on July 24th 8,910

Varistor Basics: Working Principle, Types, and Real Uses

Varistors help protect electronic devices from sudden voltage spikes. In this guide, you’ll learn what a varistor is, how it works, the types you might use, what they’re made of, where they’re used, and how to test or choose one. Whether you're fixing a device or building a new circuit, this article gives you simple, clear answers to help you get it right.

Catalog

Figure 1. Varistor

What’s a Varistor?

A varistor is known as a voltage-dependent resistor, or VDR. Its name comes from a blend of the words “variable” and “resistor,” which hints at what it does. Unlike a regular resistor, the resistance of a varistor changes depending on the voltage that’s applied to it.

When the voltage is low, the varistor’s resistance stays high. But as the voltage increases, the resistance drops. If the voltage becomes too high, that drop becomes very steep. This change in behavior is what makes varistors useful in electrical systems that need protection.

You’ll often find varistors used as a simple, built-in defense against unexpected voltage increases. They’re a common component in many types of electrical and electronic equipment.

The most widely used type is called a metal oxide varistor, or MOV for short.

How Does a Varistor Work?

Figure 2. Circuit Diagram of Varistor

A varistor works by changing how much it resists electrical current based on the voltage across it. At normal voltage levels, it has a very high resistance, so almost no current flows through it. It just sits quietly in the circuit and doesn’t affect anything. But when the voltage suddenly increases, like during a surge or spike, its resistance drops sharply.

This drop in resistance happens very fast. The varistor becomes conductive and provides a new path for the high current to flow away from the sensitive parts of the circuit. In most designs, this means the excess current is safely redirected to ground or across power lines where it won’t cause harm.

The key idea is that the varistor only reacts when the voltage goes beyond a certain level. It doesn’t trigger from normal fluctuations, but it jumps into action if something dangerous shows up. Once the voltage drops back to normal, the varistor resets on its own, returning to a high-resistance state, ready to protect the next time.

Types of Varistors

Metal Oxide Varistor (MOV)

Figure 3. Metal Oxide Varistor (MOV)

The most commonly used type is the metal oxide varistor, often referred to simply as an MOV. It's made from zinc oxide combined with small amounts of other metal oxides, all compressed into a disk and placed between two metal plates. At normal voltage levels, the MOV behaves like an insulator. But when a sudden voltage spike occurs, it quickly becomes conductive, redirecting the excess energy away from sensitive parts of the circuit. Once the voltage returns to a safe range, it stops conducting and goes back to its original state. You’ll usually find MOVs in power supplies, surge protectors, home appliances, and many types of electronic devices. Their quick reaction time and reliability make them a go-to choice in both consumer and industrial electronics.

Silicon Carbide Varistor (SiC)

Figure 4. Silicon Carbide Varistor (SiC)

Before MOVs became widely adopted, silicon carbide varistors were often used for surge protection. These are made from silicon carbide, a material known for its durability and ability to handle high temperatures. One difference with SiC varistors is that they tend to have more leakage current, meaning they allow a small amount of current to pass even when there’s no surge. Because of this, they’re often paired with a spark gap to help manage the response more effectively. While they’re not as common in modern devices, you might still come across them in older equipment or in systems where thermal stability is a bigger concern than switching speed.

What Are Varistors Made Of?

Figure 5. Varistor Internal Structure

Varistors are made from ceramic-based materials that respond to changes in voltage. The most common type, the metal oxide varistor (or MOV), is built using a mix of zinc oxide and small amounts of other metal oxides like bismuth, cobalt, and manganese. These materials are pressed together into a solid disk and placed between two metal electrodes, which are usually made of silver, nickel, or aluminum. Once assembled, the disk is often coated with a protective layer, like epoxy, to help seal it from moisture or damage.

Inside that disk, something interesting is happening. The zinc oxide grains create a network of tiny boundaries, which act like countless microscopic barriers. At normal voltage levels, those barriers hold back the flow of electricity. But if the voltage spikes, they quickly break down, allowing current to pass through and protect the rest of the circuit.

Other types, like silicon carbide varistors, are made from a different ceramic mixture. These use mostly silicon carbide and are designed to handle high temperatures and stronger electrical surges. You might still find them in older or more rugged systems, especially where heat resistance is more important than fast recovery.

How to Test a Varistor

Testing a varistor is a practical way to make sure it can still do its job. Here’s a step-by-step guide that walks you through the process clearly and carefully.

