On Sat, 15 May 2004 03:52:20 +0000, Ernest Christley wrote:

Jay wrote:
From what I've read, the bond wire that connects the semiconductor die
to the leadframe (the part you solder) can be the limiting factor on
pulse current. It has a lower thermal time constant that the chip
itself. You can smoke that little gold wire if you try to shove too
many e-'s through it at once.

Yeah. What he said.


I'm not sure what you're doing with your alternative application, but
in general, you pulse them to multiplex them for moving signs or to
lower the observed brightness (via PWM) for brightness controls.

You can also overrate them and pulse higher current for short time
periods. As you noted above, this technique definitely has its limit.
The human eye is a peak detection device for time constants greater than
about 50ms (it'll average anything shown less time than that). So, you
can drive more current, then turn it off at the right rate to get a
brighter LED without burn it up. Again, this technique has its limits.

FWIW I heard that in the olden days, LEDs worked most efficiently at
currents that were high enough to burn them out if these currents
were applied continuously. But it meant that if you drove an LED at 1A
for 1/20 duty cycle, you could get more light out than if you put 1/20 amp
in continuously (hypothetical numbers). I gather that newer LEDs tend to
work most efficiently with a steady drive current.

I assume that for a strobe application it's important to pack the
maximum power into a pulse that's equal to or shorter than the eye's
integration time. I have to say I don't know what that integration time is
- you can certainly see 20hz flicker at photopic levels, but at scotopic
(night-vision) levels the frequency can be much lower.

On the other hand, given that we can see that Mars is red (only photopic
vision lets us see colour) at night when adapted to scotopic light
levels I assume that we use photopic vision to sense small point sources
such as stars or strobe lights - perhaps someone out there actually knows
what the optimal pulse duration is?

AC