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Subject: Re: [geda-user] 5630 LED footprint
From: John Doty <jpd AT noqsi DOT com>
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Date: Mon, 14 Sep 2015 15:06:21 -0600
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On Sep 14, 2015, at 2:04 PM, Larry Doolittle <ldoolitt AT recycle DOT lbl DOT gov> wrote:

> Nicklas -
> 
> On Mon, Sep 14, 2015 at 09:35:41PM +0200, Nicklas Karlsson (nicklas DOT karlsson17 AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
>>>  * From experience, I think polymer insulates much more than metal
>>> So If the above is true I think the polymer would squash the dominant form
>>> of dissipation and bare metal is better.
>> Yes but layer is thin and emissivity is different.
> 
> Run the numbers and I think you'll find otherwise.

Nope. Standard NASA derating for surface mount components on a board cooled only by radiation is 0.6, so a 100mW resistor gets a 60mW rating. Back before this standard was established, I “ran the numbers” and obtained a similar result.

> 
> Cooling by radiation scales as the fourth power of
> temperature, or really T_object^4 - T_environment^4.
> It's hugely important above 1000 Kelvin, not so much
> when you're talking about a 320 Kelvin object and a
> 290 Kelvin environment.

The Earth cools by infrared radiation alone. That means that at terrestrial temperatures, the cooling effect of infrared radiation is comparable to the heating effect of incoming solar radiation. Things you leave in the sun can get rather hot.

At higher temperatures it’s much more effective, of course.The emission of a football field-sized area at 100,000,000K can be detected 30,000 light-years away by an instrument you could hold in your hand: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?db_key=PRE&bibcode=1981ApJ...243..257D&letter=.&classic=YES&defaultprint=YES&whole_paper=YES&page=257&epage=257&send=Send+PDF&filetype=.pdf

> 
> Heat carried away by moving air (convection) is all that matters
> for electronics.  Unless you can get the cooling you need through
> direct contact of aluminum, since the conductivity of aluminum
> is 9000 x greater than that of still air.

But aluminum heat sink makers often dye their anodized surfaces with an emissive substance. It’s often also black in the visible, but, of course, it’s the infrared that counts.Radiation helps.

> 
>  - Larry
> 

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd AT noqsi DOT com