1 watt and 5 watt LED for Nav lights?

With a properly biased LED stack and resistor there is no thermal
runaway problem. I've never seen or heard of this happening to
anyone. The slope of the diodes once they go past the knee in
combination with the series resistor more than makes up for a small
negative temp coefficient of the diodes. Maybe what you're saying is
that you need some minimum resistance as a percentage of the total
drop to guarantee stability, and this I would agree with.

And I totally agree, prototype one leg of the circuit, and run it over
voltage and temp (hit your corners) before you build it into your
wing.

Fuses- I'd still go with the standard central fuse/breaker panel.
Since its a non-inductive load, you should be able to use a
fuse/breaker without much over rating.

I'm a big believer in keeping things simple. But often times a simple
final product requires complex analysis and testing. The most complex
implementations often reflect design with less than well understood
inputs.



(BllFs6) wrote in message ...
Hi Guys...

Havent followed this thread carefully...been away for awhile...so forgive me if
this has been covered already...

There is one disadvantage to using mostly the forward voltage drop of a LED to
"span" the battery voltage ...as opposed to say a big resistor taking a good
fraction of the voltage drop...

An LED's voltage drop/resistance is pretty temp dependent....

And here is what can happen....the LED gets warm.....resistance drops....more
power flows through....LED gets warmer still...still more power
flows....warmer,more power....can easily become a runaway situation...leading
to burnout or even worse a fire somewhere in the electrical cicuit..

Now, this isnt much of a problem in very low powered LEDS, but in high powered
ones that get warm to touch just sitting on the bench, its something you need
to worry about....

If you have a circuit in mind...you need to make it and THEN run it in absolute
hottest environment at the absolute highest voltage it will ever see (ie the 14
or whatever volts is MAX from the alternator rather than the nominal 12 from
the battery)....then if that works and current limits werent exceeded back the
power flow down say 10 to 20 percent just to be safe....

What the max temp of a "lexan lighting housing" at the tip of a wing in August
on a no wind day sitting on a black asphalt runway at high noon in Texas?

I can easily imagine it being 60 degrees or more than the temp you tested it in
on the bench.....so just because it didnt suffer thermal runaway on the bench
doesnt tell you much about how it"ll behave in the plane....

Another related problem is if you have multiple parallel LED/resistor paths
running from the same voltage source....at room temp you may test the setup and
each path is taking the same power....but then there is that pesky
temp/resistance property of the LEDs...if they are different enough, when your
setup is run in a higher temp or poor cooling conditions....one path may become
a significantly lower resistance path than the other (the resistors can cause
it as well)...and all of a sudden you have way more power flowing through one
path than was intended to....and you have another thermal runaway situation...

Which brings up fuses....lets say you 3 parallel LED resistor paths, running
off of one wire pair that runs out the wing tips....

You design it so each branch takes a bit under 5 amps, so you have that wire
pair fused at 15 amps....

If you suffer something like one dead branch in combo with a thermal runaway of
another branch (or even perhaps just a thermal runaway of one branch)....you
can have a situation where one branch is WAY exceeding its design limits...yet
the total power isnt enough to trip the fuse back in the cockpit...

So, EACH branch should be fused seperately in addition to the main overall wire
fuse...

Now, if youve designed and tested your circuit worth a hoot the chances of one
branch going amook are pretty low, so something like "semi permanent" circuit
board fuses soldered into each branch probably make more sense and would
provide better reliability than removeable fuses...

Another thing....precision resistors, temp stable resistors, and even resistors
with a significant negative temp coefficient ARENT that expensive compared to
other expensense for an airplane/high powered LED project....so the might be
worth investigating, especially since your only talking a handful of them and
they could significantly ease your engineering problems as well as improve the
reliability of the setup...

Just my 10 cents....

take care

Blll

(Bill) wrote in message . com...
To Jim Weir and Group

I am interested in using the new (to me) 1 watt and 5 watt LEDs for
nav lights.
Not having to worry about changing bulbs or the power requirements are
very attractive.

I have seen some posts about leds but would like to tie it all
together.
I know of two sites showing their versions and one is using 9 LEDs
while the other is using more.

What it boils down to is this: how many leds for the the right and
left wings, how many for the tail and can the leds be strobed or any
thing else I don't know?

Thanks
Bill


Lots of good stuff here. Now if I just knew what it all meant?

Can someone sum it all up and tell a "I can solder a wire" person what
they can do to build these?

Thanks
Bill