Date: Mon, 06 Mar 2000 23:45:49 +0100 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: Peter Jeremy <peter.jeremy@alcatel.com.au> Cc: thyerm@camtech.net.au, freebsd-current@FreeBSD.ORG Subject: Re: current lockups Message-ID: <11149.952382749@critter.freebsd.dk> In-Reply-To: Your message of "Tue, 07 Mar 2000 09:32:53 %2B1100." <00Mar7.093254est.115229@border.alcanet.com.au>
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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! To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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