Cheap chineese power supply missing short-circuit protection - Page 1
Forum section: electronics / beginners

author: dominicM
author_rank: Regular Contributor
timestamp: May 12, 2018, 07:58:54 pm
content: So I got this S-400-12 power supply to hack into a variable supply. The hack did sort of work as I get 5V-15V bit I noticed something. Looking at theschematicmy PSU seems to be missing all short circuit protection components. It's even missing thermal cut out for the fan so it's on all the time. Looks like they just took everything out that's not absolutely essential.My question is this how bad is this in practice and why exactly? Is it reasonable to use it at all? Populating the components isn't really worth it as it would cost same as a new PSU. In that case I may just try my luck with a UK seller this time and hope for the best. I would appreciate some general thoughts and reasoning from someone with some experience with power supplies.

author: Jwillis
author_rank: Super Contributor
timestamp: May 12, 2018, 11:57:47 pm
content: Did buy that out of the back of a van? Just asking.

author: floobydust
author_rank: Super Contributor
timestamp: May 13, 2018, 12:18:59 am
content: Sorry that looks like an elcheapo power supply. It may have been a decent design but every possible part stripped out to save pennies.Missing fan control and short-circuit protection parts, and filter inductor L1 etc. Electrolytic capacitors C21, C22 are much smaller than the PCB footprint and some junky resistors there.I would not invest much more time and money with it.

author: Brumby
author_rank: Unknown
timestamp: May 13, 2018, 12:55:05 am
content: Quote from: floobydust on May 13, 2018, 12:18:59 amIncluding a very obviously missing heatsink.Not getting any good vibes on this unit.

author: Eka
author_rank: Regular Contributor
timestamp: May 13, 2018, 03:25:38 am
content: There has been a problem with very low cost stripped down power supplies missing the safety circuitry they passed UL, CE and other safety standards with. More than a few fires have been caused by them.

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 10:01:46 am
content: Quote from: Jwillis on May 12, 2018, 11:57:47 pmEbay, so internet equivalent of a back of a vanNot aware of any decent source for these supplies.QuoteI knew it was gonna be cheapo so that's not in itself an issue for my 200W max occasional use case. L1 is not actually missing just round and is just outside the image edge. Didn't notice the caps were smaller though.QuoteThere are actually 2 small heat sinks screwed to the side of the case, removed in image.I would still love to know exactly what issues it can cause and the reasoning behind the logic for general knowledge. If there is a real danger of fire then ye I will try buying a new one for a different supplier.

author: bob225
author_rank: Frequent Contributor
timestamp: May 13, 2018, 11:07:48 am
content: Meanwell are decent supplies for the money, buy cheap, buy twice

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 11:10:12 am
content: Quote from: bob225 on May 13, 2018, 11:07:48 amYes I looked into Meanwell but I couldn't even find where to buy the model I needed. I also wanted this exact model as I know it can be modded into a variable supply very easily.

author: kony
author_rank: Regular Contributor
timestamp: May 13, 2018, 11:14:52 am
content: Delta is almost as cheap as Meanwell, yet with ~ three times longer MTBF. Stocked in Mouser, if you don't have cheaper local distributor. But if you need variable PSU, go buy variable/lab PSU (Manson comes to mind as economic option with not so bad quality), don't try to wing it with something not designed for the purpose.

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 11:35:35 am
content: Quote from: kony on May 13, 2018, 11:14:52 amI already have Variable bench supply, this is just for semi-temporary uses, it's just good to have something that can do 5-15V. Will keep Delta in mind in the future but problem is that even Meanwell costs 10-15x times the el cheapo and that's for static 5V not variable. If it was for something important like house lighting then sure I would just buy it without thinking. Come to think of it I would probably just use a decent PC PSU for that since it's so much cheaper and still excellent.

author: janoc
author_rank: Super Contributor
timestamp: May 13, 2018, 12:52:26 pm
content: These switchers are not designed to work with variable output, you could have a lots of issues with stability, regulation, parts overheating, etc. The same for PC ATX supplies.If you want a variable supply, just buy it. Getting stuff to blow up in one's face because some component was marginal already and didn't survive the changing operating conditions when you hacked it is not fun.

