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nuckolls.bob(at)aeroelect
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 Posted: Sun Nov 01, 2020 6:33 am Post subject: DIY LED landing light -- Wing skin as heat sink? |
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At 07:46 AM 11/1/2020, you wrote:
| Quote: | --> AeroElectric-List message posted by: Finn Lassen <finn.usa(at)gmail.com>
That would introduce drag.
Name of the game here is no drag and lowest possible weight for max light output (lumens) directed in a narrow beam that can reach 3,000 feet or more down a deer infested runway. |
Save yourself a LOT of fuss and bother.
Consider adapting a COTs LED assembly
to your task. There's a boat-load of
really capable emitters with built
in power supplies and cooling. I got
'em on all my vehicles.
Dirt is more expensive . . .
https://tinyurl.com/y5kky5f6
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Sun Nov 01, 2020 3:44 pm Post subject: DIY LED landing light -- Wing skin as heat sink? |
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| Quote: | | Quote: | Are we talking about landing lights?
Bob . . . |
Yes landing lights.
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Okay . . . in the right pew now.
Is there an engineering data spec sheet available
for the device(s) you're considering?
Do you know the thermal resistance from mounting
surface to LED junction?
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Mon Nov 02, 2020 8:08 am Post subject: DIY LED landing light -- Wing skin as heat sink? |
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| Quote: | | Looking at the Cree XM-L2 with a thermal resistance of 2.5W/C from junction to mount surface. |
Watts per degree C? Thermal resistance is degreesC/Watt.
Thermal resistance is normally cited as Degrees C (voltage analog)
per Watt (current analog). If the data sheet does indeed call
it 2.5W/C then it's 0.4C/Watt which is rather nominal.
| Quote: | | Add to that a conservative 0.5W/C for heat sink compound . . . |
don't you mean 0.5C/W? which added to the assertion
above gives us 0.9C/W . . . a rather nominal and
believable value.
| Quote: | Max die temp is 150C. But unlimited life is 35C. It appears that when you go
above 35C you start reducing life of the LED. |
35C isn't very hot . . . 95F? Anything
higher is detrimental to LED service life?
| Quote: | | The LEDs are "binned" (color and efficiency determined) at 85C. |
Okay 85C die and say 25C ambient gives us
a total differential target of 60C.
If the die is pumping out 60W then the
target thermal resistance die-to-ambient
is 60C/60W or 1.0 C/W so your blast cooled sink seems
just adequate.
| Quote: | | So lets set die temp of 85C as the design goal.With built-in temp limiting, I think we can probably consider wing skin to air thermal resistance as 0W/C (air flowing over the skin in flight). |
okay
| Quote: | Consider 32C ambient we get 85 -3 -32 = 50 allowed temp difference from mount plate to skin.
Each LED has a mount surface of about 20x20mm. |
not relevant
| Quote: | | If we have eight 10W LEDs at 30% efficiency, we need to get rid of 56 watts. |
okay
| Quote: | | So max thermal resistance of mount plate and mount late to wing skin should be 56/50 = 1.1W/C. |
or 0.8 degrees C/Watt . . . we're in the same ballpark.
| Quote: | If we use heat sink compound between plate and wing skin, we're down to 0.6W/C max
thermal resistance of the mount plate. |
Thermal resistance of pasted joints are not automatically
0.5C/W . . . a common rule of thumb carried over from
the good-ol-days of TO-3 transistor cases. Thermal resistance
is both area and media dependent. Twice the area drops
thermal resistance by half. Almost anything placed
between two metallic surfaces will drop the thermal
resistance across the joint.
| Quote: | If aluminum has thermal resistance of 0.2 W/C we need 3 of something.
What is the something? Thickness vs area? This is where I bog down. |
Thermal resistance becomes a real number only for
specific cases like:
Resistance of a doped interface of TO-3 transistor
to heat sink surface
Resistance of a specific configuration of
heat sink with mass-to-ambient transfer
of heat at a specified airflow.
Resistance of semiconductor junction to it's heat
conducting surface.
The material is irrelevant . . . i.e. aluminum
has no specific thermal resistance until it
becomes part of a fabrication . . . it has
a generalized thermal conductivity value but
doesn't become a component of your thermal
resistance study until after it is fabricated
and then MEASURED or predicted. There
are computer programs not unlike finite element
stress analysis that will do similar studies of
thermal dynamics in an assembly.
https://tinyurl.com/kygh2vo
Here's a skip across the wave tops of a very
complex study
https://tinyurl.com/zsme6t6
Here's a .pdf of a thermal resistance study
I published 4 years ago:
https://tinyurl.com/yxfb9je4
How do you proposed to manage the optics of
this project? Does the natural emission profile
of the LEDS put the light where you want it
most?
I've demonstrated hand held flashlights that
offered sufficient illumination to effect a
graceful arrival with the earth . . . but
they put ALL of their 2 or 3 Watts of output
in the right place.
Headlights on cars generally shine 55W of
incandescent energy or 20W or LED
energy down the road . . . and they'll
light up a road sign a mile away.
Be wary also of too much light where you
don't want it. Incidental light in your
eyes is not conducive to skilled pilotage
1 foot above the runway.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Mon Nov 02, 2020 8:49 am Post subject: DIY LED landing light -- Wing skin as heat sink? |
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At 06:07 AM 11/2/2020, you wrote:
| Quote: | | The XM-L2 are Surface mounted. Are you planning to do double-sided PCB and add via's under each LED, or do you plan on a CNC pattern in aluminium to allow direct contact to the LED base? The via's have their own thermal resistance to account for. |
You bet . . . and they're not going to be 0.5C/W
conductors. You'll need to fasten these critters
down with solder paste and a heat gun. The back
side of the ECB can be solid copper but it's going
to be THIN. Achieving a low resistance thermal
pathway from junction-to-ship's-skin will not be
a trivial task.
Bob . . .
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