Resistor Check

[Mineral]

In my defense: I did the math myself and I searched the forums before asking.

If the LED's forward voltage at 1A is 3.7V (as listed for the single green Cree XP-E2) then no resistor is necessary. Correct?
Math:
Ω = (3.7V - 3.7V)/1A = (0)/1 = 0
W = 0Ω x 1²A = 0 x 1 = 0

On the Tri-Cree listing it says 3.58Vf for green, which leaves me at 0.12Ω at 0.12W.
Math:
Ω = (3.7V - 3.58V)/1A = (.12)/1 = .12
W = 0.12Ω x 1²A = .12 x 1 = .12

That's a much smaller resistor than anything I've seen anybody use. Am I doing something wrong, or do I really need a teeny-tiny (or no) resistor for a green Cree?

[Generic Jedi]

Your math is correct. I used two 0.47 ohm 0.5 watt resistors (one on each green) for my green/green/white tri-Cree LED.

Someone else will have to comment on the forward voltage difference between a single green Cree and a green tri-Cree.

[Silver Serpent]

Teeny-tiny is correct. There's a .47 ohm .5 watt resistor in the store that will serve your needs. It won't affect the brightness, and will provide protection to your LED.

If you're building a stunt saber (no sound), then you can skip the resistor for this case. If you're using the Nano Biscotte board, you will want the resistor to prevent odd behavior from the sound board.

[Mineral]

Thanks guys!

[NanoRex]

I wonder where the number 3.58 came from...Cree's XP-E2 datasheet says 3.7V, and there is no other colour or listed current with a forward voltage of 3.58.