Testing MOVs, etc. - Page 1
Forum section: electronics / general

author: rdenney
author_rank: Regular Contributor
timestamp: March 25, 2025, 07:36:22 pm
content: My shop has four Zap 8800 Series 3 door openers. These are unlike most garage door openers, in that they rectify incoming AC to DC in the control box and drive DC motors on the mechanism. The DC motor is connected to the jack shaft of an overhead door using a belt. The current draw on the motor is what regulates opening and closing, and also what detects an obstruction. Because they are belt-driven, they are quiet. The manual override is a clutch on the belt that when release allows the door to move without the motor moving. They are not cheap but they are so much better than the usual chain-drive opener.That said, they have a flaw. My shop is new, built as a Covid project, and the panels are protected with panel-size surge protectors. These are mounted in the first position of the panel and properly terminated into the neutral bus with the shorted wire possible (in this case, about two inches). The openers are on GFCI circuits, though I doubt that matters at all.We had a power outage last year during a storm that caused a significant surge when power was restored. I lost most surge protector strips, the motor control board in an (expensive) treadmill, an electronic-timer light switch for an outdoor light, a battery charger for cordless tools, the PCB in the shop's boiler for the radiant floor heating system, and ALL FOUR of the PCB's in the shop's garage door openers. Some of the damaged items were in the house, and some were in the shop. Both the house and the shop have whole-panel surge suppressors. The shop is powered by its own metered service, though the house and the shop share the power company's step-down transformer that is in our yard (we have a six-acre lot).On all the failed devices, the MOV's used for surge protection were literally blown apart, as were fuses in the garage door opener circuits. In all cases, the destruction of the MOV release copious amounts of magic smoke.The insurance company sent a guy out and he photographed everything. They sent me a check for replacement value for all the devices--not just the repair costs but for the whole device. The check was substantial. I figured I was being well-paid for my ability to (sometimes) fix things, and some of the new bits I've bought for the bench were acquired with that money to effect those repairs. I bought new PCB's for all the devices, $400 each for the openers and boiler, and $250 for the treadmill.(Not much) time passes. Another storm a few weeks ago caused another outage, with another surge. Again, we lost several power strips in the house, but that's all. In the shop, nothing was damaged except for (again) ALL FOUR door openers. Again, the MOV's were blown apart.Each door opener PCB has two MOVs and two 6.3-amp fuses. The fuse for the opener circuit is a time-delay type, and for the lighting circuit a fast-blow. Each seems to be in series with an MOV. On each board, only one fuse was blown and only one MOV was destroyed. Clearly, whatever voltage came in overpowered the MOV, shorted it, fried the fuse, and then blew the MOV apart. The damaged MOVs now show open.I have used a capacitance tester to test the MOVs, and they each measure about 2 nanofarads. The MOVs are S20K140's, 20mm disk style with an AC voltage rating of 140 volts.I was able to get two boards working by salvaging undamaged MOVs and fuses from the boards I removed (and, of course, saved) from last year. So, only the MOVs and fuses are damaged--they did their job. I bought a couple dozen of the MOVs from Mouser, and was able to repair all eight boards, leaving me with four working spares for the next time. I'm also putting the openers on single-outlet Tripplite surge protectors in the hopes of sacrificing something easier to replace instantly without tools so the wife can get her car out of the end of the shop where she parks it.But I can't get past the curiosity of why the MOVs in devices would blow up and leave the panel surge protectors undamaged? I can't figure this one at all. I can say that the power strips (apparently) protected what was plugged into them, while the panel protectors did not.Also, how does one test MOVs the correct way? I used a capacitance meter simply because these don't conduct low voltages and I didn't really know how to construct a proper test. Which begs the question: What is a proper testing procedure for a an MOV? This much I've figured out: If they measure capacitance around 2nF, they work, and the failed ones were open and visibly damaged.Rick "sorry for the long backstory" Denney

author: BILLPOD
author_rank: Frequent Contributor
timestamp: March 25, 2025, 07:52:24 pm
content: Good Morning RdenneyTopic, Lots of hits on a Google search. Here's one:https://www.homemade-circuits.com/testing-mov-metal-oxide-varistor-surge/

