3mm LED Question

Saw some 3mm leds on ebay. Description stated they ran at 2.5v. Could these be direct driven with 2AA batteries [1.5v x 2=3v]? Would you need a resistor?

you could probably overdrive them a bit, that doesn't seem like too much. but then again, the only leds i have used extensively are luxeons.

You need some good air flow in there if you want to run it at 3 volts. Or sink the LEDs to something conductive.

The 3mm LEDs can easily be damaged due to their small size, do not run them above the rated current, stay below 20ma

Think I'll stick with the 5mm's. Is there a simple way to step up the voltage from 2 AA's?

We have done extensive work on this subject our original design did use a step-up convertor

There are several chips out there that boost the voltage from batteries to a higher voltage, but they all come with a price - eff; most of them are 50-70% meaning you "throw away" 30% in the conversion process

To boost the voltage, you need to store the energy into a inductor and then let the inductor release its energy with the power supply to raise the voltage

There are several chips out there on the market, but you will have to "squeese" the chip, a couple of caps, a couple of resistors and a small inductor inside the TINY area of the flashlight, not an easy feat

http://www.borealis.com/~winnie/LED_BOOST/

With some of the lower power LEDs such as the 5mm, you could use a switched cap solution as well

http://www.national.com/appinfo/power/0,1768,988,00.html

Inretech, I hear what you're saying , but I believe that your statement is in reference to using step up and led in a conventional flashlight housing, ie-tube with batts and head with assembly. My personal little project runs into a different configuration where the power supply is connected to the led attachment outside of the conventional design. It isn't anything groundbreaking but a fun little time consumer for me to work on.

Overall I am really confused about the voltage thing. I have a white led at home that I put into a housing and is running off of 2 AA batts.
The led originally came from a little keychain light powered by two 3v coin cells with a resistor. I attached it without a resistor to the 2 AA's and it is working. Appears to be at same brightness as when used in the original keychain light. I just wish I knew what kind of led this one was, it would make my home project alot more easier for me to accomplish.

Do not think of operating LEDs as "voltage devices", think of them as current devices

A semiconductor diode such as an LED changes its "resistance" as the voltage changes, so running each LED at a fixed voltage is meaningless

Connect up the LED and measure the current across the LED, that is what is important

For proper operation of the 3mm and 5mm LEDs, keep the current below 20ma

When working with LEDs always have a current meter handy