Weird LED

Discussion in 'General Electronics Chat' started by specc, Jul 17, 2008.

  1. specc

    Thread Starter New Member

    Apr 27, 2007
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    0
    Hi all,

    I took apart some christmas ornaments. They were 100 blue LEDs. I am trying to make a LED array. I am not sure why a few of them wouldn't operate, but when I put in a RED LED into the same slot it would lit up. Somehow when I connect these broken LED to a 5V voltage regulator with 1.5A loading. The broken LED will work again. Can anybody explain what is happening? I know that LED should lit up with approximately ~50mA.


    Thanks

    SpecC
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    Red leds light up at a lower voltage than blue, sounds like you have some in series and don't have voltage across them. Check out the link below and it'll explain better than I can. You must also have a resistor in the circuit to limit the current. The current should be closer to 20 ma, not 50 ma unless they're some kind of high brightness led.

    http://led.linear1.org/led.wiz
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You should be using a constant current source. It's easy to make one with an LM317 voltage regulator and a resistor. Connect the resistor between the OUT and ADJ terminals, supply IN with 12v, and take the current output from the ADJ terminal.
    Iout = 1.25/R1
    R1 = 1.25/Iout
    If you wanted 20mA out, then 1.25/0.02 = 62.5 Ohms.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    Sgt, you can always dip into my albums for the schematic and drawing...

    [​IMG]

    I suspect this is going to be a popular one.
     
  5. specc

    Thread Starter New Member

    Apr 27, 2007
    3
    0
    Thanks for the replies let me give it a try
     
  6. specc

    Thread Starter New Member

    Apr 27, 2007
    3
    0
    hmm for some reason whenever I try to measure the current with my multimeter the LED wont work afterward. did I burn out something? but when I plug it directly into the 5v voltage regulator, LM7805. It will lit up again.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    I'll bet you fried the fuse in your multimeter, because you had it on the low mA scale, and accidentally touched the probes across your supply voltage. Those fuses are typically 250mA GMA-type. Radio Shack carries them, but they charge around $2.75 for a pack of 4 or 5.

    It's safer to measure the voltage drop across a resistor instead of measuring mA.
    Use Ohm's Law: I=E/R
    or: Current = Voltage / Resistance
    So, if you put a 10 Ohm resistor in series with your LED and your current limiter circuit, and measured 0.2 volts across the resistor, what would be the current throught the resistor (and also the LED and the current limiter circuit, because the same current flows through the entire circuit) ?
    I = E / R
    I = 0.2V / 10 Ohms
    I = 0.02 Amperes = 20mA
    If you're using a 62.5 Ohm resistor between the output and adjust terminal of the LM317, and taking your current supply from the adjust terminal, that's what you should read across the 10 Ohm resistor - 0.2v.

    You should be using an LM317, not an LM7805. I have no idea how you have things connected then. An LM7805 won't work as a current regulator. While you're at Radio Shack picking up some more fuses for your meter, get one or two LM317's - they carry them, too. And pick up a few resistors, too :)
     
    Last edited: Jul 18, 2008
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Oh, if you are trying to connect LED's in parallel - I suggest against it, unless each LED "string" has its own current limiter.

    As BroadBand AC implied, the Vf (forward voltage) of your LEDs can vary significantly. I have seen the Vf of LEDs vary by as much as 10%, even in the same batch.

    The way to combat that is to use a constant current source (like the LM317 current circuit we keep mentioning) and measure the Vf on each LED. Write the Vf on a piece of tape or stick-on label (use at least 3 digits), and attach it to the LED as you go.

    Then you can match them up. Take the LEDs with the lowest and highest Vf, and make them a series pair. Then the next lowest, and the next highest. You may wind up with a few mismatched LED's at the end, but your lighting will be much more even.
     
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