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 01:04:14 pm
content: Quote from: janoc on May 13, 2018, 12:52:26 pmI couldn't find a suitable variable PSU so I went the DIY route. I am following this guide which seems pretty reasonable to me:http://imajeenyus.com/electronics/20151028_smps_variable_voltage/index.shtml

author: madires
author_rank: Super Contributor
timestamp: May 13, 2018, 01:38:41 pm
content: I'd be careful with modifying such a SMPSU to support a variable voltage output. 20-30% lower or higher voltage should be possible without any issues, but more is asking for trouble with the regulation in many cases. Modifying one those extremely cheap industrial type SMPSUs is tempting and there are also tons of guides in the internet. Anyway, if you you need a cheap lab PSU then buy a cheap lab PSU. Much better than a hacked nasty surprise.

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 01:45:43 pm
content: Quote from: madires on May 13, 2018, 01:38:41 pmI already said that I have a linear bench supply. This mod would be for non-voltage sensitive semi-temporary uses. Things like powering led strips or electrolysis rust removal etc... That guide seems to have tested it reasonably well so unless there is something specifically wrong with that guide it seems safe enough. That's assuming the reference design with no missing components though...

author: madires
author_rank: Super Contributor
timestamp: May 13, 2018, 02:12:31 pm
content: A lot of assumptionsPlease ask some SMPSU greybeard to explain the issues (I'm ain't one).

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 02:19:34 pm
content: Quote from: madires on May 13, 2018, 02:12:31 pmThat's true but I think a good PSU would be safe (not necessarily reliable) even when modded this way. This one doesn't even have short-circuit protection so that's the more important question here. Hopefully someone with a great bushy beard comes along to school me on these issue

author: janoc
author_rank: Super Contributor
timestamp: May 13, 2018, 06:00:33 pm
content: Quote from: dominicM on May 13, 2018, 02:19:34 pmEeeh ... Ever thought about what will happen when the control loop fails because you have pushed the supply beyond its original design limits? Keep in mind that there is mains voltage there, with a ton of current readily available from the outlet. Big bang would be the least bad situation ...You are making somevery dangerousassumptions there. Even good quality supplies are not designed nor guaranteed to be safe when operated outside of their spec.And given that the supply you are trying to modify is dodgy at least (it is right there, in the article you are using as a guide - that creepage distance "fix" is ... ouch!)

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 06:16:34 pm
content: Quote from: janoc on May 13, 2018, 06:00:33 pmWell the upper limit is only exceeded by 3V (12V to 15V) and from what I've read going lower should be safe enough. I mean components won't blow up due to lower voltage... As far as dodgy supplies, I bet most cheap supplies somewhat dodgy. How many hobbyists are going to pay 150€ vs 15€ for a PSU for a non-critical project... I've seen plenty of tear downs of seemingly respectable widely used hardware that have same creepage and other issues so I don't know how worried you can be when it's all around you anyways. I can't imagine for example those 8€ laptop replacement adapters are any better. In the end I don't really know, that's why I asked. In any case I won't be using this particular PSU, will try and get one that at least has all it's components.

author: bob225
author_rank: Frequent Contributor
timestamp: May 13, 2018, 06:43:00 pm
content: RS, Mouser And Farnell all stock meanwell be aware some models are country specific, What spec do you want ?

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 06:54:55 pm
content: Quote from: bob225 on May 13, 2018, 06:43:00 pm5V up to 200-400W max but 5-12V variable is appealing since I may not need the 5V after some time... RS or even Farnell is the best for my location as Mouser shipping was like 25€ last time.

author: bob225
author_rank: Frequent Contributor
timestamp: May 13, 2018, 07:08:14 pm
content: So around 33 amps at 12v or 8 amps @ 5v ? thats a fair bit of current

author: janoc
author_rank: Super Contributor
timestamp: May 13, 2018, 07:41:02 pm
content: Quote from: dominicM on May 13, 2018, 06:16:34 pmComponents won't blow up because of voltage but the controller IC may stop regulating properly or even start oscillating. If that happens, the supply will likely destroy itself and take out anything connected to it as well.Keep in mind that that 15€ PSU can become a much more expensive problem once it goes bang and you have a fire. 400W supply is no sneeze, that's a serious amount of energy thatwill blow shit up and set it on fireshould anything go wrong with it. You do get what you pay for here and given that you need a high power supply, trying to go cheap could seriously cost you. When it comes to mains that is a very bad place to be cheap.I don't know how about you but I don't have any 8€ adapters anywhere (and not buying them neither). I value my life and my property more than that.

