvoltage protection applications, the varistor voltage should be greater than the actual circuit voltage, and the following formula is generally used for selection: VmA=av/bc where:

a is the circuit voltage fluctuation coefficient; v is the circuit direct current working voltage (effective value for AC); b is the varistor voltage error; c is the aging coefficient of the component; thus, the calculated VmA actual value is 1.5 times the direct current working voltage, and in the AC state, the peak value should also be considered, so the calculation result should be multiplied by 1.414.

In addition, attention must also be paid to the following when selecting:

(1) It must be ensured that the continuous working voltage does not exceed the maximum allowable value when the voltage fluctuation is at its maximum; otherwise, it will shorten the lifespan of the varistor.

(2) When using a varistor between the power line and the ground, sometimes due to poor grounding, the voltage between the line and the ground may rise, so a varistor with a higher rated voltage than that used between lines is usually adopted.

The surge current absorbed by the varistor should be less than the product's maximum flow capacity.

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