Metal Oxide Varistor: Specifications, Working and Construction

Catalog

Ⅰ What is a Metal Oxide Varistor?

metal oxide varistor

The blue or orange-colored circular part that you can normally find on the AC input side of any power supply circuit is a Metal Oxide Varistor or MOV. Another type of variable resistor that can change its resistance depending on the applied voltage through it can be called the Metal Oxide Varistor. Its resistance value reduces and functions as a short circuit when a high current flows through a MOV. Therefore, to secure circuits from high voltage spikes, MOVs are typically used in parallel with a fuse. In this post, we'll hear more about working with MOV and how to use it to shield your circuits from voltage spikes in your projects.

Ⅱ MOV Specifications 

A variety of parameters need to be considered when picking a varistor for a particular application. Below are some of the main varistor specifications listed:

Clamping voltage: This is the voltage at which the noticeable conduction of the varistor begins to show.

Rated voltage: The highest voltage at which the unit may be used is this voltage, as defined as AC or DC. A reasonable margin between the rated voltage and the operating voltage is typically better, but this may need to be balanced against the clamping voltage and the appropriate amount of safety.

Response time: This is the time that, after the pulse is applied, the varistor starts conduction. This is not a concern in many cases.

Capacitance: There is a comparatively strong capacitance of the metal oxide varistor through the unit. Although this may not be a concern for low-frequency systems, as it is used for lines carrying data, etc., it may present problems. For any circuit where this may be a concern, it is also important to verify the value of the capacitance through the system. While low capacitance variants are possible, standard metal oxide varistors can have capacitance levels between 100 and 1000 pF.

Standby current: The standby current is the current level that is drawn when the varistor runs below the clamping voltage. Normally, this current is defined through the system at a given operating voltage.

Ⅲ Working Principle

The MOV resistance would be strong under standard working times because they may draw very little current, so when there is a spike in the network, the voltage will rise above the knee or clamping voltage and they can draw further current, dissipating the surge and shielding the devices. The MOVs can only be used to defend against brief bursts, they can not cope with prolonged surges. Their properties can slightly degrade if the MOVs are subjected to repetitive surges. Whenever they feel a surge, the clamping voltage decreases marginally lower, which may also lead to their death for some time.

Varistors are used in many locations, usually in many areas where they are mounted over the lines to be covered, or down from the line to the ground for surge protection. They draw no current in usual conditions, but when a surge arrives, the voltage rises to above the knee or clamping voltage and they draw current, dissipating the surge and shielding the devices. The real surge is part of the varistor's absorption and part is performed down.

As the grain boundaries between the grains of the material serve as small PN junctions, the metal oxide and silicon carbide varistors feature. In series and parallel, the entire part behaves as if it were a huge mass of small diodes. Very little current flows while a low voltage is applied, so the junctions are reverse biased and the leakage current is the only current. The diodes undergo avalanche failure as a pulse occurs through the device that reaches the clamping voltage and a large current is allowed to pass through the device.

Varistors are sufficient only for bursts of short length and are not suited for handling prolonged surges. Their size reduces the amount of energy that they can dissipate. Exceeding the rated time or voltage can cause the devices to burn out or they can burst in serious cases when the energy they are needed to dissipate is much too high. Therefore, running them within their ratings is important.

Another aspect to watch is that MOVs, metal oxide varistors that are subjected to repetitive spikes, will subtly alter their properties and degrade them. The clamping voltage moves a little lower as they undergo spikes, and this will ultimately lead to their death.

MOVs are also connected in series with a thermal switch/fuse as a part of this failure mode, which will activate if too much current is drawn.

Ⅳ Construction

The Metal Oxide Varistor is a voltage-dependent resistor consisting of metal oxides ceramic powders such as zinc oxide and some of the other metal oxides such as cobalt oxides, manganese, bismuth, etc. A MOV consists of around 90% zinc oxide and a limited number of other metal oxides. Between two metal plates known as the electrodes, the ceramic powders of the metal oxides are left intact.

A diode junction between each immediate neighbor is created by the grains of metal oxides. So, a MOV is a large number of series-linked diodes. A reverse leakage current occurs through the junctions as you add a slight voltage to the electrodes. The produced current would initially be small, but due to electron tunneling and avalanche failure, the diode boundary junctions break down when a high voltage is applied to the MOV. In the image below, the internal configuration of a MOV is shown.

When a given voltage is applied over the connecting leads, the MOV varistor begins conducting and ceases conducting when the voltage falls below the threshold voltage. MOVs are distributed in different formats, such as disk formats, devices with axial lead, blocks and screw terminals, and devices with radial lead. For improved power handling capacity, the MOVs should always be linked in parallel and you should link it in series if you want to get a higher voltage rating.

Ⅴ Capacitance of MOV

As we have already understood that the MOV is built with two electrodes, it serves as a dielectric medium and has capacitor effects that, if not taken into account, could influence the system's functioning. Based on the region that is also inversely dependent on its thickness, each semiconductor varistor would have a capacitance meaning.

When it comes to a DC circuit, the capacitance value is not a big deal, because the capacitance will remain nearly constant before the device's voltage exceeds the clamping voltage. When the voltage exceeds the clamping voltage, there will be no capacitance effect as the varistor continues its regular work.

The capacitance of the MOV could influence the overall body resistance of the MOV that triggers the leakage current when it comes to AC circuits. The leakage resistance of the varistor decreases rapidly as the frequency increases as the varistor is attached parallel to the system to be covered. The MOV reactance value can be determined using the formula

Xc=1/2πfC

Where Xc is the capacitive reactance, and the AC supply frequency is f. The leakage current, as seen in the non-conducting leakage area of the V-I characteristics curve described above, would also increase as the frequency increases.