lm317 as current source

Discussion in 'The Projects Forum' started by mahboob2, Aug 2, 2013.

  1. mahboob2

    Thread Starter New Member

    Aug 2, 2013
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    Hi every body.
    I have a problem about designing a current source.
    I want to design it with a lm317 and potentiometer.
    But when I change value of potentiometer, the output voltage doesn't change.
    whereas I want to have a constant current.
    what shoul I do?
    please give me a picture for this.
    THX alot
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,965
    744
    see page 11 of datasheet for a variable current regulator, or constant current,

    If you have Constant current then the Voltage will vary,

    If you have Constant Voltage then the Current will vary,

    you can only make one fixed.!
     
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  3. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    I would guess you don't have a load resistor on the current supply circuit. If there is no load, the output voltage will always be as high as it can go.
     
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  4. mahboob2

    Thread Starter New Member

    Aug 2, 2013
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    I don't use load in output. I decide to when I change my potentiometer, the current doesn't change, (ie. by changing potentiometer, output voltage change, but cuurent don't change). what should i do?
     
  5. BobTPH

    Active Member

    Jun 5, 2013
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    Read #12's post
    Repeat until you understand it.

    Bob
     
  6. wayneh

    Expert

    Sep 9, 2010
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    So using a circuit you have not shown us, not wired as recommended in the datahseet, you're getting a result you don't like, which you've described first as the output voltage NOT changing and now it does. :rolleyes:

    How can we possibly help?
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Makes me nostalgic for the days I worked in the Applications group at Nat Semi answering customer calls about why our parts "didn't work".......

    When all else fails: read the data sheet.
     
  8. mahboob2

    Thread Starter New Member

    Aug 2, 2013
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    dear Dodgydave,:
    I designed my circuit like the second circuit on page 11 of datasheet, but when I vary potentiometer , voltage don' vary. I need a current source, what should I do?
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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  10. mahboob2

    Thread Starter New Member

    Aug 2, 2013
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    dear wayneh:
    I want to change voltage by changing value of potentiometer. Is it possible?
     
  11. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    In the first quote, you say the voltage changes but the current does not, in the second you say you want to make the voltage change.

    Make up your mind.

    Bob
     
  12. mahboob2

    Thread Starter New Member

    Aug 2, 2013
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    Do you mean I put a load after potentiometer?
     
  13. mahboob2

    Thread Starter New Member

    Aug 2, 2013
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    no, voltage does not change,and current change,this is my problem!!!!
    ,I like to voltage will be changed by varing potentiometer,
     
  14. mahboob2

    Thread Starter New Member

    Aug 2, 2013
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    0
  15. #12

    Expert

    Nov 30, 2010
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    YES. If you do not attach any load after the potentiometer, nothing will ever happen. It will not light up an LED if you do not connect an LED.
     
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  16. wayneh

    Expert

    Sep 9, 2010
    12,087
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    That circuit is for a constant current that can be adjusted, and will produce whatever voltage is needed to drive that current through a load. (Of course, not over the supply voltage less the drop out of the regulator.)

    With no load, the circuit cannot function and will be at maximum voltage. Changing the setting of the variable resistor will make no difference.

    You've said you want to control voltage with the potentiometer. Choose the typical application circuit for this IC, the adjustable, constant voltage supply. That is meant to do exactly what you said.
     
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  17. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    With a fixed load resistance, a change in the constant current source will produce a change in voltage. Lower current being sourced will give lower output voltage into the fixed load. If, on the other hand, you vary the load resistance, a higher load resistance, with a fixed constant current source, will result in a higher voltage. Ohms law applies.
     
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  18. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I think the O.P. really wants an adjustable voltage supply, using an LM317, but due to language diificulties has got stuck on some "current" label and has got the wrong circuit.

    To Mahboob; here is a circuit google found very easily for an "adjustable voltage regulator with LM317";

    [​IMG]

    It was on this page;
    http://www.circuitstoday.com/few-lm317-voltage-regulator-circuits
     
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