Power brick failure : 500 ohm between phase (or neutral) and earth - Page 1
Forum section: electronics / beginners

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 08:56:36 pm
content: Hi,I ordered many power supply bricks (230V AC -> 12V DC) from a chinese supplier, and I'm very satisfied with them except that very few units (about 1 of 200) are defective.I'm ok with that but am curious about what goes wrong in the defective units. Both have the same issue : plugging them immediately triggers the mains breaker. Checking the AC input with an ohm meter shows a resistance (about 500 ohm) between one of the AC pins (phase/neutral) and earth. Resistance only shows up in 1 direction otherwise it's infinite (like a diode).I haven't opened the plug pack yet since they're welded.Do you know what common failure in SMPS power packs usually causes this ?

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2023, 09:02:44 pm
content: That's unacceptable. Return all of them, if possible, otherwise junk them. They're not safe. Blacklist that supplier and never buy from them again.

author: nightfire
author_rank: Frequent Contributor
timestamp: February 08, 2023, 09:07:22 pm
content: Could be the bridge rectifier that is faulty. Otherwise there is not much on the primary side of a SMPS that would exhibit such behaviour- maybe also one of the Y-capacitors between L-N or N-PE is somewhat broken too...

author: wraper
author_rank: Unknown
timestamp: February 08, 2023, 09:14:13 pm
content: Satisfied? You effing got a batch of death traps. Most likely there is an insulation failure after a rectifier. Likely in transformer between windings and output GND is connected to earth. So you are you are measuring a diode in input rectifier through a faulty transformer. Ordering so many PSUs from china without taking one apart and testing insulation at high voltage is insanity.QuoteOnly a very severe safety critical failure can cause this. So it's extremely rare in devices that actually passed a real QC and were made according to safety standards.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2023, 09:17:14 pm
content: What else is wrong?Have you looked at the PCB? Does it have sufficient creepage and clearances? Have you dismantled the transformer? Does it have sufficient separation between the primary and mains windings? Is the tape strong and heat resistant enough not to melt?It's not your job to do this. Return them all and never buy from this seller again.

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2023, 09:25:42 pm
content: Quote from: ratatax on February 08, 2023, 08:56:36 pmPlease cut open a defective unit and post good photos. I'm sure you'll get plenty of feedback here.I have not seen SMPS failures like this at all, and I've poked, prodded and repaired a fair number. This type of failure should never happen, and I think it is the second worst possible failure to have with signficant fire (as in potentially burning down your house) being the worst. I don't what you are doing with these, but if you are selling these as a commercial product, your potential liability is extraordinarily high.

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 09:26:43 pm
content: Mmmmh OK. I'll open one and do more investigation. By the way I checked a sample before ordering many units, I'm not that crazy. I opened it and checked the construction looked OK, but I didn't remove the transformer to check its insulation.If there is a safety issue I really want to take it seriously. I'll open the defective unit and post pictures of it

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 09:41:44 pm
content: OK I cracked it open. It's the faulty one.To my not-so-experienced eye it looks fairly well constructed, with good isolation distance and not so cheap-looking components.What should I check first? Transformer? Safety capacitor?

author: Monkeh
author_rank: Super Contributor
timestamp: February 08, 2023, 09:46:16 pm
content: Perhaps the obviously crushed earth wire across the live heatsink?

author: wraper
author_rank: Unknown
timestamp: February 08, 2023, 09:49:01 pm
content: Quote from: Monkeh on February 08, 2023, 09:46:16 pmYeah.

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 09:50:35 pm
content: Oh wow, you're right. They cut a slot in the heatsink ON PURPOSE to leave some space for it, but for some reason the wire isn't inside the slot. Now I really want to open a working unit to see if the wire is also mispositionned.

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 09:54:17 pm
content: I confirm, the wire was so crushed it had its copper in contact with the heatsink.

author: wraper
author_rank: Unknown
timestamp: February 08, 2023, 09:54:32 pm
content: Construction looks OK on a first glance. The main safety concern normally would be a transformer. And there is a design flaw which allows earth wire to be jammed into the heatsink if it is dislocated from the cutout. And likely no proper Hi-pot test in production to catch that. However insulation may be pierced also during usage if someone steps on it or it is dropped.

author: wraper
author_rank: Unknown
timestamp: February 08, 2023, 09:57:09 pm
content: I suspect earth wire was supposed to be shorter so it could not be put outside the slot, or somehow fixed in the slot.

author: Monkeh
author_rank: Super Contributor
timestamp: February 08, 2023, 09:57:49 pm
content: It does look vaguely okay for a low cost supply. Some signs of, uh.. cost optimisation.The total lack of QC and hi-pot testing is a concern. Alternatively, they do both.. and just sell the failures cheap on the grey market to you.

author: floobydust
author_rank: Super Contributor
timestamp: February 08, 2023, 09:59:48 pm
content: This Big Clive cheap brick failure is worth watching, because the cause would be caught if safety tested and it's a no-brainer for EE's.You save nothing as these are powering gear worth 100's of dollars and can electrocute as well.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2023, 10:06:54 pm
content: Quote from: ratatax on February 08, 2023, 09:54:17 pmIs there a burr on the heatsink which is chafing on the cable?It's good you've learned about this, but you should send them all back and demand a refund.

