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Linear Power supply

Discussion in 'Electronic Basics' started by David Griffith, Apr 1, 2004.

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  1. Hi all,


    I am trying to construct a variable power supply from the schematic
    listed here.

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#317.gif

    Basically I wanted to know if the author has got the value for the
    filter capacitor correct or not, as it seems very high (36 millifarads)

    I have tried plugging in the maths and it gives me a more realistic
    value, but I would appreciate any insight into which of us has got it
    wrong...

    "Electronic Devices (2nd Ed)" by Thomas L Floyd gives the equation for a
    filter capacitors size to be roughly as follows

    C = 0.0024 / (R * r)

    where R is the load resistance and r is the 'ripple factor' or ripple
    voltage over DC voltage.

    The transformer I am using is not marked, when connected to a supply
    measuring 240V rms exactly on my DVM measures 13.0V rms output on the
    secondary.

    I therefore take the peak voltage to be 13 * sqrt(2) = 18.4V peak

    The LM317T regulator seems to require r to be 1/18.4 or less giving a
    maximum actual ripple of 1V , and can output up to 1.5A setting R to be
    V/I = 18.4/1.5 = 12.3 ohms

    C = (0.0024 * 1.5) after cancelling voltage from both sides of the
    equation = 3600uF not 36,000uF as stated in the schematics.

    Is my maths sound?


    Thanks in advance

    David Griffith
     
  2. The supply with the 36,000 uf cap is a 4.3 amp version that needs lots
    of voltage reserved for the pass transistor (5.5 volts at full load).
    That big cap is there to keep the ripple from falling below the
    required minimum. It is not a very good design, but that is what it
    needs.
     
  3. cpemma

    cpemma Guest

    Using the simpler formula C = load current/(frequency x ripple voltage),
    with 1.5A, our full-wave 100Hz and 1V pp ripple, gives 15,000uF, and with
    Bill's 4.3A and US 120Hz it comes out at 35,833uF.

    Formula from the guide at http://my.integritynet.com.au/purdic/power1.html
     
  4. Rich Grise

    Rich Grise Guest

    Probably, since he says:
    "To keep the ripple voltage below 1 volt at 4.3 amps, a 36,000 uF or greater
    filter capacitor is needed."

    Have Fun!
    Rich
     
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