Mixing SMD Leds Red and Green

Discussion in 'The Projects Forum' started by Bru, Feb 23, 2014.

  1. Bru

    Thread Starter New Member

    Feb 20, 2007
    Is there any reason why you can't mix red and green SMD Leds in parallel? An elderly colleague wants some running lights for several model boats he's made, and asked if I could help. I wired up half a dozen of each colour - using SMD 0805 Led, all 3.2v 20ma, wired with 36swg armature winding wire. Tested them singly with a 3v button cell and everything was fine.
    However - he couldn't get a red and green led to work at the same time, wired in parallel and using 3v from two AAA batteries.
    I use white 0805 and 0603 SMD leds in parallel all the time as a source for miniature lighting in models, but I've never mixed colours.
    Has anybody got any hints about this. I have only the most basic of electronics knowledge and would appreciate any assistance offered.
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Do you use a series resistor with each LED? Since you don't mention them I assume you don not, and that leads to your problem (and it's the reason the battery doesn't seem to last as long as it should).

    LEDs of different color need a different voltage across them to work, and in a limited sense work as a voltage regulator to maintain that voltage: they will greatly increase their current when an attempt to drive with a higher voltage.

    So if LEDs of different colors are in parallel, the one with the lower voltage will steal the current and keep the other off. Typically red LEDs have the lowest voltage.

    The solution is as simple as using a series resistor with each LED as you should have done from the start.
  3. burger2227


    Feb 3, 2014
    According to the datasheet below, red, yellow to green-yellow use 2 volts. Green and blue typically can use 3 volts.


    Thus the lower voltage LED will work, bypassing the ones requiring more voltage.
    The typical current is 20 to 30 ma so use resistors to lower the supply voltage:

    R = (Vpower - VLED) / .020

    wired in parallel. Series LED's get the same current, but part the voltage. Parallel LED's get the same voltage, but part of the current


    PS: If any LED calculator tells you that it needs a 1 ohm resistor, you don't need any. I've never had to use one.
    Last edited: Feb 23, 2014
  4. Bru

    Thread Starter New Member

    Feb 20, 2007
    Thanks very much for the help and assistance; using two presets of 47ohm and 64ohm inline with the red and green LEDs worked perfectly Most grateful. Simon.