How to Test a Varistor?

How to Test a Varistor?

A varistor, or voltage-dependent resistor, is an electronic component used to protect circuits from damage caused by overvoltage. They are commonly found in power supplies, surge protectors, and other electrical devices. If you need to test a varistor, it’s important to know how to do it properly. In this article, we will discuss the different ways to test a varistor and provide some useful tips on how to get the most accurate results. Let’s get started!

What is a Varistor?

A varistor is a semiconductor device that is used to protect electronic circuits from overvoltage and transient voltage events. Varistors are also called voltage-dependent resistors (VDRs), because their resistance changes in response to an applied voltage.

There are two types of varistors: metal oxide varistors (MOVs) and silicon carbide varistors (SICs).

What is a Varistor?

MOVs are the most common type of varistor. They are made from a mixture of metal oxides, such as zinc oxide (ZnO), manganese dioxide (MnO), and lead dioxide (PbO). MOVs are typically used to protect against transient voltage events, such as lightning strikes or power surges.

SICs are made from silicon carbide (SiC), which is a hard, brittle material that is resistant to thermal shocks and has a high breakdown voltage. SICs are typically used to protect against overvoltage events, such as electrical faults or power outages.

Varistors in general are commonly used in consumer electronics, such as televisions, computers, and cell phones. They are also used in industrial controls, telecommunications equipment, automotive electronics, and other applications. Varistors are manufactured in a variety of shapes and sizes, depending on the field of use. [1], [2]

How do Varistors Work?

The resistance of a varistor decreases as the voltage across it increases. This makes varistors ideal for protecting electronic circuits from overvoltage and transient voltage events. Varistors are typically used in parallel with the circuit to be protected. When a transient voltage event occurs, the increased current flow through the varistor causes its resistance to decrease, which limits the amount of current that can flow through the protected circuit. This prevents damage to the electronics in the circuit.

Varistors are also used to protect against lightning strikes. In this application, a large number of varistors are connected in parallel and installed on top of or near an electronic device that is susceptible to lightning damage. When a lightning strike occurs, the high current flows through all of the varistors, which limits the current flowing through the device and prevents damage.

They can also be used to protect circuits from voltage transients caused by switching power supplies and inductive loads. In these applications, the varistor is placed in series with the circuit to be protected. When a transient voltage event occurs, the increased current flow through the varistor causes its resistance to decrease, which limits the amount of current that can flow through the protected circuit. This prevents damage to the electronics in the circuit.

How do Varistors Work?

The ability of a varistor to protect a circuit from transient voltage events is determined by its clamping voltage. The clamping voltage is defined as the maximum voltage that can be applied across a varistor without causing it to break down and short circuit. The clamping voltage is a function of the varistor’s material composition, geometry, and rated current. [1], [[2]

Testing a Varistor

Testing a varistor is a process that can be carried out for a number of reasons. One of the most common reasons to test a varistor is during the manufacturing process, as it can help to ensure quality and compliance with specifications. In addition, testing a varistor may be necessary if there are any concerns about its performance or reliability. Finally, periodic testing can help to catch any potential issues before they become bigger problems.

No matter what the reason for testing, it is important to understand how to properly do so. Let’s get started!

The most common method of testing a varistor is resistance or voltage measurement. In this type of test, the varistor is placed in a circuit and the resistance or voltage is measured across it. This can be done using a digital multimeter. The results of the test can then be used to help determine the varistor’s characteristics, such as its clamping voltage and rated current.

Now we shall proceed through the steps for this process.

Disconnect it from the outlet

The first step in testing a varistor is to disconnect it from the outlet. This will help to avoid any accidental shocks and will also ensure that the device is not powered on during testing.

Disconnect it from the outlet

When you open the outlet cover, you will see the varistor located somewhere on the circuit board. It is important to be aware of which side of the device is connected to the ground and which side is connected to the live wire. To avoid any potential confusion, it may be helpful to label each side of the varistor before disconnecting it from the outlet. [2], [3]

Disconnect and desolder varistor’s lead

Once you have identified which side of the device is connected to each wire, you can begin to disconnect and desolder the lead. To do this, you will need a soldering iron and some solder.

Be very careful when disconnecting and desoldering the leads, as they may be very small and delicate. If possible, use a magnifying glass to help you see what you are doing. Disconnect the lead with pliers but be careful not to damage the lead.

Once the leads have been disconnected and desoldered, set them aside for later use and remove the varistor. [2], [3]

Now that the varistor is disconnected from the outlet, we can move on to testing it.

Use a multimeter to test the varistor

A multimeter is a device used to measure electrical properties such as voltage, current, and resistance. Multimeter indicates the resistance in ohms (Ω).

When testing a varistor, it is important to set the multimeter to the correct range. The most common range for testing a varistor is choosen based on the nominal value of the resistance you plan to measure.

To test the varistor, connect one probe of the multimeter to each lead. Make sure that the probes are connected to the same side of the device (e.g. ground and live wire).

If you are getting a reading on the multimeter, it means that the device is functional and passing current. The result does matter, for example, a varistor that has a low resistance could be considered still functional but faulty. So be sure to take some measures to solve the issue.

If you are not getting a reading on the multimeter, it means that the device is not functional and is not passing current. This could mean that the device is damaged or has failed.

Once you have finished testing the varistor, it is important to reattach it to the outlet cover and reclose the circuit board. Be sure to reconnect and solder the leads properly so that there is no danger of accidental shocks.

If your varistor is faulty, it’s important to replace it as soon as possible. A faulty varistor can be a safety hazard and may also cause damage to your electrical system. [2], [3], [4], [5]

Read more guides to improve your understanding in electronics:

FAQ

How do I test a MOV varistor with a multimeter?

Testing a MOV varistor with a multimeter is relatively simple. First, set the multimeter to measure resistance (Ω). Next, connect the leads of the multimeter to the two terminals of the MOV varistor. Finally, observe the reading on the multimeter. If the meter displays a value other than zero, then the MOV varistor is functional and will offer some level of protection from electrical surges.

What happens when a varistor fails?

If a varistor fails, it will not offer any protection from electrical surges. In addition, the failed varistor may become very hot and could potentially cause a fire. For this reason, it is important to test MOV varistors regularly and replace them if they fail.

Do varistors fail open or short?

Varistors can fail open or short, but they are more likely to fail short. When a varistor fails short, it will cause an immediate power surge and could potentially damage electronic equipment. When a varistor fails open, it will not cause an immediate power surge, but the failed component may still become very hot and could pose a fire hazard. For this reason, it is important to test MOV varistors regularly and replace them if they fail.

Useful Video: How to test varistor

Conclusions

So, now you know a little more about varistors and why they’re important. We hope this article has helped to clear up any confusion and that you feel confident in your ability to test them. So, how do you test a varistor? By using a multimeter! A multimeter is your key to measuring the resistance of your varistor. It will help you to indicate the resistance of your varistor and whether it is functioning properly or not. Be sure to take necessary measures depending on the result to prevent some major and dangerous problems!

References:

  1. https://www.circuitstoday.com/varistor-working
  2. https://www.utmel.com/blog/categories/resistor/varistor-definition-function-working-and-testing
  3. https://www.ehow.co.uk/how_7675090_test-varistor.html
  4. https://www.electronics-notes.com/articles/test-methods/meters/multimeter-resistance-measurement.php
  5. https://eepower.com/resistor-guide/resistor-types/varistor/