MOVs in Parallel (Varistors) - Page 1
Forum section: electronics / projects

author: TheRuler8510
author_rank: Regular Contributor
timestamp: December 17, 2014, 01:41:33 pm
content: To the experts...How well do MOVs in parallel work? If there are small threshold voltage differences, will one of the MOVs take the surge, leaving the other (higher voltage) MOVs unused?What is the best way to do MOVs in parallel?Thanks!

author: tom66
author_rank: Super Contributor
timestamp: December 17, 2014, 02:01:20 pm
content: Depends on the MOV....For SiC, I'd feel OK with connecting them in parallel, but ZnO would make me a little uneasy.One issue is the temperature coefficient. If increasing temperature causes the threshold voltage to fall, it's not good. If it causes it to rise, it's a self stabilising system.MOVs in series... I can't see any major issues with that, I'd recommend that over parallel MOVs.It's quite hard to design systems with MOVs in so consider other devices like TVS diodes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2014, 02:21:59 pm
content: I don't know that I've seen an MOV which identified itself as ZnO or SiC. All the ones I have seen curves for, the surge range is fairly squishy so there will be no problem connecting them in parallel.What's the intended purpose? Lower profile assembly? Enhanced lightning protection? Greater operational life?If you were using them for 'zener' applications, paralleling just for increased power dissipation, no, probably not going to work. Down at low currents, the threshold voltage makes all the difference.For the same reasons, avalanche diodes are kind of dubious to connect in parallel. They have sharper response than MOVs, so I would expect an increase in surge capacity, but I wouldn't expect a perfect doubling (or whatever), unless you use matched diodes only.If your aim is simply to improve transient tolerance of your circuit, but aren't concerned with increasing surge capacity or life -- you can use multiple stages of TVS devices, separated by impedances (usually resistance or inductance). Just as you'd build a multi-stage LC lowpass filter. Perhaps you need to withstand a 100A, 10/1000us surge: on, say, a 120VAC line, you first start with enough MOVs to handle the surge itself, which clamps it at, say, 300 or 400V peak, then follow this with a small resistor (a few ohms?) and another MOV, then maybe a few more ohms and an avalanche TVS to get the voltage clamped even more precisely. The circuit would then have to withstand only 200 or 250V peak. Or the front line protection could be a gas discharge type, followed by a moderate inductance: the inductance allows the input voltage to surge high enough to trip the GDT, and the MOV, or whatever is following it, prevent that pre-ignition spike from reaching the circuit, and clamps any residual.Tim

author: splin
author_rank: Frequent Contributor
timestamp: December 17, 2014, 05:25:05 pm
content: Consider the failure modes of the MOVs. Eg. seehttp://communities.leviton.com/docs/DOC-2374They can fail short circuit, in which case the other MOVs won't provide any additional resilience.Also the voltage threshold of a MOV subject to a surge may decrease such that the lowest voltage MOV increasingly takes more of the surge as the number of surges increases.

author: TheRuler8510
author_rank: Regular Contributor
timestamp: December 17, 2014, 05:39:39 pm
content: All,Thanks for the replies. I don't have a particular application. Dave mentioned on the AH, a circuit with extra MOVs, so I started to think how that might be implemented. Circuit protection is a interesting side subject for me.Regards,

author: DanielS
author_rank: Frequent Contributor
timestamp: December 17, 2014, 07:58:16 pm
content: One way to share load between multiple MOVs is to use small inductors (+wire resistance) in series with live and neutral before the load, then connect one MOV from filtered live to unfiltered neutral, another from unfiltered live to filtered neutral and a third directly across filtered live and neutral. This way, the inductance and resistance prevent either "cross-wired" MOV from hogging all the surge energy and the third MOV catches what the two others didn't. You may also want to add additional MOVs from filtered live/neutral to ground to prevent either filtered output from rising too high. This is the arrangement APC uses in their SurgeArrest Professional bars if we're talking mains surge suppression.