LED Array Question

Discussion in 'General Electronics Chat' started by jappy, Dec 24, 2008.

  1. jappy

    Thread Starter Member

    Nov 28, 2008
    13
    0
    Hello all and happy holidays!
    I have 8 groups of six 2volt leds in series powered by 12v (please see attached). I would like to run the leds @ 30ma but I am perplexed with the idea that once the voltage has run through the leds there is 0 volts left. With that said if I just plugged this straight into the 12v power supply how would I predict what the millamps are of the leds? Thanks for any insight.

    [​IMG]
     
  2. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    You don't power leds with a fixed voltage, you use a series resistor that limits the current through the led's to there recommended forward current flow. Each of the strings of 6 leds in your picture should have a resistor in series with that string. The value is calculated as voltage source - total diode string voltage drop divided by desired current (.030 amps in your case). So if each diode has a 1.5v drop (find out from a data sheet what drop your specific diodes have) then the resistor for each string should be 100ohms. The fact that you have eight parallel strings of LEDs has not effect, it's the same with one string to a million strings in parallel, just the total battery current drain changes.

    Does that all make sense?

    Lefty
     
  3. jappy

    Thread Starter Member

    Nov 28, 2008
    13
    0
    Plenty of sense, thank you. The forward voltage of the leds are 2volts. I bought this module off the internet and I'm curious why it was set up this way. :confused:
     
  4. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    Well if they are 2volt drop each then you can't use them safely in a 12 volt system as 6X2 = 12 and there would be no voltage left to limit current through a resistor.

    If you can't drive them with a higher voltage then I'm not sure how you can use them safely at 12vdc. The problem is that LEDs vary a little from each other in exact voltage drop so one string is bound to draw too much current and burn out, there has to be a form of current limit or regulation to work correctly.
    Lefty

    Do you have a link to a data sheet or the ad you bought them from?
     
  5. jappy

    Thread Starter Member

    Nov 28, 2008
    13
    0
    Ah ok. I'll try and find the data sheet. So I can throw 14volts at them but my only option is to put a resistor inline with the V+. And if I do this and one row burns out this increases the amount of current on the other rows leading into a failure cascade, correct?
     
  6. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    Yes, 14vdc would work. Each series string would require it's own 66 ohm resistor (closest standard value = or higher then 66 ohms) and there is no cascade failure scenario as each string in limited to 30ma no matter how many strings are wired in parallel. You can't use just one resistor for the whole array as the LED's don't share current well.

    Lefty
     
  7. jappy

    Thread Starter Member

    Nov 28, 2008
    13
    0
    Thank you Lefty.
     
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