Help: simplest/cheapest 4AA NiMH 2x Cree Q5 light

[dsvilko]

Hi all!
This is my first post so :wave:
This will also be my first DIY led light so be gentle
I am planning to build a simple and cheep bike light with a one (or preferably two) Cree Q5 emiters powered by 4 NiMH AA batteries (probably Sanyo Eneloop). It has to be 4 batteries (or at least an even number of batteries) as I don't have a 4-channel charger but a standard 2x2 AA charger.
So, what's the simplest (best?) way to drive them? Can I safely direct drive the two leds in parallel with one (or better two?) resistors?
I am looking for a strong light (1A?) for a minimum of 30 minutes. As I will be building my own housing with a star emitters mounted on a small copper CPU heatsync (overkill?) heat build-up probably won't be a problem.
Would I be better off using a current regulator? I have been looking through the DX site in search of one but it seems to me that they are either not available or better suited for a higher or a lower Vcc.
What about a reflector? I would like a bit weider beam (30-60degrees?). Can you recomend such a DX lens/reflector for the Cree emitter?

Thanks! Hope to learn much from this site!

 

[Drewfus2101]

Sounds like your already moving in the right direction. As far as a reflector/optic, I really like these:
http://dealextreme.com/details.dx/sku.1920

They are cheap but work very well with Crees.

As far as a driver board, with that input voltage (~4.8V) these will work well.
http://dealextreme.com/details.dx/sku.1885

They say to only run them up to 4.5V, but that is only because they will get hot above that. They are spec'd to 6V. So for your purposed, you would need to make sure they are mounted to the heat sink with some thermal glue:
http://dealextreme.com/details.dx/sku.4579

With this setup, each Cree would need a board, and each board would be fed by the 4 batteries. So that would mean that the batteries would see 2 amps draw. The Eneloops have (I think) around 2.5A, so you would get (in a perfect world) an hour and a half. Realistically you would see an hour or fun time.

Post up more info when your building it.

 

[dsvilko]

Thanks for the quick reply!
How's the beam with the http://dealextreme.com/details.dx/sku.1920 reflector? Won't it be to tight for the bike light?

As for the regulator circuit, that's exactly the one I was considering! Unfortunately, it's on back order and also quite expensive (as I only need two). Also, don't know how efficient it would be with the >5V input voltage.

As I am trying to keep the price as low as possible what do you think of something like this:
Freshly charged Eneloops, at 2A draw have about 1.35V (for a very short time) before falling to 1.2V at which point voltage stays more or less constant (for a 1500mAh). Let's say I calculate the needed resistors for a Vf 3.7V and If 1A at Vcc 4x1.35V=5.4V ... i get a 1.8ohm resistor for the each led. As the battery voltage drops to 1.2V the If will be drastically reduced (if I am not mistaken?). But I could then, by use of a switch, add another resistor in parallel (to both 1.8ohm resistors) and reduce the resistance to 1.2ohm thus ensuring the voltage drop on the led closer to 3.7V and the matching 1A current.
What do you think? Could it work?
Has anyone driven high-power leds with 4NiMH with only a resistor?

EDIT:

What do you think about http://www.dealextreme.com/details.dx/sku.3256 ?
Almost 90% efficiency in my Vcc range sounds very good
It's more of a 700ma driver but it's cheap and efficient. Now I am even thinking of driving 3 Cree R2 leds from a 4NiMH batteries

 

[Drewfus2101]

Here is some info I put together on the AMC boards.
http://www.candlepowerforums.com/vb/showthread.php?t=192925

I've got some better charts at home, but you get the point. At your voltage, the efficincy isn't great.

I've also used the other drivers that you posted with great success. I don't usually use them with such low voltage. Plus I like pushing the Cree to a full amp instead of 800 mA. But they do work great and are more efficient.

Those lenses still do provide some spill. I think you would be fine, but even if you were not, you wouldn't be out much money.

On the AMC drivers, what you can do is order the 1400 mA version, and remove one of the AMC chips from it to get 1050 mA. Or go the other way around and get the 700 mA and add an AMC chip to the emtpy area.

I think the resistor idea, while cheap and easy, is a bad one. It is very inefficient. Also, to be building such a nice and advanced setup and then use resistors is just a shame.

You can get the AMC drivers from kaidomain. Check and see if they are backordered as well. Don't let that be the reason you go to resistors.

Hmm......3 Crees driven at 800 mA as opposed to 2 driven at 1A is interesting. With the fact that the second drivers you posted would be more efficient at that voltage than the AMC drivers and that the Crees would also be more efficient at a lower voltage, I'd say that would be a pretty smart choice.

 

[dsvilko]

You are right, the AMC board does not look too good. It acts like a 'smart' resistor (Iin=Iout). The sku.3256 board seems much better! It's on back order but according to DX should be available in a few days.

Let's see the difference between the two setups @Vcc=5V:
AMC board can drive the full 1A current through the LED but also draws 1A. So, for 2 leds that's 2A draw and about 550 lm (I read somewhere that R2 can produce 275lm@1A)?
For the 3x sku.3256 setup we have about 3x0.53A=1.6A Iin with about 3x0.7A Iout that produces about 650 lm (if my calculations are correct)!
The second option, with 3 leds driven at 0.7A draws less power from the batteries and produces more light!
As I am planning to make a separate switch for each led, with 4 Eneloops I can get:
1h runtime @ 650lm (3 leds)
1.8h runtime @ 430lm (2 leds)
3.4h runtime @ 216lm (1 led)
For $35 (already have the batteries) I think it's going to be one sweeeet light!

