Lightning protection, overvoltage protection, when the varistor is not working, it has a high impedance (several megohm). When the transient surge voltage appears, the impedance of the varistor becomes low (several ohms), which makes the original circuit short circuit and protects the subsequent circuit. Once the varistor is broken down and short circuit cannot be restored, it must be replaced.
Use of varistor:
The varistor is used in parallel with the protected device or device and is often used in conjunction with the fuse. When overvoltage occurs, the varistor shorts if the overcurrent fuse in the circuit is disconnected from the protective stage circuit.
Selection of varistor:
The selection of varistor has two important parameters, the nominal varistor voltage V1mA and current capacity.
1, nominal varistor voltage, that is, breakdown voltage or threshold voltage. V1mA refers to the voltage measured when 1mA dc current enters the varistor.
V1mA= 1.5VP =2.2VAC, where Vp is the peak value of the rated voltage of the circuit and VAC is the effective value of the rated AC voltage. The selection of voltage value of varistor is very important, it is related to the protection effect and service life.
2, the so-called current capacity, refers to the ambient temperature of 25℃, for the specified impulse current waveform and the number of impulse current, voltage sensitive voltage change does not exceed ± 10% of the maximum pulse current value.
Starting from the protection effect, it is better to choose a large flow rate. In many cases, the actual flow rate is difficult to calculate accurately, so use 2-20kA products. If the flow rate of the product at hand cannot meet the requirements of use, several varistors can be used in parallel. The varistor voltage after parallel operation remains unchanged, and the flow rate is the sum of the values of each varistor. The voltage ampere characteristics of the varistor in parallel should be the same as far as possible, otherwise it is easy to cause uneven shunt and damage the varistor.
The difference between pressure-sensitive resistor, gas discharge tube and TVS tube three kinds of anti-surge devices:
1, reaction speed, varistor (NS) between gas discharge tube (dozens of NS) and TVS tube (PS).
2, current capacity, varistor (KA to 10 KA) between gas discharge tube (10 KA can be conductive many times) and TVS tube (several hundred A).
3. In principle, varistor is the barrier effect between zinc oxide grains, avalanche effect of TVS diode and gas breakdown discharge of gas discharge tube.
4, voltage range: 5.5V~550V TVS 10-9000V gas discharge tube 70V~3500V.
5. The varistor is bi-polar. TVS tube can be bi-polar or unipolar.
6. TVS tube has high voltage resistance, good ESD effect and high cost; Low voltage resistance of varistor, poor ESD effect, low cost.