How To Repair A Switch Mode Power Supply Without Blowing It Up!

Sometimes you can replace the faulty components in a switch mode PSU only to plug it in and watch your shiny new parts self destruct. I recently have this problem with a power supply that blew up even though it was connected to a current limiter (light bulb). Find out why this happens and how to successfully repair this type of fault
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Thank you
Richard

[@sivoltage]

Really enjoying all your videos. I’ve leaned so much from them. Our lives are very similar. Im about the same age as you. We did similar things as kids. Playing out in old scrap yards in Leeds. Pulling televisions apart and messing about. We used to break open transformers and pull out the metal E shaped things. We called them E flyers and threw them about, they would glide for miles. 😂 Constantly getting told off by my old mam for taking everything apart. Where I lived in Hunslet in the 70s the council were pulling down loads of old back to back houses. There was no fences back then so we would clamber about in them and messing with old wooden fuse boards. I’m amazed I’m still alive. Would love to hear more tales of those days.

[@sivoltage]

Brilliant thank you, Looking forward to the next one.

[@Chief_Engineer]

There is a universally accepted symbol for a light bulb - a circle with a diagonal cross inside. It doesn‘t have to look like a chef‘s hat (although I admire the artwork). 😂

[@grahambambrook313]

Hi Rich, Decades ago my old dad who was a 'sparky' for the SWS which became the Midland Electricity Board if you remember them. He showed be a way of using a lamp holder on flying clip leads to help diagnose this sort of problem and to this day, I keep a 15W or 25W, 240V pigmy bulb set up like this in my tool collection. Most of his job involved transmission @11kV and above so I guess he did not wave his trusty test lamp about too often at work 🤣 but it came in handy on domestic duties. I found through my teenage years that this can be a useful technique for finding automotive issues, too! Incidentally, for many years I could not figure out how I got a nasty 'belt' from my (12VDC) car horn on one occasion, having had one or two from ignition coils and the like it was a bit of a shock (pun intended)! Far more recently, through programs like yours and others, I discovered the mystery of back EMF as the horn operated when shorted to earth through a relay as there is quite high current through a horn circuit!! We live and learn, as 'tis said!🤯

[@francismannion7075]

Thank you for a very interesting lesson.

[@viniciusvbf22]

Amazing content! I was never able to confidently fix PSUs. Unless it was something pretty simple, like replacing capacitors, they've always shorted again just after powering them up. I've change every component in the PSU (but the transformer) once, and it still shorted again right after powering it up 😅🤷‍♂ To me, people like Haseeb are magicians.

[@Arachnoid_of_the_underverse]

Nice explanation thanks for that.

[@ralphj4012]

It will be interesting (probably, just to me) to see whether the lamp reacts faster than any dodgy mosfets and if it withstands the 320V DC (albeit for a very short period).

[@LearnElectronicsRepair]

I think this is something we can play with on the next live stream 😉

[@robtitheridge9708]

Takes me back to the days of the earley PACE set top boxes chopper transitor shorts change it and anything that looked burnt turn on and bang .thank god somone bought out a repaire kit .

[@justmc62]

Great as always Rich.. Looking to set up a permanent isolation transformer on my bench. Will appreciate your thoughts on the alternate solutions you spoke about ...Thanks

[@Bibekanandaroy-tn5jm]

Many many thanks sir ❤❤

[@Gonzi-ze1sk]

i learning a lot from you

[@LearnElectronicsRepair]

You're welcome

[@blindkeith]

looking forward to the next vid on this .

[@jkmac625]

I'd be interesting in watching a video about mains isolations transformers. Most of the ones I've found online seem to be 240 to 120 step down transformers rather than 240 in 240 out. Thanks.

[@c1q3]

thanks for the video.

[@andymouse]

Maybe bring it up on a Variac and look for hotspots or find the short on the board, But it will be interesting to see the bulb inserted in like this as I haven't seen it before ! I look forward to part 2.....cheers.

[@fey4949]

It would be nice if you show us the all of the different methodes as you suggested. We could see the pros and cons. Thanks Ritchie.

[@LearnElectronicsRepair]

Part two mostly recorded 😉

[@amirfathirad2965]

easiest way to add a light bulb to the mosfet side of the circuit is to use the capacitor leg, take out the electrolit capacitor and use wires to attach it to the board and light bulb etc, just you have to be careful with soldering electrolite capacitors, they are a little bitexplody when getting warm from soldering iron

[@Dutch_off_grid_homesteading]

Heya, love the trouble shooting thought. as with a accident I sorted my Mppsolar inverter the MPPT pcb probbebly gone try to repair it my self already openend the inverter (wouw what a lot of wires in there took foto so I later can see hohw everything is connected thinking of filming it ut don't know jet ( don't want to look as a fool if I can't repair it) also looking at youtubers who already did some repair work on these imverters

[@Foobar_The_Fat_Penguin]

26:56: Instead of cutting traces to insert the light bulb, couldn't you also desolder one of the big filter caps and reattach it with a pair of wires and then a bulb in series? If the "blow-up current" comes from the caps, it should suffice to limit their output, no? You'd probably still need the limiter on the input for other faults (e.g. the bridge rectifier suddenly going short) but you don't have to damage the PCB or bend mosfet legs or stuff like that.

