Spot the problem - Page 1
Forum section: electronics / repair

author: TerraHertz
author_rank: Super Contributor
timestamp: December 12, 2016, 12:59:36 am
content: This was a trivial repair of a dead power supply, not worth mentioning. Till I noticed something surprising. Spot the problem in this photo. The power supply is a 12V 20A switcher. From China, and there's your clue.That wasn't what killed the supply.Actually this was one of the finest examples of 'observer effect' I've ever encountered. The supply is what came with that low-cost 3D printer I bought. The supply was working fine for the last few weeks. Then on Saturday a lady friend who's a teacher, and among other things gives lessons in 3D printing, came around to have a look at this one. And 'swap lessons' - I teach her how to solder and use a multimeter, in return for some tutoring in 3D CAD software.So, the power supply worked fine just the night before. Turn it on while she's in the room - nothing. Not even a flicker. Ha ha.Later, disassembling the culprit, the fix was easy. Several really bad solder joints, including one to a pin of one of the pair of HV DC storage caps. It was actually completely loose, not soldered at all. I think it must have been touching till now, then went semi-open, with the resulting arcing creating enough transients across the main switching transistors to kill one of them.So, replacement transistors ordered, meanwhile just run the machine off a bench supply. No problem.Then I thought I'd derive the circuit diagram of the power supply, since others using that printer may find it useful. While doing that, I noticed that 'wonderful' instance of Chinese cost cutting. Love it.If no one spots it quickly, I won't keep you in suspense.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: December 12, 2016, 01:08:28 am
content: Looks like the holes are not plated through.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 12, 2016, 01:11:10 am
content: Quote from: ManateeMafia on December 12, 2016, 01:08:28 amThat's not it. Single sided low cost board, no plate-through is the norm and not an issue.The problem I'm talking about is nothing to do with the crap soldering quality.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 12, 2016, 01:18:05 am
content: Quote from: blueskull on December 12, 2016, 01:13:56 amBingo! The unsleeved MOV and fuse, who cares, that's just safety overkill. But the caps! I thought I was seeing things at first. "No, they can't have? Oh, they really did!"Needless to say, those are getting replaced.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2016, 01:18:52 am
content: The fuse doesn't exactly match the silk...Not necessarily an illegal choice, but inferior for sure.Blueskull: I'm thinking that may be an NTC, in which case, sleeving is still fairly common, but not really important for the reasons it might be on a MOV I think.Tim

author: tautech
author_rank: Super Contributor
timestamp: December 12, 2016, 01:21:35 am
content: Quote from: blueskull on December 12, 2016, 01:13:56 amThat'll be a Thermistor, you don't want to sleeve them.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 12, 2016, 01:27:08 am
content: T3sl4co1l and tautech are correct, it is an NTC. In series with the mains input, for inrush reduction. I knew it, just forgot when replying to blueskull.The fuse definitely is pathetic, tiny and glass. It probably wouldn't actually quench a 240V arc. I suppose I should replace it, but don't much care. That's why we have circuit breakers.

author: tautech
author_rank: Super Contributor
timestamp: December 12, 2016, 01:30:03 am
content: Quote from: TerraHertz on December 12, 2016, 01:27:08 amLeaded glass fuses are very common in consumer items, I've seen a few fused ones but never one ruptured.Unless you're fussy about these things I'd leave it be.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 12, 2016, 01:40:00 am
content: Quote from: tautech on December 12, 2016, 01:30:03 amAgreed. As you probably knew by now, I'm not fussy.However, it is easily the tiniest glass fuse I've seen used for mains. About 2.5 mm gap between the metal end caps.Another interesting thing about this supply struck me while circuit tracing. The LV supply for the regulator circuit comes from a secondary winding of the main transformer. Huh, so how does it start?A: the drive transistor bases are transformer coupled from the IC, but there's also a positive feedback winding from the primary current to the main transformer. So the circuit self-oscillates enough to power up the regulator loop, which then overrides the self-oscillation. (One hopes.)Schematic nearly done, then I have to redraw it for neatness. I'll post here later. Maybe tomorrow, depending on other chores.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 12, 2016, 01:42:10 am
content: As for the capacitors, I've replaced a lot of CapXon due to capacitor plague. Replace them all.

