All,
I had to brush up a little on this as it's been a long time since I took any
thermodynamics
classes, and I wasn't really good at the then, hi hi! Too many parties, etc,
and I thought the
class was totally boring. Anyhow, the black color has nothing to do with heat
being taken
away by the air, only radiation. Radiation accounts for only around 3% of the
heat that leaves
the heatsink, and convection accounts for the rest. When convection comes into
play and
air is moved across the fins, the radiation cooling really become almost moot.
It doesn't take
much air to do this either. I read what Avvid had to say, which wasn't much. It
just mentioned
using black helped, but didn't say how much, and by how. However, when reading
texts from
engineering books, and several places on the net, I seen what I thought all
along about using
black on heatsinks. Also, the use of paint does make it worse, not better as it
acts as an
insulator. Anodizing is used as it makes the thinnest insulator coating out of
all. There's also
other things that come into play about using a black color for it to be of any
benefit. I quote
this from a website;
"Heat sink color"
"If your heat sink will work in the air flow of a fan, the contribution of
radiation will be extremely
small, compared to the effect of conduction. So, it's best to leave the
aluminum bare, as any
layer of black paint, with its lower thermal conductivity, will hamper the
conduction of heat from
aluminum to air more than it may help by increasing radiation.
On the other hand, if you bolt your transistors to the back panel of a box, by
all means paint that
panel flat black! A flat panel dissipates more heat by radiation than by
conduction, and here a flat
black surface helps a lot! But it helps only if it looks at other objects that
are dark, and cooler than
the panel, or if it looks at free space. If you place such a black heat sink in
the sun, it will absorb
heat rather than radiating it, and get very hot! Likewise, placing a black heat
sink inside a shiny
aluminum box is useless, because its radiated heat will reflect back onto
itself. For that reason,
paint the inside of aluminum boxes flat black too, so that the electronic parts
inside the box can
cool themselves by radiation into the aluminum box!
Do you want another table? Well... here is one about the radiation constant of
different materials.
This is expressed in (10-8)W/(m2 K2) , at 20°C.
Perfect black body: 5.67
Matted steel: 5.4
Matted zinc: 5.3 (that's why zinc roofs get so hot in the sun!)
Oxidized copper: 3.6
Polished copper: 0.28
Matted aluminum: 0.4 (that's why aluminum roofs are much fresher in summer than
zinc ones!)
Polished aluminum: 0.23
Polished silver: 0.17
There is a simple pattern: Shiny, light surfaces emit and capture very little
radiation, while
reasonably dark surfaces, specially if matted, are almost perfect radiators and
capturers".
End quote.
From: http://ludens.cl/Electron/Thermal.html
http://en.wikipedia.org/wiki/Black_body
http://sound.westhost.com/heatsinks.htm
Check this about using silver paint;
http://www.overclockers.com/tips684/
Anyhow, I thought all here may be interested in this.
Best,
Will
*********** REPLY SEPARATOR ***********
On 8/17/06 at 9:19 PM Tim Long wrote:
>You are actually pretty close. If you want to equate thermodynamics to
>electronics, temperature equates to voltage and temperature difference
>equates to voltage difference, heat flow equates to current, thermal
>resistance equates to electrical resistance. Use the right units and
>ohms law works with heat transfer. BTW mass equates to capacitance.
>Have fun & 73, Tim, W2UI
>
>Jim Tonne wrote:
>
>>>Making the heat sink heavier by making it a larger block
>>>doesn't necessarily make it a better heat sink.
>>>
>>>
>>
>>I look "mass" (weight) as a time-constant. Bigger sink
>>stays cool longer while it is soaking up heat. But then too
>>it retains that heat for a longer time than a light-weight sink.
>>
>>
>>
>>> it is surface area that gets rid of the heat.
>>>
>>>
>>
>>- JimT
>>
>>
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>>
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