LED tail strobe

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.

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.



anonymous coward wrote in message e...
On Tue, 11 May 2004 17:04:22 +0000, Ernest Christley wrote:

anonymous coward wrote:

BTW, do you need to use zener diodes / transorbs with aircraft power
systems as you do in cars, to avoid problems with voltage spikes?

AC


In one form or another, I'd say the answer is a qualified yes. Again, I
didn't add any sort of regulation to the LED array. As I remember it,
and it has been quite a while since I looked at it, the larger LEDs can
absorb rather large transients themselves. Their construction is not
that far removed from zeners or transorbs after all.

Do you know where I might find any links to material about this? I'm
building a computer controlled LED flasher device (not for a tail strobe
- something unrelated) and naturally I would like to make the flashes as
bright as possible. Luxeon reckon that when pulse-width modulating their
LEDs, the current should never exceed 500-550 mA (for the 1W versions).
Given that their normal current is only 350 mA this isn't much of an
increase. Also, this is meant to be at pulse-width-modulation frequencies
of 100Hz or more. The pulse duration I need is 1/10 of a second.

On the other hand, your experience echoes my experience. My Luxeons aren't
heat-sunk, yet due to software faults I've unintentionally passed 1.5A
through some of them for several seconds. I won't be surprised if their
life expectancy is greatly reduced but they have lasted the 'development'
phase of my device surprisingly well.

As I understand it, the limiting factor of the LED's ability to suck
down transients is their ability to dump the internal heat is the reason
why 'overdriving' them with higher but pulsating current works.

What worries me is how quickly the die can dump heat to the aluminium
casing.

AC

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.

--
http://www.ernest.isa-geek.org/
"Ignorance is mankinds normal state,
alleviated by information and experience."
Veeduber

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