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PWM Power

  1. Feb 24, 2009 #1
    Hi
    I have a question to do with using PWM
    If for example I had a 1ohm load and a voltage supply of 4JC-1, I would get a power draw of 16JS-1, now if I had a duty cycle of say 50% I should get an average power output of 8JS-1. However if i thought of it as having an average current of 2CS-1 and a average voltage of 2JC-1 then the power output comes out to be 4JS-1, which is 4 times less and not half. If it should be the half what does the actual voltage and current average out to?

    Thank you very much.
     
  2. jcsd
  3. Feb 24, 2009 #2

    berkeman

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    Staff: Mentor

    (4V)^2/1 Ohm = 16W continuous = 4A * 4V

    50% duty cycle

    8W = 4A * 4V * 0.5

    You can't average both the current and voltage, without taking an RMS average. RMS averages give you the right answer -- can you see why?
     
  4. Feb 24, 2009 #3
    I can see that it does seem work. However I thought that RMS using route 2 was for sine waves? Am i missing something?

    Thank you
     
  5. Feb 24, 2009 #4

    berkeman

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    Staff: Mentor

    No, it's used quite generally for power calculations. You can calculate the RMS value of an arbitrary function:

    http://en.wikipedia.org/wiki/Root_mean_square

    .
     
  6. Feb 24, 2009 #5

    Averagesupernova

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    Gold Member

    Be sure to keep in mind that .707 * Vpeak is the RMS equivalent for sine waves only. But real actual RMS doesn't care what the waveform is.
     
  7. Feb 25, 2009 #6
    Thank you both very much for your help. I understand it alot better now.
     
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