[rootbeer]

While playing with some LEDs i have laying around i noticed some things i can't explain.
I have 3 LEDs, a red, an orange, and a green. Before I say what I did I will say how I'm differentiating the LED leads. I look inside the LED cap and follow what lead corresponds to the cathode and anode by noticing that one lead connects to a straight piece, and one to a L shaped piece. Now.... When forward biasing both the orange and green LEDs with the use of a multimeter on the DIODE testing setting the LEDs light. And if i reverse bias the orange and green they do not light as expected. Now when the do the same test with the red LED the exact reverse happens, forward bias results in no light, and reverse bias results in light??? What gives?

 

[error401]

Quote:

Originally Posted by rootbeer /img/forum/go_quote.gif While playing with some LEDs i have laying around i noticed some things i can't explain. I have 3 LEDs, a red, an orange, and a green. Before I say what I did I will say how I'm differentiating the LED leads. I look inside the LED cap and follow what lead corresponds to the cathode and anode by noticing that one lead connects to a straight piece, and one to a L shaped piece. Now.... When forward biasing both the orange and green LEDs with the use of a multimeter on the DIODE testing setting the LEDs light. And if i reverse bias the orange and green they do not light as expected. Now when the do the same test with the red LED the exact reverse happens, forward bias results in no light, and reverse bias results in light??? What gives?

Not all LEDs are constructed in a manner such that your visual indication would be correct. There's nothing about the way LEDs work that requires the configuration you suggest. The 'L' shaped piece has a cup that holds the semiconductor die. The straight piece connects to a bondwire that goes to the same die. I think it's easier to construct most LED types with the cup as the anode, but nothing requires it. Obviously your meter and your eyes aren't lying

.

Other common cathode indicators that would be more reliable are a flat side on the case or a shorter lead.

 

[xmokshax]

^^ yep. (and in much more detail than i would have been able to give.) i ran into an example of this the other day - the larger, L-shaped piece within the LED's head was attached to the longer lead. as error401 suggests, and as it sounds like was the case for you, the lead length was a better indicator than the internal connectivity. the short lead turned out to be the negative one, despite being connected to the smaller internal piece.

 

[rootbeer]

Thanks for the help!