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.
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 Iβ
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.
In the next article we'll further discuss regarding how to select MOVs and learn the same in detail through charts and tables.
Ho can I protect with an mov a fridge compressor with 15 LRA and 1/3 hp? Standard 120v house currently?
Yes, to some extent, against extremely short voltage spikes.
The circuit in your diagram is dangerous.
The switch AND fuse should be in the live side. Assuming switch is in Live side, a blown fuse would leave the circuit at mains voltage.
Thank you for notifying the issue, I have corrected the diagram accordingly.
Hi sir, I’m curious about purchasing. When I am trying to buy mov there is too many different Voltage filters which I am drowning among them. For example in your example we found we need the MOV’s AC and DC voltage values. But there is Varistor Voltage, Clamping Voltage. I these values need to close to operating voltage. For example I found Mauser 331KM(E,N)20 for your circuit. It’s Vac 210v Vdc 330v. But It’s Varistor Voltage is 330V Clamping voltage is 550V. So is this Clamping Voltage is the maximum voltage it can handle or the value which lowered to after peak. Also in max continuous oe allowable voltage is lower than nominal voltage so I am really confused. Isn’t the nominal or rated voltage equal to systems operating voltage? I hope I have made myself clear and understandable. Love this site and your great work btw.
Sincerely
Hi Bahadir,
The crucial aspect while selecting an MOV is its clamping voltage which must be slightly higher than the peak input voltage of the application. For example, the 220V AC is the RMS voltage and its peak voltage will be around 310V, so the MOV clamping voltage could be around 330V.
can MOV be placed in power extension board to save Airconditioner by voltage higher than 260 V ac
MOVs can stop high voltages which occur only for milliseconds, anything higher than that can burn the MOV itself.
I built a power strip/extension for my hifi/pc systems and I want to add a MOV in each power socket. There are 4 of them in a star configuration.
The voltage is 220v where I live and the the current reaching the power strip is 16A.
What varistor should i use for this application? voltage, amperes and diameter.
You can use standard MOVs having a clamping voltage rating of 350V
I have luminous shakti charge 1450 inverter
Please tell me which MOV to be used in input line or ele.supply line.
If your inverter output voltage is 220V, you can use a 350V MOV.
How can I build a DC battery charger from AC source without using transformer
You can use an SMPS power supply.
Can you please tell me which MOV is suitable for G3MC202-PL SSR??
Where do you want to place this MOV?
Sir i want to connect some mov s in my inverter line input and outputs also some points of inverter line can you send the application circuit with MOV numbers
Jose, MOVs are placed only at the input side of an AC source. I can suggest the MOV values only if you can tell me the peak voltage value of your inverter input/output voltages.
Sir .max voltage is 240v ac and output 220-240v ac sine wave – load is 500watts also explain the connection method of mov
OK, then you can use a 400V MOV at the input side of AC 240V
Hi there, I also like very much your designs, and I have used some 🙂
I have a question here, I want to build a surge protection for an AC unit (230v in my country), and I live in an area where the voltage fluctuates a lot. I’m not sure if a 400v MOV would protect the electronics, I’m thinking max 275V. Am I correct? Also, since an iTMOV takes 12 weeks delivery time(and I need some feedback that it is no longer working) I would put in series a thermal fuse with a standard MOV. Can you provide an example calculation for the temperature or the fuse? Thank you.
Thank you, and Glad you found my designs helpful!
For 275 AC the peak voltage will be 390 V, so the MOV clamping voltage must be rated at 390 V.
Sorry, I haven’t yet investigated thermal fuses, so I do not have a the details about their working principle.
How do I size my MOV to act as a lightening arrestor on a sprinkler system? I have 12 feeder control circuits spread throughout the yard and they all run into the controller in the house. I have installed a fuse link on each feeder circuit but want to add the additional protection of a lightening arrestor. Also, should the MOV tie into the circuit before(source side) or after the fuse (load side) or does it really matter. I’m taking one side of the MOV to the high side of each circuit and the other side directly to ground. 24 volt AC circuits..
It can be difficult to calculate the size of an MOV for lightening control, because we do not know the voltage level the lightening may have. The MOV must be always connected before at the source side, not the load side.
Thank you for your reply. Does clamping at 100V do me any good? Obviously, lightning will probably blow it to pieces, but does it really provide any protection other than from a slight surge from a lightning strike far away?
BTW, your picture in the article above shows the MOV on the load side of the fuse:)
I don’t think a 100V clamp would be helpful in any manner, because a far away lightening might not have any effect on your circuit unless it is in direct contact with your system. And if it is in direct contact then no MOV can save your system. It is my assumption though, I may be wrong.
Yes the MOV must be installed after the fuse so that if the MOV short circuits or malfunctions, the fuse can blow and safeguard the wires from burning. I probably misunderstood your question, when you said load-side I assumed it to be right across the load, which may be situated after an electronic system such as a relay or mosfet etc.
Thank you for your extremely informative and to the point articles and advice.
A power extension strip burnt out today and when I opened it I found that one of 3 MOV’s had flashed and burnt out, along with the adjacent switch.
I want to replace the MOV and would be grateful for your advice. The equipment in use from the extension are a flat screen TV, Amazon Firestick, laptop charger etc. Would a clamping voltage of 300 or 350 volts be sufficient, or is that too low? Current rating of one amp?
Thank you
Thank you very much, and glad you liked the post. Every MOV have a time limitation in which it can tolerate a certain amount of maximum current, if this current stays for too long, even for millisecond periods, the MOVs can get burned. Therefore to ensure the MOV lasts longer, you may have to select a higher current rated MOV.
I think the dealer of the part will know better regarding the current rating of the device.
Yes the clamping voltage can be around 350 V
Alternatively you could go for the industrial type MOVs for more robust performance
https://www.homemade-circuits.com/high-power-industrial-surge-suppressor/
Hi Swagatam,
Interesting article and good explanation. But I still have problems choosing the right replacement MOV Varistor for my circuit board (it is a chiller-airco unit on board of a boat). The Varistor is blown because of a voltage spike from the power socket in the harbour (probably more than 260V). I can read on the green-coloured Varistor a few numbers and letters: 430NR and Sp and like 73 or 76 or 78, it is about 15mm dia and 5.5mm thick. The airco unit is automatic fused at 35amps. The harbour delivers 230V but I can see that it is most of the time around 240-250V and sometimes 250-260. I was looking at replacement Varistor Bourns MOV-14D431K but I am not sure if I have the correct one. Or VDRUS14X175BSE VARISTOR, MOV, 275V, DISC 14MM VISHAY?? Can you help me out here? Thanks, Jeroen
Thank you Jeroen, according to the chart provided in the following article, the varistor VDRS14T275xyE seems to be a good choice. It is rated at 275 V continuous voltage and 430 max clamping voltage. The current is specified at 50 amps but I am not sure what is duration of this current supposed to be.
https://www.vishay.com/docs/29081/vdrs.pdf