3 watt MXDL How does it work?

I have bought a couple of the 3w "Luxeon" lamps.
Can anybody tell me where the resistor is to reduce the forward voltage? Is the body used as the resistor? By my calcs I would expect a resistor of appx 1 ohm to reduce the voltage to 3.7 volts? From 4.08V running at 700 mA.
I also tried increasing the voltage form 4.08V to 5.47V (4 rechargables) The LED brightness increased. Then tried 6.45V - Blew the LED!!! Should have checked the calcs before experimenting....
With 550 mAh AAA batteries I am getting appx 1Hr "bright" run time. Not quite sure how this works....
I=V/R therefore:-
I=4.08/(0.8+1.142)= 2.21A -Dynamic Resistance of LED + "resistor".
With 550 mAh batteries this should give a run time of 0.55/2.21 = 15 minutes run time !!!!
Could somebody check/correct my calcs???
I will now check the actual run times on my new 850 mAh AAA batteries.
Thanks For any help.

you can't find the resistor because it it's not there...
i have a couple of these lights, there areno resistors in mine either.

So how do they work? /ubbthreads/images/graemlins/confused.gif
Is the current from 3 AAA btteries below 1000 mA?
I thought the Forward voltage had to be lower than the supply voltage???? /ubbthreads/images/graemlins/confused.gif

these flashlights don't have any resistor (except the little of the body/switch but i think can be about 0,1 ohm...)

you must consider the voltage drop of the batteries: 3 alkaline AAA without load have about 4,8 volts, but under 1 amp of load about 3,6 volts

3 ni-mh AAA without load about 4,2v but under 1 amp about 3,3v

How can I measure the current draw of the LED?
I have a multi-meter but it doesn't register any amps when put in line and the LED hardly lights up.

Slime--When you say "in line" you mean that you have cut the electrical path and then jumped that cut with the ammeter. Right? (I don't mean to patronize, but a lot of people don't understand the difference between measuring current and voltage.)
Are you sure that the fuse in the meter is good? Most DMMs have a fuse to protect the amp scale in the event that it is accidentally connected to a voltage source.
As far as why there is no resistor, the LEDs in these lights are protected by a concept called "internal resistance". No battery can supply infinite current. Therefore both Voltage and current produced by the battery are finite. Since R = V/I (Ohm's Law) then there is a finite Resistance associated with the battery. It is as if all batteries are built with an internal resistor that limits their maximum power output.---Marc

Marc,
Thanks for your reply.
Yes I have broken the path to measure the current. I guess it is someting with the meter.
Is there any way of finding the max current for any given aaa battery?
Thanks,
Perry

The manufacturer's spec sheets are the best place to start. Both Duracell and Energizer post technical data on their websites. Everybody else should be quite close to their specs for equivalent products.
The "maximum current" that you are asking about is a fuzzy item.
If you really are looking for people that are experts in batteries you should start a thread in the "Electronics" forum here at CPF. A lot of guys typically design lights that draw more than the manufacturer's recommended current from batts.---Marc