Direct drive XML question, what resistor?

[freeloader700]

I am trying to direct drive and XML-U2 on 3D cell maglite. It is very bright but at about 15 sec it starts to smoke a bit from the heat. There is not blue or anything. Just a bit too much current. So now I think I need a resistor but what wattage and what value I don't know. I need some help please. Here is a beamshot of it. Barn is about 40 yards away

Very bright spill

http://img.tapatalk.com/d/12/11/03/meraqyha.jpg


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[Thr3Evo]

Honestly, no need for this thread. Ever heard of ohm's law?
Also, spend a few min looking around this forum and you will find ALL about direct driving high power LEDs. Answers are out there ppl don't be lazy.

 

[freeloader700]

I looked around for quite a while today and I didn't see anything with using 3-D's. I seen threads using AA's, li-ion and what not for direct driving. I know how to use the search function. I would have not posted this thread if I had received my answer.


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[Thr3Evo]

One again.... Ohm's law, friend.

 

[kuksul08]

V=IR
3 D's=4.5V
LED=3.35V
I=3A max
R= (4.5-3.35)V/3A
R=.383 Ohms minimum

P=IV
I=3A
V=4.5-3.35= 1.15V
P=3.45 Watts minimum

Now run with your new found knowledge and apply it to 99% of electronics.

 

[yazovyet]

unfortunately he didnt specify batteries (i think?). alkilines may not hold 1.5 volts at 3 amps (they sag pretty bad at high current) and NiMH will only be in the 1.25 area (give or take). then you have the fact that there is voltage drop off as the energy level is reduced in the batteries and the relativly high (1.35 volt for NiMH) voltage on freash batteries compared to the somewhat flat area of teh discharge curve.

in the intrest of getting more than 30-45 min runtime outof a set of alkilines he may wish to use a resistor to have a current lower than 3 amps, like 1 amp so eh might get 5+ hours.

 

[Gtamazing]

I run an xml direct drive off 3D's I never used a resistor, I used alkalines and nimh. I even ran one dd off 4 "Duraloop's".

 

[moderator007]

What kind of heat sink did you use freeloader700. Did you use a good thermal adhesive. Sounds like you may have a bad thermal path somewhere. The heat can't get away from the led.

 

[freeloader700]

I used artic silver and am using H22a heatsink. Led is on a 16mm star. Had to widen the bottom of the reflector to fit it.

So it looks like use a .4ohm resistor? Thank you guys for your help

And I am using alkalines


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[Vesper]

Honestly, no need for this thread. Ever heard of ohm's law?
Also, spend a few min looking around this forum and you will find ALL about direct driving high power LEDs. Answers are out there ppl don't be lazy.

If you have the knowledge, how about a little more help and a bit less belittling.

 

[Thr3Evo]

If you have the knowledge, how about a little more help and a bit less belittling.

If ohms law is that hard to figure out, perhaps one should not be doing things that require it. And if that question hasn't been answered here on CPF before I would agree with you. "Lead a horse.... Teach a man how to fish...." Get me?

 

[Illum]

If your power input allows you to direct drive I would recommend a linear CC regulator... a resistor only limits current when the input voltage is stable. Battery voltage will sag and eventually deplete. Regulating any current above 100ma I would not recommend using a resistor

 

[Old-Lumens]

If ohms law is that hard to figure out, perhaps one should not be doing things that require it. And if that question hasn't been answered here on CPF before I would agree with you. "Lead a horse.... Teach a man how to fish...." Get me?

If it's so hard on you to read a thread like this and not be able to respond in a nicer manner, maybe no response would be better. The simple fact is that searching is less than desirable and one can spend a long time searching for the correct answer The way the search results come up, one can search for what they want and come up with no correct answer. As far as Ohms law, one does not have to understand it, to use a flashlight, nor to mod one. That has been shown for many years now and though the ones that do understand so well may not like the fact, you don't have to be knowledgeable in all things, to make do, or to mod a flashlight. There have always been members here that are willing to accept that fact (and share the information), and some that can't.

 

[Thr3Evo]

I'm pretty sure I gave a correct instruction for the question posted. No where in the op was there anything about modding lights. Question was about a resistor value, knowing voltage and also about its power dissipation....how do you find those out old-lumens.

 

[Gtamazing]

I'm pretty sure I gave a correct instruction for the question posted. No where in the op was there anything about modding lights. Question was about a resistor value, knowing voltage and also its power dissipation....how do you find those out old-lumens.

Vulgar comment removed.

 

[Gtamazing]

I'm pretty sure I gave a correct instruction for the question posted. No where in the op was there anything about modding lights. Question was about a resistor value, knowing voltage and also its power dissipation....how do you find those out old-lumens.

Vulgar comment removed.

 

[Hoop]

Just to add some thoughts: a three way toggle switch could allow for two modes, with one side of the switch using a resistor for the low mode, and the other direct drive for high. Direct driving off of a single lifepo4 is an option too, or multiple lifepo4 cells arranged in parallel, which can be done in a single mag tube with creativity. 32600 is the same size as a D cell and they do make lifepo4's in this size. 26650's are easier to find and can be used in a D mag with a tubular spacer made from PVC pipe. The voltage curve of lifepo4 remains very flat at 3.2ish volts for most of the discharge. Let the lifepo4 cell rest for a day after charge and it should be sitting close to nominal at 3.3 volts or so. You wouldn't need an expensive charger for this setup, something around the $20 price point would suffice quite well.

 

[Robocop]

The OP asked a valid question and the point of this entire forum is for us all to share information. Some know more than others however again we can all benefit from discussion. Maybe the OP will learn about resistors, proper heatsinking,drive current or other solutions to his problem however he will gain nothing if others stir up drama.

Thr3evo you are fairly new and while your intentions may be good your delivery style is very harsh. Welcome to CPF and enjoy yourself however please try to understand that you can share your information here without making others feel unwelcome. To the OP I hope you find your answers and to the others involved do not say anything if you can not say something nice or at least civil.