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Metal Oxide Varistors: Everything You Need to Know

Are you hunting for a tool to protect your circuit from a voltage spike? Perhaps you have suffered from voltage shorts in the past and need to use a component to prevent the issue from occurring again. If so, you have come to the right place. A metal oxide varistor is either an orange or blue circular component often present on the AC input side of a power supply circuit.

A varistor is a variable resistor that can change the resistance based on how much voltage you apply. This article will show you how to protect circuits from voltage spikes using a varistor and handling your circuit’s energy capability with these circuit breakers.

What is a Metal Oxide Varistor?

Simply put, a metal oxide varistor is a variable resistor. However, unlike potentiometers, a metal oxide varistor can vary its resistance in electronic devices depending on how much voltage you apply.

If the transient voltage applied increases, then the resistance will decrease. This aspect ishighlyy beneficial as surge protection is often implemented to protect electrical networks.

blue metal oxide varistors

blue metal oxide varistors

Metal Oxide Varistor Construction

A metal oxide varistor is a voltage-dependent resistor. A metallic oxide varistor is around 90% Zinc oxide and 10% other metallic oxides. Typical metal oxide varistors get made with ceramic powers, such as zinc oxide grains.

metal oxide varistors

The metal oxide varistors

Metal Oxide Varistor Features

The Metal Oxide Varistor features include:

  • Insulation resistance is 1000 Mohn
  • DC voltage ranges from 175V to 1200V
  • AC voltage goes from 130V to 1000V
  • Operating temperates between -55 and +85°C

Metal Oxide Varistor Protection Circuit – Design Tips

Let’s look at how you can use a metal oxide varistor to protect circuits in the event of voltage spikes. Here are some design tips for designing a course that offers complete protection.

  • Firstly, when selecting a metal oxide varistor, you must determine the continuous working voltage. You can opt for a varistor with a higher operating voltage to achieve the lowest possible leakage current. Choosing a varistor with a 10-15% higher maximum rated voltage for circuit protection technologies is common.
  • Next, you need to select a varistor that has the power to dissipate more energy that is more than the energy required dissipation throughout a surge shift. You calculate by the maximum load of the varistor throughout a surge, factoring in the environment and the specifications. 
  • Next, factor in the surge or peak transient current through your varistor, and choose a varistor with an actual surge rating slightly higher than the current rating needed by an event.
  • It would help if you chose a model that offers the necessary clamping voltage. You can select the clamping voltage based on the approximate max voltage value you want to allow the circuit’s output or input to appear during an event.
metal oxide varistors

metal oxide varistors

How to use a Metal Oxide Varistor in your circuit

Varistors frequently get used within electronic circuits in parallel with a course that you need to protect.

When the voltage is within your rated limit, the varistor resistance will be high, and the current follows through the circuit and not through the varistor.

However, when a voltage spike happens, it will appear across the varistor parallel to your AC mains. The high voltage decreases the resistance value of your varistor, and it will appear similar to a short.

This action causes a large current to flow through your varistor, which could disconnect the circuit or blow a fuse. When voltage spikes occur, the faulted high voltage will go back to normal quickly. The duration of the current flow won’t be high enough to cause a fuse to blow, and then the circuit goes back to normal.

green circuit board

green circuit board

Metal Oxide Varistor: Summary and Key Takeaways

Now you have all the information you need about choosing the right varistors to handle any surge current. 

Handling voltage ratings is essential to ensure electronic components remain operational and do not get damaged. Excessive voltage can be very detrimental, but the maximum voltage will no longer be an issue if you manage peak voltage with a varistor.

Don’t hesitate to contact us if you have any questions.

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Emma Lu
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