From owner-freebsd-current  Mon Mar  6 14:46:22 2000
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To: Peter Jeremy <peter.jeremy@alcatel.com.au>
Cc: thyerm@camtech.net.au, freebsd-current@FreeBSD.ORG
Subject: Re: current lockups 
In-reply-to: Your message of "Tue, 07 Mar 2000 09:32:53 +1100."
             <00Mar7.093254est.115229@border.alcanet.com.au> 
Date: Mon, 06 Mar 2000 23:45:49 +0100
Message-ID: <11149.952382749@critter.freebsd.dk>
From: Poul-Henning Kamp <phk@critter.freebsd.dk>
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In message <00Mar7.093254est.115229@border.alcanet.com.au>, Peter Jeremy writes
:

>> How about these Peltier (sp ?) cooling devices I have heard about ?
>
>A Peltier cell is just a semiconductor heat pump.  It effectively just
>reduces the junction-to-heatsink thermal resistance, allowing you (in
>theory) to use a less efficient heatsink (or have the CPU run cooler
>with the same heatsink.

This is actually not true, quite the contrary in fact:  You need
a better heat-sink with a Peltier because of the significant
electrical power you pump into it.

As a general rule you can expect to *raise* your CPU temperature if
you put a peltier under anything less than a *very good* heat-sink.


Example:

	A Celeron 500 disipates about 25W

	An average heatsink is about .8 C/W

	delta-T becomes 25W * .8C/W = 20C

	At 30C ambient that becomes 50C CPU temperature.

	Now, add a peltier.  To remove 25W and keep a 25C
	temperature difference we need to feed it about 50W

	Now the heatsink has to deal with 25 + 50 W and the
	delta-T becomes: (25W + 50W) * .8C/W = 60C

	Subtract the 25C difference from the peltier and add
	the ambient temperature and we find:

		30C + 60C - 25C = 65C

	We just raised our CPU temperature about 15 C :-(

--
Poul-Henning Kamp             FreeBSD coreteam member
phk@FreeBSD.ORG               "Real hackers run -current on their laptop."
FreeBSD -- It will take a long time before progress goes too far!


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