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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, let's learn 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.


MOV metal oxide varistor 330V 10mm


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


characteristic of an MOV


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.


MOV connections details

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.

In the next article we'll further discuss regarding how to select MOVs and learn the same in detail through charts and tables.

Need Help? Please leave a comment, I'll get back soon with a reply!




Comments

  1. I didn't knew about MOV and seen them only on PC SMPS and PC UPS but now i know what it is and how to use it, thank you very much sir, i am learning a lot from your great site.

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  2. hi sir i just want to ask on how to replace an mov that burn on the board an no details?

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  3. Please tell me the technical details of the unit, based on that I can suggest the method...

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  4. Dear Swagatham

    The clamping voltage can be understand from the printing on the body.(eg:300v in the above image).
    But, how can uderstand current rating of a MOV.....?

    Thanks

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  5. Dear Anil, Current is not printed on the body but can be found from the available charts, the current is normally fixed at 1 amp for regular MOVs, because since the average fluctuations are in microseconds a 1 amp value is able to sink it comfortably, even if the input current is much above this value, meaning if the input voltage rises to 600V with 100 amps for 1 microsecond, the 1 amp MOV will be able to tolerate it and sink it safely...

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  6. Dear Swagatham, thanks a lot for the reply...

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  7. Dear Swagatam,

    A very informative & great site for learning a lot with simple approach.

    SsinivasanBashyam

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  8. Thank you dear Srinivasan, I am so glad you liked my site!! Please feel free to comment for any circuit related doubts or queries...

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  9. Your site is a gift from God which has a lot of very useful informations very easy xplanations and much more :

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  10. Thanks Charles, I am glad you liked my site...keep up the good work!

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  11. Thank u sir tis sure i will continue to be in your site to learn as much as possible, sir may i know which country u belong to

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  12. You are welcome Charles, I am from india...

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  13. Sir, thank u very much for the response am very glad to hear that we are in India, ur english is extreme, sir will u pl guide me where i can make or get pcb's for ur ckts, thank u.

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  14. Thanks Charles, I am sorry I won't be able to help with the PCB, I think you should inquire this in your local market and see if anybody is involved with this job....

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  15. is it logical to put mov in the output of an alternator going to the voltage reg. if the alternator is frequently switch on and off. and also in the input of a 3 phase motor if also frequently switch on and off. or what can i put to protect the voltage reg and battery from unwanted voltage spikes and also the speed controler if frequently switch on and off this motors. ur site is very helpfull to us. more knowledge to you and good health

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  16. Thank you Ralph, I am glad you are finding my website useful.

    Yes definitely an MOV can prove useful at the output of an alternator, just as it does across Mains inputs of common electrical gadgets.

    Additionally you can put combinations of high value and low value capacitors at the input and output of the voltage regulator to provide extra protection against switching spikes...

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  17. Thanks for the reply. Is there a specific value for mov to b use in alternator. What value will i use. Will the motor or alternator not be damage In frequent on and off the switching timing is around 10milisec

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  18. You can use a value that may be slightly higher than the peak value of the alternator, for example if the peak AC is 330V from the alternator, you can select an MOV rated at 360V

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