I recently purchased a "Jumbo - Super Bright Red LED" from Radio Shack. It says on the package "Rated 2.4V, 20 mA." On the back of the package, it gives the following information:
Absolute maximum ratings: Forward current: 40 mA max. Forward voltage: 5V DC max. Reverse voltage: 6V DC Power dissipation: 100 mW
Electro-optical characteristics: Forward voltage: 2.4 V +/- 0.03 V Peak emmission wavelength: 660 nm Luminous intensity: 5000 mcd (typ. at 20 mA)
note: Short lead is cathode.
I gave it a try today. I used it with my Arduino's 3.3 V output without a resistor (I thought I would be fine because I was well within the absolute maximum ratings). It was never nearly as bright as my Jumbo blue LED, but it seemed fine. Then when I unplugged it, and plugged it back in later, it didn't work. It only gave a very dim glow. I made sure to never get the leads mixed.
Am I misunderstanding something about absolute maximum ratings, or was this just a faulty LED?