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: March 25, 2025, 08:19:55 pm
content: I would suggest you to procure some thermally protected MOVs like the Littelfuse TMOV and iTMOV series.The iTMOV devices even have a third leg where you can attach a resistor, diode and LED as shown in the datasheet to indicate whether the device is still functioning or not.https://www.littelfuse.com/products/overvoltage-protection/varistors/thermally-protected/tmov

author: rdenney
author_rank: Regular Contributor
timestamp: March 25, 2025, 09:10:43 pm
content: Those thermally protected MOVs look pretty good--I just ordered a supply of them. Not cheap!Still wondering why the panel surge protectors didn't protect these.Rick "the fuses are more expensive than the MOV's" Denney

author: thm_w
author_rank: Super Contributor
timestamp: March 25, 2025, 11:06:29 pm
content: What is the part number of the "panel sized surge protector"? Its possible the clamping voltage is higher than the S20K140, though I don't know exactly what that is.For low budget, this tool might show where the MOV starts to clamp:https://www.eevblog.com/forum/reviews/completely-non-sketchy-mini-high-voltage-transistor-tester-(emeco)/

author: erikfriesen
author_rank: Newbie
timestamp: March 26, 2025, 12:09:53 am
content: 140v isn’t realistic for mov due to nuisance trips. Something more like a v230la which would be more like 160vac. Of course it’s possible downstream stuff won’t like it.MOV’s are intended to have some resistance in line, so for example a short run of 12 ga won’t allow enough drop to protect the mov.Also, make sure your neutral and ground are correct, not disconnected anywhere including the transformer. In our part of the USA they put copper pads on the bottom of every pole that tie to the lower hv wire.

author: johansen
author_rank: Super Contributor
timestamp: March 26, 2025, 05:38:38 am
content: Household panel surge protectors typically have 275 to 330vac rated line to line movs in them.I would be surprised if they have 140 or 150vac rated line to groundTypical clamping voltage at 25,000 amps is 700 volts.This can be accomplished with just 6, mov's that cost like 50 cents a piece and sell for 100$With the damage you report i am skeptical your panel surge protector is actually doing anything, or working.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 26, 2025, 07:05:01 am
content: Surge protectors are for short time surges, such as from lightning or upstream shorts.If you get a storm and the cable is cut there can beseveral cyclesof overvoltage present.Surge protectors don't help for this. Their internal fuse blows open and they cease to function.Their indicator will show and will need to be replaced.

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 26, 2025, 11:31:32 am
content: Sounds like supply connection issues somewhere.Insure that any sub panels in your installation do not bond the ground and neutral buses together at the sub panel, bonding is only done at the main panel where service enters. This is usually accomplished with a large green screw on the neutral bar, they need to be fully backed out or preferably removed altogether.

author: rdenney
author_rank: Regular Contributor
timestamp: March 26, 2025, 02:53:28 pm
content: Quote from: thm_w on March 25, 2025, 11:06:29 pmHere's the panel surge protector I'm using. My shop has two 200-amp Homeline panels, each of which has one of these.https://www.se.com/us/en/product-range/63127-homeline-hom2175sb-surgebreaker-spds/?parent-subcategory-id=86168&filter=business-4-low-voltage-products-and-systems#productsRick "the house has crappy GE panels and their branded plug-in protectors" Denney

author: bdunham7
author_rank: Super Contributor
timestamp: March 26, 2025, 03:14:21 pm
content: Quote from: rdenney on March 25, 2025, 09:10:43 pmThe panel surge protectors kick in at a higher voltage if there is a lightning strike or something like that. The circuit board designers obviously chose something that activates at a lower voltage to provide greater protection to the board and apparently this was successful. If you have a prolonged overvoltage (many cycles worth) rather than a microseconds-long transient then things get ugly. While a MOV can absorb very high power levels for a short time, they can't keep that up for very long. I'm not sure the thermally protected MOVs are the way to go because they'll just quit rather than remaining shorted and blowing the fuse. They're less likely to catch on fire, but not as likely to save your components. The only way to protect against prolonged overvoltage is to disconnect and that can be fuses or an interactive UPS. Fuses seem to have worked for you. Panel surge protectors don't come into play because you hopefully will not see an overvoltage high enough to trip them. If you do, that might be a pretty catastrophic situation.

