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Cheap chineese power supply missing short-circuit protection
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janoc:

--- Quote from: dominicM on May 13, 2018, 09:44:47 pm ---Wouldn'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...

--- End quote ---

The 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 pm ---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.

--- End quote ---

That'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?  :palm:

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 it despite being told of the risks to 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?
dominicM:
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.
janoc:

--- Quote from: dominicM on May 14, 2018, 10:00:50 am ---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.

--- End quote ---

That 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 am ---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.

--- End quote ---

I 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.pdf

The 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.
dominicM:
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.
glarsson:
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?
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