Big LED-Clusters, direct-driven, howto?

Hi,

okay, i will forget about that "LED-array with Step-Up" idea, seems like impossible.

so i now plan to run *many* LEDs on 4 or 8 nimh cells.
some questions:

1) how do i do it? just a bunch of resistors? doesnt sound good, i waste lots of energy and it gets dimmer fast (i guess?). at 4x1,2v = 4,8v and 3,3v LEDs it would be quite a lot.

2) how much can AA nimh cells output? would 4 (or better 2x4) cells in series be able to output 10w (100 LED) plus resistor loss? how about 40w? (damn, those UV LED are unhealthy cheap!)

3) any info about the difference in wavelength in near-uv? are 395nm "okay"? big difference to the really expensive lower ones? (i just know about the visible spectrum, huge differences between 650 and 635nm for example)


thank you all for your patience, i just didnt find much info about LED-arrays, unfortunaly.
well, if i finally get this built, you dont have to expect me asking questions for a while (just following the topics then) ;-)

manuel / krutzbeuazen

Most of the acrylic/expoy "plastic" ones have a short life time. They metal can/glass lens ones last about 10X longer (1000 vs. 10000 hours or more)

Define many, 2000 LEDs?

the point for me is the price.. over here in ger at least, 100x 3mm uv 395nm 10° go for 33€/$.. i consider that cheap, compared with the money i spent for 5 uv-checkers when they came out freshly ;-)
lifetime on epoxy/metal, i guess its the better heat dissipation with metal housings? perhaps i manage to get some metal between the leds. anyway, as a "flashlight" to look for insects, stones , uranium-oxide (okay, i name it, to play around), even 100 hrs lifetime would be acceptable, imho.
"many" like either 100 or 400. would be 3x3 or 6x6 cm² (if soldered tightly).

well, obviously it would be better to use 3 rechargeables (3,6v) instead 4. so i would waste 0,3v in resistors when fully charged, shouldnt be much.
actually the calculator says 0,2 Ohms. since the cells will drop voltage fast, would i need any resistor at all? or would the resistance of the whole setup be enough.. again, if its 100hrs instead of those 1000hrs, no big problem ;-)

thanks for your help and hints!

First off, the short life span is because the UV attacks the epoxy. It degrades and light output drops. The harder you drive them the sooner this happens. Glass is the solution.

I wouldn't count on either 1.2 Volts per cell or 3.6 Volts for Vf. The first is sure to be lower under heavy load (especially at the end of battery life) and the second is probably higher (3.6 Volts is OK for a blue based LED, but UV has higher energy per photon).

I'm still a big fan of proper regulators. The cost to do so here can be fairly small (a fraction of LED costs) and the benefits worthwhile (at least IMO). That said, yes, at one C (say 2100 mA for a 2100 mAh NiMH AA cell), four will give you 8.4 Watts or so for a bit under an hour.

I'd get over the notion of 'waste' in terms of energy. There is always some inefficiency (after all we only get a few percent overall in terms of light out, almost all the 'battery energy' comes out as heat, and the ratio gets worse the harder we drive it), it's simply part of 'the cost of doing business'.

Doug Owen

hi Doug,

oh, of course i would prefer some kind of regulation as well, i just dont see any solution for these loads.. speaking of 3A or even 12A at 100 or 400 LED (when all parallel at 3.3v), 10 or 40 watts. If this is even possible for NIMH cells, other question..

perhaps a linear regulator which keeps it at 3.3v, then i would use 4 or 2x4 cells?
found one, even for 3.3v, 7A max.

get 2 or 4 of them, 8 cells behind, 4.8v in, exactly 3.3v out, 400 LED in front?
that easy?
they take care of everything and keep my lamp warm in winter? Would the LEDs be save without resistor and at same brightness untill the cells have less than 1v?
Datasheet PDF 280kb

again, that all sounds too easy, too good ;-)

krutz