ripple voltage for capacitor filter

Discussion in 'Homework Help' started by vvkannan, Dec 17, 2008.

  1. vvkannan

    Thread Starter Active Member

    Aug 9, 2008
    138
    11
    Hello,
    My text says the peak to peak ripple voltage of a capacitor input filter is

    I/(f*c) (I -> dc load current ,f->ripple frequency ,C->capacitance)

    But it doesnt provide the exact derivation.How do we get it?

    Thank you
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Assume that the ripple voltage is a triangular wave and that it takes half the period (T/2) to fall from the upper peak to the lower peak.

    Voltage across the capacitor=charge/capacitance

    thus

    Vc=Q/C (also is the ripple peak-peak voltage Vr)

    Q=current*time=I*T/2

    thus

    Vc=Vr=Q/C=(I*T)/(2C)

    T=1/f

    thus

    Vr=I/(2*f*C) and not I/fC
     
  3. vvkannan

    Thread Starter Active Member

    Aug 9, 2008
    138
    11
    Thank you mik3.
    i saw it as I/fc in malvino's 'electronic principles' and thats why the doubt.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I have the same book and it is correct. If you read the following lines it says that for a full wave rectifier f=120 Hz and for a half wave rectifier f=60 Hz. This is the same as the formula I told you because in the formula I told you set f=supply frequency (60 Hz for example) and not twice the supply frequency. Also, the formula I told you is true for full wave rectifier only.
     
  5. kevincamilleri89

    New Member

    Jan 31, 2010
    1
    0
    hi,
    When is Vr=I/(2*f*C) used and when is I/fC used?
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Vr=I/(2fC) is used when you put f=equal the supply frequency

    The other is used when you put f=equal the ripple frequency
     
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