# need LED resistor help

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#### divxdude

##### Newly Enlightened
http://i13.photobucket.com/albums/a284/photoduude/LEDs.jpg

the red ones in the photo are like 3mm and the rest yellow and green seem to be a tiny 1mm?

so i'm wondering what the volt requirements are
to calculate the resistors needed.

i know some leds are as low as 1.8v so i dont want to cook them.

i will be using single LEDs in parellel with a resistorfor each
the par. array formula for a 13.4v supply
with different volt drops on the LED at a safe 17mA.

R = (13.4 - 1.85) / .017 = 679
R = (13.4 - 2.00) / .017 = 670
R = (13.4 - 2.40) / .017 = 647
R = (13.4 - 2.80) / .017 = 623

so what volts do 1mm LEDs from a couple years ago use? these are from a surplus store and may be several years old.
and am i doing the math right?

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#### EvilLithiumMan

##### Enlightened
Your math is dead on, no problem there. Most red LEDs of that size are typcially 1.7-1.8 volts. Naturally, once you get your particular LED, you'll be able to measue the actual Vf.

I wouldn't worry too much about "cooking" the LED. Putting 30ma through a 20ma will degrade it's total life, but it won't "cook" it. I've played around a lot with LEDs and have run 100ma through 20ma rated devices for many continuous hours without a failure.

And just last week, I put a RGB flashing LED into a PR2 socket for use in a flashlight. The design spec on the LED is a Vf of 3.4v at 30ma. But I used only a 10 ohm limiting resistor as I wanted maximum illumination and know the kid will only run the light a few minutes at a time. I put 4 new alkaline cells into the host and let it run for four days (96 hours). 6 volts - 3.4 = 2.6 volts, limited with a 10 ohm resistor is a whopping 260ma. I thought for sure the LED would smoke in a few minutes, but it did the entire run. (Obviously, the cells weren't at 6 volts after four days of running. I wish now I had measured the voltage before handing the light over, but it was still quite bright).