metal oxide varistor

[joshua richard]

hellow fellows i was wondering .lets say you are scavenging electronic components (we all do that right ) . the problem is what is the best way of identifying mov may be there is set of numbers or characters that is unique in mov eg capacitor its  µF  , so what makes mov unique

[bills]

start here. http://electronics.stackexchange.com/questions/55546/parts-identification-y-cap-mov-or-ntc
and remember google is your friend .

[wraper]

I would strongly suggest against salvaging MOVs. They degrade with each transient they clamp. Clamping voltage decreases over time until they just blow up because start shorting mains voltage under normal conditions.

[Pjotr]

I would strongly suggest against salvaging MOVs. They degrade with each transient they clamp. Clamping voltage decreases over time until they just blow up because start shorting mains voltage under normal conditions.

That is only true if MOV's are overloaded. And any component will be damaged or wear excessive in that case. You need to size them in accordance to how "dirty" your mains is.

[dom0]

MOVs do age, even at the rated voltage. You will also get transients all the time even in a good grid. It's just normal and caused by many things, including everyday load switching.

And since they're so cheap I just wouldn't bother with salvaging them, but buy new ones with proper specifications for <.2 €.

[wraper]

That is only true if MOV's are overloaded. And any component will be damaged or wear excessive in that case. You need to size them in accordance to how "dirty" your mains is.

Even tiniest transients cause degradation and you never know how much wear they received in their lifetime. Even if they received severe transient only once (lightening) they might be ready to blow up in any moment. Also there is not such a thing that there is written on the mains socket how clean it is. Also, usually you won't know history of the used equipment.

[DanielS]

Even tiniest transients cause degradation

Not quite, but close.

If you check Littlefuse's CIII MOV datasheets, they contain a graph of endurance vs peak current and their 20D devices' endurance is rated as infinite below 180A and 20µs duration. This would be 5-10J per pulse, not large but not tiny either.

That said, I agree with everything else you wrote. There is no way to know the MOVs' history and exactly how much might be left in them, so reusing them anywhere where reliability and safety might be an issue would not be a good idea.

[Pjotr]

I am using 20mm MOV's for some 20 years now for mains spike protection in PSU's in an industrial environment. And over that time we have had 2 or 3 broken ones over more than 500K units.

But I agree, never re-use them and use proper thermal protection (thermal fuse in series). When they break they become very hot and are a real fire risk. And believe me, you don't like the smell of a burnt epoxy PCB for sure

[ruffy91]

Wrong thread.. :-/

[Seekonk]

Years ago I built a MOV tester at work, like 1200V low current voltage multiplier to find the 2ma knee.  A lot of fun if you don't kill yourself.  Don't remember why, but we put everything in epoxy and sometimes they weren't sure what they used.  Had to build a number of hidden component testers.

Fine to use old MOV, always use a fuse.  The more you put between them and the voltage source, the better they work.  Ever notice how short the leads are in the test spec?

[Paul Moir]

I am using 20mm MOV's for some 20 years now for mains spike protection in PSU's in an industrial environment. And over that time we have had 2 or 3 broken ones over more than 500K units.

But I agree, never re-use them and use proper thermal protection (thermal fuse in series). When they break they become very hot and are a real fire risk. And believe me, you don't like the smell of a burnt epoxy PCB for sure

You know, it's funny but you see them used all the time for inductive spike supression in industrial gear.  I'm thinking pneumatic solenoid valves specifically but I'm sure I've seen them guarding relay contacts in PLCs.  Clearly in this case they're being used per cycle, and millions of cycles are typical.
I use TVS because of this but MOVs must be up to the task somehow.

[Seekonk]

I remember back when UL would no longer allow 130V MOV and had to change to 150V for line use because too many blew up.  A used MOV will last longer than any project here.

[Pjotr]

.................. A used MOV will last longer than any project here.

Thank you for the compliments 

Keep in mind that MOV's can be rated and marked either for DC or AC.

[DanielS]

I use TVS because of this but MOVs must be up to the task somehow.

As i wrote earlier, Littlefuse's CIII-series 20mm MOVs are rated for infinite lifespan under 180A/20µs pulse current. Most PLCs do not switch anywhere near this much current so their MOVs used as inductive clamps will likely outlive the rest of the PLC.

[Pjotr]

Even the smallest 8mm will do for clamping solenoids and relay coils. But a TVS clamps "harder", is more suited at low voltages and is smaller, cheaper and more available in SMD.

[SeanB]

I have had a few that objected to spikes and failed safely, as a short circuit. I have a nice red GE G4 250L40B MOV in front of me, which objected strenuously to the generator output rising high when under low load, and which stalled it out in going low resistance and cooking the plugtop it was in.  Another did the same when the Izinyoka came around and stole the neutral and earth cabling from the substation across the park, resulting in the mains rising to 300VAC plus. They did this 3 times in a row.......

Must add some to my next order from RS, I need some capacitors......