What size resistor?

[Bigandrewgold]

So I am making a rf mod for a cg/cg2 controller.  What ohm resistor should I use for the led's?

[SetpicLemon]

As far as I'm aware for resistance, you use Ohm's law which is volts divided by amps...

In this case, you need to find out what voltage you're sending to the LEDs, what voltage the LEDs run at, and then what amperage the LEDs run at...

The typical red LEDs that I sell at my work run at about 2.5 volts, and have a have amperage of about 30 milliamps.

Assuming that you're sending 5 volts to the LEDs, seeing as the 360 works on USB you would use this equation.

forward voltage of source minus forward voltage of LED, divide by amperage of each LED multiply by how many LEDs you're using...

(5 - 2.5) / (0.03 * 1)  (Lets say you're only using one LED)

Which equals about 84 ohms. However you LED will be bound to have a different voltage, a different amperage, and your source voltage is more likely to be different too. So as long as you get your figures you can use the equation above to work out what resistor you need.

[Modded Matt]

Very good answer, your explanation was right on. Except for the info on the xbox, while logically because it uses usb, it would be 5v, it is not the controllers are powered from 2 each 1.5v aa batteries or battery pack @3v. Even plugged in, you wont see 5v.

Redo the math with a supply voltage of 3V and you will have the correct resistor.

There are calculators that will do this for you if you know the values that are posted here. Below is my fav:

http://ledz.com/?p=zz.led.resistor.calculator

[SetpicLemon]

Ah ha! Good point, the AA batteries!

So...  it'll go like this...

(3 - 2.5) / (0.03 * 1)

Which equals 16.6(recurring) Ohms. So best to stick with 20 ohms, but only if you're buying Red 5mm LEDs from Maplin Electronics.

Again you'll need to know what LEDs you're using before you find out how much resistance you need.