The varistor is one of the necessary electronic components in the hands of engineers. The varistor can often be seen in LED switching power supplies or high-power power supplies. It is very important that the varistor provides instantaneous overvoltage protection for the circuit system during operation. In fact, in addition to the technical parameters of the overvoltage protection varistor, the following problems should be considered in the actual selection. How to properly use overvoltage protection varistor?
The varistor (MOV) is a non-linear resistance element with zinc oxide (ZnO)
as its main component. The surge current tolerance and nonlinear coefficient of
this component are very large. When the threshold voltage is below the threshold
voltage, the resistance is very high and there is almost no current flow. If the
threshold voltage is exceeded, the resistance is drastically reduced, and the
amplified current can be discharged. Due to this characteristic, as a protective
element for electronic and electrical equipment, the absorption of abnormal
voltage and the absorption of lightning surges play a large role.
The varistor is generally used in parallel in the circuit. When the voltage across the resistor changes abruptly, the short circuit of the resistor blows the current fuse to protect it. Varistors are used in circuits and are often used for power supply overvoltage protection and regulation.
1. Varistor voltage UN (U1mA)
A flag voltage that is normally turned on by a voltage of 1 mA DC current across a varistor. This voltage is called a varistor voltage UN. The varistor voltage is also commonly referred to by the symbol U1mA. The error range of the varistor voltage is generally ±10%. In test and practical use, the varistor voltage is usually reduced by 10% from the normal value as the criterion for varistor failure.
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. General Uac≈0.64U1mA, Udc≈0.83U1mA
3. Maximum clamp voltage (limit voltage) VC
The maximum clamp voltage value is the voltage appearing on the varistor when a specified 8/20 μs wave impulse current IX (A) is applied to the varistor.
4. Leakage current Il
Current flowing when a maximum DC voltage Udc is applied to the varistor. When measuring the leakage current, the voltage of Udc=0.83U1mA is usually applied to the varistor (sometimes also 0.75U1mA). Static leakage current is generally required to be ≤ 20μA (also required to be ≤ 10μA). In actual use, it is not the magnitude of the static leakage current value itself, but its stability, that is, the rate of change after the impact test or under high temperature conditions. The rate of change is not more than double after the impact test or under high temperature conditions, which is considered to be stable.
As a wide range of varistor applications, overvoltage protection varistors are often used to protect thyristors. How do we choose the appropriate overvoltage protection pressure depending on the flow capacity? What about the varistor? In general, the 3kA varistor model is usually used for surge absorption of electrical equipment, while the 5kA resistor model is used for overvoltage absorption of lightning strikes and electronic equipment, 10kA. It will be used to protect against lightning strikes. Here we use lightning protection as an example to illustrate.
Usually in the test, we will choose the common integrated wave (the generator open circuit output produces 1.2/50μs voltage wave; the short circuit output produces 8/20μs current wave; the generator's internal resistance is 2Ω) to online test equipment against lightning strikes The ability to surge interference. In the 4kV test, the overvoltage protection varistor can sink up to 2kA, and for the 6kV test, the sink current is 3kA. However, in actual selection, the flow capacity of the selected overvoltage protection varistor should also be appropriately increased.