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Odd multimeter readings with AC->DC wall adapter

  1. Feb 12, 2015 #1

    Nugatory

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    Is there any reason why my basic Radio Shack multimeter might consistently read high when I use it to test the output voltage of an AC->DC wall adapter? I try it on a 4.5V adapter rated for a few hundred mA and it reads close to 10V... I try it on a 6V one and it reads around 12V... but then I try it on a known-good charged car battery and get the 12.6V I expect. (And no, I am not messing up the range setting :) as all three readings come off the 0-25 scale without touching the range setting).

    I am testing the adapters with no load. Is it possible that they rectify the output badly enough to confuse this multimeter?
     
  2. jcsd
  3. Feb 12, 2015 #2

    nsaspook

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    Bad transformer regulation (sometimes designed in as a safety measure) and bad power filtering makes for some pretty wide voltage swings from unloaded to fully loaded in the small non-switcher warts.
     
  4. Feb 12, 2015 #3

    berkeman

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    Use the AC setting to see if there is noise that is confusing the DC setting. Are these wall adapters just transformers + rectification, or are they switching bricks?
     
  5. Feb 12, 2015 #4

    berkeman

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    Also, can you re-check their output voltages when they are running into a nominal or heavy load?
     
  6. Feb 12, 2015 #5

    dlgoff

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    There is the
    thing, if you need good accuracy and linearity.
     
  7. Feb 12, 2015 #6

    Nugatory

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    Ha - yep, tens(!) of volts of AC indicated (and zero volts of AC on the car battery which is a simple no-nonsene dc source). Yes, this is transformer+rectifier (if "to rectify" means "to make right" there may be a false advertising claim in here somewhere :) ) so this is consistent with:
    I should, but I'm pretty sure that the mystery is resolved. I'm going to put this multimeter back to doing what it does well, which is diagnosing the 12V DC stuff in old cars. Thanks to all.

    The backstory here: Someone in my family had a device that wanted 6V DC and a random adapter that was labeled for 6V DC but didn't specify the polarity. I said I'd check the polarity, and did... multimeter does that just fine... but then I noticed that the indicated voltage was way over the 6V that I expected, which sent me off measuring other voltage sources and wondering what was going on.
     
  8. Feb 12, 2015 #7

    davenn

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    You never said and no one asked .... if the wall adaptors were regulated or not
    If they are not regulated, as many are not ( unless stated otherwise on their casing)
    The no load DC output is going to be quite a few volts higher than what is stated on the plugpack label
    and the figures you stated would be about correct give or take a couple of volts
    So no fault with your multimeter :)


    Dave
     
  9. Feb 13, 2015 #8

    jim hardy

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    Yea Dave !

    Transformers are rated for full load. So they'll make ~10% more when unloaded.

    Rectified DC with no filter capacitor will read the average of a sine wave, about 64% of peak.
    With a filter capacitor you'll read the peak.

    What i suspect you have is a rectified and unfiltered wall adapter. They are common for charging batteries.
    And an inexpensive digital meter that has a small averaging filter at its DC input. It's a low pass RC filter to give an average reading.
    A meter like that might well report the peak voltage of your unloaded wall adapter because its averaging filter , well, averages;
    BUT
    when the averaging capacitor can't discharge back into the source it'll act like a filter capacitor and capture the peak.
    Peak is 1/.64 X nominalDC = 1.56 X nominalDC . Add another 10% for unloaded vs loaded .
    So, 4.5Vnominal X 1.56 X 1.1 = 7.7V
    See if above arithmetic agrees with a more precise reading.

    You didn't say whether you meter is analog or digital.
    Myself i prefer inexpensive analog ones because i know how they work.
    A fellow needs to know what's inside his test equipment lest he get fooled by its limitations.

    For 99% of what i do I'd rather use this
    http://demandware.edgesuite.net/sits_pod26/dw/image/v2/AASR_PRD/on/demandware.static/Sites-radioshack-Site/Sites-master-catalog/default/v1423868431532/images/02200109_00.jpg?sw=350&sh=350&sm=fit

    than this
    http://demandware.edgesuite.net/sits_pod26/dw/image/v2/AASR_PRD/on/demandware.static/Sites-radioshack-Site/Sites-master-catalog/default/v1423868431532/images/02200813_00.jpg?sw=350&sh=350&sm=fit


    let us know what you find?

    old jim
     
  10. Feb 13, 2015 #9

    davenn

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    @Nugatory
    OK just for an example
    I just tested an unregulated AC-DC adaptor I have in the shack. Its rectified and has smoothing capacitor, just no regulator IC

    The pack output is labelled 12VDC @ 430mA
    its measured output ( No load (other than the meter)) is almost 17V (16.85)
    This is a standard sort of reading :)

    The general inference is that with a load drawing 430mA, its output voltage will be 12V
    and any variation in the load will change the current and therefore the output voltage

    cheers
    Dave
     
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