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 10:09:58 pm
content: I will alert the manufacturer on this issue and check a few working units. Now my stress is at max level, I really hope the working units do not have a crushed wire waiting to failI selected this manufacturer (XINSPOWER) after checking five different PSU's from different suppliers, this one was the most quality-looking and also provided the best efficiency and output ripple compared to the others.

author: floobydust
author_rank: Super Contributor
timestamp: February 08, 2023, 10:11:24 pm
content: You want hard proof they have safety approvals, always ask for the product's certificate.P.S. - where is this thing's fuse? First thing to look for.

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 10:21:00 pm
content: Quote from: Zero999 on February 08, 2023, 10:06:54 pmWhat do you mean by burr ? Heatsink looks normal. Doesn't look sharp to cut anything so the pressure against the case during assembly must be huge.Opened a working one. Wire is correctly positionned. It's held in position with silicone against the capacitor. I guess this is a manual step done in their factory, which is subject to mistakes. Disappointed that they didn't test the units. Unless they tested it and it failed during shipping with vibrations causing just enough damage to the wire to make contact with the heatsink.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2023, 10:34:03 pm
content: Quote from: ratatax on February 08, 2023, 10:21:00 pmhttps://en.wikipedia.org/wiki/Burr_(edge)If it's not sharp, then it's purely an assembly error, as you've said.

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 10:35:34 pm
content: Quote from: floobydust on February 08, 2023, 10:11:24 pmI checked it has a proper fuse inside heatshrink tube.The capacitor between primary and secondary is jd222my1. 400V rated but I don't know if it's a proper safety one with the peak handling of several kV

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 10:45:15 pm
content: Quote from: Zero999 on February 08, 2023, 10:34:03 pmYup! Thank you everyone for the super quick replies on this thread! Not sure about what to do yet with these PSU's, it's quite hard to return anything to China. And if I switch to another manufacturer, it can be worse than that...

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2023, 11:05:09 pm
content: Now you don't know if the remaining ones are assembled OK or if they just haven't broken the insulation through to make a short yet. Do you know anyone with an X-ray machine? If you got the right angle, you might be able to confirm that the wire is cleanly and entirely in the slot.

author: srb1954
author_rank: Super Contributor
timestamp: February 08, 2023, 11:10:50 pm
content: Quote from: ratatax on February 08, 2023, 10:21:00 pmIt might have passed the factory test but with pressure on the wire, and some heat from the heat sink, the insulation on the wire would have flowed away from the pressure point over time leading to the conductor eventually being exposed.The application of the silicone looks to pretty haphazard to me and shouldn't have been relied upon as additional insulation. I would have expected that where the wire passes through the slot in the heat sink it should have been protected by another insulating sleeve that is both temperature and abrasion resistant.

author: srb1954
author_rank: Super Contributor
timestamp: February 08, 2023, 11:15:02 pm
content: Quote from: ratatax on February 08, 2023, 10:35:34 pmIf it is a proper safety capacitor it would be marked as Class Y and have various symbols printed on it attesting to it having been approved by various safety authorities.But, then there is the question if those markings are genuine or not or whether it is even a counterfeit capacitor.

author: thm_w
author_rank: Super Contributor
timestamp: February 08, 2023, 11:22:18 pm
content: A few failed parts is not really a "huge deal, return everything to the supplier" issue as people are making it out to be. Its completely normal even with western companies. Until we get some actual proof of intentional poor construction.Which leads me to the question, why are these tripping the mains breaker? Shouldn't they have an internal fuse? I would have assumed that is a requirement for these.Maybe check that the mains lead is wired correctly, and that there is some fuse or fusible resistor on that board.

author: Monkeh
author_rank: Super Contributor
timestamp: February 08, 2023, 11:32:09 pm
content: Quote from: thm_w on February 08, 2023, 11:22:18 pmRCD (GFCI).

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2023, 11:45:17 pm
content: Quote from: thm_w on February 08, 2023, 11:22:18 pmIt depends on the failure. 1 out of 200 "just don't work"? OK, warranty replacement. 1 out of 2000 bursts into flames or 1 out of 200000 electrocutes the user on the first use? Total product recall.

