Among electronic components, varistors are also relatively common. Compared with ESD electrostatic diodes and TVS tubes, varistors play a large role in circuit protection. Many people may not be familiar with them, but regardless of their role in circuit design There should be a way to quickly select a varistor during selection or selection.

Overview
The varistor is connected in parallel in the circuit. It is relatively sensitive to voltage (up to a certain abnormally high voltage). At normal operating voltage, it is equivalent to an insulator. When the voltage is abnormally high, the resistance value instantly becomes very small. , the current flows back to the front end through the varistor and pulls the fuse. If the voltage is relatively high for a long time, it will breakdown instantly, protecting the back-end circuit.

Tips for quickly selecting a varistor:
1) Working environment:
It should be within the range specified by the technical conditions, ambient temperature: -40C ~ +85℃.
2) Select the varistor according to the power supply voltage:
The so-called varistor voltage is the breakdown voltage or threshold voltage. Refers to the voltage value under a specified current. In most cases, the voltage value is measured when 1mA DC current is passed into the varistor. The varistor voltage range of its products can range from 10-9000V. The correct choice can be made according to specific needs. Generally V1mA=1.5Vp=2.2VAC. In the formula, Vp is the peak value of the rated voltage of the circuit. VAC is the effective value of the rated AC voltage. The selection of the voltage value of ZnO varistor is crucial, as it is related to the protection effect and service life. For example, the rated power supply voltage of an electrical appliance is 220V, then the voltage value of the varistor is V1mA=1.5Vp=1.5×1.414×220V=476V, V1mA=2.2VAC=2.2×220V=484V, so the breakdown voltage of the varistor is optional. Between 470-480V.
3) Selection of traffic flow:
The so-called current capacity, that is, the peak value of the maximum pulse current is the maximum pulse current when the change of the varistor voltage does not exceed ±10% for the specified inrush current waveform and the specified number of inrush currents when the ambient temperature is 25°C. value. In order to extend the service life of the device, the amplitude of the surge current absorbed by the ZnO varistor should be less than the maximum flow rate of the product given in the manual. However, from the perspective of protection effect, it is better to select a larger flow rate. In many cases, it is difficult to accurately calculate the actual flow rate, so products of 2-20KA are selected. If the flow rate of the product on hand cannot meet the usage requirements, several single varistors can be used in parallel. The varistor after parallel connection will remain unchanged, and its flow rate is the sum of the values ​​of each single varistor. The volt-ampere characteristics of parallel connected varistors are required to be as consistent as possible, otherwise uneven shunting may occur and damage the varistors.

4) Selection of clamping voltage:
The pressure-sensitive clamping voltage should not be greater than the voltage that the protected circuit or sensitive equipment can withstand.
5) Parallel connection of varistor:
When a single varistor cannot meet the nominal discharge current requirement, two or more should be used in parallel in the circuit. Sometimes in order to reduce the limit voltage, multiple varistors are connected in parallel even if the nominal discharge current meets the requirements. Special attention should be paid to the fact that when varistors are used in parallel, they must be carefully selected and paired with consistent parameters to ensure even distribution of current.

More information
You can view more product line information of varistor through the following method: https://semiware.com/pro-cat/varistor/. If you have technical questions, please contact our technical support engineers: fae01@semiware.com.