MOV protection - Page 1
Forum section: electronics / beginners

author: IvoS
author_rank: Frequent Contributor
timestamp: May 17, 2023, 07:17:00 pm
content: I have 120V 6VA transformer powering my logic and startup board for my power amp. What kind of MOV protection is a good practice to put on 120V mains? What else should I put there? I have seen a lot of people put 0.1uF X2 cap accross the mains as well.

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2023, 07:29:31 pm
content: First- the fuse is important, what are you using? At that low VA rating, the primary wire guage is tiny and ends up being the fuse, but one is still required.I wouldn't bother with the MOV because the transformer impedance limits transients. I would expect say 2x the output voltage though on a long-term surge.The X-cap is for EMC with SMPS and no use for a power transformer, and a safety issue as it's across mains.

author: IvoS
author_rank: Frequent Contributor
timestamp: May 17, 2023, 08:29:57 pm
content: Yes, of course I will put a fuse, I forgot to mention that. 5mm 100mA T should be fine. Thanks.

author: wraper
author_rank: Unknown
timestamp: May 17, 2023, 08:36:04 pm
content: In such application you'd get way more cases of MOV going short and exploding than it actually protecting anything. MOV can only protect against very short transients without exploding and transformers are quite robust to those to begin with.

author: wraper
author_rank: Unknown
timestamp: May 17, 2023, 08:42:50 pm
content: Quote from: floobydust on May 17, 2023, 07:29:31 pmNot really. If small transformer is built according to certain safety requirements (which I'm not an expert at) and has thermal fuse or is "inherently short-circuit proof" like thishttps://www.block.eu/en_US/productversion/vb-035215/then fuse is not required. That's why you will have a very hard time to find any transformer based power brick with a fuse.

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2023, 09:13:02 pm
content: Yes. For transformers purchased from distributors (as a component) I find North American vendors sell low VA (say <25VA) transformers as "non-inherently limited" with it being a rare option for any integral protection. We don't know what transformer OP is using. Small European power transformers come with it (thermal fuse or impedance protected) but not North America so much. It might be the difference in regulatory approvals UL/CSA 5085 vs IEC 61558.i.e. looking at some:https://www.tamuracorp.com/clientuploads/pdfs/engineeringdocs/3FS-4XX.pdfoptionalhttps://www.belfuse.com/resources/datasheets/signaltransformer/ds-st-14a-series.pdfoptionalhttps://catalog.triadmagnetics.com/Asset/FS16-800.pdfClass II are really used for doorbell and HVAC, generally pretty uglyhttps://www.hammfg.com/electronics/transformers/class-2Popular low VA isTalemabut what is at disti's has no protection.So I suggest to have some fuse, somewhere lol

author: bdunham7
author_rank: Super Contributor
timestamp: May 17, 2023, 09:16:40 pm
content: Quote from: wraper on May 17, 2023, 08:42:50 pmThat's true for a commercial wall-wart or power brick where the main concerns are reliability, low cost and not catching on fire. In those cases, a blown fuse and an entirely ruined set of innards are equivalent results--the unit goes in the trash. For a one off hobbyist project or anything where repairability might be an option, I like the MDL fuses for regular transformer setups. For this case I'd use an MDL 1/10 fuse that has a cold resistance of 15R, thus limiting fault currents quite a bit.I agree the MOV is unnecessary, but if installed after such a fuse it isn't a problem. There's probably little need for any surge suppression, but if he's concerned about things downstream, a TVS across the bridge rectifier in the secondary is a much nicer solution.

author: IvoS
author_rank: Frequent Contributor
timestamp: May 17, 2023, 09:22:36 pm
content: I'll be using this transformer:https://www.digikey.com/en/products/detail/signal-transformer/LP-24-250/1020890