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 11:46:27 pm
content: Quote from: thm_w on February 08, 2023, 11:22:18 pmThe only threat I can see is if one of my customer get a defective unit AND for some reason has no earth connection in his house. The heatsink is at 100+VDC and gets connected to the negative output terminal if that earth wire touches it (then I don't know if the PSU destroys itself or keeps a dangerous voltage at the output)It shouldn't be possible to plug my power supply into an earth-less outlet, since it has that prong that will only fit earthed sockets.So imagine if some accident occurs, who is liable ? Me because the power supply is defective, or the customer because his electrical installation isn't right ?

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2023, 11:53:21 pm
content: Quote from: ratatax on February 08, 2023, 11:46:27 pmI rarely venture any legal advice outside my home territory, but I'm pretty sure you'd be liable and you'd have other issues as well, like the product non-conformity. And if it were shown that you previously knew about the defect or non-conformity, you might have criminal liability in some jurisdictions.

author: ratatax
author_rank: Regular Contributor
timestamp: February 08, 2023, 11:57:13 pm
content: I could test the faulty unit without the earth connection, to see what's the failure mode in case that very rare case happens (blown fuse ? boom ? flames ?), but even if I take every measure to do it safely I'm not very comfortable doing that experiment

author: bdunham7
author_rank: Super Contributor
timestamp: February 09, 2023, 12:24:16 am
content: Quote from: ratatax on February 08, 2023, 11:57:13 pmWell than I'd hope that you would be equally uncomfortable shipping the units as-is for your customers to do the experiment themselves! I would assume that even if it doesn't combust, the defect would cause the output to be at high voltage relative to ground if the power plug were not earthed.It's a crappy deal and TBH, I think it is likely that the vast majority of the non-failed devices are just fine. If it were me I'd sleep better if I could at least X-ray them. Have you notified your supplier of this issue?

author: thm_w
author_rank: Super Contributor
timestamp: February 09, 2023, 12:33:14 am
content: Quote from: bdunham7 on February 08, 2023, 11:45:17 pmThe output is earth grounded in this case, its not going to electrocute the user.

author: ratatax
author_rank: Regular Contributor
timestamp: February 09, 2023, 12:35:13 am
content: Quote from: bdunham7 on February 09, 2023, 12:24:16 amI don't know anyone with an Xray machine, but that would be very interesting to see how frequent is that wire issue.I notified the supplier and am waiting for his response.

author: jonpaul
author_rank: Super Contributor
timestamp: February 09, 2023, 02:27:27 am
content: don't buy cheap ChiCom junk in a safety related devicej

author: bdunham7
author_rank: Super Contributor
timestamp: February 09, 2023, 02:30:46 am
content: Quote from: thm_w on February 09, 2023, 12:33:14 amThe question was what happens when the device is plugged into a socket that doesn't have a working ground.

author: wraper
author_rank: Unknown
timestamp: February 09, 2023, 07:36:53 am
content: IMHO failure happened during enclosure welding. If you are intending to keep them, inspect all PSUs if they don't have a slight bulge in heatsink area. Maybe try pressing on enclosure and see if plastic can be pushed inside slightly on OK units. This way you could find bad ones without taking apart. Also certainly get an insulation tester like thishttps://www.tme.eu/en/details/ut512/insulation-resistance-meters/uni-t/and check them all. I also suggest taking apart a transformer to check if there is a sufficient insulation between windings.

author: wraper
author_rank: Unknown
timestamp: February 09, 2023, 07:56:04 am
content: Quote from: bdunham7 on February 09, 2023, 02:30:46 amYes, absence of earth is way too common, especially in older houses.

author: wraper
author_rank: Unknown
timestamp: February 09, 2023, 08:14:41 am
content: Quote from: ratatax on February 08, 2023, 11:46:27 pmFrom your profile you're in France where they have no Earth prong on the plug side. As of Europe and many other places, unearthed sockets (Type C) accept earthed Schuko plugs. Also many older houses have earthed sockets installed but no actual earth connection. Not to say user can bring the device to other country and replace the cable.

author: Zero999
author_rank: Super Contributor
timestamp: February 09, 2023, 08:25:19 am
content: Quote from: thm_w on February 08, 2023, 11:22:18 pmA failure which results in the output being connected to mains is a big deal. As mentioned above, unearthed sockets are all too common. It should be designed not to give someone a dangerous shock, even if it is plugged into an unearthed socket.Quote from: ratatax on February 09, 2023, 12:35:13 amThere is no way to test for this, because the insulation on the earth might fail during use.You could potentially open them all up and rectify the fault, by ensuring the earth wire is in the cut-out and is properly protected from abrasion, but is it worth your time?

author: wraper
author_rank: Unknown
timestamp: February 09, 2023, 10:55:55 am
content: Quote from: Zero999 on February 09, 2023, 08:25:19 amX-Ray will show dislocated wire. Also you could look for a bulge on enclosure. If insulation is pierced but wire is not making connection Hi-pot testing will show it too.

