Overview

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

The varistor is connected in parallel in the circuit. It is sensitive to voltage (reaching a certain abnormally high voltage). At normal operating voltage, it is equivalent to an insulator. When the voltage is abnormally high, the resistance value becomes very small instantly. 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 instantly break down and protect the back-end circuit.

Tips for quickly selecting varistors:

  • Working environment: It should be within the range specified by the technical conditions, ambient temperature: -40C ~ +85℃.
  • Select varistor according to power supply voltage: The so-called varistor voltage is the breakdown voltage or threshold voltage. It refers to the voltage value under the specified current. In most cases, it is the voltage value measured when 1mA DC current is passed through the varistor. The varistor voltage range of its products can range from 10-9000V. It can be selected correctly according to specific needs. Generally, V1mA=1.5Vp=2.2VAC, where Vp is the peak value of the rated voltage of the circuit. VAC is the effective value of the rated AC voltage. The voltage value selection of ZnO varistor is crucial, which is related to the protection effect and service life. If the rated power supply voltage of an appliance is 220V, then the varistor voltage value V1mA=1.5Vp=1.5×1.414×220V=476V, V1mA=2.2VAC=2.2×220V=484V, so the breakdown voltage of the varistor can be selected between 470-480V.
  • Selection of flow capacity: The so-called flow capacity, that is, the peak value of the maximum pulse current is the maximum pulse current value when the change of the varistor voltage does not exceed ± 10% for the specified impulse current waveform and the specified number of impulse currents under the condition of an ambient temperature of 25℃. In order to extend the service life of the device, the surge current amplitude absorbed by the ZnO varistor should be less than the maximum flow of the product given in the manual. However, from the perspective of protection effect, it is better to select a larger flow. In many cases, the actual flow is difficult to calculate accurately, so 2-20KA products are selected. If the current flow of the product at hand cannot meet the use requirements, several single varistors can be used in parallel. The varistor voltage after parallel connection remains unchanged, and its current flow is the sum of the values ​​of each single varistor. It is required that the volt-ampere characteristics of the parallel varistors are as similar as possible, otherwise it is easy to cause uneven shunting and damage the varistor.
  • Selection of clamping voltage: The varistor clamping voltage should not be greater than the voltage that the protected circuit or sensitive equipment can withstand.
  • Parallel connection of varistors: When a single varistor cannot meet the nominal discharge current requirements, two or more varistors should be used in parallel in the circuit. Sometimes, in order to reduce the limiting voltage, multiple varistors are used in parallel even if the nominal discharge current meets the requirements. It should be noted that when varistors are used in parallel, they must be strictly selected with consistent parameters for pairing to ensure uniform current distribution.

More information

You can view more varistor product line information at: https://semiware.com/en/product-line/varistor/