Re: High-output, low-duty cycle LED strobe circuit

On May 4, 2:45 am, D from BC <myrealaddr...@xxxxxxxxx> wrote:
On Sun, 3 May 2009 21:21:54 -0700 (PDT), mj <eluc...@xxxxxxxxx> wrote:
I'm looking for ideas on how to make an LED flash so brightly at a low
duty cycle that it's reasonably bright--maybe even close to what it
would be if it were on DC.

I'm building a project where I need to flash white LEDs very brightly
30-50 times a second at about a 0.4% duty cycle. (I'm strobing a
spinning disk, and want to freeze images near the LED--too high a duty
cycle, and the image blurs.) 0.4% is not much time for an LED to be
on. I've heard that you can drive LEDs to up to 10x their normal
forward current without damage (though I guess lifetime is shortened)
if you keep duty cycle to <= 1%.

To try to get a bright enough flash, I got some 0.5W white LEDs that
can take a max DC forward current of 150 mA, and have about a ~3.6
forward drop, producing an intensity of 130k mcd. (Not too clear on
the mcd part.) And dem suckers is bright when you're pumping even 100
mA through them. Like squint-to-look-at-it bright. They're in a
standard 5mm package (T-1 3/4, what is up with that package name?),
though it's sturdier than most you've probably seen. The LED looks
like it's been lifting weights, and the leads are shorter and fatter.

Anyway, I have two 2n2222's hooked up as a Darlington, with +5 Vcc,
driving the front Q's base with ~15mA (with a microcontroller pin).
There's NO current-limiting resistor on the back end, where the second
transistor's collector is attached to +5v, and the emitter goes
through the LED to ground.

Since the LED is only on 0.4% of the time, max, it still simply isn't
very bright. The strobe works--I can see the frozen image on the
spinning disk--but the light is simply anemic.

So, I'm wondering if anyone here knows how to design a circuit that
can dump an amp and a half through an LED for, say, 200 microseconds
at a time or less, at 20-50 Hz. Or if anyone else has ideas about how
to make LEDs look bright even if they're only on half a percent of the
time.

I'm hoping I won't have to dump the LED idea and go with tiny xenon
strobes--not really into figuring how to design a 50Hz photo strobe at
the moment.

Thanks for anyone that wants to provide some ideas.

mosfet driverhttp://ixdev.ixys.com/Data***/99061.pdf

D from BC
myrealaddress(at)comic(dot)com
BC, Canada
Posted to usenet sci.electronics.design

14 amps, well now that ought to do it. :{)

Can someone explain to me why a MOSFET would be better here than a
bipolar? Seems I should be getting enough current through the LED with
my 2n2222 darlington arrangement, but my transistor design skills have
always been modest. And when I learned it, MOSFET drivers were still
exotic, believe it or not. We did only bipolars.
.