Large regulator capacitor

Discussion in 'General Electronics Chat' started by Highlight, Jun 27, 2012.

  1. Highlight

    Thread Starter New Member

    Nov 26, 2010
    28
    0
    Can someone explain what the 220uF capacitor at the input of the LM317 is for? I dont think this is for filtering am i wrong?
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    Yes it is filtering. If the LM317 is more that 6 inches away from the normal powersupply filter the capacitor is recommended.
     
  3. Highlight

    Thread Starter New Member

    Nov 26, 2010
    28
    0
    I thought thats what the .1uf capacitor was for.
     
  4. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I looked a three data sheets and two had an electrolytic capacitor on the input. Both wrote it is for transcient protection. Only one mentioned the
    6" rule.
    I mostly use the 317T with AC/DC wall warts and the cable from the wart to the box where the 317T is is about 4 feet, so I insert a 100uF right at the input pins and don't use a 0.1uF.
     
  5. Highlight

    Thread Starter New Member

    Nov 26, 2010
    28
    0
    I did some more research and from what i can infer the 220uf cap is serving as a "reservoir" cap, while the .1uf is the filter cap. I think the reservoir cap is there to provide extra energy to keep the input voltage from drooping when a current surge occurs at the regulators output. The reservoir cap will recharge the filter cap when it becomes depleted. This would make sense as to why the "reservoir" cap is large-so it can store a lot of additional energy. Again i dont know if this is right but it kinda makes sense.
     
  6. BSomer

    Member

    Dec 28, 2011
    433
    106
    Both the 220uF and the 0.1uF capacitors are for filtering. The 0.1uF cap is generally a ceramic capacitor and has a faster response than the electrolytic. At least this is my understanding of them.
     
  7. Highlight

    Thread Starter New Member

    Nov 26, 2010
    28
    0
    Yes, i think the 220uf cap is serving a dual purpose as a filter cap and reservoir cap.
     
  8. ArmbrusterJo

    New Member

    Jun 27, 2012
    5
    0
    I looked a three data sheets and two had an electrolytic
    [​IMG]
     
  9. DMahalko

    Senior Member

    Oct 5, 2008
    175
    14
    If the input is rectified AC such as from a 12v AC power transformer, the 220uF stores enough power for continuous draw, across the gap when the input AC voltage drops below the output, on the trailing downward curve of every sinewave pulse.

    Normal AC to DC conversion with a bridge rectifier and regulator:
    [​IMG]


    If the input is DC, then the 220uF is simply storing "reserve power" for intermittent-load cases where the output amperage temporarily exceeds the input. When the output current is equal or less than input, the capacitor always stays fully topped off.


    Output power handling capacity is only as large as the capacitor's current storage limit, and when that limit is hit the output voltage will decline.

    For AC this at first results in a jagged stairstep that pushes back up on every sinewave rise to maximum voltage that tops off the capacitor again, until the transformer's output limit is also reached, and then the overall stairstep voltage declines.

    Output current too high or input filter capacitor too small, input sinewave amperage not exceeded:
    [​IMG]


    For DC when the capacitor is depleted, the output voltage just drops off as long as output current exceeds input.

    Input amperage will go up as the input voltage declines, but will be limited by the regulator, wire sizes, may blow the device fuse, etc.
     
    Last edited: Jun 28, 2012
  10. DMahalko

    Senior Member

    Oct 5, 2008
    175
    14
    Oh good, the fact that no one is replying suggests to me that I'm not making any mistakes with this description of how it works. :)
     
  11. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    The small ceramic capacitor filters high frequency noise that the input wire picks up acting as an antenna.
     
Loading...