Measuring PC power with clamp meter

Discussion in 'Electronic Equipment' started by Robert Downes, Mar 3, 2004.

  1. Out of curiosity (and so I can tell whether the bigger power supplies
    are actually necessary next time I built a machine) I would like to
    measure how much power my PC is currently using, e.g. during 3D games
    that place demand on the 3D card, etc.

    I would also like a multimeter for general use (checking power supplies
    are still behaving, and so on).

    Can a cheap clamp meter do both? I'm under the impression that a clamp
    meter for active appliance measurments really needs to be "True RMS"
    (because of distortion introduced by noisy drives). But is it possible
    to measure power by measuring the voltage on the power cord, then
    measuring current at different times? Or would the voltage change
    significantly as the demand changed, making current-only calculations
    useless?

    Can a clamp meter perform the same tasks as a standard digital
    multimeter (assuming you have test leads with it)?

    Basically, is it possible to closely (within 10% of true power
    requirement) measure and calculate appliance power use with only a
    standard (well, True RMS) clamp meter, or would the full-whack,
    expensive "Clamp-on Power Meter" be required to do this?

    Thanks for any advice you can offer.
    --
    Bob
    London, UK
    echo Mail fefsensmrrjyaheeoceoq\! | tr "jefroq\!" "@obe.uk"
    Robert Downes, Mar 3, 2004
    #1
  2. "Robert Downes" <> wrote in message
    news:40453127$0$1157$...
    > Out of curiosity (and so I can tell whether the bigger power supplies
    > are actually necessary next time I built a machine) I would like to
    > measure how much power my PC is currently using, e.g. during 3D games
    > that place demand on the 3D card, etc.
    >
    > I would also like a multimeter for general use (checking power supplies
    > are still behaving, and so on).
    >
    > Can a cheap clamp meter do both?


    You're right that there are some issues with measured versus actual power
    consumption for switch-mode power supplies.

    But perhaps the biggest problem you'll run into is that the power rating of
    the supply has not much to do with how much power it can actually produce;
    and it also has not much to do with how much it draws from the power line.

    PC power supplies are rated in terms of DC output power: for instance, if a
    supply is able to produce 20 amps at 5 volts, plus 3 amps at 12 volts, plus
    1 amp at -12 volts, it would be rated at (20*5 + 3*12 + 1*12) = 148W. It
    might draw 190W from the power line, because it's not totally efficient.

    So, suppose you see your supply drawing 180W from the power line. Should
    you conclude you need a 150W supply? (I'm making these numbers up, by the
    way; real supplies these days are rated higher than that.) No, because
    maybe you're drawing 30 amps at 5v and nothing at all from the 12v supplies.
    (Unlikely, but not impossible.)

    The most accurate way to rate a PC power supply is to add up the amps of the
    various devices inside the PC - each hard drive, the motherboard, each CD,
    and so forth - and do the math. Short of simultaneously measuring the
    actual OUTPUT current of each power supply line, you're not going to get
    much better.

    The best way - not the most accurate - is to decide whether you've got an
    average, above-average, or below-average system, and buy an average,
    above-average, or below-average power supply accordingly. After all, 10
    million PC users can't be wrong.
    Walter Harley, Mar 3, 2004
    #2
  3. Robert Downes

    Sharpy Guest

    > I would also like a multimeter for general use (checking power supplies
    > are still behaving, and so on).


    Try the following link for a ATX POWER TESTER...
    http://www.dansdata.com/quickshot018.htm

    Sharpy

    ----------------------------
    "Robert Downes" <> wrote in message
    news:40453127$0$1157$...
    > Out of curiosity (and so I can tell whether the bigger power supplies
    > are actually necessary next time I built a machine) I would like to
    > measure how much power my PC is currently using, e.g. during 3D games
    > that place demand on the 3D card, etc.
    >
    > I would also like a multimeter for general use (checking power supplies
    > are still behaving, and so on).
    >
    > Can a cheap clamp meter do both? I'm under the impression that a clamp
    > meter for active appliance measurments really needs to be "True RMS"
    > (because of distortion introduced by noisy drives). But is it possible
    > to measure power by measuring the voltage on the power cord, then
    > measuring current at different times? Or would the voltage change
    > significantly as the demand changed, making current-only calculations
    > useless?
    >
    > Can a clamp meter perform the same tasks as a standard digital
    > multimeter (assuming you have test leads with it)?
    >
    > Basically, is it possible to closely (within 10% of true power
    > requirement) measure and calculate appliance power use with only a
    > standard (well, True RMS) clamp meter, or would the full-whack,
    > expensive "Clamp-on Power Meter" be required to do this?
    >
    > Thanks for any advice you can offer.
    > --
    > Bob
    > London, UK
    > echo Mail fefsensmrrjyaheeoceoq\! | tr "jefroq\!" "@obe.uk"
    Sharpy, Mar 3, 2004
    #3
  4. Walter Harley wrote:

    > The best way - not the most accurate - is to decide whether you've got an
    > average, above-average, or below-average system, and buy an average,
    > above-average, or below-average power supply accordingly. After all, 10
    > million PC users can't be wrong.


    Ah, damn. I'm already doing that. And I've no idea what capacity UPS
    (uninterrupted power supply) I need to put both my machines, and monitor
    on a battery backup line.
    --
    Bob
    London, UK
    echo Mail fefsensmrrjyaheeoceoq\! | tr "jefroq\!" "@obe.uk"
    Robert Downes, Mar 3, 2004
    #4

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