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# What is MOV!

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#### Suraj143

##### Active Member
Hi I want to know what is the meaning of MOV.I have seen them on power supplies.What is the purpose of using them?

OK thanks.

Let say I have a transformer power supply.

230VAC/12VAC out.I use a bridge,smoothing cap & a 7805 to regulate & take 5V DC out

Where is the best place to add a MOV?I'm mounting this power supply to a plastic box.

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It doesn't have any diagram how to connect to a power supply.

If you can't figure out where a MOV might be useful in connecting to a circuit for surge suppression after reading the available information on Google and Wikipedia you have no business working with line voltage.

I am curious why you are asking this question… why do you think you need them?

You’re playing in what could be a dangerous area. You typically put them in three places:
The + to Common,
The + to Earth and
The Common to Earth.
(Some people just put them only across + to Common)

Now having said that YOU MUST BE CAREFUL OF THE VALUES YOU SELECT AND THE DISSIPATION YOU ARE ANTICIPATING TO SURPRESS.

Remember basics here. At 230 VAC = 260v PEAK. So without surges, Your voltage peaks at 262.43V ( 230 x 1.414).

Hope this puts you in the right direction.

Floyd

Thanks Floyd,

But I thought they were putting to 230V side.I'm thinking like this.

Live to Earth
Neutral to Earth.

Is this correct?

Yes you can omit the one across Live to Neutral if you like.

That is up to you.

You should research your needs to decide how much protection you will require. Remember these MOVs are simply to protect your circuit. You could overkill protection needs. Safety is a primary goal and protection of you circuitry is secondary.

Floyd

I've knocked up a drawing to show how I use MOV's and suppression caps.
All quite simple. The MOV goes across the Live and Neutral, and the caps across Live and Neutral, and also Live to Earth and Neutral to Earth.

Be careful playing with line voltages, you rarely get a second chance if you do things wrong!!!

#### Attachments

• MOV.GIF
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I guess its time to ask the question, 'what are we trying to do?'...

if you want to clamp voltage surges, you will need more MOV's and perhaps other clamps with more power capabilities depending on your needs. There are so many methods to clamp surges. A true power surge will eat most MOV's.
If you want to surpress RFI, then the caps will help. The caps will do nothing for power surges.

There are two items to keep in mind when clamping.... POWER DISSIPATION and SPEED
I guess this question goes to Suraj.

I am bowing out of this thread... lots of luck to all.
Floyd

Now having said that YOU MUST BE CAREFUL OF THE VALUES YOU SELECT AND THE DISSIPATION YOU ARE ANTICIPATING TO SURPRESS.
correct.
Remember basics here. At 230 VAC = 260v PEAK. So without surges, Your voltage peaks at 262.43V ( 230 x 1.414).
WRONG.

230*1.414 = 325V. You need to take into account that the "230V" may differ depending on your country. If it's actually 240V, then 240*1.414 = 339V. You also need to leave a significant margin there will be brief increases due to loads being switched on nearby customers.

OOps, fat fingers on the calculator strikes again....
Now your on the right track... actually a few % is not going to make much difference, just give enough margin to stay from Maximums voltages an minimum part tolerances.
Good luck to you.
Floyd

Mesa, that's not a few percent error, that's 30% voltage error on the peaks... Inductive loads are only typically rated with a 10% error, and you can NOT violate the maximum voltages on a device like a MOV, or it conducts; Not a good area for mistakes. You're talking the difference between a perfectly normally operating functional device and a cloud of vaporized material.

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Forget the 'error' as you put it.
calculate your worse case condition and design around it.

Shax thanks for your diagram.That's the one I was thinking.

After reading Floyd's post I got to know there are two things that will effect on power supplies.They are "Power surge" & "RFI".

If I need only protect to a power surge then I'll need only one MOV connecting from Live to Neutral.So I can omit the capacitors.

Can you tell whats the reason for connecting that two capacitors in series & common pins to earth?

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