Metal Oxide Varistor – The Basics

varistors

Metal Oxide Varistor (MOV) is that orange or blue-colored component commonly found in the input of a power supply circuit. According to the voltage applied, the Metal Oxide Varistor can change its resistance. Thus, it is considered to be one kind of variable resistor. If any high current flows through a Metal Oxide Varistor, it decreases the resistance value, acting as the short circuit. Thus, MOVs are connected in parallel with a fuse to save circuits from any high voltage surges. In this article, we shall be talking about the basics of the Metal Oxide Varistor.

MOV Specifications

  • Energy Absorption

It refers to the highest energy that can be dissipated for a particular waveform without any interruptions. MOV energy absorption is usually below 1 kJ. 

  • Surge Shift

It refers to the variation in the electrical voltage after a surge. When a surge current is applied, this shift happens, and the clamping voltage reduces. 

  • Maximum Working Voltage

It is the voltage in a DC steady-state where the leakage current is lesser than your given value. 

  • Response Time

The time taken for the varistor to begin soon after the surge happens is called the response time. Its standard is said to be near 100nS. 

  • Maximum AC Voltage 

The RMS line voltage offered to MOV constantly is the maximum AC voltage. Its sine value should not be the same as the peak voltage of the minimum varistors. Otherwise, the lifetime of your electrical components will be reduced. 

How MOV Functions

MOV resistance usually remains high during the standard operating conditions. They tend to draw in little current at that moment. However, when the surge occurs, the voltage spikes well above the clamping or knee voltage, and they start drawing more current. 

Therefore, it saves the equipment and emits the surge. For short surge guards, the MOVs are helpful. 

They cannot take in sustained surges. If they were to be exposed to constant surges, their function would slightly deteriorate. 

During surges, their clamping voltage shoots down. It would lead to the MOV’s destruction. A fuse in parallel protects this from happening. 

Electrical Properties of MOV

Now we shall see three electrical properties of the Metal Oxide Varistor. Let’s dig in:

  • Static Resistance

As per the static resistance curve, the voltage where resistance remains at the peak is known as the normal voltage. The graph will show that as the voltage increases, the MOV resistance gradually decreases. This curve is used to determine the resistance at various voltage levels. 

  • V-I Properties

The curve of a variable resistor changes with the change in voltage as MOV can function in two directions; the curve has symmetrical bi-directional features. It shows a linear relationship when the current flow is zero. As the voltage increases, the resistance falls, and MOV starts to conduct. 

During the stated clamping voltage, the current flow is maximum. 

  • Capacitance

It remains constant till the graph reaches clamping voltage. The capacitance in AC circuits will influence the overall resistance. It can be connected in parallel to protect the device when the resistance increases or falls too rapidly. 

Final Words

That calls for a wrap. We hope you have acquired knowledge about Metal Oxide Varistor – the basics. Just be a bit careful with the semi-conductor usage.

The views expressed in this article are those of the authors and do not necessarily reflect the views or policies of The World Financial Review.