Hi there folks,
I was recently given about 50 10mm dual(Red/Green) colour LED's as the control boards they came off had failed and were going to be discarded.
Being a person that doesn't like stuff seeing stuff like this go to waste, I said I'd like the LED's, for some future project.
Now, here is the problem, these LED's have 3 wires on them, Red, Black and Yellow, I tried a 1.5V AAA cell on one of them, between the red and black wires and got a faint red glow from the LED, so far, so good.
I connected the yellow(+ve) and black wires to the battery and....nothing.
Is the voltage I'm using too low or too high?
I can only find data for 5mm LED's on the Net and it states Vforward should be about 1.4Vdc.

Cheers,
Mike.

A green LED takes slightly more forward voltage than a red one. Probably in the neighborhood of 1.8 V or so.

An LED will not put out any light unless you apply enough voltage to forward bias the junction. A single 1.5V dry cell will not be able to light a green LED.

An LED also needs a series resistor to limit the forward current. If you exceed the forward current rating, the LED will quickly overheat and die.

http://www.rohm.com/products/databook/led/pdf/spr-54.pdf
Hi Mike,
NJ beat me to it but I agree that the battery that you used was a little light for the job. A Fluke on diode check is a great way to check LEDs. You'll find that it takes slightly more voltage to bias green Vs red as shown in the above link. That is also reflected in observed meter readings. As long as you are sure of LED polarity, you can use a 9V transistor radio battery and about 700 ohms to establish around 10mA as a safe test current. But 9 volts could exceed the reverse breakdown voltage on an LED that you are not sure of. In that case, better to use 2 of your dry cells in series with about 100 ohms as a safe test setup. I haven't noticed an LED with < 5 volts reverse breakdown voltage. In short, take the voltage source you have, subtract 2V for the LED. Divide the remaining voltage by the biasing current that you desire (10mA is good) to get the ballpark resistor value. Reds or greens will boldly reveal their true identity, while whites or blues might seem a liitle shy with their higher forward voltage. (Note: 3 volts wouldn't be enough to check blues or whites)
Joe
Of course, now I see that the datasheet I linked to shows a max reverse voltage of 3V. Also, here is a link to 1, 10mm Red/Green LED
http://media.digikey.com/pdf/Data%20Sheets/Lumex%20PDFs/Ssl-lx100139IGW.pdf