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Demystifying Varistors: Function, Working, and Testing Explained

Varistors are essential components in modern electronics, protecting against voltage spikes and transient surges. This article delves into the intricate world of Varistors, uncovering their function, working principle, and the methods used to test their efficacy.


Inside the global of electronics, protection against voltage fluctuations is paramount. Varistors, which are voltage-dependent resistors or VDRs, play a vital role in safeguarding touchy components from temporary voltage spikes. This article aims to demystify Varistors by exploring their function, working principle, testing methods, and more.

What are Varistors?

Quarktwin Electronic Varistors exhibit varying resistance based on the applied voltage. They primarily protect electronic circuits and components from voltage surges by diverting excessive current away from sensitive parts. Varistors act as voltage clamps, limiting the voltage across a circuit within a safe range.

How Do Varistors Function?

Varistors function based on a non-linear voltage-current characteristic. At low voltages, they exhibit high resistance, preventing significant current flow. However, when the voltage exceeds a certain threshold, the varistor’s resistance decreases dramatically, allowing excess current to pass through and dissipate the energy as heat.

Understanding the Working Principle

The operating principle of Varistors is rooted inside the semiconductor’s capacity to transition between insulating and conductive states. Varistors are commonly made from ceramics with zinc oxide (ZnO) as a primary thing. The ZnO grains shape crystalline, and the limits among these grains act as junctions. When a voltage spike occurs, these junctions transition, leading to a sudden drop in resistance.

Types of Varistors

Varistors come in various types, each tailored to specific applications. Some common types include:

  1. Metal-Oxide Varistors (MOV): These are the most common varistor type, featuring a ceramic body with a metal oxide layer. MOVs are versatile and widely used for general surge protection.
  2. Semiconductor Varistors: These are constructed using semiconducting materials and are often faster in response than MOVs. They find applications in high-speed protection circuits.
  3. Multi-Layer Varistors (MLV): MLVs are surface-mount components known for their compact size and suitability for high-frequency applications.
  4. Transient Voltage Suppression Diodes (TVS): While not traditional Varistors, TVS diodes serve a similar purpose by quickly diverting excess voltage away from sensitive components.

Advantages of Using Varistors

The utilization of Varistors offers several key advantages:

  • Voltage Spike Suppression: Varistors effectively suppress voltage spikes, preventing damage to sensitive electronics.
  • Fast Response Time: Varistors have quick response times, reacting to voltage fluctuations almost instantaneously.
  • Reliability: Varistors have a long operational life and can endure multiple transient events without degradation.
  • Cost-Effectiveness: Implementing Varistors can reduce the need for more complex protection circuitry, making them a cost-effective solution.

Varistor Applications

Varistors find applications in various industries and devices, including:

  • Power Supplies: Varistors protect power supplies from voltage surges, ensuring stable and safe operation.
  • Telecommunications: They safeguard communication equipment against lightning-induced surges and other transient events.
  • Consumer Electronics: Varistors shield consumer electronics like TVs, computers, and audio systems from power fluctuations.
  • Industrial Equipment: Industrial machinery and automation systems benefit from Varistors‘ surge protection capabilities.

Factors Affecting Varistor Performance

The performance of Varistors can be influenced by factors such as:

    • Operating Voltage: The right varistor with an appropriate clamping voltage is crucial for adequate protection.
    • Response Time: Faster response times are essential for sensitive equipment.
    • Energy Absorption Capacity: Varistors must be able to absorb and dissipate the energy from transient events.
  • Environmental Conditions: Extreme temperatures and humidity ranges can affect varistor performance.
  • Testing Varistors: Importance and Methods

Regular testing of Varistors is vital to ensure their reliability and effectiveness. By identifying faulty Varistors, potential equipment damage can be prevented. There are two main methods of testing Varistors:

In-Circuit Testing vs. Bench Testing

In-circuit testing involves testing Varistors while they are still connected to the circuit. This method provides real-world conditions but may not accurately determine varistor health. Bench testing, on the other hand, requires the varistor to be disconnected and tested under controlled conditions.

Step-by-Step Guide to Varistor Testing

  1. Safety Precautions: Ensure proper safety measures, such as disconnecting power sources.
  2. Remove Varistor: If performing bench testing, carefully remove the varistor from the circuit.
  3. Resistance Measurement: Use a multimeter to measure the varistor’s resistance at a specific voltage.
  4. Clamping Voltage Test: Apply increasing voltage to the varistor until it reaches the clamping voltage, observing its response.
  5. Response Time Test: Apply a fast voltage spike and measure the varistor’s response time.
  6. Energy Absorption Test: Apply a surge and calculate the energy the varistor absorbs.
  7. Compare Results: Compare test results to varistor specifications to determine its health.

Common Varistor Testing Issues

During varistor testing, several challenges may arise:

  • Degraded Varistors: Varistors can degrade over time, leading to inaccurate test results.
  • In-Circuit Interference: In-circuit testing may provide skewed results due to the influence of other components.
  • Limited Accessibility: Bench testing may be hindered by limited access to the varistor.

Varistor Replacement and Maintenance

If a varistor is found to be faulty, prompt replacement is crucial. Additionally, routine inspection and testing of Varistors can prevent unexpected failures and ensure continuous protection.

Choosing the Right Varistor for Your Application

Selecting the appropriate varistor involves considering operating voltage, response time, and energy absorption capacity. Consulting varistor datasheets and seeking expert advice can aid in making an informed decision.


Quarktwin Electronic Varistors are unsung heroes in electronics, shielding delicate circuits from the unpredictable nature of voltage spikes. Their ability to rapidly respond to transient events makes them an invaluable asset in various industries. By understanding their function, working principle, and testing methods, you can harness the protective power of Varistors to safeguard your electronic investments.


Q1: Can I reuse a varistor after it has absorbed a surge?

A: In most cases, Varistors can withstand multiple transient events and continue functioning. However, replacing a varistor is recommended if it has experienced a significant surge.

Q2: Are Varistors suitable for outdoor applications?

A: Yes, Varistors designed for outdoor use are available. These Varistors are built to withstand harsh environmental conditions.

Q3: Can I test a varistor without disconnecting it from the circuit?

A: In-circuit testing is possible but may not provide as accurate results as bench testing, which isolates the varistor from other components.

Q4: What is the typical lifespan of a varistor?

A: Varistors can have a long operational life, often enduring thousands of transient events before needing replacement.

Q5: How do Varistors compare to surge protection devices like TVS diodes?

A: While Varistors and TVS diodes offer surge protection, their response times and applications may differ. Varistors are ideal for slower, more significant surges, while TVS diodes are better suited for faster transient events.

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