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Wall warts

Discussion in 'Troubleshooting and Repair' started by Somethingrandom, Jan 20, 2014.

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  1. Somethingrandom


    Jan 20, 2014
    Hi all. Had a wall wart ie, plug in power pack fail on my voip phone today. Tested with my meter, read 0v at the tip when plugged in, jiggled the cable around a bit just to be sure, got nothing. Decided to crack it open and found a small transformer, a bridge rectifier circuit a cap and a resistor. Plugged it into the mains power, set meter to ac and tested the secondary ( the coil connected to the bridge rectifier circuit and dc output), secondary coil read 0v. Unplugged wall wart, tested secondary continuity, heard the beep, so all good there. Tested continuity on the primary ( the side feeding into mains power) which tested as an open circuit. Decided primary winding was damaged and transformer was screwed.

    However... Just for the heck of it got out the meter and tested continuity on some working wall warts, and found that they also failed continuity testing on the primary ie, testing continuity directly from pin to pin. Also checked resistance on all settings which remained open on every setting. Is this normal? Is anyone able to explain why working wall warts fail continuity? I would expect to at least be able to read off some resistance if continuity fails...

    Oh this is my first post, Cheers guys!
  2. duke37


    Jan 9, 2011
    An open circuit primary is a common fault and could be due to a thermal fuse failing. I have never been able to repair these.

    The transformer is very small so the primary winding resistance will be quite high and the correct range on your meter must be used or you may get erroneous results. See if you can measure the value of a 1k resistance and a 10k resistance. I have a cheap digital meter which goes bananas when asked to measure the resistance of an inductor so I use my old analog meter.

    More recent wall warts use a switch mode control, not a mains transformer, these weigh less. These rectify the mains and charge a capacitor so there is no DC connection.
  3. Somethingrandom


    Jan 20, 2014
    Thanks Duke. Blown thermal fuse seems sounds likely.
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