How to Select MOV – Explained with a Practical Design

MOVs or metal oxide varistors are devices designed for controlling mains switch ON surges in electrical and electronic circuits. Selecting an MOV for a particular electronic circuit might require some consideration and calculation, I have explained the procedures here.

What are MOVs

Metal oxide varistors or simply varistors are non-linear surge suppressor devices which are used for suppressing sudden, high abnormal voltage transients or surges, especially during power switch ON or thunder lightening situations.

These are mostly used in sensitive electronic circuits for safeguarding against such catastrophic occurrences.

MOVs are basically non-polar, voltage dependent devices, meaning these devices will react to changes in voltage conditions.

Therefore MOVs are specified to trigger ON whenever the rated magnitude of voltage across their connections is exceeded.

This voltage rating at which an MOV may be rated to fire and short the transient to ground is called its clamping voltage specification.

For example, if suppose the clamping voltage rating of an MOV is 350V then it will switch ON whenever the voltage across it surpasses this limit.

When an MOV switches ON or is triggered by a high voltage surge it shorts the voltage spike across its terminals, preventing it from entering the vulnerable electronic device attached on the other side.

This action protects the electronic circuit from such accidental voltage surges and transient spikes.

And since the above reaction is sudden, MOVs are characterized as non-linear devices, which implies that these will not vary their characteristics gradually but suddenly when the specified parameters is exceeded.

The best characteristic of an MOV is its ability to absorb high current content accompanied with the voltage surge . Depending on the MOV specification the current absorbing capacity of an MOV could be anywhere between 1 amp to a massive 2500 amps

Current-voltage characteristic waveform of a typical zinc oxide MOV

However the duration of the current handling feature of an MOV may be limited to a few microseconds only, which means the activation of an MOV under such sever situations can not be more than a few microseconds, otherwise it could burn the device and damage it permanently.

Therefore it is advised to use a fuse in series with the mains line in conjunction with the attached MOV for ensuring safety to both the electronic circuit and also to the MOV under a possible extreme catastrophic conditions.

ELECTRICAL CHARACTERISTICS

Typically the V/I characteristic of a ZnO varistor (MOV) can be understood with the following explanation:

The relationship between voltage and current of a varistor can be roughly estimated with the following formula

V = C x I^β
where:
V = Voltage
C = Varistor voltage at 1 A
I = Actual working current
β = Tangent of angle curve deviating from the horizontal

Practical Example

When:
C = 230 V at 1 A
β = 0.035 (ZnO)
I = 10-3 A or 102 A
V = C x Iβ
so that for current of 10^-3 A: V = 230 x (10^-3)^0.035 = 180 V and
for a current of 10² A: V = 230 x (10²)^0.035 = 270 V

Source: https://www.vishay.com/docs/29079/varintro.pdf

How to Select an MOV

Selecting an MOV for a desired application is actually easy.

First determine the maximum peak safe operating voltage of the electronic circuit which needs the protection and then apply an MOV specified to conduct near about this voltage limit.

For example, suppose it's an SMPS device with a max capability of 285V RMS from the mains input, implies that the unit would be able to handle a peak mains surge of not more than 285/0.707 = 403V

The 403V figure provides us the max peak mains handling capacity of the SMPS circuit which must be avoided under any circumstances and therefore an MOV rated with a clamping voltage of around 400V could be applied to this SMPS safely.

The current rating of the MOV could be twice that of the SMPS rating, meaning if the SMPS wattage is rated at 24 watts at the secondary, then the primary could be calculated as 24/285 = 0.084 amps, therefore the MOV current could be anywhere above 0.084 x 2 = 0.168 amps or 200mA.

However a 200mA MOV could be difficult to obtain therefore a standard 1 amp device could be used for serving the purpose with utmost efficiency.

Note: Many countries have adopted a harmonized standard for AC power grid voltage typically around 230V with a tolerance of +10% and -6%. This means, the acceptable voltage range can fluctuate between 207V and 253V. Previously, some regions might have had a nominal voltage of 240V with an upper limit of 264V. However, under the new standard, the upper limit would be 253V. Therefore varistors rated for 270V or 275V could be suitable for protecting against voltage spikes in these regions.