How do I keep a LED glowing constant from a variable DC voltage

Discussion in 'The Projects Forum' started by rigerman, Mar 29, 2016.

  1. rigerman

    Thread Starter Member

    Apr 25, 2014
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    My problem is to keep a LED glowing constant as the output of my circuit varies from 2.9V to 10V. The LED now glows according to the output voltage. I would like to keep it glowing constant. I have tried using LM317T but, for the range 2.9V to 3.7V, the LED remains off as LM317T needs a min voltage of 3.7V at Vin.

    Any suggestions?
     
  2. rigerman

    Thread Starter Member

    Apr 25, 2014
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    I think I need around 2.1v constant at LED pin no matter what the output voltage is. (2.9 to 10) Is there any sort of regulator for it?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    You can use a low dropout regulator, such as the LT3080 (350mV dropout), in a constant current configuration (see data sheet).
     
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  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You could use a Jfet constant current circuit, but your led will need 2.1v and the fet or regulator will need 1v.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Here's a simple circuit which will give the appearance of a reasonably constant light. In this example, the LED current varies from about 9mA-12mA for the 2.9V-10V input range specified.
    NearlyConstantCurrent.PNG
     
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  6. crutschow

    Expert

    Mar 14, 2008
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    Here's a two transistor variation of Alec's circuit, which has the advantage of better regulation.
    Its minimum voltage drop is <0.8V.
    The current varies from 9.6mA to 10.7mA over a 2.9V-10V input range with a 2.2V LED, which likely will give very little change in the observed brightness.
    The nominal current is ≈ 0.67/R1.

    upload_2016-3-29_12-28-7.png
     
    Last edited: Mar 30, 2016
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  7. rigerman

    Thread Starter Member

    Apr 25, 2014
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    Thanks. It works!

    I am having a slight problem with the V at 2.9v. The LED dims down significantly. I measured the voltage at it's pins. It is around 1.43V. My output voltage also drops at times to 2.3V which might be causing the issue.

    I will provide some more feedback once I stable the output voltage.
     
  8. RichardO

    Well-Known Member

    May 4, 2013
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    You never said what kind of LED you are using. For instance, a red LED might only need 1.7 volts to light to maximum brightness. A blue or white LED might take as much as 4 volts to be at full brightness. Other colors of LED's require different voltages.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    aka see the "forward voltage" rating in the datasheet of the specific LED you are using
     
  10. crutschow

    Expert

    Mar 14, 2008
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    What is the voltage at the collector of the transistor when the input voltage is 2.9V?
     
  11. dannyf

    Well-Known Member

    Sep 13, 2015
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    The 2.9v can be hard to hit, depending on the leds used. A jfet CCS or a CCS-built off a LDO may be your best chance.

    Or a boost converter.
     
  12. rigerman

    Thread Starter Member

    Apr 25, 2014
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    My output voltage is dropping to a stable 1.9V. So if I can provide around 1.85V to the LED constant, I think the glow will be stable. The V range is 1.9V to 10.1 V.
     
  13. crutschow

    Expert

    Mar 14, 2008
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    Below is a CC circuit that requires a minimum operating voltage of only about 0.2V above the LED forward voltage to operate, which is likely about as low a dropout voltage as is practical.
    It uses a low voltage op amp that will operate down to 1.2V, single supply, and has a built in 0.2V reference.
    The two 10kΩ resistor dividers reduce the reference voltage to 100mV , giving a constant-current of 100mV/R1.

    upload_2016-3-31_1-6-34.png
     
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  14. rigerman

    Thread Starter Member

    Apr 25, 2014
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    I am sorry for the delayed reply. I will test the circuit (I do not have the opamp in my arsenal), and feedback.

    Thanks a lot for your time...
     
  15. John P

    AAC Fanatic!

    Oct 14, 2008
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    It seems simplest to just make the circuit's output turn on a transistor, and use it as a switch to power the LED from some other source.
     
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