New to LED's..need help

Discussion in 'The Projects Forum' started by mvp, Jan 22, 2014.

  1. mvp

    Thread Starter New Member

    Jan 21, 2014
    I customize video game consoles, and i have always purchased LED strips for ease

    I now want to start making my own LED's
    I picked up some white 5mm LED lights, 3.6v, 20ma.
    My transistors are 1/2w 680ohm
    my power source is 12v

    i was looking at using 2 LED's in a console. I wired each LED individually, with their own transistor. Is this ok to wire to 12v, or will it be too much power?
    Will i need to install 2 more LED's? Is it ok to wire them individually the way i did? I do this for the ease of flexible placement for better lighting.

    Here is a picture of how i wired them. The positives from each LED wired to the positive wire going to the power supply +, that was tapped into. The ground wires from each LED are wired into the ground from the power supply that was tapped into.. I can wire this route or hard wire solder + ground on the motherboard. In the picture the ground i labeled -..

    Sorry, i'm a NOOB

    • LED.jpg
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  2. alexfreed


    Oct 8, 2012
    1. Those are RESISTORS, not "transistors".
    2. Connect 2 LEDs in series. 3.6V * 2 = 7.2V will be needed. You have 12.
    12 - 7.2 = 4.8V is extra. (3 LEDs would be even better)
    With 2 LEDs you need to drop 4.8V at 20 mA
    4800mV / 20 mA = 240 ohm. So wire 2 LEDs and one 240 ohm resistor in series.
    The resistor will dissipate 20 mA * 4.8V = 96 mW meaning that 1/4 W or 1/8 W is OK.
  3. mvp

    Thread Starter New Member

    Jan 21, 2014
    sorry, i meant resistors

    So i can wire 3 LED in series with each having their own resistor, or 1 resistor? If it's one, can i use the 1/2w 680 resistor i have?

  4. alexfreed


    Oct 8, 2012
    If you wire 2 LEDs in series you need to add ONE resistor of 240 ohm as I said, not 680.
    With 680 ohm the current will be only 7 mA - not very bright. If you really need to use what you have, 3 parallel resistors 680 each is almost 240, but will overload the LEDs just a bit.

    With 3 LEDs in series (3.6*3 = 10.8) you only need to drop 1.2 V with a 60 ohm resistor.
  5. k7elp60

    Distinguished Member

    Nov 4, 2008
    My comment is that even though the LED's are rated at 20mA, you do not need to run them at this current. Most LED's are adequate intensity at less then the rated current. True they are the brightest at maximum current, but that intensity may not be required. I am using some blue ones that have intensity of 4600mcd at 20mA. I am running them on 10mA and the intensity is 25% less, so there is a savings on power besides them being
    bright enough.