white led array revisited!

Discussion in 'The Projects Forum' started by russ3270, Nov 2, 2009.

  1. russ3270

    Thread Starter New Member

    Apr 19, 2009
    7
    0
    hello again,
    i ran a couple of test strings of leds.
    3 white leds running at 30ma(using appropriate resistor with lm317lz to limit current),and 3 x25ma again using appropriate resistor with a second lm317lz
    i put them on test for around 200 hours during 6 week hols.
    the ref voltage was steady 1.25v on one and 1.26v on the other but after 200 hours on test its fair to say that 4 out of 6 had lost there initial brightness and there was some yellowing of the white led's.
    Can you think of anything im doing wrong?
    would you perhaps run at 20ma by changing resistor value?
    is my circuit design too basic? just an lm317 with each string of three leds using a res to set current?
    initial inrush shortening life?capacitor required?
    trying to connect 10x3 at 20,25.30ma using lm317lz on 12v constant inline laptop type ps.
    once again your input would be greatly appreciated to try and help me sort out what should be the most basic of all circuits.:)
    pretty much as attached drawing except i am sourcing output from adj terminal!! user48582_pic416_1240174761.jpg
     
    Last edited: Nov 2, 2009
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What value was that resistor?
    What value was the 2nd resistor?
    LEDs get dim as they age. Running them with excessive current makes them bright at first, but ages them much more rapidly, causing them to get dim much more quickly.
    Try a set at 20mA and another set at 18mA.
    That's pretty basic, but pretty standard.
    No and no. OTOH, if your line from the supply to IN is more than a few inches long, use a small capacitor (say 0.1uF) from IN to GND. No cap on the output or ADJ terminals though - you're regulating current, not voltage.
    Constant current supplies should have a very high output impedance.
    Constant voltage supplies should have a very low output impedance.
    A capacitor on the output lowers the output impedance.
     
    Last edited: Nov 3, 2009
  3. russ3270

    Thread Starter New Member

    Apr 19, 2009
    7
    0
    hi and thanks for speedy reply,
    i used an online lm317 calculator to find res values to set current
    cant remeber exact values,one at 42Ω or just greater and the other at 50Ωohms or just geater to acheive < or=30ma and ,< or=25ma
    i will try and run strings at 18ma and 20ma respectivley to see if i have any better results.
    good to know that basic as the circuit is,it should be fit for purpose and ive not done anything too wrong:)
    how long should the leds stay bright and white for when being driven correctly with lm317, 50,000 hours ish?
    these 5mm leds are just for background illumination, diffused,with 120 deg viewing angle, they arent super high lumen output 55000 mcd jobs.
    what would you be tempted to run them at bearing in mind i have no current at a specific point on voltage line? and do you think 25 or 30ma would of shortened life.
    thanks once again russ.
    ps(i will post results after a couple of hundred hours! lol)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Well, most LEDs nowadays are generally safe to run at 20mA without affecting their life much. A long time ago, LEDs were limited to 15mA. You may have some of these older LEDs. But, set up tests as you see fit. Keep notes on the tests that you run.

    It would help if you could quantify your results better, like using a light meter at a set distance from a single LED at a known current to measure/record it's light output level both when new and after x amount of hours at y current.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    You can guess at the age of an LED by measuring its Vf. The lower the Vf, the older the LED.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You have the ADJ and the OUT pins of the LM317 connected backwards so the current in the LEDs is extremely high.
    This schematic is shown in the datasheet:
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Our OP acknowledged that error in his drawing in his original post; and on the last line indicated that he was sourcing current from the ADJ terminal instead of the OUT terminal.
     
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