LM317 Caps necessary?

Discussion in 'The Projects Forum' started by anode505, Apr 15, 2016.

  1. anode505

    Thread Starter Member

    Apr 21, 2009
    12
    0
    Want to use a 3c Lipo to replace the AA batteries on a RC toy (4.5VDC).

    All the sample LM317 schemas I see have caps in them.
    Are they needed or just there to smooth out the ripple from rectified AC?

    (I have the caps in stock, but trying to keep things as simple, small and light as possible)

    TIA
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    yes they are highly recommended.. for decoupling/reduce ringing,etc..
    use them
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    The input capacitor (0.1μF) is recommended if the device is very far (more that a few inches) from the supply.
    The output capacitor is only for improving the transient response to sudden changes in load current.
     
  4. #12

    Expert

    Nov 30, 2010
    16,267
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    It seems to be a very common belief that engineers add unnecessary parts just for fun.
    Those capacitors put the chip in an operating condition where it can't oscillate.
     
  5. recklessrog

    Member

    May 23, 2013
    338
    102
    As #12 has pointed out, if you don't want an oscillator, you need the capacitors. You also need to look up the manufactures spec for the type of capacitor, some only work properly under all conditions with the specified type. If they say use a Tantalum, use one, if they say use a ceramic, then take their advice. After all, they designed and manufacture the thing and as previously stated, they don't add components for fun.
    I learned to my cost some time ago that even the same type from different manufacturers sometimes need different external considerations such as capacitor type, recommended max lead lengths etc.
     
  6. anode505

    Thread Starter Member

    Apr 21, 2009
    12
    0
    Thanks guys! Will add them in. Explanations make total sense.
     
  7. dannyf

    Well-Known Member

    Sep 13, 2015
    1,782
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    I think for questions like that, the datasheet offers the best answer: it exists for that reason.
     
    recklessrog likes this.
  8. crutschow

    Expert

    Mar 14, 2008
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    According to this data sheet (pg 10) the capacitors are not necessarily needed for stability, but the input capacitor is recommended if the regulator is any distance from the supply..
     
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