(yet another) Need help with LED wiring and controller selection

Hi all and thank you for taking your time and looking in!

I need some help in setup I have. Don't want to bore anyone with detail of why and where. I will just go into the basic facts:

a.) LED: I have 4 x LEDs (all identical). All details can be found here: http://www.led-tech.de/en/Light-Emitting-Diodes/0.5W-Power-LEDs/0.5-Watt-Power-LED-LT-1026_1_86.html
Most important (relevant specs are): 100mA, V_typ:3.4V (forward voltage I think??), V max: 4V, , 0.34W.

b.) Source: I have a battery pack of 8 x 18650 (3400mAh Panasonic) cells in 2S4P configuration so output = 8.4V~5.5V (that is full charge to cutoff range) Avg: 7.4V (your typical bike light packs).

And yes, you guessed it. I would like to drive a.) with b.). with preferable to most optimal (as in least waist of power) way possible.

To quickly get some obvious questions out of the picture. I mean like "why 2S4P? why not 1S8P? on the battery". Well the answer is simple. I have Consumer bike light (actually 2 of them) that need the 7.4V source (they have the usual control and protection circuit built-in). So the 2S setup is needed. Take it as a given.

QUESTIONS:
What I need help with is sorting out the connection and regulations. I know the basics of parallel vs serial LED connection. Yet I still managed to complete confuse myself when I started considering the following:

Should I go for 2S2P with the LEDs to get the setup closer to the source? Or is that a stupid idea? What connection (serial or parallel or hybrid) is most recommended in my case, especially taking the question below into considerations (regulator options)?
I want to use a controller but not sure which one. Please look at some of my options here: http://www.led-tech.de/en/LED-Controlling/Constant-Current-Power-Supply-c_118_119.html
(the LEDs I already have, but controller is an open question, so I can order from ebay if you have a better suggestion or non of those fit my actual needs).

I really don't want to use resistor. I see BUCKs are much more expensive than the Mini Constant Current Power Supply (not pretending that I know what the real difference is), but if you say I am better of with a BUCK then I will get that.

And finally allow a very stupid question: Those the fact that I have other things (the above mentioned consumer bike light) hooked up to the same source (obviously parallel) make any difference in the solution? That is choice in regulator and LED wiring. I say stupid question because I know that the answer is NO, however I have gotten myself so confused in the mini project that I am not sure anymore.

PS: I am not lazy and asking anyone to help me lookup the right regulator for me on ebay. What I need help with is the wiring recommendation and the spec calculations.

Options I came up with so far (please let me know if I am wrong or right):

1. Buy 4 x this: http://www.led-tech.de/en/LED-Contr...upply--100mA,-up-to-37V--LT-1220_118_119.html and 1 connect to each led. Then parallel them all to the source.

or

2. Connect 2 LED in series and 2 in parallel and find a Step Down BUCK with 200mA output, 5V-9V input. Haven't been able find one, only circuit diagram: http://designs.digikey.com/library/4294959899/4294959898/163

or

3. Connect all 4 LEDs in parallel and same as No.2 only 100mA output. Still looking for actual product, but would be something like this I guess: http://www.linear.com/product/LTC3631

(if all 3 is correct, then which should I do, which is most efficient and viable?)

Thank you very much in advance.

Option 1 will work, but is the least efficient. That's a linear regulator, which means it will regulate better than a resistor, but has the same efficiency. The efficiency of a linear regulator is Vled/Vbat.

Option 2 would work if you could find the right regulator, but when the battery voltage gets low enough, the output current will start to drop (the regulator "comes out of regulation"). Buck regulators need higher input voltage than output voltage. Some only need a fraction of a volt, some need more than a volt, and all have specific minimum input voltages that may be even higher than that for 1S LED configurations.

Option 3 would work if you could find the right regulator, but what you want is 400 mA, not 100. When you put LEDs in parallel, the currents add and the voltage stays the same. When you put them in series, the voltage adds up and the current stays the same. I think this is probably your best option, because you will get the best efficiency, and it will stay in regulation until your battery is dead flat. On second thought, maybe the LED dimming would be a good warning that your battery is running down.

There are loads of 350 mA drivers around, you should be able to find one that would be suitable, and you'd just get a little less light than at 400 mA. There are also some 700 mA regs that can be dimmed, which would be suitable. For option 2, you might find a 350 mA reg that can be dimmed.

The BuckPuck line of regs are available from many retailers, and they offer a 350 mA one that would be great for option 3, as it will operate with an input voltage down to 5V. The ones you linked to only run down to 7V. For a little more money, they offer a dimmable one but the input voltage range only goes down to 7V. Below that it may well continue to operate, but it might not perform to spec. At some point it would just not work, but there's no telling where that is.

DIWdiver said:

The BuckPuck line of regs are available from many retailers, and they offer a 350 mA one that would be great for option 3, as it will operate with an input voltage down to 5V. The ones you linked to only run down to 7V. For a little more money, they offer a dimmable one but the input voltage range only goes down to 7V. Below that it may well continue to operate, but it might not perform to spec. At some point it would just not work, but there's no telling where that is.

Click to expand...

