What Capacitor for LM317

Discussion in 'The Projects Forum' started by kaushizcute, May 8, 2012.

  1. kaushizcute

    Thread Starter New Member

    Feb 16, 2012
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    I have made a constant current source using lm317T. Input voltage is 12V to 20V.
    How shall I determine the capacitor value at the input and output side??
    What is the purpose of using a capacitor??
     
  2. #12

    Expert

    Nov 30, 2010
    16,266
    6,778
    Read the datasheet.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Don't use a capacitor on the output when you are using the LM317 as a constant current source.

    The cap on the input more-or-less depends upon the impedance of your voltage source, maximum load current, and maximum desired ripple. You should have a 0.1uF-0.33uF cap very close to the LM317 input terminal to compensate for the parasitic inductance of wiring.
     
  4. kaushizcute

    Thread Starter New Member

    Feb 16, 2012
    24
    0
    SgtWookie

    what will happen if i use a capacitor at the output end?

    what is ripple??
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    A theoretically ideal current source has an infinite output impedance, whereas a theoretically ideal voltage source has an output impedance of zero.

    If you use a capacitor on the output of your current source, then your "real world" current source is further away from the theoretically ideal current source, which is not a good thing.

    For voltage, it's like waves on the surface of water.
    For current, it's like constant changes of flow volume.
    The polarity or direction of voltage or current does not change, just the pressure or volume. If the direction or polarity changes, then you have AC instead of ripple.
     
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