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LM317 as current regulator

Discussion in 'General Electronics Discussion' started by conntaxman, Sep 4, 2011.

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  1. conntaxman

    conntaxman

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    Jun 17, 2011
    Im just learning so bare with me. I want to use leds for a small light. I know that the supply voltage is very low and you will Burn out the led it you go above that. Now im confused a little with the LM317. Link http://www.reuk.co.uk/LM317-Current-Calculator.htm
    they are saying that all you need is that one resistor to light the led with about 20ma. So now if the input voltage is 12vdc wouldn't that come out as about 9 volts seeing that the 317 looses about 3 vdc. I have two small batteries 3.2vdc x 1000ma. and Im going to make a solar charger also useing the LM317 . I will be putting the 2 batteries in seires ,6.4 volts..
    I know that the first question said that I was going to feed the leds with 12vdc.
    I just want to know about the voltage and the amp. part for the OUTPUT pin. I thought it would have to be about 2 volts x 20ma for the led
    tks
    John
    I know dumb question. but if you don't know the answer it isn't.
     
  2. shrtrnd

    shrtrnd

    3,703
    462
    Jan 15, 2010
    The LM317 is and ADJUSTABLE voltage regulator.
    You'll need more to your circuit than a resistor and an LED.
    Google some LED circuits using the LM317 to get the voltage ADJUSTED to what you need.
     
  3. conntaxman

    conntaxman

    89
    0
    Jun 17, 2011
  4. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    they are not using the LM317 as it was intentionally designed for. They are using it at its minimum output voltage to regulate current through a LED.
     
  5. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Led LM317 voltage regulator

    Hi there.
    Why not build the LM317 as a supply, use current limiting resistor's single led or series with a sensible in put to the LM317 your get the 1.5 amps, the resistor's will limit the current, single or series.

    LED current limiting resistor: 12 volts - 2.5 volts = 9.5 volts divided by the led's current 0.020 miliamps = 475 ohms resistor. Dave. :) PS check your led's forward voltage and operating current to determine the resistor.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,309
    2,738
    Jan 21, 2010
    An adjustable current source is probably exactly what you want to vary the brightness of a LED.

    Another (more complex) alternative is a PWM driver. This is something to consider if you're going to dissipate a lot of heat in a linear constant current source.

    Along that route, a constant current switch mode regulator is another option.

    The very last option is a variable voltage supply to a LED (or LEDs) with a series resistor. The reason I put this last is that the change in current will not be linear with voltage. It's not a very "nice" solution, especially with higher powered LEDs.

    AN LM317 set up as a current source is great, except for the fact that no value of resistance will give you zero current (i.e. the LED will never be off). If you only want a fixed value of brightness, that's OK. If you want it variable you have other problems.
     
  7. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    LED constant current

    Hi conntaxman.
    I find the RE UK site really interesting lots of projects ideas, i found this a bit more ( complex ) but if you can work with copper strip board these might be the way to go, ive not had the need yet to have an led at a constant luminous brightness, most of my led's are in panel indicators, if i do an led project i will, iam thinking of high power led's for security lights in the yard with dawn to dusk activated switch. Dave. :)

    Write the post forgot the link, ok edit, link below.

    http://www.reuk.co.uk/LED-Dimmer-Circuit.htm
     
    Last edited: Sep 5, 2011
  8. conntaxman

    conntaxman

    89
    0
    Jun 17, 2011
    lm317

    I want the led's to turn off 100%, so I guess that I'll be looking for something else.I just want a simple cir. I guess the same as the "garden" lights have. It's just to Light up a Flag for the night, because if you want to leave you flag up over night it must/should be in a light or a light shineing on it.
    so i have two 3.2vdc x 1000ma batteries.
    All ideas excepetd.
    tks
    John
     
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