Varistor: Definition, Function, Working, and Testing

Introduction

Varistor, also known as Voltage Dependent Resistor (VDR), is a nonlinear resistor element whose resistance changes with the voltage applied to its two ends. The main function of varistor is to protect electronic circuits from voltage transients, and is usually used for overvoltage protection of power lines, communication lines and various electronic devices.For detailed information about varistors, including their specifications and applications, you can visit the Varistor Product Page on the DRex website. This page provides a comprehensive overview of various varistor models offered by DRex, helping you select the right component for your circuit protection needs. If you’re interested in exploring other circuit protection components, the Circuit Protection Components Category Page offers a wide range of products designed to safeguard electronic circuits from overvoltage and other electrical issues.

Definition

Varistor is an electronic component made of metal oxide semiconductor materials (such as zinc oxide). Its resistance value changes nonlinearly with the applied voltage within a certain range. When the voltage is lower than its threshold voltage, the varistor exhibits a high resistance value; when the voltage exceeds the threshold voltage, the resistance value of the varistor drops rapidly, thereby forming a low-resistance channel, shunting or short-circuiting the excess current, and protecting subsequent circuit components from damage.

Function

The main function of the varistor is to provide overvoltage protection to prevent the circuit from being damaged by voltage fluctuations, surge currents or lightning strikes. Specifically, varistors have the following main functions:

3. Overvoltage protection:In a circuit, when the voltage exceeds the action voltage of the varistor, the varistor will quickly turn on, reduce the voltage, and protect other components.
4. Current shunt:When the voltage is too high, the varistor will absorb and shunt the excess current to prevent the components in the subsequent circuit from being subjected to excessive current.
5. Anti-interference: Varistors can also be used to filter out voltage spikes and noise on power lines and reduce the impact of electromagnetic interference on sensitive components.
6. Lightning protection: Varistors are often used to protect equipment from transient high voltages generated by lightning strikes.

Working principle

The working principle of the varistor is based on the nonlinear relationship of its voltage-resistance characteristics. Varistors are usually made of zinc oxide as the main material, which has excellent nonlinear volt-ampere characteristics. The specific working principle is as follows:

1. Low voltage state:When the voltage in the circuit is lower than the rated voltage of the varistor (also called threshold voltage or breakdown voltage), the varistor is in a high resistance state, its resistance is very large, and it is almost non-conductive.
2. Overvoltage state: When the voltage exceeds the threshold voltage, the varistor enters a low-resistance state, and the resistance drops sharply, forming a low-resistance channel. At this time, the varistor quickly shunts or shorts the excess voltage by conducting a large amount of current, thereby protecting the circuit from overvoltage damage.
3. Recovery state: When the overvoltage phenomenon disappears and the voltage returns to normal, the resistance of the varistor increases again, returns to the high-resistance state, and continues to protect the circuit normally.

Test method

The test of the varistor mainly includes its appearance inspection, breakdown voltage test, and leakage current test. The specific test method is as follows:

1. Appearance inspection: First check whether the appearance of the varistor has obvious defects such as damage, burning or cracks to ensure that it is intact.
2. Breakdown voltage test:Connect the varistor to a DC regulated power supply, gradually increase the voltage, and record the voltage value when the varistor starts to conduct, which is the breakdown voltage of the varistor. The voltage value should be close to the nominal value. If the deviation is large, it means that the varistor may be damaged or aged.
3. Leakage current test:Under the condition of less than the breakdown voltage, use a multimeter or micro-ammeter to measure the leakage current through the varistor. Under normal circumstances, the leakage current should be very small. If the measured value is too large, it may indicate that the insulation of the varistor has been reduced and is no longer suitable for continued use.
4. On-off test: Use the resistance range of the multimeter to measure the resistance of the varistor. Under normal voltage, the varistor should show a high resistance value; if the measured resistance value is significantly low, it may indicate that the varistor is short-circuited internally and needs to be replaced.

Summary

As an important overvoltage protection component, varistors are widely used in various electronic circuits. By understanding the definition, function, working principle and test method of varistors, we can better select and use varistors to protect electronic equipment from overvoltage. For engineers and technicians, mastering the relevant knowledge of varistors will help improve the reliability and safety of circuit design.For additional resources, the Varistor Overview by Littelfuse provides a deep dive into the working principles and applications of varistors. If you’re looking to test varistors, the Testing Varistors article on Electro Schematics explains the methods and tools necessary for effectively evaluating these components, ensuring they function properly within your circuits.