author: tautech
author_rank: Super Contributor
timestamp: December 12, 2016, 01:44:01 am
content: Quote from: blueskull on December 12, 2016, 01:35:21 amAcknowledged.But really there's little point of fusing a SMPS if you consider the overcurrent response time of silicon compared to a fuse....the silicon always wins and usually save the fuse.

author: wraper
author_rank: Unknown
timestamp: December 12, 2016, 01:52:40 am
content: Quote from: Tomorokoshi on December 12, 2016, 01:42:10 amFrankly they are not as bad as 10 years ago when lot of crap was made by many manufacturers. Also not all Capxon all series were that bad. IIRC GF and KF were the worst.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2016, 03:42:32 am
content: Quote from: TerraHertz on December 12, 2016, 01:27:08 amHrm, well, Idunno--It might not quench a hard arc (i.e., the circuit fails shorted so hard that fault current (low ~kA) flows), but then the breaker should take care of that. There may be unusual scorching of the board, maybe a bit of a flameout too?A softer failure (say the mains side electrolytic starts cooking off..??) might blow the fuse quietly, but wouldn't be enough to trip the breaker (since we're talking amps, not tens or hundreds of amps). If the fuse weren't there at all (i.e., depending on just the breaker), heat is able to build up, and you get an explosion, or fire, or something like that...It might take a contrived situation to better illustrate this, but stranger things can happen in reality, so bear with me.Example:I was working on a prototype switching supply, working behind an isolation transformer. The trans was fairly undersized, so added significant series resistance to the supply. A shorted secondary wouldn't even trip the breaker! Now, the work was fused adequately, but the sequence of events that transpired was:1. Supply working nicely like it should. Yay. Getting kind of warm though.2. Main transistor (flyback topology) fails shorted. DC+, drain-source, and shunt resistor to DC-, draws mondo amps.3. Transistor doesn't actually fail completely three-way-shorted, but blasts plasma for a while, dropping say 100V between pins. Erupts in flame.4. A couple of seconds later, the equipment fuse (2A I think it was) burns out. But it doesn't really burn, it kind of splatters inside the glass tube, with much vapor residue. (The glass didn't crack or rupture.)So the energy delivered was much greater than if the fuse had blown out promptly.There might be a situation where this could happen with breakdown of insulation, and odd transistor failure modes (not quite the usual melt-into-short-circuit behavior), so I wouldn't discount it as purely a consequence of my crappy transformer.Tim

author: edy
author_rank: Super Contributor
timestamp: December 12, 2016, 03:56:50 am
content: Quote from: TerraHertz on December 12, 2016, 01:18:05 amThat's the first thing I saw as well.... Why? Because of this issue which happened to my 2 year old Samsung TV a few years ago. Turns out they were using 10V caps on a 12V rail:https://youtu.be/XJTfdDTAQpo?t=2m56sI subsequently switched those caps with 25V versions using same uF rating and the TV is still working fine now for the past 5 years. And I'm not the only one who got screwed. The Samsung TV issue became the center of a class-action lawsuit which is why my above video has over 700,000 views! Little did they know that they helped me make back the cost of the TV (and then some) on the Google Adsense revenue just on that video alone.Another user noticing same thing:http://redpushpin.blogspot.ca/2011/08/repairing-samsung-lcd-tv-with-clicking.htmlSamsung finally settled in the USA but never admitted any guilt. I was not involved in the suit because as a Canadian, there was no lawsuit in Canada at the time and would have to be filed separately (and the Canadian system sucks for consumer protection). Samsung eventually did honor repairs in Canada even though the settlement only involved the USA, possibly to avoid another lawsuit knowing they would probably lose with the US-setting a precedent, but by then I had already fixed my TV.I'm not sure what happened and what moron in either the engineering team or supply-chain decided to use the lower V-rated caps and for what possible reason, and not know it was going to potentially cause millions of TV's to have premature failure. Was it intentional? Or just stupid oversight? Or to save a few pennies per TV on parts?https://www.cnet.com/news/samsung-settlement-warrants-older-tvs-with-faulty-capacitors/https://topclassactions.com/lawsuit-settlements/lawsuit-news/1654-samsung-tv-capacitor-class-action-settlement/http://www.pcmag.com/article2/0,2817,2400675,00.asphttps://hdguru.com/samsung-settles-hdtv-lawsuit-millions-affected/http://miami.cbslocal.com/2012/02/09/more-than-7-million-samsung-tvs-plagued-by-possible-power-defect/

author: djos
author_rank: Unknown
timestamp: December 12, 2016, 03:59:45 am
content: Maybe it's just the angle or lighting, but the Diode closest to the transformer appears to be installed incorrectly when compared to the screen print under it?

