Boost Converter help..

Ok, so if anyone had read my post about needing a buck or DC-DC converter to power some 20-40ma 3.6v LED's:
http://www.candlepowerforums.com/ubbthreads/showflat.php?Cat=&Number=1051429&page=3&view=collapsed&sb=5&o=&fpart=1
then they know my dilema... however, I figured I may be going about it all wrong and actually want to use a boost converter. The ones I've seen are the Badboy and the Wizard2. Would they be capable of a Vin of 1.2v or 2.4v and a steady Vout of say around 3.6-3.8v? Did I miss something with these? There is not much info on them. Any help would be greatly appreciated! Thanks!

I think those convertors really need about 2.8 volts or better to run. They're designed for two AA cells, or a single CR123. The Wizard will also convert down, so you could use maybe three or four AA cells, or two CR123.

Try a MadMax converter. It runs great on a single AA rechargeable cell (1.2 volt).

How much current can the Madmax have on the Vout? I mean, trying not to get too technical.. what is the efficiency if I were to use say a 10000mah D 1.2v NiMH to power a total of say 600-800ma at 3.6v's? Would I still get around 10000mah or less?

How many LEDs will you be using?
What type of battery?
This will help decide whether series or parallel or combination is best.

To run 6 or 8 white or UV leds in series at a current regulated 20mA from 3 AA NiMh's, this Linear Tech IC LT1932 works well.

This is possible if you are up for the build-it-yourself challenge, but other solutions abound. Many of us thrive on this.

Ok, thank you. I'd like to get about 30-40 of these UV's. I need them to stay pretty constant current, that's way I was looking at a voltage regulator. But after looking at some battery specs, I think at least the rechargable lithium-ions hold a pretty steady voltage. What would be wonderful is to run them off a one or two D 10000mah NiMH... or heck, even an F 12000mah.

Now you're getting out of my league. I have a Madmax, so I know it works well with 1.2 volts. But, I've never built my own circuits with it.

You might want to search around the Sandwich Shoppe forum. They might know something there. Also, check out Wayne's help page:
http://home.comcast.net/~theledguy/technical/converter_comparison/index.htm

It looks like he says the Madmax Plus can drive 900ma out. That would be forty-five LEDs at 20ma each, I suppose. Efficiency is claimed at 78%~88%. The rest of the math is an exercise for the reader.

Yeah.. I can't find any datasheet or anything for the Madmax Plus, I was considering that.. and theledguys website won't work for me.. it says Technical Information on the page for each product.. but then it's blank? Hmmm....these look interesting too.. but maybe over my head:
microsemi's lx1994 and lx1993xa http://www.microsemi.com/catalog/part.asp?ID=56584

Forty LEDs @ 20mA is almost three watts, that is challenging to get from a 2.4V source (2 NiMh). The NexGen 750 or BadBoy from The Sandwich Shop might be able to do it. Three cells would work much better, and then a Fatman from TaskLed would be an option. The Fatman would allow easy adjustability of LED current.

I have not worked with either convertor board, so I can't be of much help there. I'd contact the vendors to get a better feel which would work. Is there anyone with expirience who could shed some light on this?

Hmm. Well, this isn't a datasheet, but it does describe some specs of the Maxim 1674 and 1675 parts used in the Madmax. He even has a section near the bottom about using it with typical 5mm LEDs (at about 20ma).

http://home.comcast.net/~wayney8888/hobby/max167x_/Installation_instructions.pdf

[ QUOTE ]
Crux said:
Forty LEDs @ 20mA is almost three watts, that is challenging to get from a 2.4V source (2 NiMh). The NexGen 750 or BadBoy from The Sandwich Shop might be able to do it. Three cells would work much better, and then a Fatman from TaskLed would be an option. The Fatman would allow easy adjustability of LED current.

I have not worked with either convertor board, so I can't be of much help there. I'd contact the vendors to get a better feel which would work. Is there anyone with expirience who could shed some light on this?

[/ QUOTE ]

If I were to use 3 cells at 1.2v each, that would give me a 3.6v nominal voltage and the fatman wouldn't work well when they are over the 3.6v because it says that the Vin must be lower than the Vout and the LED's are 3.6v's. I'm thinking the Madmax Plus or Badboy with 2.4v's (2 Series NiMH) might do the trick.. too bad they are pretty expensive just to test... anyone have a surplus they might want to sell or trade?

Check out this little goofy thing...
http://www.zetex.com/3.0/pdf/ZXSC300.pdf

Most boost convertors can tolerate Vin slightly higher (0.7V) than Vf, your results may vary. Typically Vin sags when the convertor in ON, which helps keep things in the right direction.

However, the Fatman can drive two LEDs in series, which makes Vf = 2*3.6V Just parallel twenty strings of two series LEDs and set output current to 400mA. Thats 20mA for each string.

That makes sense in theory.. thank you. I may contact the maker and see if the Fatman would be capable of this...for each string of 2 LED's though I would need a resistor to make sure it was at 20ma correct? Basically, what I mean is if I set the max output to 400ma, would the ma be dispersed evenly across the 20, 2 LED strings without a resistor to regulate each string?

The Fatman is advertised as a constant current regulator and has an adjust input to set the current (verify this with George). That being said, a 10 to 25 ohm resistor (each the same value) in each string will certainly help balance out any differences in Vf.

I think I'm going with the LM2623.. looks like what I need... I ordered some samples.. so we shall see.

The LM2623 is a voltage regulator not current, so current limit resistors will be needed. You could probably string (13 sets of) 3 leds in series with a 60 ohm resistors and set the output at 12V. Power dissipation in the resistors would total 0.3W with 2.8W going to the 39 LEDs. But thats just one senario...

Another voltage booster choice is LTC3425 but it is limited in output to 5.5V

Keep us posted.

Hmmm.. wouldn't it be best though to set the voltage at 3.8v with the regulator and then just use a resistor to limit the current across say 6 strings of 5 led's in parallel? I know there would be several ways to go about the configuration with tsay the LM2623.. but what would be the advantages of say setting the output to 12v and using LED's in a series rather than setting to 3.8v and using resistors on a string of parallels? Thansk for the LTC3425 link.

I have a LM2623 driving a Lux-3 in a 2-D Maglight mod. It works great. I just adjusted the output of the LM2623 to hit the point on the voltage/current curve for my particular Lux-3 where I wanted it to be driven. In my case it was 3.6 volts. The LM2623 fits easily inside the 2-D Mag.
Right now I am waiting for the new Lux-K2's and I will pull the Lux-3 and replace it with the K2.

The advantage to setting the voltage to 12V is the convertor can be more efficient. The LM2623 uses a schottky diode rather than a synchronous rectifier. As output current increases power losses in the diode increase. Three LEDs in parallel draw 3 times more current than three LEDs in series. The amount of loss is still low, but it is avoidable.

If all else fails, you could try 1 of these pucks.
Jim