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[SOLVED] RGB LED combination..

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lockman_akim

lockman_akim

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i bough a RGB LED..n i start to do some experiment..
i know that if we combine some color such as BLUE + GREEN = CYAN
but the problem is if i combine RED + BLUE = RED, GREEN + RED = RED..
i saw at the led, every time i i turn ON RED, the other cant turn ON..
Why?
 

If you combine red and blue you should get violet for sure. There should be some thing wrong with the connections or led, checkout
 

What is the LED part number and how are you driving it? If you just put the LEDs in parallel then the one with the lowest ON voltage will prevent the others turning on. They need individual resistors.

Keith
 
keith: im not sure what the code is..

but this the way i connect it..
 

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OK, I see the problem. You need to remove R1. The 100 ohms resistors are then a bit low. I don't know the foward volts drop or maximum LED current (and I guess you don't) but 20mA is probably a safe maximum. So, make the resistors at least 680 ohms.

You could then measure the volts drop and/or current of each LED and adjust the resistors. The resistor for the blue could probably be lower as the volts drop will be higher.

Keith
 
Last edited:
can u suggest me the RGB LED part no that can use same resistor value for each color..
 

No such part will exist because of the intrinsic voltage drop differences imposed by the device chemistry. Practically all useful circuits employing RGB's make use of pulse width modulation (PWM) to control the output color mix. Either a fixed resistor for each color must be used to set a maximum current level or each color must have it's own current source.

I usually just hook up a 1k potentiometer set at maximum and measure the current as I decrease the resistance to get 20mA. Then I measure the potentiometer and replace with the next higher standard value resistor.
 
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