Led backlight resistors

[michaelmeissner]

I just got one each of the 3 LED backlights, and I was wanting to calculate the resistor to use. It wasn't clear if these backlights already have a resistor inside, or if I'll need to provide on.

The datasheet for each light gives the same electrical characteristics:

• For normal use, forward voltage of 3.3v, and 15mA forward current;
• For minimal use, forward voltage of 2.8v, and 10mA forward current.

Now, I want to drive it from both 3.3v microprocessors (Teensy 3.0, Gemma) and 5v microprocessors (Trinket, Digispark, Uno, raw ATtiny85). If I plug the numbers into a resistor calculator, it gives a 1ohm resistor if I power it from 3.3v, and a 15ohm resistor if I power it from 5v.

Are these numbers in the right ballpark? I assume I still need a resistor to prevent too much current from from flowing to the LEDs, or at these levels, can I just leave it off?

[adafruit_support_mike]

Running 3.3v LEDs from a 3.3v supply is always an iffy proposition. I'd suggest 10 ohm resistor and seeing how that works before going any lower. That will put the forward voltage at about 3.1-3.2v and the current somewhere around 13mA.

You'll want about 120 ohms at 5v.. 120ohm * .015A = 1.8v. 5v - 1.8v = 3.2v.

You always want some kind of external current limiter for LEDs though.

[michaelmeissner]

Thanks.