author: ratatax
author_rank: Regular Contributor
timestamp: February 09, 2023, 11:42:54 am
content: Quote from: wraper on February 09, 2023, 08:14:41 amThe only problem is if earthed sockets are installed but not connected. Replacing the cable won't allow them to get rid of earth (IEC 3-pong, it always comes with an earthed plug on the other end, and these wont' fit into Type-C outlets, they really only fit into earthed ones like type F or type E).QuoteAbrasion cannot happen really if it's inside the cut-out. Wire is fairly rigid and the heatsink is thick.I checked for bulges but I can't see anything and I can press on the faulty unit without problems. I think the wire got squeezed when the "press" assemble the units. Once it's assembled, there is some tolerance between the case height and the heatsink (heatsink = 23mm high, while i measured the inside case at 23.5mm, just enough place for the copper wire once the insulation is cut)

author: wraper
author_rank: Unknown
timestamp: February 09, 2023, 12:46:26 pm
content: Quote from: ratatax on February 09, 2023, 11:42:54 amWhat? Any plug that fits into earthed type F/E socket will also fit into unearthed type C. There are compatibility problems between type F and E unless plug is made universal to fit both socket types but any type E and F plug will fit into type C socket.https://en.wikipedia.org/wiki/CEE_7_standard_AC_plugs_and_sockets

author: wraper
author_rank: Unknown
timestamp: February 09, 2023, 01:00:05 pm
content: Another method how to check these came to my mind. You could pass high current from the mains earth terminal to negative output and look with somewhat decent IR camera. If wire is jammed it will be closer to enclosure and in the wrong location. So hotspot in a wrong place should be noticeable.Again, don't even think shipping these to your customers without checking every unit with insulation tester with at least 2.5kV test voltage.

author: ratatax
author_rank: Regular Contributor
timestamp: February 09, 2023, 02:15:36 pm
content: Thanks I will buy that UNI-T insulation tester. At the end, no matter how is the wire, if it survives high voltage insulation test then it's fine.I have an IR camera but I need a REALLY high current to make such a thick wire to warm up and warm the plastic case around it

author: ratatax
author_rank: Regular Contributor
timestamp: February 09, 2023, 02:17:02 pm
content: Quote from: wraper on February 09, 2023, 12:46:26 pmYes sorry I was wrong on that

author: thm_w
author_rank: Super Contributor
timestamp: February 10, 2023, 01:01:14 am
content: Quote from: wraper on February 09, 2023, 01:00:05 pmHopefully the device has no L-E MOVs, and OP can tie L-N together then hi-pot to earth at 2.5kV.Quote from: ratatax on February 09, 2023, 02:15:36 pmYeah I can't really see that working.

author: thm_w
author_rank: Super Contributor
timestamp: February 10, 2023, 01:16:00 am
content: BTW ratatax, how much are you paying per adapter? How many watts?

author: ratatax
author_rank: Regular Contributor
timestamp: February 10, 2023, 02:41:33 am
content: Adapters are 24W (12V 2A). They cost me about $4.4 per unit

author: wraper
author_rank: Unknown
timestamp: February 10, 2023, 07:52:44 am
content: Quote from: thm_w on February 10, 2023, 01:01:14 amThe main concern is if negative output wire can handle the current. You don't need some extreme current, if the wire heats up by a few degrees C, IMHO it should be enough to see it from the outside. Good enough IR camera will show a fraction of a degree difference. If earth wire is AWG 18 or 16, I see no problem heating it up enough. Of course it needs some time, like 5-10 minutes to be visible from the outside. Output wire must be out of IR camera view to not screw up the measurement range..

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2023, 12:20:47 pm
content: Quote from: wraper on February 10, 2023, 07:52:44 amA toroidal transformer and variac can be used to make a cheap variable high current power source.I still don't think it's worth it, considering how cheap they are. It would make more sense to buy some new ones of a reputable brand. Meanwell are a decent Chinese brand and are reasonably priced.

author: wraper
author_rank: Unknown
timestamp: February 10, 2023, 12:54:06 pm
content: Quote from: Zero999 on February 10, 2023, 12:20:47 pmLower end Meanwell is no better than this if you look at construction and components used. The main concern here is how this passed QC. Meanwell is not Chinese but Taiwanese BTW.

author: ratatax
author_rank: Regular Contributor
timestamp: February 10, 2023, 01:28:44 pm
content: I contacted the manufacturer to explain the issue and they strongly believe it's an isolated mistake during production, that isn't wide-spread. I suggested them to make the wire shorter to make it stay in place or manufacture the heatsink with a hole in it so it forces the wire to pass through.Other than that, build quality seems high to me, it was the best I could find while staying affordable. That manufacturing issue is fortunately quite easy to fix I think, if they are willing to take it seriously.