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 09:44:47 pm
content: Quote from: janoc on May 13, 2018, 07:41:02 pmWouldn't PSU just turn off if the voltage is too low resulting in a on-off loop? That's what I learned with a different PSU I tried to mod. I asked in another thread and there was no suggestion of any danger. Perhaps you can elaborate on exact scenario etc...Most people aren't on EEVBlog forum so that no indicator of what people use. Phone chargers and such can be cheap crap too even if it's genuine. Even genuine adapters would have issues if you tear it down. It's could be made in the same factory, it's just that chances of fault are lower due to some oversight and QC. Again I don't really know, but it's hard to take someones advice when you don't know the reasons behind that advice that's why I asked.

author: dominicM
author_rank: Regular Contributor
timestamp: May 13, 2018, 09:48:28 pm
content: Quote from: bob225 on May 13, 2018, 07:08:14 pmIt is. It's the max draw possible though, I probably I will only use it at below 100W but would be nice not to be limited or accidentally draw more than PSU can handle. Best I can find on RS is 65€ for 200W Meanwell PSU. Pretty expensive considering PC PSU prices.

author: abraxa
author_rank: Frequent Contributor
timestamp: May 14, 2018, 05:18:16 am
content: It's all about risk management. If you know what you're doing, you can judge whether what you're planning on doing has a risk of causing major damage or not. Comparing that to the risk of a potential worst-case outcome of said damage (burnt-out apartment that you'll have to pay out of your own pocket, assuming no one dies in the fire) allows you to then judge whether it's worth saving $100 and performing the modification or whether the potential for a serious cost penalty is too high and not worth the gamble. Even if you're insured against accidental damage/fire/loss of life, the insurer will be more than happy to hand you the bills once they discover that a PSU that you modified is the origin of the problem.Since you don't even know what properties the components used in the power supply possess and already being well-aware that the manufacturer of this power supply is cutting cost in scary ways, you're unable to make a proper assessment of how likely the device is to cause major damage. That's the point when you should be jumping ship.Quote from: dominicM on May 13, 2018, 09:44:47 pmLow duty cycles at high filter cap discharge rates (i.e. high output current) cause high charge current spikes. What happens if the output filter caps aren't up to the challenge and fail short-circuit? I think you know what a power supply without short circuit protection is going to do then.

author: janoc
author_rank: Super Contributor
timestamp: May 14, 2018, 09:29:06 am
content: Quote from: dominicM on May 13, 2018, 09:44:47 pmThe problem is not low voltage! What I am talking about is a modification of the feedback control loop - the part that actually tells the IC controlling the supply whether to increase (output voltage too low) or decrease (output voltage too high) the duty cycle of the PWM signal driving the switching transistor(s). This you need to modify if you want to change the output voltage of such supply.This is a non-trivial matter because the response of this loop dictates a lot of the behavior of the supply - things like voltage regulation, handling changes in the load, even overload and short circuit protection could be tied into this. If the control loop is not designed well or you mess with it without knowing what you are doing, then you can easily make it unstable. And a 400W mains supply breaking into oscillation is a very bad news.Even when that doesn't happen, the loop may have some limits you aren't supposed to exceed because components may overload, transformer saturates, etc. This has been optimized for the best performance/efficiency at the fixed voltage the supply has been designed for. If you start messing with it, you are in an unknown territory relying on design margins that may or may not be there for things to not blow up on you.There are many things that could go wrong if you don't know how the supply was designed and what the margins are (if any, especially on a cheap supply where every penny that could have been pinched has been).Quote from: dominicM on May 13, 2018, 09:44:47 pmThat's complete BS. The factory QC is completely irrelevant here. Or you think that the missing components on your unit were a failure of the quality control on the factory floor and not a deliberate decision of the manufacturer to save a few bucks?What matters is that most of the genuine good quality supplies have the design and sample units actually tested by third party test labs - almost nobody who is serious about their business will dare to put a mains supply on the market that isn't UL or TuV certified, it would be a huge liability should anything happen. Even the fairly useless CE mark which doesn't actually require you to have stuff tested (you self-declare but good luck if something happens!) has requirements re safety.Look at the sticker of a good supply and you will certainly find the UL logo there, along with the ubiquitous CE. UL certified devices are also published online, so you can easily verify whether or not the logo on the device is actually genuine (unlike CE that anyone puts on without any testing):https://www.ul.com/ul-databases-and-directories/If someone buys cheap crap, it is their problem. Using people doing stupid things out of ignorance as an argument that this is somehow acceptable is strange when we are talking safety - there have been several reports just last year of people being electrocuted, of fires being started by these poor quality chargers. I can understand that lay people are ignorant of the issue but insisting on doing itdespite being told of the risksto save a bit of money is not understandable for me. However, if you want to join that grim statistics to save a few bucks, be my guest ...As they say - penny wise pound foolish?

