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Selection and precautions of varistor

Selection and precautions of varistor

Before choosing a varistor, you should first understand the following related technical parameters: Nominal voltage refers to the voltage value across the varistor under the specified temperature and DC current. Leakage current refers to the current value flowing in the varistor when the maximum continuous DC voltage is applied under the condition of 25°C. Grade voltage refers to the voltage peak value that appears at both ends of the varistor when it passes 8/20 grade current pulses [1]. The flow rate is the peak current when the specified pulse current (8/20μs) waveform is applied. Surge environment parameters include the maximum surge current Ipm (or the maximum surge voltage Vpm and the surge source impedance Zo), the surge pulse width Tt, the minimum time interval Tm between two adjacent surges, and the preset value of the varistor During the working life, the total number of surge pulses N etc. Generally speaking, varistors are often used in parallel with the protected device or device. Under normal circumstances, the DC or AC voltage across the varistor should be lower than the nominal voltage, even when the power supply fluctuates the worst. It should not be higher than the maximum continuous working voltage selected in the rated value, and the nominal voltage value corresponding to the maximum continuous working voltage value is the selected value. For the application of overvoltage protection, the varistor voltage value should be greater than the voltage value of the actual circuit. Generally, the following formula should be used for selection: VmA=av/bc where: 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 actual value of VmA calculated in this way is 1.5 times of the DC working voltage , The peak value should be considered in the AC state, so the calculation result should be enlarged by 1.414 times. In addition, you must pay attention to: (1) It must be ensured that the continuous working voltage will not exceed the maximum allowable value when the voltage fluctuation is maximum, otherwise the service life of the varistor will be shortened; (2) When a varistor is used between the power line and the ground, sometimes the voltage between the line and the ground rises due to poor grounding. Therefore, a varistor with a higher nominal voltage than the line-to-line use is usually used. The surge current absorbed by the varistor should be less than the maximum flow rate of the product.
Release time : 2020-04-13
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