Does this RGB LED really need 220 ohm resistors per color?

[Chuoi]

I have an arduino kit (5v) and instead of just following their simple instructions I want to know why they are using certain resistors for their RGB LED's.

The RGB LED that comes with this is identical to this one:LED Indication. And I'm assuming the forward voltage and current is the same.

The directions call for three 220-ohm resistors, but is that necessary? My math tells me that Red needs a 100-ohm and the Green & Blue need 160-ohm's. Won't using more resistance decrease the luminosity of the LED?

Or maybe the resistors have something to do with the affect they are trying to achieve with the LED? Here's what the book states, which I don't really understand:

"By creating a voltage difference between the cathode and voltage coming out of the Arduino's PWM pins (which are connected to the anodes through 220-ohm resistors), you'll cause the LED to fade between its three colors."
Do the resistors have anything to do with this fading, or isn't it just the PWM controlling that?

[Tristan]

I am also researching this for my Raspberry Pi. The forward voltages are different from colour to colour (RGB) and manufacturer to manufacturer, best check your datasheet. Yes it will decrease your luminosity.

You are correct in your assumptions with your resistor calcs but other factors also come into play. The red is usually a way lower voltage, mine is 1.95V and yours seems to be 1.90V looking at the datasheet attached below. My Green and Blue are the same voltage at 3.3V.

As long as you use higher and not lower resistors you will be fine. Most tutorials simply state to use the same rating resistor for ease, in reality for best output results you would do the calcs like you have done, and then make corrections to balance the colour intensity between red, green and blue. A simple test is to try set all colours at 100% and see if it is pure white, if not, and say you can see a tint of say blue, then the resistor on Blue will require a higher rating.

Yes, resistors control the fade/intensity of the RGB LED but the same effect can be achieved via PWM. If the LED is to be used at a set full colour and no fading then resistors are perfect. If you wanting to fade in and out, then PWM is you choice.

http://www.mouser.com/ds/2/678/V02-0373 ... 909029.pdf