On Fri, 12 Aug 2005 04:45:39 -0700, Larry Jaques
wrote:

On Thu, 11 Aug 2005 21:48:36 -0500, the opaque Don Foreman
clearly wrote:

On Thu, 11 Aug 2005 18:32:52 -0700, Larry Jaques
wrote:


Excellent. Thanks, Don. I picked up several white LEDs and haven't
yet experimented with them. I suppose Lithiums would be a good power
source for those, too.

Yes.

You'll need a dropping resistor (or elex) to limit LED current. A
nice thing about lithiums is that their voltage doesn't droop over
life nearly as much as alkies do. Alkies go from 1.5 to 0.8 volts
per cell. CR123's go from 3 volts to 2.5 volts at about 90%
depeleted, to 2 volts in the last 10% of life.

White LED's typically run somewhere around 3.4 volts, so you'll need
two 123's. They'll then droop from 6 volts new to about 5 volts
nearly gone. Resistor voltage (hence current) goes from 2.6 to 1.47,
a change of about 38%. WIth 3 alkies, voltage goes from 4.5 volts
to 2.4 volts -- but the light goes dark at 3.4 volts (1.13 volts per
cell) so you discard batteries that are only a bit over 1/2 depleted
when the light has diminished from full brightness to no brightness
during that time.

That's how some flashlight mfrs get their lifetime claims with
alkies. They may produce light for xx hours, but it will be
significantly less than half output for most of those xx hours.
With 3 AAA's (and resistor current limiting) you're down to half
output (and half drain) at 1.31 volts per cell which is only about
25% depleted, down to 25% when the batteries are about 39% depleted,
and so on.

White LED's (other than Luxeons and a couple of others) are typically
rated to run at somewhere between 20 mA and 50 mA. Overdriving them
just shortens their life, which may be acceptable. They'll still last
much longer than incandescant bulbs. If you want to run several
LED's, each should ideally have its own dropping resistor for
operation from a 6-volt source or less. Some flashlights just rely
on the internal resistance of alkie cells -- so lithiums may
overdrive the hell out of them.

For 2 CR123's and 50mA per LED, a 51ohm resistor is about right.
For 100 mA per LED, about 27 ohms is about right.

There's a ****-simple elex circuit (about 3 bux worth of parts) that
can supply dead flat current (constant brightness) over battery
lifetime -- and it could drive several LED's in parallel if they were
fairly well-matched in forward voltage drop. Email me for details
if interested. It's also on the web somewhere.

Check out one of those 3W luxeon lights to go with your P-11. A
3-watt Luxeon puts out about 60 lumens, comparable to your tactical
krypton light -- and the bulb won't ever burn out. Some Krypton
bulbs last about 10 hours -- the bulb, not the batteries!

Have fun!