Step 1: Disconnect from the Circuit

Before you begin any tests, it’s important to isolate the varistor from the rest of the circuit. You can either fully remove it or simply desolder one of its leads.

STEP 2: Check Resistance with a Multimeter

Once the varistor is disconnected, set your multimeter to the resistance mode. Touch the probes to the varistor’s two leads.

Optional: Verify Clamping Behavior

If you have access to a variable power supply and know the varistor’s rated clamping voltage, you can test its response under increasing voltage. Connect the varistor in series with a current-limiting resistor and slowly raise the supply voltage while monitoring both current and voltage. A functioning varistor will begin to conduct current when the voltage approaches its rated clamp value. If it starts conducting too early or doesn’t react at all, it may be degraded or out of spec.

Where Are Varistors Used?

Figure 6. Varistors Mounted on a Circuit Board

Home and Office Electronics

In everyday devices like TVs, computers, and kitchen appliances, varistors protect internal circuits from power line disturbances and lightning-induced surges. You’ll often find them inside power strips and surge protectors, where they act as the first line of defense.

Power Supplies and Industrial Equipment

Varistors are commonly built into power supplies to manage high-voltage spikes caused by switching or inductive loads. In industrial systems, they help protect motors, control panels, and automation equipment from transient voltage events.

LED Lighting and Electrical Loads

LED drivers and lighting fixtures use varistors to guard against electrostatic discharge and voltage fluctuations. In circuits with motors or coils, they also absorb the surge that happens when the current suddenly stops.

Communication and Signal Lines

Low-capacitance varistors are used in telecommunications, networking hardware, and cable systems. They prevent electrostatic or surge damage while preserving signal quality across phone lines, antennas, or data ports.

Automotive and Energy Infrastructure

In vehicles, varistors help shield sensitive components like ECUs and sensor modules from voltage spikes, especially during load dumps. In larger infrastructure like substations or railway systems, block-type varistors manage high-energy surges and help maintain system stability.

Pros and Cons of Using Varistors

Pros	Cons
Fast voltage clamping	Wear and aging over time
Handles high-energy surges	Short lifespan under harsh conditions
Affordable and easy to install	Not reliable for long-term over-voltage
Reliable in normal conditions	Risk of overheating if unprotected
Compact and space-efficient	May cause signal distortion
Works across wide temperature ranges	Limited protection against large surges
	Requires derating and backup components

How to Pick the Right Varistor

Figure 7. Varistor Sizes and Types for Different Protection Needs

Choosing a varistor means more than just matching specs. Use these simple tips to find the right fit for your circuit.

• Choose a voltage rating with headroom

The varistor’s continuous voltage rating should be slightly higher than your circuit’s normal operating voltage. Around 15 to 20 percent higher is usually a safe margin. This keeps the varistor inactive during everyday fluctuations but ready to respond when needed.

• Check surge current and energy ratings

Look at how much surge energy the varistor can absorb and the peak current it can handle. If your system faces strong or repeated voltage spikes, pick a varistor that can withstand those conditions without wearing out quickly.

• Pay attention to clamping voltage

Clamping voltage is the maximum voltage the varistor allows through during a surge. It should be low enough to protect your components but still above the regular voltage range. If it’s too low, it may trigger unnecessarily. If it’s too high, it won’t protect effectively.

• Consider size, leakage current, and lifespan

Larger varistors tend to have lower leakage current, absorb more energy, and last longer. If your layout allows for it, a slightly larger varistor can offer better performance and added reliability over time.

• Match the varistor to the environment

If your application involves heat, moisture, or unstable conditions, choose a varistor rated for that setting. Some include thermal protection or can be paired with a fuse to prevent overheating if failure occurs.

• Use low-capacitance varistors for signal circuits

In circuits that carry data or high-frequency signals, high capacitance can cause interference. For those designs, choose a varistor with low capacitance to protect the line without disrupting performance.

Conclusion

Varistors help protect your circuits by stepping in during unexpected voltage spikes. They react fast, absorb extra energy, and reset themselves when things go back to normal. You’ve seen how they work, what they’re made of, and the different types available like MOVs and SiC varistors. They’re found in everything from home electronics to industrial systems and even vehicles. You also learned how to test them, where they’re used, and how to pick the right one based on voltage, clamping level, and environment. Whether you’re dealing with power surges or signal lines, the right varistor adds a layer of safety without taking up much space. Keep these points in mind when designing or repairing a circuit, and you’ll have a stronger, more reliable setup.