Driver Help Needed - 4AA LED Lantern projects

[Tinkerer54]

I sure could use some help on selecting an efficient driver board. I just recently purchased 2 LED lanterns as mod projects. One is a clone of the original 4AA River Rock lantern, the other is
clone of the Osram Golden Dragon (also 4xAA). Both have had the original high power LEDs replaced by 5MM leds.

As a starting point, I'm looking at using the Cree XR-E Q5, although I do have a XP-G2 R5 coming in from KD. I'm shooting for approximately 6-8 hours runtime on high, so it looks like I'm talking a Buck driver. Batteries would be 4xAA either Alkalines or LSD NiMh. Currently, I have Tenergy Centura LSD AAs @ 2100Mah; though I'm eyeing the new Eneloop LSD 2500 Mah AAs.

I've studied this post on Drivers: Flashlights>Flashlight Electronics - Batteries Included>Flashlight Electronics - Batteries Included - Threads of Interest
"Drivers, how leds are adapted for different battery voltages" by HKJ November 2009
(sorry, but I don't know how to link to another post on this forum)

A single 7135 driver board seems an obvious initial choice. However, studying the graphs HKJ's post, it seems that when using battery voltage above about 3.6V, any excess voltage is just burned off as heat. Not too efficient; it seems like I'm just wasting the 4th AA battery as so might as well just short across the 4th battery contacts and use only 3AA. Also, I've read that KD recommends not going about 4.5V with these drivers, in spite of the 2.7 - 6 Volt rating due to heat issues. Am I correct on this?

Other buck drivers I see on KD, DX, and Fastech with higher voltage ranges like 3V - 8.4V put out far too much current and use unknown type and number of driver chips. If It's possible to reduce the current, I haven't found out how.

Can anyone give me some pointers on this?

You can safely assume I'm familiar with Ohm's Law and Kirkoff loop equations, and have read most of the CPF posts about the River Rock and Osram Golden Dragon lanterns, including mods. But drivers are still somewhat of a mystery. I'm an ex PC overclocker, so I'm familar with heatsinks and heat issues and have a good stock of heatsinks in sizes down to those fitting motherboard voltage regulators and ram chips. The Modding posts I've read were retaining the original River Rock or Osram driver boards, and so are not much help.

Any comments or suggestions would be greatly appreciated.

 

[VegasF6]

Sounds like you will want a switcher as you say. Probably ~400ma drive current. And as effacious an led as possible, your xpg2 is a good choice as low forward voltage too especially at those currents. You could look at the "kenan" driver also sold as "highly efficient flashlight driver 3-18v"
it's based on ax2002 ic and can change current by set resistor simple formula. Sku 26110 at DX.
Also it's only 2 bucks so order spares.

 

[Justin Case]

I would definitely use an LED that has low Vf, as well as decent output at lower drive currents.

If you assume AA NiMH cells with 2000mAh each, then to get 6 hr run time, you basically can draw 2000/6 ~300mA from the battery stack.

Let's assume that you have a buck driver that is 85% efficient. Thus, the battery stack delivers 4.8V * 0.3A * 0.85 ~1.22W. The 1.22W point on the XP-G2 datasheet graph of forward current vs forward voltage occurs at about 450mA forward current and 2.8V Vf.

At that drive current, the emitter lumens could be about 200 lumens for an R5 bin.

As VegasF6 suggests, I'd get an AX2002-based buck driver from DX or KD and mod the driver for 450mA drive by replacing the sense resistor. For the DX 26110, the stock sense resistor is R22. Replace it with an R56 SMD resistor. Probably something like a 1206 1/4W SMD. That will give you a 450mA buck driver that should be able to run in full regulation for basically the full run time of your NiMH cells (starting at say 4.8V and going down to 4.0V when nearly depleted). Even when empty, the cells should have a Vbatt that is 1.2V above Vf.