author: dominicM
author_rank: Regular Contributor
timestamp: May 14, 2018, 10:00:50 am
content: I wan't referring to QC control for this PSU but rather in general. Point is that so many devices used everyday would be called unsafe on this forum, genuine or Chinese crap alike. Granted the PSU I got is worse than most but the point still stands. Certificates of any sort don't protect against manufacturers changing the design by omitting components like with this PSU.I don't understand the point of the last paragraph since I already said I will not be using this PSU. Enough doubt was cast so it's not worth the worry. I might still try to buy a reference design of this PSU for occasional use but for permanent use I will probably end up getting a Meanwell/Delta or a used industrial PSU perhaps. I was hoping to learn the reasons and probabilities so I have a good idea just how important these missing circuits are so I know how important it is and why as opposed to just repeating generic warnings.

author: janoc
author_rank: Super Contributor
timestamp: May 14, 2018, 10:32:24 am
content: Quote from: dominicM on May 14, 2018, 10:00:50 amThat would be fraud and such certificate would be instantly revoked once this has been discovered. With possibly a lawsuit following too - these test labs take their reputation seriously.If a manufacturer wants to change the design, they have to ask for recertification.Quote from: dominicM on May 14, 2018, 10:00:50 amI believe that you still wanted to modify a supply, even though a bit more reputable one?Also the problem with "occasional use" is that it still may go bang on you. The supply doesn't know you aren't planning to permanently install it somewhere (and it doesn't really make any difference). Just hopefully you will be around to shut it off should it happen and don't get electrocuted when it happens - as opposed to having it in inside of a project somewhere.Re reasons and probabilities - well, there is a lot of theory behind the design of these things, I don't think someone is going to be able to explain it here on the forum. There is just too much of it. So the warnings may sound "generic". If you want to deep dive, there is e.g. this document from Microchip:http://ww1.microchip.com/downloads/en/DeviceDoc/dsPICSMPS%20AC_DC%20Users%20Guide.pdfThe probabilities can nobody give you because a lot depends on how the supply in question is constructed - if e.g. the transformer is poorly made because the manufacturer skimped on insulation, the device could kill you. But you won't know that unless you check the construction of the transformer (and destroy it in the process).You can only make some semi-educated guesses about the quality when you see the missing components, the creepage and clearance, etc. Still better to err on the side of caution, though.

author: dominicM
author_rank: Regular Contributor
timestamp: May 15, 2018, 01:16:39 pm
content: Yes but it has to be discovered first. I would not have known about missing components if I hadn't opened it up. Manufacturers may even have warranty void stickers or similar as well discouraging discovery. Companies engage in fraud constantly (not specifically with certificates), and not the small guys either.When I say occasional I also meant attended. I think you may be exaggerating the dangers a bit. I mean it's in a metal case with grounding. The plug has a fuse. Sockets have an RCD and a main RCD too. As for fire, unless it's covered in newspapers (even then) there is very little to burn inside to cause surrounding areas to catch on fire especially with RCDs etc... And then there's also short-circuit protection on a decent reference design.The most I would hope is someone with experience with cheaper supplies like these to chime in on how they tend fail and what damage they cause from experience.

author: glarsson
author_rank: Frequent Contributor
timestamp: May 15, 2018, 01:29:11 pm
content: Some cheap power supplies fail by leaking high tension from the primary side to the secondary side through a poorly designed transformer (not enough insulation, improper construction, overload damage,...)You defend this trash power supply against all comments - why?

