How Do Metal Oxide Varistors Work?

 A Metal Oxide Varistor MOV is a blue or orange-coloured circular component that can be easily seen in any AC to DC Power Supply Circuit. The word “Varistor” is a combination of words VARI-able resi-STOR. So, the Metal Oxide Varistor can be considered as another type of variable resistor whose resistance changes based on the voltage across it. Hence, It shows non-linear volt-ampere characteristics. When a voltage higher than the threshold voltage is applied across a MOV, its resistance value decreases and it acts as a short circuit. Generally, MOVs are used in parallel with a fuse, to protect circuits from high-voltage spikes.

It is known by the name of MOV when metal alloy granules are used whereas another type of varistor that is now widely used has Zn (Zinc) as the main material and it is called ZOV. 

Construction of MOV

A varistor is made from ceramic, powders of metal oxides like zinc oxide and other oxides of cobalt, manganese, bismuth, etc. A MOV is composed primarily of zinc oxide, with a small amount of other metal oxides, at about 90%. Between two metal plates known as electrodes, the ceramic powders of the metal oxides are kept intact.

A diode junction is created between each immediate neighbour by the metal oxide grains. In other words, a MOV is a lot of diodes connected in series. A reverse leakage current appears across the junctions when a small voltage is applied to the electrodes. The generated current will initially be small, but when a high voltage is applied to the MOV, the diode border junctions fail as a result of avalanche breakdown and electron tunnelling. The image below depicts a MOV's internal structure.

Choosing the Right MOV for Protection

MOVs are used in a wide range of voltages, from about 10 volts to over 1,000 volts AC or DC, so to choose a varistor for any circuit it is necessary to know the maximum supply voltage.

Varistors can withstand a maximum surge current value depending on the transient pulse width and the number of pulse repetitions. A transient pulse's width, which is typically between 20 and 50 microseconds (µs) approx. The varistor could overheat and suffer damage if the peak pulse current rating is inadequate. Therefore, a varistor must have the ability to rapidly dissipate the absorbed energy of the transient pulse and return safely to its pre-pulse condition in order to operate without failure or degradation.

Maximum AC Voltage:  Highest RMS line voltage that can be continuously applied to the varistor; the maximum RMS value should be selected to be just a little bit higher than the line voltage's actual RMS value. If the peak voltage of the sine wave crosses over the minimum varistor, it may shorten the components' lifetime. The manufacturer provides the product description like the maximum AC Voltage that can be applied to the varistor.

Response Time: In many cases, there is no precise response time; it is the moment the varistor begins to conduct following the occurrence of the surge. Usually, the response time is fixed at 100ns.

Leakage Current: It refers to the current flowing through the varistor at a specified temperature and maximum DC voltage.

Surge Shift: The variation in voltage following a surge is referred to as the surge shift. Whenever the device experiences a surge, the rated clamping voltage drops.

Static Capacitance: the inherent capacitance of the varistor itself.

At what voltage does the MOV begin to conduct and dissipate the surge current? [Clamping Voltage]

Maximum working voltage: The steady-state DC voltage at which the average leakage current will be less than the required amount.

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