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DIY LED landing light -- Wing skin as heat sink?

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nuckolls.bob(at)aeroelect
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Posted: Sun Nov 01, 2020 6:17 am Post subject: DIY LED landing light -- Wing skin as heat sink?

 	  Quote: 	
		  

 Let's say we use a 100W LED (~13,000 lumens). We'd have to remove 70 to 80W of heat from the LED.

 LED doesn't like more than 150°C and it would be better to keep it cooler. LED area 34x34mm. 	


   Don't think it's quite that bad. LED efficiency is on the order of

   35 to 40% which would dump 60 to 65 watts of heat.



  	  Quote: 	
		  I don't have a good grasp of thermal conductivity (conductance?). How thick would the mount

 plate have to be? How much area would be needed where the mount plate contacts the wing

 skin (using some kind of heat sink compound/adhesive between the plate and skin)? 	


   The flow of heat energy is analogous to the flow

   of electrical energy. Temperature rise is 'Volts'.

   Thermal resistance is 'Ohms' and Watts is still

   watts. Configuring thermal management for an

   LED is identical to that of say a power transistor.



   The working part of the LED has a known thermal

   resistance between the semiconductor junction

   and the heat sinking surface or case. It can

   be stated in degrees C per watt. So if you

   have a junction dissipating 100 watts of heat

   and it's packaged thermal resistance is say

   1C/W then the junction is going to operate

   at 100C HIGHER than the heat sinking surface.



   If the thing is limited to 150C then you have

   to maintain the case at 50C.



   There will be a thermal resistance associated

   with the mounting of the device to the

   heat sink . . . add say 0.25C/W. Adding up

   the resistances we're at 1.25C/W . . . 100

   Watts will drop 125C so now your heatsink

   has to operate at 25C.



   These resistance numbers are pulled out of

   my hat . . . but you get the picture.



   Now we get to the proposed 'heat sink'. Just

   how good is it. VERY difficult to calculate

   if complex like structure of an airplane.

   Best way to test it is bolt a power resistor

   to the LED mounting surface and cause it

   to dissipate X watts. Thermocouple the area

   under the resistor to get degrees C/Watt

   of thermal resistance.



   Now to throw a really big monkey wrench into

   the study. AIR FLOW has a PROFOUND influence

   on thermal resistance. Those tiny chips in

   your computer dissipate a bucket load of heat

   in terms of their mass. The heat sink on your

   CPU has lots of SURFACE area and a FAN.



   The only time your LED nav lights are at

   risk for thermal abuse is while sitting

   on the ground or taxi. If I were designing

   an LED fixture, I'd probably include 

   over temperature sensing to dim the lighting

   to prevent thermal damage. Once airborne,

   you'll have plenty of air flow.



   I suspect the 'big guys' in aviation lighting

   already do this. After all, there are TSO

   requirements for thermal robustness that

   have to be met and by now, they're very

   good at it. If you're looking at a DIY

   LED illumination project, start with a study

   of how the big guys do it.



   When assigned a task for development of

   a new product in years past, one of the

   first things I would do it search out

   the patents on legacy technology. HISTORY

   can give you a leg up on avoiding

   exercises in re-invention.



  	  Quote: 	
		  Am I right in assuming the plate has to be thickest were the LED is mounted and then taper down in thickness as it expands out in width and height? 	


   No . . . thermal resistance of base material,

   surface area and AIRFLOW . . . but without

   a doubt, include ELECTRONIC overheat

   protection.



 

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Nov 01, 2020 6:30 am Post subject: DIY LED landing light -- Wing skin as heat sink?

 	  Quote: 	
		  Let's say we use a 100W LED (~13,000 lumens). We'd have to remove 70 to 80W of heat from the LED. 	


   AND the associated electronics.

  

   See attached:



 



 

   Bob . . .

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