> -----Original Message-----
> From: John Woodgate [SMTP:[email protected]]
> Sent: Tuesday, September 24, 2002 10:51 PM
> To: [email protected]
> Subject: Re: Heatsinks and EMI
>
>
> I read in !emc-pstc that Georgerian, Richard <RGeorgerian@carrieraccess.
> com> wrote (in <[email protected]
> ess.com>) about 'Heatsinks and EMI' on Tue, 24 Sep 2002:
>
> > Many thanks to all those that responded. I believe I have enough to
> > information to get myself started on understanding the role heat sinks
> > play
> > in radiated emissions.
> >
> You might also bear in mind that, in spite of their relatively low
> surface temperature, they do lose a significant proportion of the *heat*
> by radiation. (If they are not bright and shiny, that is.)
> --
>
>
Out of curiosity I worked through a quick example.
Here are the results:
Looking at a simple bent metal heatsink, rated 7.1degC/W.
Approximate effective surface area 0.006194m2
Assume ambient 25C (298K)
If the heatsink is dissipating 10 watts, temperature rise above ambient
is 71C
ie 96C (369K)
From Stefan Boltzmans law: W=SB x Ax (T^4 - Tamb^4)
(where W= power in watts, SB= stefan Boltzmans constant 5.67 E-8; A=
area in m2; T is temp of body K; Tamb is temp of ambient K)
This yields power radiated by black body is 3.7W.
So for black anodised heatsink, assume emissivity is 0.95, heatsink
radiates 3.55W or 36% of the total.
If heatsink is oxidised aluminium, emissivity is approx 0.35. It then
radiates 1.3W.
If unoxidised shiny aluminium, emissivity is approx 0.025. It then
radiates less than 0.1W.
This illustrates two points 1) the effect of a blackened high
emissivity surface
2) due to T^4 dependence, if the temperature goes up a higher
proportion of the energy will be dissipated as radiation.
Regards
Andrew Wood
Land Instruments International
Dronfield, England.
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