author: tautech
author_rank: Super Contributor
timestamp: December 12, 2016, 05:31:00 am
content: Quote from: wilfred on December 12, 2016, 03:49:11 amThinking about this you may have a point.I've seen a few old NTC's crumble and it's probably because the ceramic has no glaze on it and if they're power cycled lots the power cycling and atmospheric moisture is weakening the ceramic body. I have no proof or evidence of this other than what I've observed over the years.Like most materials even ceramics don't like thermal cycling.Would they run hotter sleeved, most likely but sleeving should restrict the ingress of moisture but a negative might be it's also flammable and available as fuel should a NTC fuse open.I'd leave them be.

author: mikerj
author_rank: Super Contributor
timestamp: December 12, 2016, 09:29:05 am
content: Quote from: tautech on December 12, 2016, 05:31:00 amI used to see lots of NTC failures in the degausing circuits of old CRT TVs/Monitors, they aren't the most reliable of components. However sleeving them is not a good idea because it makes them take that much longer to cool down.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2016, 04:28:46 pm
content: Quote from: mikerj on December 12, 2016, 09:29:05 amThose are PTCs.Though I don't know enough about either device to be able to say which one should last longer, or why. (They are ceramic devices -- we're not talking cheap PolyFuse PTCs here.)Tim

author: Gyro
author_rank: Super Contributor
timestamp: December 12, 2016, 08:41:30 pm
content: QuoteActually, most degaussing circuits called for a series PTC and a shunt NTC to achieve the correct current profile.

author: technogeeky
author_rank: Frequent Contributor
timestamp: December 13, 2016, 10:55:39 am
content: It could be the somewhat out of focus picture, or my out of focus eyeballs, but isn't the topmost (closest to transformer) diode installed backwards?It looks like it's installed the same way as the second (from the bottom of the picture) diode, which is not what the silk screen specifies.

author: Rerouter
author_rank: Super Contributor
timestamp: December 13, 2016, 11:42:53 am
content: Hmm, oddly enough the 12V 21A power supply that came with my dirt cheap G2S Pro printer actually opted for 16V caps (still uncomfortably close, but much better than 10V!) and has not horrible soldering (its not good, but its not bad enough to be a problem) so glad to see, even in the dirt cheap end of china, what you pay means nothing towards what you get

author: djos
author_rank: Unknown
timestamp: December 13, 2016, 08:59:13 pm
content: Quote from: technogeeky on December 13, 2016, 10:55:39 amI wondered the very same thing inreply #19

author: mikerj
author_rank: Super Contributor
timestamp: December 13, 2016, 10:01:53 pm
content: Quote from: T3sl4co1l on December 12, 2016, 04:28:46 pmOops, err...well done for spotting my deliberate mistake

author: TerraHertz
author_rank: Super Contributor
timestamp: December 15, 2016, 03:26:21 am
content: About that diode - it looks wrong because the stripe paint is mostly rubbed off. But it is the right way round.The schematic - sigh. Various unexpected chores popped up, and I didn't finish that yet. Almost.There's another 12V DC supply built into my wiring closet. It's by the same manufacturer, though a lower current. I thought I'd better check the caps in it. They are OK.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 18, 2016, 07:59:05 am
content: Here's the schematic of that power supply, if anyone wants it.The writeup herehttp://everist.org/NobLog/20161129_3d_learning_curve.htm#psalso has a PNG version and the raw SCH file & library. In Protel98 form, ha ha, yes I know, very old tools.After tracing the diagram in the rough form (very messy, on several sheets of paper) I'd intended to do a neat hand-drawn version.But it's hard to fit on a single A4 sheet, sooo... for the first time since around 2004 I installed my old schematic/pcb tools. One of several, but I was most familiar with Protel98, so hoped I'd still remember something of how to use it. Wasn't too painful.So, good. I'd been meaning to do that for ages. It's a long story, involving some fairly traumatic events back then which kind of turned me off several things, including electronics design.