Nav Lights using Luxeon LEDs

Hello

It was my impression that mounting the LED directly to the reflector
(flashing aluminum) will not provide enough heat sinking. A large
surface area is not the only determining factor because the thermal
conductivity of the metal will limit how far the heat can spread. The
thinner the metal, the worse the thermal conductivity. Flashing
aluminum is only 1/100" thick so its conductivity is not very high. One
could construct fins on the back side to increase the surface area, but
commercial heat sinks do that much better.




wrote:
Great write up, and well documented and good composite stuff along with
the electronics.

I'll throw in my 2 cents, on top of an already good design.

1) Sometimes less is more, especially in aircraft. If one LED hits
your numbers, go that way. With a single LED, it increases the
benefits if a switching arrangement versus the linear one deployed.
The hockey pucks mentioned elsewhere might be an easy way to go. As
far as extra margin for degradation over time, this is really an issue
for applications that run 24/7, but for an aircraft and then only the
hours its used at night, these babies aren't going to see over 1000
hours in their life.
2) I hate fans/Heat sinking- airplanes and heat sinks are made out of
the same stuff. Use your reflector as the heat sink by epoxying the
LED to the reflector.

So if you went that way you'd have an off the shelf circuit, one LED on
each side, and no fans.

Andrew Sarangan wrote:
Hello

It was my impression that mounting the LED directly to the reflector
(flashing aluminum) will not provide enough heat sinking. A large
surface area is not the only determining factor because the thermal
conductivity of the metal will limit how far the heat can spread. The
thinner the metal, the worse the thermal conductivity. Flashing
aluminum is only 1/100" thick so its conductivity is not very high. One
could construct fins on the back side to increase the surface area, but
commercial heat sinks do that much better.


I think you have a beautiful design with good tradeoffs, Andrew.
However, if one were inclined to go fanless, there are heatsinks now
that are much lighter will provide much greater surface area. It uses a
much greater number of fins, and each is very thin.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

On Sun, 05 Mar 2006 05:16:51 GMT, Ernest Christley
wrote:

Andrew Sarangan wrote:
Hello

It was my impression that mounting the LED directly to the reflector
(flashing aluminum) will not provide enough heat sinking. A large
surface area is not the only determining factor because the thermal
conductivity of the metal will limit how far the heat can spread. The
thinner the metal, the worse the thermal conductivity. Flashing
aluminum is only 1/100" thick so its conductivity is not very high. One
could construct fins on the back side to increase the surface area, but
commercial heat sinks do that much better.


I think you have a beautiful design with good tradeoffs, Andrew.
However, if one were inclined to go fanless, there are heatsinks now
that are much lighter will provide much greater surface area. It uses a
much greater number of fins, and each is very thin.

Ernest, it takes cross sectional area to conduct heat; if you look at
the TRULY designed heatsinks to maximize heat transfer to air, you
will note a tapered base with tapered fins, larger and thicker fins
close, smaller and thinner further away.

I am weak in describing this, but I think you will get the picture;
think about holding a piece of aluminum foil 2 inches long in a candle
flame, then a piece of 1/4" aluminum rod in the same flame. Your
fingers won't get warm with the foil, but will burn with the rod.

For some reason I thought you'd mentioned that it was .1". If its that
thin (.01") you might need to either use thicker stock or epoxy (with
special high heat transfer epoxy) that heat sink behind the LED.