author: rdenney
author_rank: Regular Contributor
timestamp: March 26, 2025, 03:32:18 pm
content: Quote from: erikfriesen on March 26, 2025, 12:09:53 amI'm sure the clamping voltage of the panel protectors is higher than in these boxes, and that's probably the explanation. But MOVs are cheap and I'd rather solder in new ones every now and then than have other things on the boards damaged that might not be so easy to replace. The MOVs are through-hole but a lot of other components on the boards are surface-mounted, which my old shaky hands struggle with, even assuming I'd be able to diagnose the failure.The shop's wiring is correct regarding neutral and ground. The ground to which the panel is tied is a Ufer ground, connecting to the steel reinforcing in the 200 feet of foundation footing. The connection from the meter box behind the panel to the power company's transformer is underground and about 50 feet (in plastic conduit). Upstream from the transformer, the distribution lines are about 800 feet underground before going up a pole to the overhead distribution system. The poles are all owned by the power company and I expect they are properly bonded.The door openers are on four separate 20-amp circuits. I have 10 20-amp circuits in the shop's ground floor, and the openers plug into the nearest receptacle using the supplied six-foot cord. The shop only has two 15-amp circuits, and those are used for lighting only. Of course, the 20-amp circuits are wired with 12-gauge copper. The cords look more like 16-guage. One of the openers wasn't close enough to a receptacle, and instead of the cord I used a 20-foot length of #14 cord with a 15-amp plug on the end of it.Here's a picture of the PCB installed. The main power comes in through the 4-position plug at the bottom. Two positions are line and neutral for incoming power, and two are outgoing power for a lighting circuit that is energized for five minutes after the door is actuated. The ground bus is just to the left of it. The damaged MOV and fuse is just above that.I believe these are made in the UK.Rick "removing/replacing the 8 screws holding the PCB takes longer than everything else in the repair combined" Denney

author: rdenney
author_rank: Regular Contributor
timestamp: March 26, 2025, 03:34:58 pm
content: Quote from: Jeroen3 on March 26, 2025, 07:05:01 amDo you mean the indicator will show that they are working, but still need to be replaced? The indicators on mine are lit as if they are working.Rick "would have thought Schneider would think a little harder about the indicators" Denney

author: rdenney
author_rank: Regular Contributor
timestamp: March 26, 2025, 03:37:50 pm
content: Quote from: WattsThat on March 26, 2025, 11:31:32 amNo subpanels, and I'm very familiar with the need to avoid bonding neutral and ground anywhere except in the main panel. I've had to deal with that in a vintage motorhome I bought, where the previous owner (aka, PO) had bonded the panel in the coach and wondered why a ground fault at the campground could electrify his coach.Rick "the only earth ground is the Ufer ground, and it's only connected to the safety ground wiring, plus the neutral bus in the main panel" Denney

author: rdenney
author_rank: Regular Contributor
timestamp: March 26, 2025, 03:42:09 pm
content: That a local interruption caused a surge over at least several cycles was the conclusion of the group at the local Stammtisch, though the judgments there may have been impaired slightly. The group includes several EE's, of course I am not one, though after a career building traffic signals I'm pretty familiar with the basics. I figure anything longer than 80 microseconds or so could blow the MOV apart.Rick "thinking the power cooperative's infrastructure is an unknown" Denney

author: Kleinstein
author_rank: Super Contributor
timestamp: March 27, 2025, 08:30:24 am
content: Usually MOVs fail with developing leakage current aready at a lower voltage than they should. So testing would be by measuring the leakage current at a voltage a little lower than the nominal voltage. Make sure to have resistance in series to limit the current, in case the MOV is badly damages. The datasheet ideally gives a test point in the specs.

author: rdenney
author_rank: Regular Contributor
timestamp: March 27, 2025, 01:20:10 pm
content: Quote from: Kleinstein on March 27, 2025, 08:30:24 amThe tricky bit is having a voltage source that can be adjusted that finely. I have a Variac which would get me to the 123 VAC coming in from the power company, though I might be able to get to the 180 VDC rating by daisy-chaining my bench power supplies together. The leakage current charts in the data sheet don't even go down to 120VAC (assuming the chart means AC, which it doesn't say).Rick "may sacrifice an unused MOV" Denney

author: johansen
author_rank: Super Contributor
timestamp: March 27, 2025, 05:12:58 pm
content: Quote from: rdenney on March 27, 2025, 01:20:10 pmmost variacs can be connected to deliver 0-140vac, which may just barely be enough to measure some current through some 150vac rated mov'syour local hvac company will have a "junkyard" of parts and there is a 25-40va 24vac transformer with a tapped 240/208v primary in most junk furnaces and heat pumps, if you don't have anything similar on hand right now. -they actually throw away a lot of useful shit.you could daisy chain a dc supply in series with an ac signal, you could then string an ac amp meter and a dc amp meter in series to measure the leakage through the mov. -flip the dc supply around to see if the leakage is the same in both directions.