LED issues

Discussion in 'General Electronics Chat' started by trip, Nov 17, 2004.

  1. trip

    Thread Starter New Member

    Nov 17, 2004
    Allo, i have a simple question regarding LEDS, im a complete novice to electronics .

    What i want to do, is run 2 red LED's from a 12v power supply,
    but have the bulbs on thier own boards ie, board for power jack, running two seperate boards holding one bulb each

    both the led's are rated 3v thats all info i have, it was mentioned i need resisitor on board to restrict power etc etc. thats where my problem starts, what resistor do i need? or how do i work that out if the only info of the bulb i have is 3v, and no 'ma' info?

    if its easier i can use 2 12v bulbs, but still id need a resistor on board, again thats where i get lost

    hope someone can help, coz as simple as it is or sounds, its driving me nutts lol

  2. beenthere

    Retired Moderator

    Apr 20, 2004

    LED's are not too hard to use. Since yours are red, they should take about 1.5 volts to get them into conduction (ignore the 3 volt stuff). The output of a LED is related to the current through it. That's why you need a resistor. It limits the current.

    You're using 12 volts. 12 volts minus the 1.5 volts to break over the led junction leaves 10.5 volts. Most LED's like around 10 mills current. A 1000 ohm resistor will limit current to close to this level. Use a separate resistor for each LED. Any size resistor will work, but 1/4 watt is plenty.

    By the way, a bulb is its own resistor. But they usually take more current and don't last for 100,000 hours.
  3. trip

    Thread Starter New Member

    Nov 17, 2004
    Beenthere, Thank you very much. Job sorted and running fine.

    Once again Cheers ;)