[@TallyTechandTroubleshooting]

Fantastic! Is there a "current limiting lightbulb" explanation somewhere on your channel. Very interested in putting one together. Thank you for your patience, Richard and thank you for your channel!!

[@georgeprout42]

Dim light bulb tester is probably the term you need to search. A 100W old school tungsten lamp, ohms law takes ~0.4A @240V. If the unit under test is internally shorted then the lamp turns on at full brightness and the unit is limited to ~400mA. But the voltage drop is all on the lamp. The result is no bang, no damage. If it lights up dimly, the test passes and you can plug it in directly as you know it's not shorted. A brief flash can be ignored, inrush current will do that.

[@LearnElectronicsRepair]

Yeah I made at least one video about that kzbin.info/www/bejne/imO4f4mliZmaeas The one I use mostly is 65W halogen

[@TallyTechandTroubleshooting]

@@georgeprout42 Thank you!!

[@TallyTechandTroubleshooting]

@@LearnElectronicsRepair Thank you, Richard!!!

[@nasosst3092]

Thanks again Rick. But are we positive that the aliexpress replace components are good for the job?

[@LearnElectronicsRepair]

No. That is one possibility here

[@SpinStar1956]

Hi there, could I ask your source for the nice graph-paper? I’ve been looking for some with smaller graduated squares like yours. I looked on Amazon but couldn’t match yours Richard. I use it for schematics and PCB initial layout. Thanks & 73! 😊

[@ianhaylock7409]

Could also use a variac to reduce the input voltage.

[@davet3804]

To limit current ..Could you not switch big caps for low value 200v caps to check unit is running with no load ??

[@stevewebb1941]

Is the power supply going through the isolation transformer and oscilloscope

[@LearnElectronicsRepair]

Only connect the DUT to the isolation transformer, you can then connect your grounded test equipment (one at a time if you connect them to different ground points or all together if you use a common point) to anywhere you like

[@talkintech9363]

I bought a brand new 500W PSU, powers up ok but when plugged into the motherboard it doesnt power up, mother board is ok as when the old psu is plugged in it powers up normally. What should I be lookung for to repair it?

[@ciobanurivelino3844]

It's normal to you, to know that trick. As is an Old School trick like you and me. Regards!

[@Fizeeek]

Hello! Please help... I have a power board from my Amana Mytag fridge freezer, and I am trying to troubleshoot it. I have identified and replaced several bad, electrolytic capacitors, and a blown overload relay, which I have replaced. After testing the windings of the compressor and the cooling fan (all good), I installed the repaired power board and turned on the fridge. it ran for about 15 minutes perfectly. I turned it off to do something but it refused to run again when I turned it back on. I took the power board out again for further diagnostics and found that one of the inductors is reading 65 ohm, with no continuity beep. I suspect a problem here, but I am uncertain. I have removed the inductor from the board but the markings are insufficient to identify it's type and specifications. My question is, am I barking up the wrong tree? The only markings on the inductor are 333J 06339 and VEI on the side. It measures 8mm dia x 10.5mm high and had a lead spacing of 5.0mm with 0.6mm dia leads. The inductor is heat shrinked black. Please help, I'm going nuts without a fridge and since this one cost over 2 grand I would rather save this fantastic fridge than bin it and buy a new piece of crap from the far east, that's only going to last a few years!

[@Pozitiv_ion]

but why should we insert the bulb into the mosfet circuit, to see that they are burnt... isn't it easier to check them and all the connections with the surrounding resistors and so on?

[@LearnElectronicsRepair]

You are missing the point, in the previous video I replaced the shorted mosfets checked all the surrounding resistors and so on. It powered up for a few seconds then the replacement mosfets went short. So we need a method to diagnose the problem in this sort of scenario. Previous video on this repair : kzbin.info/www/bejne/a4mvpmyOmayjpZo

[@Pozitiv_ion]

@@LearnElectronicsRepair sorry, I haven't seen the previous episode. I'll look right now...

[@jimgross5]

Just my opinion but if I am repairing something for a paying customer I will never trust mosfets purchased from aliexpress or ebay go with a reputable seller like Mouser and DigiKey. Yea you pay more but they are not fake.