metal oxide varistor - Page 1
Forum section: electronics / projects

author: joshua richard
author_rank: Newbie
timestamp: April 16, 2015, 06:51:56 am
content: 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

author: bills
author_rank: Frequent Contributor
timestamp: April 16, 2015, 07:31:23 am
content: start here.http://electronics.stackexchange.com/questions/55546/parts-identification-y-cap-mov-or-ntcand remember google is your friend .

author: wraper
author_rank: Unknown
timestamp: April 16, 2015, 07:41:04 am
content: 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.

author: Pjotr
author_rank: Frequent Contributor
timestamp: April 16, 2015, 09:21:05 am
content: Quote from: wraper on April 16, 2015, 07:41:04 amThat 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.

author: dom0
author_rank: Super Contributor
timestamp: April 16, 2015, 10:46:50 am
content: MOVsdoage, 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 €.

author: wraper
author_rank: Unknown
timestamp: April 16, 2015, 11:03:06 am
content: Quote from: Pjotr on April 16, 2015, 09:21:05 amEven 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.

author: DanielS
author_rank: Frequent Contributor
timestamp: April 16, 2015, 12:09:13 pm
content: Quote from: wraper on April 16, 2015, 11:03:06 amNot 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.

author: Pjotr
author_rank: Frequent Contributor
timestamp: April 16, 2015, 08:40:58 pm
content: 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

author: ruffy91
author_rank: Regular Contributor
timestamp: April 17, 2015, 05:03:31 am
content: Wrong thread.. :-/

author: Seekonk
author_rank: Super Contributor
timestamp: April 17, 2015, 08:17:49 am
content: 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?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: April 17, 2015, 08:27:25 am
content: Quote from: Pjotr on April 16, 2015, 08:40:58 pmYou 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.

author: Seekonk
author_rank: Super Contributor
timestamp: April 17, 2015, 01:53:57 pm
content: 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.

author: Pjotr
author_rank: Frequent Contributor
timestamp: April 17, 2015, 02:08:13 pm
content: Quote from: Seekonk on April 17, 2015, 01:53:57 pmThank you for the complimentsKeep in mind that MOV's can be rated and marked either for DC or AC.

author: DanielS
author_rank: Frequent Contributor
timestamp: April 17, 2015, 02:30:36 pm
content: Quote from: Paul Moir on April 17, 2015, 08:27:25 amAs 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.

author: Pjotr
author_rank: Frequent Contributor
timestamp: April 17, 2015, 03:35:51 pm
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: April 17, 2015, 05:05:35 pm
content: 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......