How to Test a Varistor with a Multimeter: Step-by-step Guide by an Expert

Introduction

A varistor is also called a Voltage-Dependent Resistor. This electronic component has a non-linear voltage-dependent resistance. Because of this, it has high resistance when the voltage is low, which can quickly be reduced if the voltage is increased to pass a specific threshold. This operation effectively shunts excess current away.
Because of its operation, the varistor is handy in protecting circuits from overvoltage surges and transients. This property makes the varistor important in a circuit, so there is the need to ensure it is always working right. That is why we look at how to test a varistor to be sure it is working.

Why and When to Test a Varistor

Someone might want to know how to test a metal oxide varistor in their circuits for a couple of reasons. Below, we will explain why and when you should consider testing varistors.

Why Test a Varistor

• When you want to verify its capability to clamp excessive voltage, testing helps you see that the varistor actually does its job right.
• Sometimes, the varistor might fail as an open circuit or short circuit. This means it can no longer protect the devices against voltage surges.
• Testing can also allow you to see if the varistor can effectively switch from the high-resistance state to the low-resistance state in case of a surge.
• Regular testing is also recommended as a maintenance method to ensure the varistors are working as designed. Knowing that the various sensitive components are well protected gives you peace of mind.

When to Test a Varistor

• It is common for varistors to be tested during manufacturing to ensure the product is high quality before it is released to the users.
• You can do the testing as part of routine maintenance. This ensures you can quickly replace the failing varistors to keep protecting the other devices.
• You can also test the varistor while troubleshooting a connection. If the electronic device is not working correctly because of a voltage surge, testing a varistor can help pinpoint whether it is malfunctioning.
• If you have recently experienced a surge, testing the varistor can help determine whether it is still working or needs to be replaced.
• Sometimes, the circuit protection system does not function properly. Testing the varistor and other components helps to determine the issue.

How a Varistor Works

Before we can look at how to test a varistor with the multimeter, let us first understand how it works.

During normal operations, the varistor acts like a high-resistance resistor. This allows only a minimal amount of current to flow.

If there is a voltage surge, the varistor’s resistance will dramatically decrease. This creates a low-resistance path in current shunting so that the excess current can pass through it rather than going to the sensitive electronics.

Since the varistor conducts the surge current, the voltage is also clamped to a safe level to ensure it does not reach the sensitive components.

Common Types of Varistors

Learning about the types of varistors can help you know the best way on how to test a MOV varistor. There are two main types of varistors. They include metal oxide varistors (MOV) and silicon carbide varistors (SiC).

Metal oxide varistors are the most common in the market. They are made by using sintered zinc oxide combined with other metal oxides. These varistors are suitable for clamping voltage spikes to a safe level if the circuit is exposed to excessive voltage. Expect to find them in power supplies and consumer electronics.

Although silicon carbide varistors are less common, they still provide impressive performance regarding high energy absorption capabilities. They are used mostly for heavy industrial applications to provide robust protection. Examples include power distribution systems and large machines.

Applications of Varistors

Varistors come in handy in protecting sensitive devices in various circuits. Expect to find varistors in applications such as:

• Power supplies and equipment
• House appliances
• Automotive electronics
• Industrial and communication systems
• Power backup and UPS systems
• Energy storage systems
• Lighting protection
• Circuit protection

The Process of Testing a Varistor with a Multimeter

We now get to see how to test a varistor with a multimeter to check if it is still working. We chose a multimeter because it is readily available, and we will measure the resistance.

1. Safety first

Before you can start testing, you must disconnect the power supply to the circuit and ensure the circuit is discharged to avoid shock.

2. Setup the multimeter

Proceed to set the multimeter to resistance mode. It has the ohm symbol (Ω). Also, select the appropriate range of resistance. We recommend using the megohm range with the symbol (MΩ).

3. Connect the multimeter probes.

Connect the probes to the varistor’s terminals to start measuring the resistance across the varistor.

4. Interpret the reading

Whenever you get high resistance, it means that the varistor is working well. The resistance is usually a high value for this part.

When the multimeter shows low resistance close to zero, it means the varistor is likely short, and you should replace it.

5. Switch the probes

This step is optional but recommended. When you reverse the probes, we want readings similar to those we got in the above step. Consistent values show that the varistor is functioning correctly.

Safety Precautions for Testing a Varistor

Now that you know how to test if a varistor is bad, it is worth mentioning that following certain precautions can help you test the varistor safely. Here is what you can do:

• It is highly recommended that you discharge the capacitors before you can start testing the circuit. This helps prevent accidental shock.
• Always power off the power supply to the circuit before testing the varistor.
• Also, do not exceed the rated voltage of the varistor if you want to keep it working properly.
• Sometimes, you can simply inspect the unit for physical damage before you can test it. This is a clear indicator you need to replace the varistor.
• Of course, use the appropriate test equipment. This is important to get the right readings and make a better judgment on the varistor’s performance.

Conclusion

We have seen how to test a varistor in a circuit, and it is generally an easy process to follow. However, it is important to use the right tools to find the right readings of the varistor. We recommend using the right type of varistor to protect the circuit. Of course, make sure that the varistor is correctly rated for the application to work as expected. If you find a faulty varistor in a circuit, go ahead and replace it for continued protection.