Another power supply 13.8V 20A - Page 1
Forum section: electronics / repair

author: Northy
author_rank: Regular Contributor
timestamp: October 06, 2020, 08:24:26 pm
content: I found this one in the bin at an RC car racing meeting, I thought I could maybe use the case for something atleast.Anyway, I'd like to have a go at fixing it, but this will be my first mains PSU repair.Could someone point me in the right direction on where to start?How do these things work?Thanks,G

author: capt bullshot
author_rank: Super Contributor
timestamp: October 06, 2020, 09:49:07 pm
content: It's a standard crappy switcher.Do you have any experience with measuring voltages at mains potential?The mains input is rectified, the two electrolytics on the right hand side are the primary bulk capacitors.At first, I'd ask you plug it in, turn it on and measure the voltage across these - it's either near zero or some 320V (in a 230V country) or 160V (in a 110V country). If you don't know what I mean, stop here - it's just dangerous. If it's zero, check the fuse, rectifier diodes and other parts on the way from mains input to the capacitors.Next stage is a (most probably) MOSFET half bridge and the high frequency transformer (the large part with yellow tape in the middle). The MOSFETs chop the rectified mains to some 10kHz, this goes through the transfomer.The primary MOSFETs are mounted at the right hand heat sink. One could check them with a DMM in diode mode, but I suspect they're OK, because they often fail shorted and let the smoke out. This would be visible here.Mounted on the other heatsink are rectifier diodes, these rectify the high frequency into the output DC voltage. The toroid is the main storage inductor for "smoothing" the output.The IC (a standard TL494) is the control circuitry and regulates the output voltage. It's located on the secondary side, so this circuit must have some self oscillating capabilities to enable start up.You most probably won't find a fault here without a scope, as you'd have to watch if the control circuitry tries to start up.Did I mention it's quite dangerous to work with DMM and scope on a mains powered circuit? I you're unsure and don't know what I'm speaking of, better stop here.Some other method would be to test the components in a "safe" (non-powered) state and try to find a defective component. Check the discrete semiconductors, the electrolytics and the red film capacitor. Transformers and inductor typically don't fail here. Maybe you'll find an open resistor or a bad solder joint. Fix what you found, and carefully test the unit. It might explode on you.If you google for "PC silver box" schematics of the older types, you'll find diagrams of power supplies that work in a similar way.TBH, I'd just bin the innards and re-use the case. It's a crappy supply.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: October 06, 2020, 11:35:43 pm
content: HiI totally agree with Capt Bullshot, bin it before it kills you. I can see no sign of any primary (high voltage) / secondary (low voltage) insulation barrier, indeed the two appear to intermingle. NEVER NEVER EVER use a scope around a switch mode power supply unless you want to destroy the scope and possibly blind your self with molten copper, there are up to 1,000V pluses present.Keep SafeGeorge G6HIG Dover UK

author: floobydust
author_rank: Super Contributor
timestamp: October 07, 2020, 04:48:23 am
content: Where is the FUSE ?

author: ChuckDarwin
author_rank: Regular Contributor
timestamp: October 07, 2020, 05:09:58 am
content: That is some dodgy assembly. I like the inductor riding on top of the folded over cap. Salvage what you can and tip it, this is not a safe assembly.If you are going have a go at getting it working, fix a few things that aren't going to be the primary problem as well: C3 looks like it may have failed (second photo, bottom). Lots of components have excessive lead length and may be in contact with other components. Look at the little cluster of resistors just left of T1 under the cable sleeve in photo 1, for example. Don't use it after you fix it.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 09:14:33 am
content: The fuse could be a socket on the rear panel ??If you want to probe in a psu like this with a scope, you must use an isolation transformer on the scopeYep c3 seems to have failedI dont see this psu as a gimmick its an classic TL494 based design, until you have full schematics i would not say its a junk, but i dont like the toroidal coil floating around and over a capacitor, yes i call that dodgyAnd you have coupling transformers between the low power side and the main side, totally safe up to a point.And yes you have to know how to work in a psu like this, if its a 120vac unit the main dc voltage could be 200vdc and up to 350volts dc, they use a trick to double the ac to dc rectification and create a virtual ground between the 2 main capacitors, see the 2 blue resistors near them, if one of them pops, the psu could not work well.R27 seems to have heated a lot or faded with timei would change it and use longer leads to lift it away from the pcbAny branding on the casethe numbers on the pcb doesn't give anything on google

