Need some help regarding 10 - 20 5mm LED driver.

[Deleted member 684]

Can anyone build a driver that is current regulated and will drive a string of 10 - 40 5mm LEDs @ 20ma? 40 may be a bit too much I think but 10 or 20? Voltage would come from 4 or 8 AA nimhs. No, don't ask what I'm using it for. /ubbthreads/images/graemlins/grin.gif

TIA! /ubbthreads/images/graemlins/smile.gif

 

[Doug Owen]

As any real hot rodder will tell you, 'sure, speed costs money, how fast can you afford to go'?

A *string* of 20 LEDs would have over seventy Volts Vf to deal with, that's a very hard boost off twelve Volts from your battery. If OTOH, you do it with series parallel (say strings of three?) you're probably much better off.

Otherwise a serious inverter project is in your future, something with a proper transformer in it. Best also keep a total power eye on those poor AA cells, for sure four aren't up to it alone with inverter and regulator losses driving all those LEDs.....

And I didn't ask....wanted to, but didn't. Hope it's legal...

Doug Owen

 

[Steelwolf]

If you set up the entire string as parallel, you could use a simple linear voltage or current regulator off 4 cells (if using 8 cells, set them up for 2 parallel packs of 4 for doubled runtime). I believe a simple circuit like that can be built up from a LM317. IIRC, the circuit you need is included in the complete set of device specification data sheets.

Quite inefficient, but simple.

 

[georges80]

[ QUOTE ]
Nerd said:
Can anyone build a driver that is current regulated and will drive a string of 10 - 40 5mm LEDs @ 20ma? 40 may be a bit too much I think but 10 or 20? Voltage would come from 4 or 8 AA nimhs. No, don't ask what I'm using it for. /ubbthreads/images/graemlins/grin.gif

TIA! /ubbthreads/images/graemlins/smile.gif

[/ QUOTE ]

For 20 check out the LT1618 data sheet towards the end you'll see a circuit for driving 4 parallel strings of 5 - 20 20mA LEDS.
Assuming 3.5V per LED Vf gives about 1.4W output. Feed it with 4 nimhs (close 80% efficiency) means 1.75W input. At 4.8V that gives <400mA current draw. Should give you about 4 hours of run time with some decent nimhs.

george.

 

[IlluminatingBikr]

I don't think this is what you are looking for, but maybe.

 

[Deleted member 684]

Doug, thanks for the advice. It's legal I think. Actually it numbers to 0.1K 5mm white LEDs. I'm using it as a bike light, as well as a macro photography light. I don't want to use a luxeon since it's a point light source and would give lots of shadows in macro photography. I want a really wide source of light so that I can avoid using the flash on my camera which causes harsh shadows. Maybe I should add 5 red leds to boost the red output.

Steelwolf, I won't really worry about inefficiency in the circuit. But if possible, I'm looking for adjustable brightness. Meaning I can feed anywhere from 1 ma to 30 ma per LED. Dang.. tell me, am I dreaming too far? It's good to dream though. Thanks for your input in this. /ubbthreads/images/graemlins/smile.gif

georges80, thanks for the link. Are you refering to the high power white LED driver in the PDF? That looks good, even comes with pwm brightness adjust. Now build 5 of that for 100 LEDs... /ubbthreads/images/graemlins/help.gif What would it likely cost? I probably will be able to solder the parts together if they don't burn out from prolonged soldering first. Can anyone build me 5 of this? Size isn't really a concern so long as it can fit into a coke can. /ubbthreads/images/graemlins/smile.gif

IlluminatingBikr, that isn't really what I'm looking for. But it does give rise to some ideas. Thanks for your help!