How to use SMD Patch varistor correctly?

SMD chip varistoris an essential electronic component. It often appears in LED switching power supply or high-power power supply equipment. The transient overvoltage protection provided by SMD patch varistor in the operation of circuit system is very important. In fact, in addition to the technical parameters of overvoltage protection SMD patch varistor, the following issues should also be considered in the actual selection

Varistor (MOV) is a kind of zinc oxide (ZnO) Non-linear resistance element with main component. The surge current tolerance and nonlinear coefficient of components are large. When the threshold voltage is below, the resistance is very high and there is almost no current flow. If the threshold voltage is exceeded, the resistance will drop sharply and the amplified current will discharge. Because of this characteristic, the protection element of electrical equipment, as an electronic element, plays a great role in absorbing abnormal voltage and lightning surge

Varistors are usually used in parallel in circuits. When the voltage at both ends of the resistor changes sharply, the short circuit of the resistor will blow the current fuse and play a protective role. Varistors are often used for power overvoltage protection and voltage stabilization in circuits

1. SMD patch varistor voltage UN (U1mA): usually indicates whether it is turned on by the voltage when 1mA DC current passes through SMD patch varistor. This voltage is called SMD patch varistor voltage UN. The voltage sensitive voltage is usually represented by the symbol U1mA. The error range of varistor voltage is generally ± 10%. In the test and actual use, the failure criterion of the varistor is usually that the voltage of the varistor is 10% lower than the normal value

2. Maximum continuous working voltage Uc: refers to the maximum AC voltage (effective value) UAC or maximum DC voltage UDC that the varistor can withstand for a long time

3. Maximum clamping voltage (limiting voltage) VC: The maximum clamping voltage value refers to the voltage applied to the varistor μ 8/20. is the voltage on the varistor under S-wave pulse current IX (a)

4. Leakage current IL: the current when the maximum DC voltage UDC is applied to the varistor. When measuring leakage current, usually add UDC=0.83u1ma voltage (sometimes 0.75u1ma) to theSMD patch varistor. Normally, static leakage current IL ≤ 20 μ A (≤ 10 is also required) μ A)。 In practical applications, more attention is paid to the stability of the static leakage current, that is, the change rate after the impact test or under high temperature conditions, rather than the size of the static leakage current itself. If the change rate does not exceed two times after the impact test or under high temperature conditions, it is considered stable

As a widely usedSMD patch varistor, overvoltage protection SMD patch varistor is usually used to protect thyristor rectifier

In general, the surge absorption of electrical equipment usually adopts the 3KA varistor model, the overvoltage absorption of lightning and electronic equipment usually adopts the 5kA varistor model, and the lightning protection generally adopts the 10kA varistor model. Here we take lightning protection as an example