What should I pay attention to when selecting SMD patch varistor?

Before selecting a rheostat, you should first understand the following technical parameters: nominal voltage refers to the voltage value at both ends of the rheostat at the specified temperature and DC current. Leakage current refers to the current value flowing through SMD patch varistor when the maximum continuous DC voltage is applied at 25 ℃. Grade voltage refers to the voltage peak at both ends of the rheostat when the rheostat passes 8/20 grade current pulse. Apply the specified pulse current (8/20) on the waveform through the flow rate μ s) Peak current. The surge environmental 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 between two adjacent surges TM, and the total number of surge pulses within the expected service life of the SMD patch varistor

In general, the varistor is usually used in parallel with the protected device or device. Normally, the DC or AC voltage at both ends of the SMD patch varistor should be lower than the nominal voltage. Even if the power fluctuation is the most serious, it should not be higher than the maximum continuous working voltage selected in the rated value. The nominal voltage corresponding to the maximum continuous working voltage value is the selected value. For overvoltage protection applications, the voltage value of the capacitor must be It must be greater than the voltage value of the actual circuit. Common formula: VMA=AV/BC, formula: the loop voltage fluctuation coefficient is 1.2. V is the working voltage of DC circuit (effective value at AC), B is the voltage sensitive fault, usually 0.85; C is the element aging coefficient, which is generally 0.9; The actual value of VMI calculated by this method is 1.5 times of the DC operating voltage. The peak value should also be considered under AC conditions, so the calculation result should be increased by 1.414 times. In addition, it must be noted that:

(1) When the voltage fluctuation is maximum, the continuous working voltage must not exceed the maximum allowable value, otherwise the service life of the rheostat will be shortened

(2) When using the varistor between the power line and the ground, sometimes the voltage between the line and the ground increases due to poor grounding, so the SMD patch varistor with the nominal voltage higher than the voltage between the lines is usually used

The fault mode of SMD chip varistor is mainly short circuit. However, the short circuit will not damage the rheostat, because the resistance is located at the positive and negative input terminals of the power supply; The insurance is good, and it is proved that it is not caused by short circuit or overcurrent. It may be that the surge energy is too large to exceed the absorbed power, which causes the SMD patch varistor to burn out; When the overcurrent is too large, it may also cause the valve disc to burst and open circuit

PhotoformingSMD chip varistorWhat is the cause of damage

1. Number of overvoltage protection

2. Ambient working temperature

3. Whether SMD patch varistor is squeezed

4. Whether it has passed the quality certification

5. The surge energy is too large to exceed the absorbed power

6. Insufficient withstand voltage

7. Excessive current, surge, etc

In addition, the service life of the rheostat is short, and the performance decreases after repeated shocks. Therefore, the arrester composed ofSMD patch varistorhas the problem of maintenance and replacement after long-term use