Flashlight: Cree LED loses power overtime

Hello everyone!

I modded a flashlight I had around here and placed a 1Amp 3.7v Cree LED inside of it, with a voltage regulator to regulate a 9V battery to 3.7v and thats pretty much it.
Thing is, even though it works and its quite bright.... if I leave the flashlight on for about 5/10minutes it gets realllllyyyyy weak.

Not sure whats happening, but i suspect that the 9V is getting drained really fast even thought im limitin the voltage the LED is sucking as much amps as it can from the 9V battery.

since i dont need 9V, and amperage would be much more nicer... would replacing the 9v with ... dunno, 3x 1.2v AA 2700mAh, fix this problem?

So did you do anything to limit the current? You can't just apply a fixed voltage to a power LED and hope it will work. You need a constant current driver, even a resistor dosn't cut it with a power LED.

Having said that then yes you are draining the battery like mad. You need a switching regulator to be more efficient.

I think the voltage regulator is just acting as a voltage regulator. So i guess there isn't any current limiting thingy going on.
But the 9V battery should be something like 2xxmAh, limiting it to 1Amp (to avoid LED damage) wont help the battery from losing all of its current, and limiting to less than a Amp it will affect how bright the LED will burn.

Would the best option be, add a current limiter, and switch batteries?

A switching regulator will reduce the current taken from the 9V battery to below the current taken by the LED.
You can have a switching constant current LED driver that will not only protect the LED but get the maximum power from the battery without wasting much in heat like you are doing now.

What does "2xxmAh" mean?

AWOL: 200 and something mAh (im not sure about this but the battery has nothing written on it about mAh) and last time i saw a 9v with indicated mAh it was 280mAh. So this is a guesstimate

Grumpy_Mike: That sounds great! The voltage regulator does have a small heat-sink that gets a bit warm. I just cant remember how did I set up the LM317, as a voltage regulator or current limiter. (Sorry i modded the flashlight last month)
Using the LM317 as a current limiter would be a decent Constant Current driver? Or do you suggest some other electronic component?

The constant current regulator is like a voltage regulator in that it burns off the excess power as heat. A switching regulator on the other hand does not, it transforms the power into a magnetic field and then that field is used to generate the lower voltage you need. It does this with about 90% efficiency so the current draw from the battery is much lower than the current draw from the LED. So look for a switching power LED driver.

If you are using a small square 9V battery then use something else, those are useless.

Isn't that way too big to fit on a flashlight?
http://www.google.pt/search?gcx=c&q=switching+power+led+driver&um=1&ie=UTF-8&hl=pt-PT&tbm=isch&source=og&sa=N&tab=wi&biw=3068&bih=1582

Maybe i can build my own circuit with some kind of IC.
This seems to be something that might help me out: http://www.national.com/pf/LM/LM3405.html#Overview
Any suggestions?

thanks for the help Grumpy_Mike :wink: Always a gentleman

PS: Yeah I was using a regular square 9V battery. Tight fit on the flashlight!

SgtOneill:
Any suggestions?

There's a wealth of information on DIY flashlights, and mods, at candlepower forums.

Yes the LM3405 is exactly the sort of chip I was talking about.
It should increase your battery life by at least three times.

Going back to the original post. If you are using a linear regulator, isn't it basically dropping the extra voltage as heat. Thus a 1A draw from a 200mAH battery equates to about .2 hours of run time. Which is exactly what is being seen.

For efficiency you want to use a current regulator of some sort. Typical for a LED flashlight is something like this at Deal Extreme that uses multiple AMC7135 current regulators.

that uses multiple AMC7135 current regulators.

These are no more efficient than voltage regulators, they still burn the excess power off as heat. And there is still the same amount of excess power as in the original circuit.

Ill try the LM3405.
Also found some need Capacitive Touch Switch tutorial with the QT113 wich would be an awesome way to turn on the flashlight :wink:

guess ill need a new flashlight body for all of that.

I have a very nice Ring Cyba-Lite Orion flashlight which has a Cree LED and a DC-DC converter. It's very bright and the DC-DC converter keeps it working at good brightness until the battery is too low, then it suddenly fails. You could try modifying such a flashlight. What I don't know is the maximum voltage you can put into the DC-DC converter.

The alternative is to roll your own constant current switching regulator using a 78S40 chip or something more modern. The tricky bit is designig the inductor.

[EDIT: forget the 78S40, I've just looked at the datsheet for the LM3405 and it is far better suited to the job.]