I have I few more questions, if I may?
Will an old aluminium 50x50x35mm, 11 fin CPU heatsync be enough to cool 3 R2 leds @ 0.7A? I am guessing it should be enough.
Do I need a heatsync for the regulators? With 0.2W heat dissipation on each, probably not.
What about the batteries? With the Iin of 1.6A, how hot will they get? Can I just put them in a plastic tube or do I have to provide a better cooling?
Thanks again!

 

[Lynx_Arc]

depending on how much it cost you could just about buy 2 flashlights and mount them both on your bike and use one or both of them. It may not be quite the same as a 4AA 2 emitter light but it has redundancy and no worrying about heatsinking and circuits.

 

[dsvilko]

Thanks for the suggestion but there are a few serious disadvantages to buying a few complete flashlights:
- it's no fun
- it would probably cost a lot more than $30
- it would not be as bright (>600lm)
- most of them use a non-AA batteries (so add extra $$ for the batteries and the charger)

I will consider it but I like building things myself

 

[Lynx_Arc]

Thanks for the suggestion but there are a few serious disadvantages to buying a few complete flashlights:
- it's no fun
- it would probably cost a lot more than $30
- it would not be as bright (>600lm)
- most of them use a non-AA batteries (so add extra $$ for the batteries and the charger)

I will consider it but I like building things myself

good luck with it, sounds like a project for sure I hope you can figure out on how to make it weatherproof. I would be concerned about overdischarging your batteries because 4x 0.75v =3v would still drive LEDs too.

 

[Jagge]

Note, you will no get 5V for long. After few minutes cell voltage will be 1.2V under load. And you will have losses in battery holder (contact resistance), wires (especially if you have remote pack), switches and connectors. Driver may soon get something like 4.5V or less.

My simple bike light has one P7 driven at 2.1A using one simple 6 x AMC7135 board. About 1h from 4 x eneloop remote pack. Also about 650 lumens. AMC7135 based drivers aren't as bad as one might think.

 

[dsvilko]

Thanks! That's a good tip about over discharging the NiMH cells. I am thinking that the regulator circuit has some voltage drop over it so with the almost depleted cells the voltage over the led should be 3V or even lower. With the Vf of 3V the current should be about 180mA producing about 75lm - hopefully noticeably dimmer than the full (1 led) brightness of ~220lm.
How is that problem usually handled? Not all flashlights use LiION protected cells.
As for the weatherproofing... I am not planing on using it in a wet weather so a complete waterproofing will not be my primary concern but I will try to seal the most obvious holes.

 

[Lynx_Arc]

I am not sure how it is handled... one consideration is to go with 6AAs in 2 pairs of 3 in series that would be closer to Vf of the LED you could almost direct drive them at that rate and have more runtime.

 

[dsvilko]

The average voltage of a NiMH cell under 2-3A load is about 1.1V. Isn't 3.3V (3AA) too low for a 3W white led? I know it would light up but nowhere near it's maximum brightness.

As for the driver, I have decided to go with the http://www.dealextreme.com/details.dx/sku.3256 after receiving a very informative info from the DX user isotco with the Iout for my Vin range:
Uin 4.0 Iin 0.88 Iout 0.88
Uin 4.1 Iin 1.02 Iout 1.02
Uin 4.2 Iin 0.83 Iout 0.87
Uin 4.3 Iin 0.85 Iout 0.90
Uin 4.4 Iin 0.77 Iout 0.86
Uin 4.5 Iin 0.76 Iout 0.87
Uin 4.6 Iin 0.76 Iout 0.89
Uin 4.7 Iin 0.75 Iout 0.89
Uin 4.8 Iin 0.75 Iout 0.90
Uin 4.9 Iin 0.75 Iout 0.92
Uin 5.0 Iin 0.75 Iout 0.94
Uin 5.1 Iin 0.73 Iout 0.90
Uin 5.2 Iin 0.74 Iout 0.95
Uin 5.3 Iin 0.73 Iout 0.95

So, the Iout is almost as good as the AMC board but with a significantly higher efficiency.

 

[Lynx_Arc]

two sets of 3 in parallel splits that load in half.....

 

[aranyak]

The average voltage of a NiMH cell under 2-3A load is about 1.1V. Isn't 3.3V (3AA) too low for a 3W white led? I know it would light up but nowhere near it's maximum brightness.

As for the driver, I have decided to go with the http://www.dealextreme.com/details.dx/sku.3256 after receiving a very informative info from the DX user isotco with the Iout for my Vin range:
Uin 4.0 Iin 0.88 Iout 0.88
Uin 4.1 Iin 1.02 Iout 1.02
Uin 4.2 Iin 0.83 Iout 0.87
Uin 4.3 Iin 0.85 Iout 0.90
Uin 4.4 Iin 0.77 Iout 0.86
Uin 4.5 Iin 0.76 Iout 0.87
Uin 4.6 Iin 0.76 Iout 0.89
Uin 4.7 Iin 0.75 Iout 0.89
Uin 4.8 Iin 0.75 Iout 0.90
Uin 4.9 Iin 0.75 Iout 0.92
Uin 5.0 Iin 0.75 Iout 0.94
Uin 5.1 Iin 0.73 Iout 0.90
Uin 5.2 Iin 0.74 Iout 0.95
Uin 5.3 Iin 0.73 Iout 0.95

So, the Iout is almost as good as the AMC board but with a significantly higher efficiency.

Hi dsv, can I ask the result of this project? I am looking at a similar one (not bike light, more like a home emergency lamp in case of power cuts). Not yet sure if I should use the SKU 3256 and whether 2xQ5 or 3xQ5 configs.