MOV protection - Page 1
Forum section: electronics / beginners

author: polis93
author_rank: Newbie
timestamp: December 02, 2016, 06:00:23 am
content: Hi everyone,here is my question: When I connect 3 MOVs in series, lets say 100V DC clamping voltage, will they behave likeone with 300V DC clamping voltage? Or is it more complicated?Thanks for answer.

author: danadak
author_rank: Super Contributor
timestamp: December 02, 2016, 10:55:52 pm
content: Yes -http://www.littelfuse.com/~/media/electronics_technical/application_notes/varistors/littelfuse_the_abcs_of_movs_application_note.pdfRegards, Dana.

author: saturation
author_rank: Super Contributor
timestamp: December 04, 2016, 06:16:53 pm
content: Another yes just a caveat, MOV are specified with10%varied tolerancesso the actual clamping voltage of 3 MOV in series will vary.If one of them is at the extreme end of your 100V, that would be 90V, it could fail prematurely. Thus because of the variation caused by manufacturer's tolerances, its better to use one MOV at 300V than 3 at 100V in series if the voltage cutoff must be precise.Striked out. Corrected by wraper and other posts above.

author: wraper
author_rank: Unknown
timestamp: December 04, 2016, 07:06:49 pm
content: Quote from: saturation on December 04, 2016, 06:16:53 pmYou are confusing MOVs with capacitors. It won't fail and imbalance can be much higher than 10% (and they don't need to be with the same clamping voltage rating at all), depending on at what voltage particular MOV has certain leakage current. If the MOV will have lower clamping voltage, it actually will be the least stressed part among all when surge happens.

author: wraper
author_rank: Unknown
timestamp: December 04, 2016, 07:22:46 pm
content: Actually when connecting them in parallel to increase clamping current capacity, then it becomes a situation when clamping voltage tolerance matters. As basically one MOV could clamp most of the transient while others stay idle. On top of that, when MOV is stressed by absorbing transients, it's clamping voltage actually becomes lower. So imbalance will only increase with transients clamped, to the point when clamping voltage becomes so low that it starts to pass high current without any transients at normal operating voltage, and in the end MOV just blows up.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 04, 2016, 08:24:25 pm
content: In series, they share current, and the voltages are only nominally mismatched. Moreover, MOVs are sloppy enough devices (they call them VDRs for a reason -- they're always at least somewhat resistive) that the mismatch doesn't result in too much difference in dissipated power, or relative heating.The same goes for if they're in parallel. MOVs in parallel I think is not recommended, but not fatal or anything. You won't get proportional performance, because any given set will not share perfectly evenly, but the mismatch will be modest. Three in parallel might be good for double the energy of a single part, or something like that.The same also goes for TVSs (silicon avalanche diodes). These are sharper (current ~exponential with voltage above the threshold), but more precise as well. And, as is the case for MOVs, when you're up in the surge-absorbing range of current flow, there's relatively a lot of resistance at work, so they will still share current in parallel -- if not ideally well.More important is designing your circuit to tolerate the remaining transient. Because of the internal resistance, an MOV will only clamp massive surges (like nearby conducted or induced lightning surges) to a peak voltage about 150% above its rating. TVSs, about 60%. So you need to handle triple or double the nominal voltage, respectively. An automotive circuit that operates at a nominal maximum of 18V can use a 18V MOV or TVS (to handle switching spikes and load dump), then the circuit needs to withstand a peak of 54V or 36V respectively (which, incidentally, is why voltage ratings of 36 to 60V are so common among power supply components targeted for automotive use!).Tim