Switched Mode Power Supply Repair Guide


[Erich] spotted a broken DVD recorder at a local amateur radio meeting and decided to see if he could restore it to working order. While he was fortunate enough that someone labeled it as having a bad power supply, things aren’t always that easy. He gives a broad explanation as to how switched mode power supplies work as well as discusses some of the reasons these devices tend to fail. He identifies a few common components and areas that one should check while diagnosing a non-functioning power supply. While obvious bulging capacitors are easily identifiable, he discusses the need for an ESR meter and uses a kit-built model to test capacitors that do not have any visual signs of damage. While some of his walkthrough might be basic knowledge for readers who have experience in recapping circuit boards, it serves as a nice guide for those who are new to the world of electronics troubleshooting and repair.

46 thoughts on “Switched Mode Power Supply Repair Guide

  1. I discourage to work on a 130-220 volts power supply. This is very dangerous. You can die or hurt your self very much wich that. A very good friend tried to repair his HDD power supply and now he cant move his left arm because the heart stopped for a few minutes and some parts of his brain will never work as normal.

  2. If the power supply is broken, there is anything wrong inside of it. If it is maybe the condensator it can get more about one THOUSAND VOLTS!!! PLEASE BE WARNED. IF IT IS NOT ON ENERGY IT ALSO DID NOT LOST HIS ENERGY. AS USAL YOU HAVE TO WAIT FOR ABOUT 25 HOURS TO WORK ON IT!!!:

  3. There is an ESR meter called the Blue ESR Meter that is the successor model to the one shown in the article. Its available in either kit form or fully assembled. It does have a controller chip in it which is specially programmed for it.

    As far as the voltages are concerned. Simply discharging the capacitors with a resistor is usually enough. And there is far from a thousand volts in most consumer electronics, usually the highest voltages is on the input side of the power supply which is line voltage (110-240V in most cases). I’ve repaired the power supply in my DirecTIVO unit, LCD monitors, desktop motherboards, and desktop computer power supplies(and in one case with the power supply still hooked up to the computer but unplugged) mere minutes after unplugging and disassembling it, the circuitry is generally sufficient to drain the voltages of the power supply.

    Now for working with HV circuits like the flyback circuits of CRT televisions, yes special procautions need to be taken, but in your run of the mill switching power supply not as much, just look at the ratings on the capacitors being replaced usually under 25 volts. – Peter

  4. @echodella
    And you’re here with us :) Just to say that… if people follow every safety rules and f*king common sense everything will be fine.
    I’m very sorry for your friend, but… what was his mistake?!

  5. It would be neat if hack sites like this had some sort of “disclaimer” pages for articles like this. Not condescending disclaimers telling you not to do it though, but articles about proper safety procedures, how to do things safely.

    Like, any article dealing with HV electronics would have a link at the top, saying “Before you play with high voltage, here are some things you should know. Please read our HV FAQ”, or the same for laser hacks or the same with hacks involving pyrotechnics, etc.

  6. I’ve got a big 42″ TV that I’ve been wanting to repair. It’s got a bizarre problem where it works fine for about 1 hour, and then fails (fuzzy screen and frightening sound). I’ll try and pop it open next weekend and replace all of the caps in the power supply.

  7. He worked on a condensator, thought there is no power(he killed the multimeter LOL) and came in touch, it was to much voltage for his body. I know. IF U KNOW WHAT YOU DO AND FOLLOW SIMPLE SAFTEY RULES ITS NO PROBLEM. But I think there many noobs which can hurt themselfs very much.

  8. do like i do
    stick your tongue across all capacitors in the circuit(you may need to re-moisten your tongue as it tends to dry up after a few discharges through it)
    95% of the electrical energy will disipate through the conductive flesh in your tongue and not go through your arm OR heart in any way shape or form

  9. @Frogz LOL

    WTH is going on with all of those warnings about using mains? If your house follows the regulations, at least here in my country, healthy people won’t die or get electrocuted with 220V. YES it will be a nasty shock (I was zapped more than once) and people with heart problems might die *BUT* for some reason your house SHOULD have several fast acting differential and overcurrent relays which will open the circuit and spare your life.

    Hack a day had plenty of high voltage stuff including tesla coils, why the ranting now? O.o

    On topic: Thanks for the article, it was the first time I saw a defective capacitor due to heat. For sure it will help me next time I find a blown power supply.

  10. The blue ESR meter will cost you $80 in kit form, which is still pretty expensive for some of us (esp tightwads like myself). I found what looks like a decent ESR project here – http://www.members.shaw.ca/swstuff/esrmeter.html

    I got all the components for about £20 (including buying packs of resistors, diodes, caps, etc, way more than I need for this project), just need to find some time to put it together.

    1. this is a good esr meter for those guys who have a limited budget… if you can’t find a 50uA panel meter, you can use your analogue meter and set it to the DCmA (50uA)range and use this as your indicator.

  11. Usually the low voltage side caps go bad. If they are swollen or have vented, replace them. Use similar value and equal or higher voltage caps.

