Easy question from new guy

I have a mini mag I want to convert to a led. Of course it is 2 aa with the voltage of 3v. If I put in a 3mm led with a drop voltage of 3.3v would I need a resistor? And would that work?

I think it would be dim even without resistor.

i was thinking same thing.

Hi jamesr, I'm not sure which direction you're looking to go with your light, but you might want to check out the sandwich shoppe forum. Buying a sandwich isn't much of a do-it-yourself project, but it's a heck of an upgrade. Welcome to the CPF!

No need to use a resistor, jamesr, but you will probably want to use lithium AA cells in that light to better match the supply voltage to the LED's rating.

Red/orange/amber/yellow LEDs typically require 1.8-2.4V, and would need a limiting resistor to keep the current level within spec, usually 20-30 mA each for high brightness units.

a little fussing around with a digital multimeter will let you know what your supply voltage sends across your LEDs. Red & yellow leds dim out when they are overdriven, but blue/green/white/UV ones grow brighter at the expense of shorter lifespan and lower power efficiency. Many white LED products overdrive their diodes to about 50mA for maximum brightness with reasonable lifespan.

Chalo Colina

Also check out Lambda's website. He has something similar to Dat2Zip's sandwiches, but for less.

Needing more work and some ingenuity at how to assemble the lot, you could get the LEDDynamics MicroPuck.

You really don't want to hook up a nominally 3.6V LED to a 3V power source. The result would not be satisfactory... to say the least.

Another way to drive a discrete LED in a AA MiniMag is to use 3 N cells. The 3 cells will be 10mm shorter than 2 AAs and will require a spacer to make up the distance. Being as these cells will deliver a combined voltage of 4.5v, a dropping resistor should be incorporated in the spacer. This spacer can be constructed out of plastic, wood, pvc tube, etc.. I hope this helps.

Dan

If you use a 3mm LED you don't need a resistor. This may be too dim depending on your intended use, though. I know from experience, the light still is a bit too bright for me to read comfortably for too long.

You can also go with 3 N cells (1.5 V x 3). Since the cells are shorter than 2 AA, you'll have to use a spacer. I've seen people incorporating resistors into the spacers, but I guess you might be able to do without the resistor and overdrive the LED. I haven't tried this myself yet (next basic project, I'm learning here).

Hola Chalo,
So, how much shorter lifespan can I expect from a 5mm, 20ma, white LED if I don't use a resistor and use 3 N cells? Or will the 4.5 V kill the LED almost immediately (fV is 3.6 if I remember correctly for the LED I have right now)?
Miguel


**EDIT**
OK, so I did some quick calculations, and this will generate 900 mA. I'm guessing I'd smoke the LED almost instantly, but I don't know yet.

How about something more realistic. If I used a 15 Ohm resistor in the circuit, I'd have 60 mA. How long would my hypothetical 5mm, 20mA, white LED last if I drove it at this current? Are we talking seconds, hours, 5 years?
Thanks!
Miguel

At 60mA the LED will last a long, long time (though not nearly the 100,000 hours predicted by the typical spec sheet.)

To put it in perspective, the Arc AAA, CMG Sonic and Infinity Ultra, and Inova X1 all drive their single LEDs to 50mA, give or take. This seems to be a sweet spot where per-LED brightness is maximized without shortening battery or diode life unacceptably much.

So 60mA is likely to be fine, though you'll get better battery life at almost the same brightness if you drop the current to below 50mA.

And if you don't already know, white LEDs deliver their best color, and the most lumens per watt, when driven below their service rating of 20mA. The implication is that more LEDs are better than harder-driven LEDs, if your packaging and budget permit.

Chalo Colina

Exactly the info I was looking for. Thanks a bunch, Chalo!

I got hold of some "Superbrite 10 Candela!" Green LEDs from Frys. With two Alk AAs, dim! I haven't tried two Lith AA (beyond me why not!). 3xN is decently bright and seems reliable.

Seem to work best in AA lights with barrel switches. I can't make it work reliably in a Minim*g.

Besides, Madmax and Opalec reside in my Minim*gs!

Just tried 2AA Liths.

3N is better as far as brightness.

I would think 2AA Lith would run longer at it's brightness.