Erratic led brightness

Discussion in 'The Projects Forum' started by EVETS, Mar 9, 2014.

  1. EVETS

    Thread Starter New Member

    Jan 29, 2011
    21
    2
    I have a 12v regulated dc supply to a lm317T regulator. Usual set-up, caps across 12v, across output, 240 ohm resistor output to adjust pin and 5k variable to ground. I put a red led and 470 ohm resistor from +12 to ground to show power on. When I adjust the voltage from the 317, at lowest voltage output the led lights bright. When I raise the voltage to +9,10 etc. the led dims. I've checked the wiring 10 times or more. Maybe I'm just brain fried, but I'm sure the led is across +12v in, which is stable. What am I doing wrong? Is something oscillating? Thanx
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Have you monitored the +12Vin while adjusting the Vout? It must be sagging.
     
  3. EVETS

    Thread Starter New Member

    Jan 29, 2011
    21
    2
    I thought so, but it remains stable at 11.88.
     
  4. EVETS

    Thread Starter New Member

    Jan 29, 2011
    21
    2
    And why would the brightness increase as the 317 output voltage is lowered. I would think it would dissipate more power at the lower voltage. Maybe I'm wrong. It wouldn't be the first time.
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,388
    497
    More brightness means more current is being provided. At least that is how I interpret it.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    As usual, post your schematic.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    This forum has a long history of people using LM317 regulators and connecting the pins wrong.

    Most people are familiar with the 7805 type regulators that have this pinout (looking from the front);
    Vin, GND, Vout

    and don't realise the LM317 has a totally different pinout;
    [​IMG]
     
    Metalmann likes this.
  8. EVETS

    Thread Starter New Member

    Jan 29, 2011
    21
    2
    Sorry, guys. Sometimes you shouldn't spend too much time on something. I looked again today, and I had the cathode of the led connected to the output bus of the 317. So of course when the output voltage was lowered it got brighter. Sorry to be a bother.
     
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