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 15, 2018, 01:33:00 pm
content: I have mentioned that previously I worked at a large PSU manufacturer.This is Deja Vu..............What I see here is something that we saw a couple of times. A Chinese hacker would take one of our designs, proceed to "Muntz it" and then start making replicas with the exact same board artwork minus a lot of components or with lower-rated components.Of course, those could be sold significantly cheaper.My two Yen on reliability on this particular PSU: Modify and test it.A simple andvery baresetup: Let it cook at full load, fully fuse-and ground fault protected, inside a metal box. The metal box will allow the temperature to rise (have a thermometer monitoring it so it does not rise that much) and catch any flying debris or flames.As long as you use power resistors as a load, nothing will happen to the load.If you have access to a Variac, apply at intervals full rated AC input voltage, followed by the minimum AC input voltage.Leave it on over the weekend. When you finish, if the unit is still working look around for signs of discoloration on resistors, brown markings on the board, or bulging electro caps.

author: dominicM
author_rank: Regular Contributor
timestamp: May 15, 2018, 01:44:05 pm
content: Quote from: glarsson on May 15, 2018, 01:29:11 pmI am not defending it. At most I am making a point that cheap stuff has it's place. If it is reasonably safe that is, hence the discussion. The problem with decent PSUs is that they seem to be unreasonably expensive (even compared to high end PC supplies) and harder to source at times.

author: dominicM
author_rank: Regular Contributor
timestamp: May 15, 2018, 03:09:39 pm
content: Quote from: schmitt trigger on May 15, 2018, 01:33:00 pmNot sure how useful that would be. I am already working under assumption that it is unreliable (with or without missing components). If it fails I can just replace it, it's not critical. Issue is with safety, including to a small degree components attached to it.I would never run these cheap PSUs at full load anyways, if I actually needed a 400W I would try to go with 800W PSU. It also has a fan so heat is probably not going to be an issue here.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 15, 2018, 06:11:15 pm
content: Hmmmmmmm.............Looking at your schematic, where is the bootstrap circuitry?For startup purposes.

author: dominicM
author_rank: Regular Contributor
timestamp: May 15, 2018, 06:28:01 pm
content: Quote from: schmitt trigger on May 15, 2018, 06:11:15 pmIt's not my schematic I just linked to it. It's also traced so irrelevant parts may have been left out. There is no better datasheet as far as I know.

author: Eka
author_rank: Regular Contributor
timestamp: May 16, 2018, 10:23:03 am
content: Quote from: dominicM on May 15, 2018, 01:44:05 pmThe main reason the good power supplies cost more is Non Refundable Engineering (NRE) costs. IE, the costs of designing them, compliance testing, and bringing them to market. Typically they are also produced in much lower volumes than the mass market power supplies like used in PCs. So those NREs need to be spread over fewer devices, and thus cost more per device.

author: glarsson
author_rank: Frequent Contributor
timestamp: May 16, 2018, 11:45:37 am
content: Quote from: Eka on May 16, 2018, 10:23:03 amNRE = Non Recurring Engineering

author: janoc
author_rank: Super Contributor
timestamp: May 16, 2018, 04:54:08 pm
content: Quote from: dominicM on May 15, 2018, 01:44:05 pmBut the problem is thatyou have no means to know it is reasonably safe !!!You are neither qualified to judge something like that nor a lot of this stuff can be just "eyeballed" but needs to be tested, sometimes destructively (e.g. transformer insulation). So the only way one can be somewhat safe is to rely on third party test labs doing their job.Deciding that something is "safe" because newspapers are not full of stories featuring dead people and burned down houses is ... insane, in my opinion. Especially when the "quality" (or rather lack of it) of these various cheap supplies is a well documented fact.QuoteRCDs will not do anything unless there is a short/leakage to ground. There are plenty of failure modes involving high currents (causing a fire) that won't trip neither a breaker nor an RCD.Also, grounded metal case, RCD, etc. will do zilch, nada against a short on the secondary side (e.g. because a crappy capacitor fails short). If the supply isn't protected properly (good luck with a cheapie), you most likely have a fire now because the supply is dumping all its current into that smouldering cap (or whatever remains of it).And re fire - you have obviously not seen the board and cables inside a supply to catch on fire yet and how fast that can spread. There is nothing to contain the flames inside and it will ignite anything flammable nearby - enclosure of the device, your desk, etc.If you want examples of what can happen with a poorly made power supply, just google "power supply fire". I have personally witnessed what kind of mess an ATX supply can make when it goes bang. We got very lucky that it has happened during the day and we have contained the fire right away. Don't want to think what would have been the result if it has happened overnight ... And that was a decent brand, not some low end 10 buck piece of crap.Feel free to think I am exaggerating. However, with this attitude to safety you are going to get hurt at some point.