How do i select the right MOV?

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unknown_entity
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How do i select the right MOV?

Post by unknown_entity »

I need to use MOVs for transient suppression of the 120V line in a power supply i am working on. I am fairly sure i know how to select the proper voltage rating but i am not sure how to select the proper “power dissapating” rating. If i remember correctly(is this correct?) i only need 3 MOVs, 1 from Hot to Heutral, 1 from Ground to Neutral and 1 from Hot to Ground.<p>
This is how i calculated the voltage i need:
120Vrms/.707 = 169.7Vpeak...
Now just by picking a number that “sounds good” i would want the MOVs rated voltage to be about 20% higher than the normal peak voltage.
169.7Vpeak * 1.2 = 203.6V...
So i'd want to use a MOV whose rating is close to 203V<p>
Im not sure what to select for the power rating. Is this spec just a measure of how much a MOV can dissipate before it destroys itself? So bigger is better right? Can anyone explain or tell me where i can find more info on this spec.<p>Thanks,
James
unknown_entity
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Re: How do i select the right MOV?

Post by unknown_entity »

Since MOVs are basically self destroying im guessing it would be better to place them behind an EMI filter to prolong their life.<p>I found a little bit of onfo on the joule ratings of MOVs.
http://www.surgex.com/library/62001.html
http://www.techsavvy.com/industry/file/ ... se_region=*
http://www.control-concepts.com/pdfs/01_005.pdf<p>The last pdf was helpful but im still not sure whats a good joule value to pick. I'm still going with the assumption bigger is better for this rating. From reading some of these im guessing a 220V-240V MOV would be better than the value i first calculated...<p>If anyone sees anything wrong that i missed let me know.
dyarker
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Re: How do i select the right MOV?

Post by dyarker »

spec sheet<p>The Panasonic ZNR, Type D, spec sheet recommends ERZVxxD241 and ERZVxxD271 for 100-130VAC lines (where xx is package size code related to joules, and 241/271 indicates varistor voltage).<p>For electronics, I would go with a 240V unit. For protecting equipment with motors, relays and solinoids, the 270V or 330V so the equipment doesn't "eat" it's own protection.<p>In general, the more joules the better. Space inside the case or on the circuit board must be considered. The minimum depends on line cord and fuse sizes (mostly fuse). Panasonic recommends at least size 05 for fuses up to 3A, size 07 for fuses up to 5A, size 09 for 7A, and size 14 for 10A. Over a 10A line fuse needs special fuses in series with each MOV.<p>The "wire in the walls", plug, line cord and fuse are the resistance that drops the surge voltage when the MOVs turn on. So with heavy line cords (16-14AWG) or slow blow fuses, increase the minimum MOV size by one.<p>Put the the MOVs ahead of the EMI filter. They will last a long time with normal surges (like your air conditioner on and off), for large surges (like lightning hitting the pole outside your house) they should be considered expendable same as the fuse. For last chance clean up after the EMI filter use zener based suppressors.<p>Layout is important too! So a large surge that blows the fuse can't arc over to protected side.<p>Cheers,
Dale Y
Robert Reed
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Re: How do i select the right MOV?

Post by Robert Reed »

entity
Movs made for 120ACV lines are usually labeled that in catolog. These begin to breakdown at 330 volt peaks. Movs for 240VAC lines have labeling the same way. These will begin to break down at 600 plus Volt peakThese are what you want respectivly. Since line transients are more likly to affect the more delicate electronic parts (IC's,etc) you shouldn't have to worry much about the cruder simpler components (power transformers,power resistors,etc).Sizing Movs to the situation can be very complex to calculate.But basically the more wiring between your equiptment and the offending source, the smaller the MOV can be in regards to its power dissapation. Remember that most transients are short duration and quick.They might last a millesecond or less with rise times as fast as one microsecond. Average house hold transients will be on the order of 1000 volts. Lightning strikes very near the pole transformer may be in the vicinity of 6KV. the further downstream your equiptment is the less voltage and consequently power dissapation your MOV would see. This is due to the lines inductive reactance at these fast risetimes. I have no idea why a manufacture would specify fuse ratings based on the MOV specs.Other than end cap spacing for arc over gap which would probably require a hefty hit such as lightning strikes in which case all bets are off any how. These transients come and go so fast, the fuse would never see them, destructive or not. In the event of equipment failure due to this, normal fusing will remove the equipment from service.Don't know what your situation is, but I usually go with the middle of the road unless space or special conditions dictate otherwise. For a ton of information --Google LITTLE FUSE. I guarantee their website will have more information than you want to know on this subject.
Robert Reed
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Re: How do i select the right MOV?

Post by Robert Reed »

Entity
One other thing of interest I did not mention in previos reply. I have experienced three separate lightning hits very near the supposedly protected equipment (with MOVS and such).
Not once did they protect the equipment. After destroying the MOVs, it tore thru the equipment randomly destroying circuits at will, with no rhyme or reason.
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Chris Smith
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Re: How do i select the right MOV?

Post by Chris Smith »

Robert<p>That’s because they are Surge Protectors, and not lightning protectors per see. <p>Lightning in most cases cant be actually measured accurately, [or stopped seriously] while 10,000 amps at 10 million volts [and not for a few nano seconds either] is the norm if not small for a standard lightning strike. <p>Lightning bolts often exceed thirty miles long, and then strike your little MOV if only for the fireworks effects and proof that man is not in control. <p>Single MOV use stops line surges quite well like we had recently when the Bonneville Dam 1000 miles away dropped the ball and sent us a switching surge. <p>Most MOVs in this town worked quite well at random, some didn’t, and some surges went past the MOV to destroy the electronics while expensive power strips protected others. <p>The power strips in question that worked best cost more than 50 bucks, and have banks of MOVs hard at work, while the built in pee wee MOVs in things like satellite boxes did well acting like a firecracker, but didn’t stop even that small of a surge. <p>But when Lightning gets serious around here and strikes the pole, nothing survives as we are already on our third transformer in ten years, and pretty soon a new power pole if it keeps up?<p>[ June 28, 2005: Message edited by: Chris Smith ]</p>
billon
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Re: How do i select the right MOV?

Post by billon »

How do MOVs fail: open or shorted? In other words, how would one know that an MOV in a powerstrip failed and is no longer protecting?
Gorgon
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Re: How do i select the right MOV?

Post by Gorgon »

Hi Billon,
You know it by the smell, at least after some major hit. Just like the old Selene rectifiers when they quit.<p>I think you can have all variants depending on what you protect. I don't think there is one rule in this game. Normally you are lucky if the MOV is present after a lightning strike.<p>We used to have four levels of protection on telecom lines in heavy weather zones. Gas filled surge arresters, MOVs and tranzorbers. On top of that normal fuses. This worked very well, and we normally only replaced the fuses. :) <p>TOK ;)<p>[ July 18, 2005: Message edited by: Gorgon ]</p>
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)
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