LED question for electronics wizards

Here is another thought. Using the math I did, if you had a 12V battery and 3 LEDS, each 3.5V, then your resistor would be 60 ohm, and you could use a 1/4 or 1/8 watt resistor. However! If your LEDs turned out to be 3.0 V and not 3.5, then the current through them would be

If you want to get fancy, you could build a current regulator and set it at 25mA. Use this in place of your current limiting resistor. Then you could use any combination of 1, 2, or 3 LEDs, and the current would still be properly regulated. An LM317 will do this quite nicely. The downside of this is that you need to understand a bit about electronics, or find a friend that does to design it for you. I could whip one out pretty quickly, but if you don't know a bit about electronics, you won't know what to do if it's not working, or how to test that it is indeed working correctly.

The ONLY reason I limited the discussed resistance to 500 ohm resistors is that the poster said that the LEDs came with them.

LEDS also vary in the voltage drop across the LED. So, you generally need to read the complete spec sheet, or do a little testing. I.E. lets say that the LED drops 3/10v at rated current. So, the resistor only drops 3.3v instead of the 3.6. Actually, the LED may have a much greater voltage drop due to internal construction. Another issue is brightness matching, esp. between different color LEDs. Maximum applied voltage may be 3,6v, or it may have been the working voltage spec. I have no way to know. Some LEDs have internal regulation and or series led chips. Yes, the dissipation of the resistor must be taken into account. Certainly series operation will result in less resistor dissapation, thus more efficiency. It may be that a combination of series and parallel leds with small value balance resistors provides the best overall solution.

This is exactly what I'm planning to do. I'm going to put one resistor and 3 LEDs in series in each rib bay, all powered by a 12 volt source. I wired up a set of 3 with a 68 ohm resistor and left it connected to a car battery for a couple of hours and everything works great.

I'm going to hit the hobby store tomorrow and see what looks good for 12 volts. Considering the number of LEDs I'm using, a 750mAh nicad pack would theoretically give me an hour of light. I'll see if he has any lead/acid gel cells that compare favorably.

Thanks to everybody for the help.

Ok. I dug out my Panasonic 5v Gel Cell (took me two days of digging around in the garage to find a no-longer-used item) and in parenthesis is this: (5.8v). So, even though the gel cell is marked as a 5v cell it is in reality closer to 6 volts. Used in an alarm system as backup for the microcontroller and relay boards. Charged with a 6.2 volt charger. After two years or more in storage, it still has a charge of 5.5 volts. Gotta love lead acid, eh?

-- Ray