 

[Tinkerer54]

Thanks to both of you! That was the magic I needed; and ID on a driver's chip, and how to modify the current output. I found the DX 26110, captured the high rez photos, and could see R22. This driver will come in handy for some upcoming 9V projects I have, now that I know how to modify output current. I'll buy several. 3 Questions came up:
1) A reviewer on DX confirmed he thought is was a AX2002 IC, and commented about modding the output current with the loop back resistor. Is that what R22 is?
2) Where can I find the formula for calculating the current given a R22 resistor value? I'd like the method for other projects.
3) This is a single mode driver. I wonder if anyone has ever tried to put a pot on the positive supply wire to the LED for a dimmer?
A lot of questions, but I'm new at this.

 

[VegasF6]

Yes when he refers to the loop back resistor it's the sense resistor. R22 in that case. Your driver may differ from the photos. I have had them come set anywhere from 800 to 1200ma. Also with 1 and with 2 parallel resistors. If you continuing reading the reviews you should find mine that includes the formula. As I recall its sense voltage/current = r.
I think 250mv. So for instance .250/.4 = .625 ohms.
And there is a long thread titled modding kenan driver or new kenan driver or something like that is like 50 pages long. (Maybe not quite). It talks about using a switch to add extra resistance, using potentiometer etc. on my phone so hard to search.

try this.
http://www.candlepowerforums.com/vb...-DX-26110-or-Kennan-from-Kai&highlight=Kennan

 

[Tinkerer54]

Thanks again VegasF6. I found the Kennan thread you were referring to, as well as stuff on dimming along the way. So now I do my homework.

I just ordered 5 of these sku 26110 controllers from DX, now to get will get a 2nd XP-G2 on the way.

*slight correction* I incorrectly gave the Osram Golden Dragon clone as 4xAA when it's really 3xAA. But I was thinking on on using a 3rd lantern, a $5 Coleman w 5mm LEDs that IS 4xAA as the base, using some optical parts of the GD clone, which has the same battery box problem (with NiHh cells) as the original.

 

[Justin Case]

Just Google "ax2002" and you should find the PDF for the datasheet, which gives you the formula for the sense resistor value to give you whatever drive current you want. The equation is (drive current in amps) = 0.25/(sense resistor value in ohms).

Putting a pot on LED+ is probably not a good idea. You are using a constant current driver and it will see some weird load. The SPICE model for the XP-G2 has a parasitic series resistance of 0.26 ohms, so if you had a pot that added say 1 ohm, the driver would simply think it has to drive 5 XP-Gs in series.

For dimming, try post 138, assuming that you find an AX2002 board like the one shown (see Kaidomain product S002982).

 

[Tinkerer54]

I really appreciate it Justin! I now have the AX2002 datasheet and found gillestugan's post 138 for the dimming mod - the x-ray photo he added to show the board trace to cut was impressive.

I'm just looking for 1 low level dimmed from full output. For gillestugan's photo showing the 10K pot coming off the AX2002 pin, I'll probably try adding an external switch inline with the pot. Once I've adjusted the pot to a dimming level I like, closing the switch will then allow me to go to the DIM LED mode from outside the finished lantern..... UNLESS it won't work on full mode with the switch open due to the cutting the board trace for the dimming mod. Waiting on the drivers coming from DX in China gives me lots of time to try to figure that out. Or I'll just find out by doing.

You guys have been a great help. I'd read and researched CPF and the web enough that I knew I wouldn't solve the problem without getting some inside information on where to look.

Incidentally, I was looking on Mouser.com for parts, and found they carry large selection of LED optics to fit about any LED model, including all the Crees and Osram LEDs. They also carry the new Osram Golden Dragon Plus LEDs at prices about the same as KD and DX charge, but without the wait on shipping from China.

 

[Justin Case]

You might experiment with a 5K pot to see if it gives you better dimming range.

The circuit is a voltage divider. You can calculate the effect to give you an estimate of the resistance setting on the pot that you might want to give you the dimming level desired.