Getting steady current with no ripples using LM317

Discussion in 'General Electronics Chat' started by RG23, Jan 7, 2016.

  1. RG23

    Thread Starter Active Member

    Dec 6, 2010
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    I am trying to use LM317 as current regulator but still could not manage to get ripple free current at its output.

    Can anyone guide me in this?

    Thanks
     
  2. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    It is used as variable voltage source.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    Post a schematic.
     
    ErnieM likes this.
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yep, post your schematic.

    How much ripple is too much?

    How much ripple is at the input? If the input is not high enough the output will drop with the input.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How big is the filter capacitor upstream of the LM317?

    How much voltage across the load?


    Enough of the questions,


    Just post the schematic!
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    We can't even be certain that there is a filter cap... 300 posts and still doesn't know to post a schematic??
     
  7. Roderick Young

    Member

    Feb 22, 2015
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    You have the LM317 wired in constant current mode, right? What size of inductor is in series with the output, if any? Do not put a capacitor on the output for filtering. As the others have mentioned, it's really to your benefit to post a schematic. Hand drawn is fine, or you can do one in a drawing program if you like. Or if you took the circuit from elsewhere on the web, give us a pointer to it.
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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    LM317's only have a finite ripple rejection ratio. If you need even more, try preceding the LM317 constant current source with a fixed DC voltage regulator. It worked for me in one product.
     
  9. dannyf

    Well-Known Member

    Sep 13, 2015
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    Your question cannot be answered unless you specify the levels of ripples you are willing to live with, as any real circuit will have ripples no matter what you do. You can minimize it but not eliminate it.
     
  10. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Interesting. But wouldn't it be better to use a voltage reference and then boost its output with a bipolar transistor?
    Of course the OP wants to use an LM317, but I wonder if he'd be open to a different approach.
     
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