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Using a multimeter to check AC adapters

Discussion in 'Electronic Basics' started by Mienie, Dec 27, 2003.

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  1. Mienie

    Mienie Guest

    I have a handy little 22-179 Radio Shack digital multimeter, or at
    least it would be handy if I knew more about using it. ;-) Just
    lost--I think--a bunch of AC adapters in a power surge. I've been
    checking them using the DC (AC for one) setting and it seems that the
    good ones measure at or above the rated voltage, and the bad ones just
    spew out a couple hundred millivolts, or in one case a 12 volt adapter
    read at about 1.5 volts. Am I correct in these assumptions or are
    there special cases I need to know about? TIA.
  2. tempus fugit

    tempus fugit Guest

    When you're measuring DC on an adaptor, a good one will usually show a
    higher than spec'd voltage (unless it's regulated).

    Any less (especially 90% less) and it's dead. You may be able to fix it, but
    something is definitely wrong with it.
  3. Sounds like you are doing it right.
  4. There is a fuse on the primary side that is there to protect against
    fire. It usually gets blown out by a surge on the AC line side. This
    fuse is sealed inside the case and often under the transformer winding
    cover. If you're into cracking the case open with hammer and
    screwdriver, and digging out the fuse and soldering another in, you
    might be able to fix it. But it's hard to find the tiny fuse, and
    sometimes the transformer itself gets damaged. So it may not work
    out. If you can find a substitute wall wart, then it's usually the
    quickest and cheapest way to go.

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  5. Mienie

    Mienie Guest

    Thanks. I doubt I'll crack any open. But I'm right that an AC adapter
    producing only a few millivolts or 1.5 volts instead of 12 would
    definitely be bad?

  6. Mienie

    Mienie Guest

    Thanks! Always gratifying to know that all my mental powers haven't
    fled completely.

  7. Mienie

    Mienie Guest


  8. Chan

    Chan Guest

    .... but he said he can measure a few millivolts. this would suggest in
    is not the fuse !?
  9. Mienie

    Mienie Guest

    I'll check the fuse, since I now know there might be one. However, I'm
    more concerned with whether I'm reading the thing correctly. I have
    virtually no experience with electronics or using a multimeter other
    thanc changing old vacuum tubes and spotting a blown cap or resistor
    from the external damage.


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