Thank you so so much! So I'll go option 3. With this: http://www.ledsupply.com/led-drivers/buckpuck-350ma-constant-current-led-driver-without-dimming or this: https://illuminationsupply.com/led-drivers-c-52_32/bucktoot-buck-350ma-p-130.html#.UqUB_-KmZL8 I guess they are just different manufacturers of the same function. I don't have to set anything right? I mean output voltage wise. I see some BUCK have trimmers for both current and volts. I too don't believe I need to drive the led at 100mA the 87mA (350/4) should still give the brightness that I need (the battery will actually last longer this way right?).

BTW: Source will never go below 5v as cell have built in protection, I don't even think they would go lower that 2.8 per cell, so more like 5.6V lowest.

P.S: I see some of the Buckpuck's have dimmer and some have trim pot http://www.ledsupply.com/buckpuck-c...alog&mode=catalog&parent=998&pg=1&pagesize=54. I don't understand what the difference is. Could you explain? Just curious.

Edit: I guess this doesn't have any advantage over the others (strictly for my case I mean). I just wonder cause it is cheaper than both of the above options. Might it have better efficiency therefor a better option or is it too much of an overkill? http://www.prodctodc.com/dc-530v-to...buck-module-with-cc-cv-p-93.html#.UqUD3uKmZL8

Best regards
Alex

A trimpot is a variable resistor that's used for adjusting the output current. The one with trimpot has this part inside, the one with dimming has pins for connecting it externally (supplied by you), and the one without has no pins to access the trim control. The one with trimpot has 40-110% range, so you could get it pretty near 400 mA. The one for external trimpot has 0-100% range.

The prodctodc one would probably be a little less efficient, but it's hard to say. They say 'around 85%' but all the pictures show it running at less than 80%. It's definitely more flexible though. There's no telling how much overhead (difference between input and output voltages) it would need. The buckpuck data sheet says it needs 2.5V, so to run your leds at 3.4V, you'd need 5.9V at the input. 2.5V is on the high side, but at least you know it will work.

My only qualm with the Buckpuck 3023-D-N-350 is with the efficiency. In the factory data sheet it only starts the graph at 10V input. Doesn't say anything about low voltages. The BuckToot DC Driver - 350mA (second one linked) has a lower input margin, That is 2V compared to 2.5V of the Buckpuck. I don't know if this might be an indication that the efficiency would be better. At least in their product sheet they don't hide the "ugly" values which occur (I guess with all such Bucks) at the low input voltages. See datasheet: http://www.superbrightleds.com/more...led-driver-bucktoot-dc-constant-current/1056/ seems to be about 73-73% efficiency at this voltage.
Anyway I am very grateful, that I know what I need. I will now try and look around for the most efficient model that I can find (hope I can find something with over 85% at said voltage). Thank you for pointing me in the right direction.

The efficiency of a buck regulator doesn't inherently go south as the output approaches the input (or vice versa). I designed one that runs on a 12V SLA, and the efficiency rises, approaching 99% as the battery discharges to the LED voltage and it falls out of regulation. Check it out here http://www.ritelite.co.uk/utilities/products/portable_lighting/supalite_k9s.htm

It would certainly be possible today to design one that would be over 90% efficient with 0.1 to 0.2V overhead at 3.4V output. It might not be cheap....

Hey DIWdriver!

I just wanted to thank you for all the help you gave me here.
I finally did go with your first recommendation (BuckPuck)

Here is the final result: http://forums.mtbr.com/lights-diy-do-yourself/finaly-ready-diy-pimp-my-ride-897813.html



So thank you so much.

Best regards
Alex

Cool! Thanks for sharing.

DIWdiver said:

Cool! Thanks for sharing.

Click to expand...

I realized I have so much spare power with the 13Amp setup that I would like to charge my smartphone too while riding (I use it as a bike computer so screen is always on). Currently it is a Samsung Galaxy Note 2 (but will probably change to Galaxy S5 once its available).

Anyway I know I probably need to connect a Step-down converter switching constant voltage buck (So many ways I see it called).
Two questions that I have.
1. Does it mater if the buck is rated 3A or even 5A, that will not cause any problems with the smartphone as the factory charger is rated 2A and I know it draws about 1.5A. Also I know I read some places that people had problem charging a particular phone with a particular buck. I even saw a buck listed on ebay and in the description it was mentioned that it didn't work well for iphones. I don't get how it matters what phone it is. Its USB ~5V what difference does it make? Also note that Samsung S5 is USB 3, but I think the voltage rating are the same.
2. Do you have any recommendation in particular for this application. So basically same 8.4V~5.5V input (6V min is quite fine) and constant 5V output and 1-2A draw. I find so many option I find it hard to decide. Altouhg many are not good for my application as the most frequent stuff is for cars (12V) to usb and many are input volt min. 7V. Here are some I found:
http://www.ebay.com/itm/4-38V-to-1-...5V-12V-5A-Car-Voltage-Regulator-/271372587932
https://www.adafruit.com/products/1385
http://www.adafruit.com/products/1065
http://www.ebay.com/itm/DC-DC-Conve...d=100005&prg=1088&rk=3&rkt=5&sd=271372587932&

Thanks again.