Keeping red under control with an RGB setup

Discussion in 'General Electronics Chat' started by littlenemo, Oct 8, 2010.

  1. littlenemo

    Thread Starter New Member

    Mar 17, 2009
    Disclaimer: I'm a noob. I've been making lightsabers as a hobby for a couple of years, and have learned enough quick-and-dirty electrical knowledge to allow me to, well, make lightsabers. Still, I do try to pry more knowledge into my tiny brain in order to make cooler lightsabers. I've made three lightsabers that use LedEngin's 10-Watt RGBA and RGGB LEDs, and which allow you to change the color of the blade by mixing and matching the different colors. (Here's my latest, and here's a video of it.) The attached ultra-simplified (?) diagram shows how I do this.
    Yes, I do know enough to put the appropriate resistor on each color, though I left them out of this diagram.

    The problem is, any time red or amber are on, they suck current (if I'm not mistaken) from green and/or blue, so while the setup in the diagram should provide 15 more or less distinct colors, red and amber tend to dominate, and I end up with an inordinate number of pinkish or orangeish shades that are barely distinguishable from each other. I have gotten vague advice to the effect that I should be able to solve this problem "with a diode." I may not know much, but I know enough to know that there are all kinds of diodes (including LEDs).

    Scouring the Internet for info, I found references to "serial resistors," but what I found has not helped me so far.

    So here I am. Without using PWM via a microcontroller, such as Pic or Arduino, how can I keep red and amber under control and get nice color mixes?

    Many thanks in advance for any help you can offer!
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    Red and Amber LEDs require less voltage to run than green or blue; they also tend to be more efficient given the same power. You just need to adjust your resistor values such that the brighness is evened out.
  3. littlenemo

    Thread Starter New Member

    Mar 17, 2009
    Thanks for the prompt response, marshall.

    Hmm. But different combinations of on/off would then require different resistor values, wouldn't they?

    Using this wiring setup, would it possible/practical/helpful to put 1000mA Buckpucks on each of the four chips?
  4. marshallf3

    Well-Known Member

    Jul 26, 2010

    Nope, just drop the appropriate voltages in each LED leg run. Start getting them all the same, ie drop 1.1V on the red with a 1.1 ohm resistor. Of course the current will go down a bit but you can just tweak it from there.

    BTW: Thinking about it, chances are that 1 ohm just for the red and 1 ohm for the amber should be close enough.
    Use at least 2 watt resistors so they don't get red hot.
    Last edited: Oct 8, 2010
  5. cumesoftware

    Senior Member

    Apr 27, 2007
    If you have the datasheets for those specific leds, you can match the brightness more precisely that way. Most datasheets have graphs relating current and luminous intensity, and current with voltage drop. I expect each led to have different relations.

    Vf is strangely high for the green led. :confused:
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    Agreed, the luminous intensity charts will make life easier for matching.

    Also, The red and amber shouldn't be current hogs. The blue/white and violet will suck a battery dry much faster.

    Putting the resistors on the led of the led, or the lead leading to the led is the way to get exacting results.

    Remember, the human eye percieves reds and blues (and greens for that fact) differently.

    You may only need 7ma of current to the red led when blending it with 20ma of blue led to get an even purple mix.

    This mix is going to be LED SPECIFIC. This all feeds back to the initial suggestion:

    Use the LED's datasheets LI (luminous intensity) charts to determine proper ratios and proper resistances to reach those numbers.