    I use capacitors stolen from PC power supplies, which are usually more conservatively rated than consumer electronic device capacitors.

  12. There is almost nothing that will keep 120v and 240v from killing you in your house. Maybe a gfci, but not every outlet has one. Everyone who says it’s not lethal is WRONG and should stop saying so. You’re just lucky.

  13. @mosheen
    “Maybe a gfci, but not every outlet has one.”

    YES, a gfci is what I was referring to as “fast acting differential relay/switch”. It works by measuring the differential current between ground and phase. In my country (and rest of Europe I believe) every house must be equipped with at least one, depending on the structure of the house and according to regulatory specifications.

    Here at home, I installed one just for my electronics desk. So.. for me to get electrocuted, the protective switch of my desk needs to fail, then the switch of my attic needs to fail and then the main switch also needs to fail – sounds improbable doesn’t it?

  14. @MrX
    Your GFI won’t protect you a single bit unless the current goes from Live through your body to Ground/Earth, then it will interrupt your power.

    If you hold the Live wire in one hand and the Neutral in the other the current will pass through your body just as any other lightbulb and will NOT disconnect.

    In a kitchen, bathroom or any other place here there’s water a GFI is definitely a good idea. But at a lab workbench you shouldn’t count on it to protect when working on modern stuff that most often doesn’t even have the ground wire connected (2-prong only).

    The GFI is not a magical bullet that will save you from harm – but it can save your ass in some situations. (So I have two installed at my workbench anyways…)

  15. @Mr. X
    You are one dumb f**k. How stupid must you be to imply that 220V wall current won’t kill you? Did you eat a lot of paint chips as a child?

    Tell me, what is the approximate resistance of the human body, from one fingertip to the other? Let’s assume your skin is not moist, but not dry. Now, how much current does it take to disrupt the electrical activity of the heart? OK, now, do ohms law assuming 220V from the wall, and the previously mentioned resistance of the human body. Compare it to the amount of current needed to disrupt your heart.

    Do you now understand how stupid you are?

    Also, FYI, the HEAT SOURCE on the board (where you’ll see the hot spots) is generally FROM the capacitor – As the ESR increases, the dissipates more and more heat (ZOMG OHMS LAW AGAIN!)… This could happen because of crappy electrolyte (CapXon, anyone?)… Another killer is ripple current being too high. That = more internal heat, and greatly decreased capacitor life.

    Sorry for the huge post, but geez man, you are freaking dumb. This post was warranted.

  16. @Mr x:
    The GFCI may save you , but you’re crazy if you rely on it.

    The breaker will NEVER, EVER trip if you’re on it. It is to protect property, not people. You will die waiting for it to trip.

  17. @Jake
    Please, first, learn something from
    and then reformulate your comment please. I don’t reply to lame posts.

    From wikipedia: RCDs are intended to operate within 25-40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.

    I have THREE fast acting differential switches between me and the mains, these will open the circuit in less than 6 to 10 milliseconds with differential currents over 30 mA. The circuit will be opened first by the switch with fastest acting time.

    The breaker (over-current protection) will trip when you are at it, but only after your are toasted with your body converted to carbon :p
    And no, I don’t solely rely on it. I just find stupid all the “OMGZZ!! 220V!” fuzz. To be honest it feels kind of strange that people that visit this site don’t even understand the basics of home’s electrical wirings.

  18. There are several steps to safely working on a supply, number one being a working knowledge of basic electronics. Two is always disconnect the unit from it’s power source, and discharge the unit before opening the case.

  19. Wattage kills. Voltage or amperage in isolation don’t tell the whole story.

    Anyway, I’ve worked on some HV stuff. Nothing to fear as long as you drain all the capacitors. You’ll have an idea of when it’s almost safe when you stop feeling the eddy current through 2+ inches of insulation. At least at that point it won’t fry your testing equipment as easily.

    It’s all a matter of how you use it. None of you would spray your own skin off with a pressure washer, would you? A little caution goes a long way, and these things generally behave predictably.

    Personally, I’m just glad to see a resource that describes methods for repairing PS modules.

  20. If you end up touching the live and neutral and are unlucky enough to not be touching an earthed surface the current will flow through your dead body until the power company switches it off due to unpaid bills. An RCD does not make you invincible.

  21. Common problem, I have repaired quite a few of them myself. And yes, of course it is dangerous, like anything else that involves mains work. But doesn’t mean I need to turn on my capslock to make a point.

  22. In EU we’ve mostly got 230V nominal voltage with +-10% tolerance, which means a typical offline SMPS can have 358VDC available before the PFC.

    Common sense is encouraged when working at these voltages.

    Or just lick the caps.

  23. Sorry, but I generally don’t approve of promoting the fear of electricity, and I think readers and visitors of HaD should be smart enough to know their way around it – or leave it be. I got a lot of people into tinkering with electronics by now, all of which were turned off before because everybody told them “don’t!”. True, SMPS are high-class electronics, but while it is wise to discuss the facts and hazards, it is not to talk about how many times something can potentially kill you in how many ways all over again, as all speculations depend on standard environment conditions which are almost never met.

