The other challenge brings back something I asked on this list a while back.
I did not get any responses I could use that time, but I like the idea of
presenting it as a challenge.  I will even give a $200 reward for an idea I
can use.

I manufacture a fiber optic instrument light for aircraft.  The illuminator
can be seen at http://www.engalt.com/newpage2.htm.  It is actually much
smaller than that now.  The LED is a 100 mA LED.  I also have a 150 mA LED I
can use, but I am currently only driving the one I use at 75 mA due to heat
dissipation (in the dropping resistors, not the LED).  The inside of the box
is about 3/4" cubed.  I actually have less space to work with than that
because the gland that the fibers go into and the connector takes up space
in the enclosure.  The enclosure is potted with potting compound.

Here is my problem.  The LED is about a 2.5V LED.  I have these units set up
for either 12V or 24V input.  Actually, they run about 13.5V or 26V in use.
With the 12V unit I am dropping about 11V at 75mA which is .825 W of heat I
need to get rid of.  I use two dropping resistors now, one on each leg of
the LED and orient them near the top of the potting compound.  That does a
good job and does not get very warm.  24V is a completely different story.
That gives me 1.76 W of heat to get rid of and the unit just overheats.  I
really want to run the unit at the full 150mA brightness, but that gives me
over 3.5W and that is really too much.

Heat sink is not an option.  External big resistor is how I am having to do
the 24V units now and that is a very bad solution.

A switching regulator is the answer, but one catch is that it has to be
dimmable.  That is easy enough to do with off the shelf parts now, but
presents a problem in this unit.  I only have a positive and negative input
and can not add a third lead for dimming.  What I need is something that can
go in the place of a light bulb and work with the existing dimming rheostat
already in the plane.  I want full brightness about 13V and dimmed to about
nothing at around 3V.  All of the off the shelf driver ICs are made to
output a constant current with a varying voltage and have a third input for
dimming.  I need something that has the output current varying linearly with
the input voltage.  It needs to be inductorless, very small and low parts
count, and cost no more than a buck or three.  I can build the boards with
different value parts for the 12 or 24V units.  Ideal would be something on
a half inch square board or less with no more than one low pin count IC and
a few supporting parts.

If you want to send me your ideas direct my email address is
brian@engalt.com.  If you think that you can offer a complete solution but
don't want to give it away for a few hundred bucks and think no one else
will either send me a proposal and we will talk.

Brian Kraut
Engineering Alternatives, Inc.
www.engalt.com


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