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converting watts to milliamps

Discussion in 'Electronic Basics' started by Carl Farrington, Dec 13, 2003.

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  1. Hi there,

    Please could someone explain to me how to convert watts into milliamps? I
    want to compare the power usage characteristics of two devices (one is a
    2.5" hard disk, the other is an IDE Flash Disk) but one manufacturer lists
    usage in watts, the other in milliamps!

    thanks,

    Carl Farrington
     
  2. Amps = watts/volts
    divide the result by 1000 for mA
     
  3. thank you!
     
  4. Assuming we're talking DC here (the AC formulae are slightly
    different): Ohm's Law gives you everything you need.

    P = IE, where P is power in Watts, I is the current drain in Amps,
    and E is the voltage involved.

    Taking this a step further, we get I = P / E

    So: As an example, assume a 12V fan draws 4 watts of power. That's
    4 over 12, which gives a current drain of about 0.333A, or 333mA.

    We can double-check this by finding the resistance, and using
    Ohm's Law again: R = E / I. Since we now know I, we can find that
    R = 12 / 0.333 = about 36 ohms.

    Now that we know resistance, we can use the formula P=I-squared
    times R to find the power figure: P = .333 squared times 36 = 3.99
    watts. Pretty close, heh?

    You can also simply do P = IE, or .333 times 12 = 3.996.

    Hope that helps. ;-)


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  5. Ian Stirling

    Ian Stirling Guest

    You may want to look at the detailed datasheets, and work out just
    what your usage will be.
    Active, sleep, read/write, spinup, ...
     
  6. KP

    KP Guest

    another way is to go to www.pawprint.net and download 'Versaverter' saves
    the brain somewhat
     
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