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2023, 09:28:55 pm
content: I've used that series before and they have no integral fusing, so your fuse is needed.I find they have terrible regulation and at light loads output voltage goes up over 20% which roasts linear voltage regulators.Surge protection you can put a MOV in after the fuse for peace of mind but transients get limited by core saturation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2023, 09:53:52 pm
content: Quote from: wraper on May 17, 2023, 08:36:04 pmTo clarify, there is a lot of paranoia out there about MOVs, even among professionals. This happens because few of them have seen MOV failures in person, so only get their information from word of mouth and exceptional reports; and the few that have seen any, have seen the graphic results stick in their mind. This results in confirmation bias, that disregards all the invisible decades of successful, reliable protection that these devices have provided.A MOV of adequate size, will pass all regulatory test levels, and will serve in commercial service reliably (expected lifetime >= 10 years) at 99.9% of all sites (or something like that). Sites with greater hazard are few and far between, but do exist, and either "oversized" protection, or alternative means (very large TVS?) may be desired.As for the transformer, I expect it will not notice a thing. Small split-bobbin transformers have more than enough primary resistance and leakage inductance that the surge waveform will simply drop across it, greatly reducing the peak current flow. Rather than a MOV shunting the surge, it'll simply ride through it. I suppose the worst that could happen is internal breakdown (surge is a few kV, it could cause breakdown of the enameled wire in the primary), but I don't think that's very likely. The pulse could also melt the fine wire within, causing an open or short failure, but that seems even less likely. As long as the secondary side has adequate filtering, you won't notice a thing.Tim

author: bdunham7
author_rank: Super Contributor
timestamp: May 17, 2023, 10:01:17 pm
content: Quote from: T3sl4co1l on May 17, 2023, 09:53:52 pmThe person that first told me about MOV issues was a fire marshal, probably 20 years ago. There have been a fair number of fires and millions of surge protectors recalled. Perhaps things are better now, but unfused or unprotected MOVs connected to mains are (or were) a hazard. Perhaps not super common, but there are plenty of pictures out there of burned surge protectors and GFIs.

author: bdunham7
author_rank: Super Contributor
timestamp: May 17, 2023, 10:05:46 pm
content: Quote from: floobydust on May 17, 2023, 09:28:55 pmApparently they're Class 3 Impedance Protected, which accounts for all of those issues--no fuse/PTC and not being very 'stiff'. I would still fuse it, but it probably isn't strictly necessary unless the transformer getting very warm causes some other issue.

author: srb1954
author_rank: Super Contributor
timestamp: May 17, 2023, 10:06:45 pm
content: Quote from: floobydust on May 17, 2023, 09:28:55 pmExcept for for fast transients which can be coupled through the inter-winding capacitance bypassing the magnetic circuit altogether.Some X and Y capacitors on the mains input can help with knocking back those fast transients before they get coupled through. In that case I like to include a MOV on the primary circuit, more to provide protection for the X capacitor than the rest of the circuitry.

author: floobydust
author_rank: Super Contributor
timestamp: May 18, 2023, 12:50:02 am
content: Quote from: srb1954 on May 17, 2023, 10:06:45 pmThese are split-bobbin semi-toroidal construction, primary and secondary are apart giving very low inter-winding capacitance and 1,500VRMS rated hipot.I say (differential) fast transient won't make it far enough and core saturation and impedance will be a good enough filter.For common-mode transients, a MOV/X-cap is of no use. It can help with back-EMF is there if a relay or small switch.For many years I used to design using a fuse, MOV, then power transformer.Fuse is sized typically 167%-300% of max. primary current, depending on the safety standard. MDL-1/4A is 3.2Ω (and $10 ea. now?) so the MOV would get an easy life.130VRMS MOV (clamp max. 340V). This works fine but I realize the MOV voltage rating is tight, too low for nowadays. Mains voltages are higher and MOV aging- although I have not seen a problem, I have several used 30 year old 20mm 130VRMS MOV's and I checked they are 50uA at 122VRMS. Today I would use a 150VRMS part for 120VAC mains.But even using a MOV you still only have it down to ~2-3x mains for transients. But I think the transformer magnetics clamp it. So I don't use a MOV anymore.OP be careful with running traces underneath the transformer, I find you can get induced hum (right hand rule), and keep primary clearances in mind.