    Ignoring that, it might be a good move to consult Sam Goldwassers page for background information before repair, it becomes very useful as soon as the culprit is not the typical output cap – or if you are designing your own smps.
    Also, you should discharge ALL parts of the device before doing any work on it as wide-range supplies use a mains voltage doubler for 110V to achieve independence of supply voltage, charging up a BIG cap to about 300V – so keep all unneeded fingers in your pockets, and think first, then measure ;-) Neither a circuit breaker nor any RCD will protect you from stored electrical charge in capacitors!

  24. First – this is HAD not Crochet-A-Day. Some risk is to be expected when working with any hack. Mechanical or Electrical.

    Second – There are a bajillion (okay maybe only a million) articles out there on how to be safe when working with both high voltage and high current. Please post links to such instead of OMGWTFBBQHIGHVOLTAGESTAYAWAY!

    Some links:





    Third – Calm the hell down people. Even if you’re concerned that people might get hurt the fact that you’re flaming about it is only going to make them ignore your warnings.

  25. Its called electricity, and its dangerous….
    Just like rocketry, Antarctic expeditions, flight, tower climbing, scuba diving, engine repair, skiing, going through the car wash and every other thing worth doing.

    Reasonable people adapt themselves to the world. Unreasonable people attempt to adapt the world to themselves. All progress, therefore, depends on unreasonable people.


  26. Getting back on topic… thanks HAD and Erich for the article, I filed away a bookmark for when I have the time to thoroughly read through the whole article. This is great as it was already on my list of things to do to understand switching power supplies better as the old school transformer types are super simple to fix.

  27. By the way, anyone that reads the article will notice that Erich has a safety disclaimer and mentions safety tips and precautions all through the article. I am not sure if he edited the article since the first post but the safety issue seems moot.

  28. Calm down.

    We come here for hacking information.

    Not to be told that things are too dangerous to be done.

    I was tickled to see there was over 30 comments on this article because I wanted to know more about testing capacitors in circuit. Instead I get preached at instead of educated to.

  29. Thanks for the feedback.

    The disclaimers were in there from the beginning… oh well… so it goes when trying to spread useful information to reduce landfill…

    If replacing capacitors, be sure to get the higher temperature, 105 degree rated, low ESR capacitors. some cheaper brands use lower temperature rated capacitors to save a few cents.

    Sometimes, the caps next to big heatsinks that look like they get hot are worth replacing too.

    If you don’t have an ESR meter, just replace the ones that look like they get the most heat to begin with, or just do them all.

    This is one of the commonest modes of failure and is easy and cheap to fix. It is harder to troubleshoot blown semiconductors.



  30. @Mr.X

    Good GOD, man. There is no help for you. I know there is a lot of idiocy in the world, but yours is a VERY special flavor!

    YES, GFCI’s *can* save lives, but if you’re depending on it to save yours, and *expect* it to work, and *expect* current that is electrocuting you to go directly to ground and cause the GFCI to trip, then you are truly a very stupid person.

    Good luck with that.

  31. A nice thing about blown semiconductors is that they short-circuit much more often than open-circuit on failure, if it is FETs or diodes. Check the resistance across D-S or C-A respectively, in-circuit works fine most of the time for the primary switching FETs. The diodes (commonly TO220 dual) should be desoldered first since the secondary winding of the transformer has very little resistance close to a short and can therefore screw up results. The secondary rectifiers are always worth a check if the output caps don’t fix the problem as they get stressed more if there is no “buffer” behind.
    Failed FETs just have a nasty habit of taking a lot more circuitry down with them if the input of the supply isn’t properly limited or fused. They are pretty easy to spot then, though ^^

  32. I’ve got a tooth abscess today, I read a nice article, to take my mind off of my pain, then read the comments. You’re anger at MR.X made my pain worse. Stop “F”ing and getting all mad like, let the bugger do what he thinks is correct, and talk about the article. What was written was a waste of Bandwidth and time. You could just discuss the article.
    Whats worse?, these bloody Amoxillin pills, ain’t working!

  33. In case anyone reads to the bottom here, DO NOT short the caps to discharge the stored energy. Ignoring all the rest of what is happening in these comments section, jump on Youtube and check out some videos of people vapourising screwdrivers by discharging caps this way.

    My preferred method is a resistor glued to a paddlepop stick. Doesn’t need to be big, and I don’t care if it heats up beyond it’s power capability. The key part is to limit the *power* being discharged over time so as to prevent sparks which will scare the crap out of you the first time it happens.

    Remember kids electrically isolate, then release stored energy in a controlled fashion.

  34. Hi, I have a radio here with a similar problem.
    Changed out the “bad” 1000uF capacitor but still no go, its tripping about once every 5 seconds and all the rails go back up to their full voltage momentarily,

    Its using a very strange 5 pin regulator on the secondary side, one of the circuit protectors had failed but replacing that didn’t help.
    Anyone ever run into something like this?

    Obviously all the capacitors are getting ESR tested as this unit has S*m*on (capxon clones) which are likely to be failure prone.

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