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What is the difference between these two formula of ripple voltage

  1. Jul 31, 2013 #1
    I found 2 equation for calculating the peak to peak ripple voltage,

    this one is the common one i see,

    Vripple p-p = I / 2fC


    And i also see this one in my book, but can't really find any other source use this formula,

    Vripple p-p = Vrect / fRLC

    Vrect is the peak voltage of unfiltered voltage
    f is frequency,
    C is capacitance
    RL is the load voltage


    What is the difference between these two formula? Are they really for counting the same parameter?
     
  2. jcsd
  3. Jul 31, 2013 #2

    sophiecentaur

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    That's Resistance, I think.
    The ripple is very much dependent upon the load resistance. If the load has infinite resistance then there will be no ripple at all because the capacitor will hold its charge from one positive supply peak to the next. The smaller the R, the faster the C will discharge and the time constant will be RC. The "I" in the first formula is a result of I = V/R, so both formulae are equivalent. They are approximations.
    See this link, as an example.

    The two formulae assume that the source resistance is low enough to ignore; the volts on the C at each peak need to 'follow the nominal sinusoidal input wave' for the simple approximation. If you use a cheapo transformer and over-load it, the output volts will never reach the open circuit maximum value. The volts can 'sag' because of the potential divider effect of the source and load resistances and you'll get less ripple and less DC.
     
  4. Jul 31, 2013 #3
    from the second equation,

    Vripple p-p = Vrect / fRLC

    = Vrect / fC x I/Vrect

    = I / fC

    but the first equation has 1/2
     
  5. Jul 31, 2013 #4

    sophiecentaur

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    I think the 1/2 factor must come from the peak to peak of the Voltage waveform and the average value of current (due to the average voltage). They are both approximations, in any case.
     
  6. Jul 31, 2013 #5
    Get it, thanks
     
  7. Aug 1, 2013 #6
    the 1/2 factor comes from the type of rectifer used in the circuit.
    For a full wave or bridge rectifier the capacitor charges every half cycle of the waveform, so the ripple frequency is 2f.
     
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