Electronic

What is a Varistor? How to use varistor correctly?

Varistors are one of the necessary Electronic components for engineers. Varistors can often be seen in LED switching power supplies or high-power power supply equipment. The transient overvoltage protection provided by the varistor for the circuit system during operation is very important. In fact, in addition to the technical parameters of the overvoltage protection varistor, the following issues should be considered in the actual selection. How to use the overvoltage protection varistor correctly?

Varistors are one of the necessary electronic components for engineers. Varistors can often be seen in LED switching power supplies or high-power power supply equipment. The transient overvoltage protection provided by the varistor for the circuit system during operation is very important. In fact, in addition to the technical parameters of the overvoltage protection varistor, the following issues should be considered in the actual selection. How to use the overvoltage protection varistor correctly?

Varistor (MOV) is a non-linear resistance element with zinc oxide (ZnO) as the main component. The surge current withstand capacity and nonlinear coefficient of this element are very large. When the voltage is below the threshold voltage, the resistance is very high and there is almost no current flow. However, if the threshold voltage is exceeded, the resistance decreases sharply, and the large current can be released. Due to this characteristic, as a protection element of electronic and electrical equipment, it plays a great role in absorbing abnormal voltage and lightning surge. .

What is a Varistor? How to use varistor correctly?

How to use varistor correctly

The varistor is generally used in parallel in the circuit. When the voltage across the resistor changes sharply, the short circuit of the resistor will fuse the current fuse and play a protective role. Varistors are often used in power supply overvoltage protection and voltage regulation in circuits.

1. Varistor voltage UN (U1mA): Usually, the voltage when 1mA DC current passes through the varistor is used to indicate whether it is turned on or not. This voltage is called the varistor voltage UN. The varistor voltage is also commonly represented by the symbol U1mA. The error range of the varistor voltage is generally ±10%. In the test and actual use, the varistor voltage is usually reduced by 10% from the normal value as the criterion for the failure of the varistor.

2. The maximum continuous working voltage UC: refers to the maximum AC voltage (effective value) Uac or the maximum DC voltage Udc that the varistor can withstand for a long time. Generally Uac≈0.64U1mA, Udc≈0.83U1mA

3. Maximum clamping voltage (limiting voltage) VC: The maximum clamping voltage value refers to the voltage present on the varistor when the specified 8/20μs wave impulse current IX (A) is applied to the varistor. ?

4. Leakage current Il: the current flowing when the maximum DC voltage Udc is applied to the varistor. When measuring leakage current, a voltage of Udc=0.83U1mA is usually added to the varistor (sometimes 0.75U1mA is also used). Generally, the static leakage current Il is required to be ≤20μA (there are also requirements of ≤10μA). In actual use, what is more concerned is not the size 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. After the impact test or under high temperature conditions, its rate of change does not more than double, that is, it is considered to be stable?

As a varistor with a wide range of applications, the overvoltage protection varistor is also often used to protect the thyristor rectifier. Then according to the different current capacity, how should we choose the appropriate overvoltage protection voltage What about the varistor? Under normal circumstances, the 3kA varistor type is usually used for surge absorption of electrical equipment, while the 5kA resistance type is mostly used for lightning strikes and overvoltage absorption of electronic equipment, and the 10kA type will be used. in protection against lightning strikes. Here we take lightning strike protection as an example to illustrate.

Usually when testing, we will choose the commonly used synthetic wave (voltage wave of 1.2/50μs when the generator is open-circuit output; current wave of 8/20μs when the output is short-circuited; the internal resistance of the generator is 2Ω) to evaluate the equipment online against lightning strikes surge disturbance capability. In the 4kV test, the current absorbed by the overvoltage protection varistor can reach 2kA, and for the 6kV test, the absorbed current value is 3kA. However, in the actual selection, the current capacity of the selected overvoltage protection varistor should also be appropriately increased.

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