Cree Direct Drive
- Thread starter chadwide
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IsaacHayes
Flashlight Enthusiast
AFAIK the cree's Vf vs current doesn't rise up like a luxeon, so just a few more milivolts could mean tons more current. So you really need a convertor. I'd at least start with a resistor of decent value, measure current drawn on a fresh hot off the charger cell (4.2v) and go from there until you get desired current. That way when throwing a fresh cell on there it won't nuke it. It will dim down as the cell settles, but at lest the led will be safe.
IsaacHayes
Flashlight Enthusiast
3 volts from a primary would be under the Vf @ 350ma yes. It will probably pull under 350ma. How much exactly will depend on the bin. Still should be some decent lumens for just a primary battery hooked up. That will work for now until you get a driver or something on there for more light and better control of the current.
chevrofreak
Flashlight Enthusiast
I thought about this a while back and took a look at the charts Silverfox made for the LG 2400mAh 18650, then compared it to Newbie's current and voltage chart. If you tried to direct drive an XR-E on a cell like the LG 18650 the LED would be eating about 2 amps, maybe more.
The voltage should sag enough on an RCR123 that it wouldn't try drawing that much power, but I'd still not try doing it without atleast a small resistor.
The voltage should sag enough on an RCR123 that it wouldn't try drawing that much power, but I'd still not try doing it without atleast a small resistor.
Ganp
Enlightened
I have an XR-E direct driven from one of AW's protected 18500 3.7V rechargables. It seems OK so far although its not left on for more than a few minutes at a time.
Colin.
Colin.
LEDite
Enlightened
Chadwide & Ganp;
You need a resistor for direct drive.
You can start with a 2 ohm 1/2 watt to get you in the ballpark.
Check the voltage across it.
Larry Cobb
You need a resistor for direct drive.
You can start with a 2 ohm 1/2 watt to get you in the ballpark.
Check the voltage across it.
Larry Cobb
Last edited:
IsaacHayes
Flashlight Enthusiast
chadwide, the flupic would work great with the cree and a li-ion. IIRC the flupic isn't buck but LDO, which does the same thing but not quite as efficiecent I believe. Vin to Vf should be close enough that this doesn't make a big deal.
IsaacHayes
Flashlight Enthusiast
Goldserve is the creator of the flupic, click on the link and you can see his signature with links to threads, and you can click "see all threads started by user" and see past threads about the flupic.
http://candlepowerforums.com/vb/member.php?u=7270
http://candlepowerforums.com/vb/member.php?u=7270
I've driven the Cree off of 3AA batteries and the color looked greenish. I think you'll need a inline resistor. A 2 ohm resistor is way too high. You only need a small voltage drop. Using a 2 ohm resistor will run it around 250mA. I think the Cree only uses around 3.2v so you will only need to drop the voltage by .5v. If you are planning on having a current of 700mA you will need a (.5v/.7A) = .7 ohm resistor. I would run it a little hot so a .6 ohm resistor rated for .5w should be good.
IsaacHayes
Flashlight Enthusiast
greenish? that can be a sign of overdrive and overheating!!! if it turns deep dim greenish blue then it's being hurt big time!