author: janoc
author_rank: Super Contributor
timestamp: October 07, 2020, 09:20:39 am
content: Quote from: coromonadalix on October 07, 2020, 09:14:33 amGood way to kill yourself, especially a newbie who doesn't really know what they are doing.Get a proper high voltage differential probe instead of isolating your scope and potentially getting lethal voltages on the exposed scope terminals.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 07, 2020, 11:33:07 am
content: ah i'm still alive loll doing this now and then if you know how to do thatAnyways critisizing is not constructive to help the OP with his psuThe OP must be aware that danger is lurking in the psu and any wrong doing could be hazardousOnce the warnings are done, what can we do to helpjust trash it ? if OP want to repair it ? you have many results on the web involving tl494 psu designs / schematics

author: Northy
author_rank: Regular Contributor
timestamp: October 07, 2020, 01:17:14 pm
content: Thanks for all the feedback and help so far!A bit of background about me. I've been an electronic engineer for 19 years, but pretty much always hand held or low power stuff. I'm just starting to potentially move into the world of doing a bit of design that involves mains voltages on PCB's so that's why I'm starting to look at the rules/regulations.One of the ideas I had was to have a look at this broken PSU that I've had in the garage for a few years! I've also got a bench lab PSU in there somewhere that needs repairing when I find it.It's interesting that people are less than impressed by the design/construction. I'm no expert in mains PSU design obviously, but it's didn't look 'nice' to me when I took the lid off. However, I think if you cut open most China wall warts they'd have similar types of construction wouldn't they?Interesting also because a few years ago this was a pretty popular PSU in the RC racing world (probably only UK). These days most people seem to use a converted server power supply. I've also got a box of those I rescued out of a skip!I don't have a scope at home, but I plan to change that soon.Thanks all.G

author: janoc
author_rank: Super Contributor
timestamp: October 07, 2020, 07:30:30 pm
content: Quote from: coromonadalix on October 07, 2020, 11:33:07 amI think I have said pretty clearly what is the correct way of doing things there. Or maybe I understand the idea of constructive criticism wrong?The risks of isolating the scope and why is that a bad idea have been discussed on this forum in the past, including a case of an engineer who got hurt doing that, despite taking a lot of precautions.See e.g.:https://www.eevblog.com/forum/projects/floating-scopedut-how-to-explain-to-my-boss-that-this-is-wrong/https://www.eevblog.com/forum/testgear/floating-scopes/If you want to use an isolation transformer, use it on the DUT, not the scope. If you don't have a differential probe, use two channels of the scope and a math function to subtract them. Then you can probe the circuit without worrying about frying your scope or shorting something out.Quote from: coromonadalix on October 07, 2020, 11:33:07 amThat's not in dispute. But explicitly giving dangerous advice to someone who probably doesn't know any better, like floating a scope, is a different matter.Quote from: coromonadalix on October 07, 2020, 11:33:07 amGiven the "quality" of the construction of that supply, that would be trash for me. Having a schematic for something doesn't mean it is worth fixing - how can you be sure that the transformers have adequate insulation given the "quality" of the rest? There doesn't seem to be any low/high voltage barrier neither, as mentioned by others already. No way to fix that, regardless of any schematic.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 07, 2020, 08:06:39 pm
content: This (the component tester of an old Hameg scope) might be a way to debug the supply safely:https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg3266896/#msg3266896

author: floobydust
author_rank: Super Contributor
timestamp: October 07, 2020, 08:45:24 pm
content: In a perfect world you would have a scope, isolation transfomer, HV probes, schematic and a full understanding of how it works.You can do alot without even powering the unit up. Look for cold solder joints, test the usual suspect components. People do it all the time using just a multimeter.The power supply is model FG-114 by ChenghuiXing Electronics (Huizhou) Co., Ltd. Many components are crooked and not fully seated, so this is hand assembled in the gutters of guangdong. I would make sure components are not touching or shorting to each other.