Precautions for use of varistors

Sensitive resistance has the advantages of fast response time, low leakage current, etc. Now it has been applied in power systems, security systems, automotive electronic systems, motor protection, home appliances and many other fields. What should we pay attention to when selecting varistors?

Select varistor precautions:

(1) Varistor voltage selection

Consider the deviation between the actual varistor voltage and the nominal voltage of the varistor (which should be taken as 1.1 to 1.2 times of the nominal voltage) and the possible fluctuation range of the supply voltage in the AC circuit (1.4-1.5 of the rated voltage should be considered 1.5 times), the relationship between the peak value of the AC voltage and the effective value (1.4 times should be considered), therefore, the varistor with a varistor voltage of 2.2 to 2.5 times the rated voltage should be used. In DC circuits, varistors with a varistor voltage of 1.8 to 2 times the DC voltage rating are often used.

(2) Selection of flow capacity

In principle, it should be selected according to the possible transient surge currents.

In the actual selection, the flow capacity of the selected varistor should also be appropriately increased. Because varistors with large flow capacity have a relatively small residual voltage drop when absorbing surge currents of the same size, there is also a large margin of protection for selected varistors.

(3) Intrinsic parasitic capacitance

Varistors have an inherent capacitance problem, which varies from hundreds to thousands of pF, depending on the size of the outline and the nominal voltage. The inherent capacitance of the varistor determines that it is not suitable for use in high frequency applications, otherwise it will affect the normal operation of the system; It is suitable for use in industrial frequency systems, such as the protection of incoming power lines, the protection of thyristors, etc. .

The instantaneous power of the varistor is relatively large, but the average continuous power is very small, so it cannot be allowed to work for a long time.

When using varistors, we can refer to the above.