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LED Wiz - RGB LEDs 3.3v/2v bank select
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Entropy42:
I just wanted to add some info to this thread, even though its really old, since other people may come across it in a google search like I did.  You should not use a 3.3V power supply without a resistor unless your power supply is current limited.  Without an external resistor, the LEDs will become extremely sensitive to small variations in source voltage, i.e. 0.05V could increase LED current by 50%.
More info here:
http://electronics.stackexchange.com/questions/12865/is-a-current-limiting-resistor-required-for-leds-if-the-forward-voltage-and-supp
RandyT:

--- Quote from: Entropy42 on March 07, 2013, 12:04:56 am ---I just wanted to add some info to this thread, even though its really old, since other people may come across it in a google search like I did.  You should not use a 3.3V power supply without a resistor unless your power supply is current limited.  Without an external resistor, the LEDs will become extremely sensitive to small variations in source voltage, i.e. 0.05V could increase LED current by 50%.
More info here:
http://electronics.stackexchange.com/questions/12865/is-a-current-limiting-resistor-required-for-leds-if-the-forward-voltage-and-supp

--- End quote ---

What you say is true, and it is always a good idea to include a resistor.  However, if the LED is rated at 4v, and the power supply is regulated 3.3, there would need to be a large and very unlikely power deviation to approach the limitation of the LED.  The link you provided questioned whether an LED and power source at the same voltage negated the resistor requirement.  In that case, it is always much safer to include the resistor as the supply outputs are never exact. 
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