Confusions with working with more than 1 LED

Discussion in 'The Projects Forum' started by solexious, Jul 16, 2008.

  1. solexious

    Thread Starter Active Member

    Nov 20, 2007
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    Hello all,

    I'm trying to get my head around this problem, and hope you can tell me the things I have to consider when doing this.

    I'm making an led "trafic light" and want to get the best group of led's for each colour. And by best I mean, using the available power supply to get the most amount of light within reason. I'm not going to be mesuring the lumen output or any thing like that!

    Only one colour will ever be on at once.

    My problem with understanding is not to draw to much from the power supply, and I think my problems before have been miscalculating when i have series wired led's in parallel groups.

    Any help you can Give would be great.

    Sol
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. solexious

    Thread Starter Active Member

    Nov 20, 2007
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  4. SgtWookie

    Expert

    Jul 17, 2007
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    Vf = the voltage drop across the LED when it is forward biased.

    The red LED's you linked to are rated for 2Vf (typical) @20mA.
    Green LEDs may be somewhat higher than that; 2.1 to 3v.
    Blue LEDs are generally 3.5 to 4.0v.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Their is a current regulator made from a LM317 (one LM317, one resistor) that will set up a current, you won't need to worrry about Vf then, as long as you have a minimum voltage, somewhere around 6-7VDC.

    [​IMG]
     
  6. solexious

    Thread Starter Active Member

    Nov 20, 2007
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    Great, and as an idiot check question, the fact that the powersupply is 12v 1amp wont matter that 16 leds in 4 groups of 4 draws draws only 72.7ma. As i can have more amps avalible than I need with out damage as you draw amps rather than there pushed into the led?

    Thank you all again

    Sol
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    LEDs do require something like a resistor to limit their current, or the magic smoke escapes, but in general a circuit draws what it needs, you can't "push" current into them. If you use a LED with a Vf of 4V and put 4 in series you will need 16V just for the LEDs (4V X 4), so you will need 2 groups of 2 for the same result on a 12V power supply.

    I've used this drawing before in other posts, I'm showing it for concept only.

    [​IMG]

    The fact the power supply has more amps than it needs just means you're not stressing, so it will likely last longer.
     
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