LED tail strobe

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

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.


Superbrightled.com (I think) has some of this info. The rest is just
general knowledge of semi-conductors. If Luxeon gave you a Imax for
pulsed current, they should have also given you a pulse duration and
duty cycle. That is, how long and how often the current flows. Again,
the limiting factor is how quickly can you get the heat from the CENTER
of the device. The surface temperature is really only a side effect.
It's the temperature of that little piece of doped glass in the center
of that large chunk of plastic that is critical, and it can melt while
the exterior is still cool. So you can briefly drive a lot of current,
but then you have to stop and let the casing suck the heat out.


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

You are well informed to be worried. All of these PC overclockers
thinking they can crank up the juice if they just add a bigger heat sink
are fooling themselves. The heat energy has a somewhat tortuous path to
traverse through the IC packaging before it can even be transferred to
the heatsink. To be most effective, a heat sink has to be as 'close' to
the heat source as possible. Close being defined as the least amount of
insulation between the two, ie. a 1/2" gap filled with copper plate is
probably better than a 1/10" air gap. The base of the LED is NOT the
heat source. (Though the leads are some nice metal heat conductors
connected directly to the glass in the middle 8*)

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