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power supply voltage sagging under load

Discussion in 'Troubleshooting and Repair' started by UnionTech, Nov 29, 2012.

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  1. UnionTech


    Mar 5, 2012
    Hi guys quick question. I have an old computer power supply I"m converting to a benchtop power supply. It was broke when I received it and after testing it found the 5 volt rail sags dramatically when I apply a load. The 5V rail is rated for 30amps and i draw about .5 amps and it sags to around 3 volts which is being sensed by the power supply so it shuts down. So my question is what component should I be looking at for repair. I thought the electrolytics but those looked good in circuit when checking with cap wizard. Will just replace them anyways if you guys agree its caps.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    *wow* that's an expensive ESR meter. I hope you didn't just go out and buy it for this project.

    It also looks fairly useless in some respects.

    One important issue is that a "good" ESR is not a constant figure, it depends on the voltage rating, capacitance, and type of capacitor.

    For example, the capacitors in a switchmode power supply are generally relatively high value, low-ESR types that have a nominal ESR that is very low.

    That meter suggests a reading under 1 ohm is good and one over 10 ohms is bad. For these power supply capacitors, a reading up near 1 ohm would have me replacing the capacitor immediately.

    Here is a page discussing a number of ESR meters. Note that all of them have some sort of guide to ESR values rather than a static scale.

    Whilst your meter may not be able to read very low ESRs, I would still recommend that you print out one of those charts and stick it on your meter.

    Even with that guide, you still have to realise that capacitors may be designed to have a certain ESR. Lower ESR capacitors are generally more expensive, and when they're used it is for a reason. Thus they will fail to operate at ESR values which may still be considered good for other capacitors of the same voltage and capacitance.

    If possible, it's always good to refer to the datasheet for the capacitor (or failing that, compare the capacitance to a known good one).

    In any case, for switchmode power supplies (especially those capable of tens of amps) an ESR of anywhere near 1 ohm is going to be a bad thing.
  3. UnionTech


    Mar 5, 2012
    Sounds like you agree its probley the caps. Ill keep in mind what you said about ESR on SMPS. The output caps were 16v 2200uF will pull data sheet and see what ESR should be.

    Didn't buy cap wizard I work in the instrument shop of a power plant so lots of nice fancy tools laying around. Unfortantly lot of the guys who knew component level repair have retired or just dont do it anymore so when I have a question I lean to you guys here ;-)

    Thanks for the help Steve.
  4. Jotto


    Aug 24, 2012
    The Capacitor Wizard only tests ESR, not a great test but still a decent tester. Price wise I think that the Capacitor Wizard and the Cap Analyzer is a good tester to have if your testing electrolytic capacitors.

    Just because the the ESR says its good doesn't mean that the capacitance is correct. The best tester is a LC103, this unit actually puts the cap under load. I will also say that it isn't 100% correct, but you get used to the ones that just don't read correctly. None of the testers is 100%.

    Example is today testing a power supply, testing two sizes of capacitor [email protected], [email protected] Cap Analyzer said they were good, all 7 caps were domed. Now I put it on the LC103.

    [email protected] = 423mf, esr 6.1ohms, absorption 100%, leakage 3.93ma 5.07k ohms
    [email protected] = 154.2mf, esr 8.4 ohms, absorption 47%, leakage 1057ma, 39.5kohms

    Just because a capacitor looks good, doesn't mean it is good. Power supplies generate a lot of heat and don't always dome the capacitors.

    If you have a DDM that can read capacitance you should be able to test them, but this doesn't put them under a load so it will read high, but low enough to show you which ones to replace. Also if you wish to get rid of the unit I will offer 100.00 for it.
    Last edited: Nov 30, 2012
  5. shrtrnd


    Jan 15, 2010
    For Trivia, I use a Sencor Z-Meter to tests caps. It's darned good for that.
    Sounds like there's agreement about the caps being suspect.
    I'd also check the semiconductors in the cap ckt (Voltage Regulators and Power
    Output Transistors).
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