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How to verify my ac-dc adapter is ok?

Discussion in 'Troubleshooting and Repair' started by khankll, Jun 8, 2012.

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  1. khankll


    Feb 6, 2011
    i have few ac-dc adapters.. those that take 220vac and gives between 1.5-12 volts a selection knob..

    they have been lying idle for almost a decade .. yesterday i took one and as soon as i plugged it in .. kaboom a spark ocure.. but luckily my cicuit breakers tripped ...
    i m 1000 % sure that these were ok when they were deserted ...
    now how do i make sure that others are ok.. i want to verify them before using them.. i opened one but there is no apparent damage inside or rust.. etc.. all parts seem new and ok.. few were working .. and i stopped checking the rest until i kaboommed this one..

    these are simple adapters .. it has a bridge rectifier a capacitor a resistor...
    here are the readings for the one that kaboomed..

    all the diodes were conducting one way with vf=490mV to 510 mV and were not conducting other way..
    resistor is 47 ohm and it came out as 47 ohm when tested..
    cap is looking ok.. no leak no buldg.. flat top..
    transformer primary has resistance of 949 ohms..
    i checked resistance between one primary winding and all secondary windings... none had any resistance .. i also checked for resistance by slecting second primary winding and the rest of the secondary widings no resistance... (resistance was checked at ranges of 20k ,2k,200 ohm) ..
    i checked the resistance on secondary side.. there is resistance between 'any two' windings..
    i checked by placing one prope of multimeter on first (?) secondary winding and then progressed the other through the rest of secondary windings.. there was some resistance between all... r b/w first and second was 1 ohm.. b/w 1 and 3rd it was fluctuating b/w 1 and 2 ohms.. ...and so on.. the r b/w first and last wass 4 or 5 ohms..

    Last edited: Jun 8, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Chances are that after years of lying around the electrolytic capacitor(s) is defect. You don't necessarily see this. You can check by desoldering the cap and measuring it with a capacitance meter. Or simply re´place all electrolytic caps as a matter of precaution.

  3. khankll


    Feb 6, 2011
    from my above tests of the kaboomed adapter do u think other readings indicate those items are ok?
    i suspect the cap.. its nivhicon 680uF 25 volts.. its 12volts 500mA adapter .. so i know voltage rating of cap should be at min 25 volts .. but how high or low can i go on uF ratings ? i hve 2200 uF 50 volts cap at hand can i plug it in ?
    also after that kaboom this cap is still hysically looking perfect ..:confused: caps are cheap .. cap meters are hell expensive so i will throw the cap and replace new one instead of cjecking it and reusing it.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The 2200µ Cap should be no problem for a simple adapter. Go ahead, try it.


    When toying with a mains adapter take any appropriate care to avoid the risk of an electrical shock. I can not assume any responsibility for damages caused by your doing.
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    If the fault took out a circuit breaker, it won't be on the secondary side. Most likely to be insulation breakdown in the primary. The primary measures 949 ohms, which sounds fine, but when mains voltage is applied, insulation can break down. Look carefully at the primary wires and all components connected to them for signs of damage caused by the spark you saw.
    Another possibility is earth leakage, if the breaker that tripped was an earth leakage circuit breaker (ELCB) (also known as a residual current device, RCD). In that case there may be moisture on the PCB causing leakage to ground, if the adapter is connected to anything that's grounded or being held by a person.
    It would help if you could upload some photos of the adapter so we can see the construction.
  6. CocaCola


    Apr 7, 2012
    Just an FYI in the US those are generally called GFCI, GFI or ground fault breakers...
  7. khankll


    Feb 6, 2011
    ok i took another measurement.. there is some resistance between the core/lamination and windings ,, infact if i place one probe of multimeter on the core and another on any of the two wires are primary i get resistance.. similar is the case if i place one probe on the lamination and another probe on any of the secondary winding wire... i get resistance.. sounds like the lamination/insulation (?) on the windings wires have gone.. and are shoritng the core.. ?

    i dont think there should be resistance between a core and any of hte winding be it primary or secondary...

    can some one tell me how to test just the transformer..?
  8. khankll


    Feb 6, 2011
  9. duke37


    Jan 9, 2011
    You have tested the transformer and found two faults.
    There should be no connection betweent the primary and the core.
    There should be no connection between the secondary and the core.

    If the voltage has been high enough to break down the spool insulation, the windings are probably damaged also. Time to visit the scrap heap.
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