LM317 adj. current limiter, how do I add an indication LED?

Discussion in 'The Projects Forum' started by Hamlet, May 7, 2016.

  1. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I've built this, but would like to add an LED that lights up when current limiting is active...

    upload_2016-5-7_15-21-35.png

    I took the schematic from the data sheet, found a 150ohm pot on ebay, and bread-boarded it
    every which way, using LM317, LM338, LM350, etc. ( I tried a 500ohm, 200ohm, a 100ohm pot, etc., but the 150ohm worked best.)
    I soldered it up, and it's been really nifty addition to my workbench. However, I'd like
    to add an indicator, to let me know when it is actively limiting current. Any ideas?
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
    1,821
    363
    The 2nd regulator (LM117 here) generates a constant current of 100ma. The LM338 maintains a constant voltage across R1 (=R1 * load current) + R2 (=a portion of R2 * 100ma).

    When the load current goes up, LM338 will attempt to lower its output voltage, thus lower the potential on its Adj pin. How "violently" that kicks in depends onthe ratio of R1 and R2 (the portion of R2 that's between the adj pin and the output).

    So one obvious way to indicate activation of the current limit is to measure the voltage across the regulator. For example, a resistor + led / leds between the input pin and the output pin of the regulator, or the input pin of the regulator and the current output nod, would work. You may need to experiment the number of leds needed. The maximum voltage there would be the input voltage - the negative / ground.
     
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