What resistor to use with luxeon3 with 4 AA

Just wondering what resistor to use to drive a luxeon 3 with 4 batteries either 6v , /ubbthreads/images/graemlins/thinking.gifor 4.8 v with nicads.
I need the maximum brightness and don't care much for run times. Also what wattage should the resistor be.
I have got a Twok and i would hate to screw it up.
Thanks guys
Ledean

http://flashlightreviews.home.att.net/articles/tiptrick_old.htm

this is from Quickbeam's site--it is where i learned how to calculate resistance!

um...direct drive might not be a great idea with nicads...i would guess that a small amount of resistance would be perfect--and heat sink well.

Bob

Depending on what sort of current/voltage you want to feed your LuxIII, you could get away with a 1W resistor, or be conservative and use a 10W resistor.

You might also get a little crazy and use a pot in series with a resistor so you can adjust your brightness as your battery voltage drops. Make sure it's not exceeding its wattage (2W pots are the biggest I've seen on a regular basis).

If you're really clever, you can use 2 resistors and the pot in a series-parallel setup.

The resistance of NiMh AA is higher than C and D NiMH's. Perhaps there will some sag in voltage, and you could get away with 1 Ohm, particularly if using a K or L binned III?

Bill

[ QUOTE ]
Bullzeyebill said:
The resistance of NiMh AA is higher than C and D NiMH's.

[/ QUOTE ]

This is simply not true. At least not to any useful degree. It's measured in hundredths of an ohm. Go look it up.

The best answer is 'measure what you get', however if we want to hold to say 750 mA, and have a four Volt Vf and a six Volt battery we'll need 2 Volts (the difference) divided by .75 (the current we want), or 2.66 ohms total. Since we're talking .75 Amps and 2 Volts, we're talking 1.5 Watts. Some of this needed resistance is in the cells and LED itself (and will change value with discharge and heat), so I'd start out with two ohms, at least two Watts, and see what I got.

For sure a proper current regulator is the way to go, but if you're going to use a simple resistor I suggest you need to use facts about the parts and established laws (like Ohm's Law) in design rather than opinions.

Doug Owen

The only way to be sure is to measure the current. Every flashlight and every set of cells and every Luxeon is different. You are beginning with the assumtion that NiMh are 1.2 volts each. That's not what I see. My Rayovac 15 minute NiMh AA's measure close to 1.5 volts. Since the NiMh are able to produce higher amperage than alkalines, they can completely change the amount of resistance you need. So if you are planning to use rechargeables, determine your resistance needs with freshly charged batteries and a multi-meter. I can just about guarantee you that 2 ohms won't be enough resistance for 4 rechargeable AA's.