BY FERNANDO GARCIA
■ FIGURE 1. Light output vs.
junction temperature.
The first challenge was to remove
the excessive localized heating which
causes a loss of luminous efficiency
(shown in Figure 1) and may even set
off thermal runaway. Additionally,
LEDs are diode junctions, and require
their current to be regulated. This is
simple, trivial resistor stuff when only
a few milliamps are required. Not so
trivial — and quite wasteful — when
dealing with high currents.
A possible solution would be a
current regulated switchmode power
supply. However, switchmode supplies do not tolerate excessive current
swings, especially when going from
zero to full blast and again back to
zero in a millisecond. (Hmmm ... who
said this would be a simple project?)
To top it all off, these LEDs have
a Lambertian light distribution;
“Lambertian” being a fancy word
meaning “all over the place.” Some
sort of lens or reflector would be
required to form a useful beam.
First things first ... which LED source
should I employ? After reading countless datasheets, I settled for Luxeon’s K2
in a “star” configuration. As shown in
Figure 2, the LED die is attached to an
aluminum star-shaped plate, which itself
is to be mounted to the heatsink. This
plate also provides the necessary solder
pads to attach the wires.
I quickly determined that an
all-aluminum box would both
house the project and serve as a
heatsink. I could then use hardware (similar to what is used to
mount TO- 3 transistors) to
attach it to the aluminum box.
But this hardware caused an
interference with the optical
lenses. As discussed above, the
LEDs require lenses to focus the
light, and as shown in Figure 3,
these are attached to the LED
assembly with an adhesive pad.
A possible solution would be to
employ an adhesive thermal co pound,
but I wanted something that is widely
available to hobbyists. A workable solution was found: Apply a tiny bead of silicon thermal compound in the center of
the star (on the flat side that will attach
to the aluminum box) and push the star
firmly against the aluminum box’s face.
If done correctly, the bead will
spread out evenly and NOT smear
out. This is important, and it make take
a couple of trials to get it right.
Then, cyanoacrylate superglue is
employed on the star’s periphery
to perform the actual bonding.
Done correctly, this provides a
good thermal interface and fairly
secure attachment.
optical, and mechanical aspects of the
assembly, then came the fun part.
How to drive these LEDs? There are
literally dozens of high output,
constant current switchmode LED
drivers available from different IC
vendors. However, they either are in
nightmarishly-tiny SMT packages, or
optimized for steady state output and/
or a single light shot such as a flash.
■ FIGURE 3. Lens assembly.
Photo courtesy of LuxeonStar.
Electrical
Considerations
Having solved the thermal,
■ FIGURE 2. K2 Star.
Photo courtesy of LuxeonStar.
January 2008 33