Xref: utzoo comp.sys.ibm.pc.misc:5059 sci.electronics:16494 Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!ubc-cs!alberta!mts.ucs.UAlberta.CA!David_Halliwell From: userDHAL@mts.ucs.UAlberta.CA (David Halliwell) Newsgroups: comp.sys.ibm.pc.misc,sci.electronics Subject: Re: Peltier effect device Message-ID: <2036@mts.ucs.UAlberta.CA> Date: 25 Dec 90 07:19:52 GMT References: <1990Dec14.213730.10078@spool.cs.wisc.edu> <5070@optilink.UUCP> Organization: MTS Univ of Alberta Lines: 63 In article <5070@optilink.UUCP>, cramer@optilink.UUCP (Clayton Cramer) writes: >In article <1990Dec14.213730.10078@spool.cs.wisc.edu>, peng@chaource.cs.wisc.edu (PENG) writes: >> I hope this has not been asked before. On the December issue of Byte (p.132), >> there is a short article on an interesting cooling device, which can be >> mounted on a CPU and is able to cool the chip down to 0 degree centigrade. >> This device, according to Byte, is a Peltier effect device, which is >> "a thermoelectric cooling system based on the principle that passing a current >> between two physically connected, dissimilar materials produces cooling on one >> side and heat on the other." I am not quite sure if I know what this > >A Peltier effect device is essentially a thermocouple run backwards. >(A thermocouple involves heating two dissimilar metals, and getting . ^^^^^^^^ . not quite!!!! >electron flow -- and therefore electricity -- from one metal to the >other. Such devices are used in places where simplicity and reliability >are more important than efficiency -- say a spacecraft). > >I can visualize the process by which heat causes electron flow from >one metal to another (since different metals have different electro- >negativities) -- for some reason, I can't picture how electron >flow causes cold. >-- . A thermocouple circuit does involve two dissimilar metals, but the current flow is generated because the junctions are at different temperatures. However, current flow is not the governing electrical characteristic - voltage is. For each unit of temperature difference, the thermocouple will output a certain voltage, in a nearly linear fashion (at least over a small temperature range). For example, a copper- constantan thermocouple puts out about 40 microvolts per Celcius degree difference. Thermocouples are often used for temperature measurement, but because they just give a difference there must be an independent method of measuring the absolute temperature at one junction. The thermocouple then gives a voltage indicating the temperature at the second junction. The reference junction can be placed in something of known temperature - e.g an ice/water mixture - or measured using a thermistor, platinum RTD, etc. . Given this basic understanding of a thermocouple, a Peltier module basically reverses the process. By imposing a voltage in a thermocouple circuit, we can force a temperature difference between the two junctions. This in itself will not create cooling, but if we cool the warm junction we will still have the cold junction at the same offset below it. The heat added to the system through the electrical power consumption must also be removed from the warm side, but the cold side should be cooler than it would be if we didn't have a Peltier module and just tried to cool the component directly. . Years ago I worked in a lab where Peltier modules were used for freezing tests on soil samples. We used a water-cooled warm side, which kept the warm junction around room temperature. It was easy to get sub-freezing temperatures from the system. The module was controlled by a rather large mass of electronics, to get the stability needed for the tests. It made a rather impressive demo during show-and-tell at the open houses, to have a block of ice sitting on a module at room temperature showing no signs of thawing... Dave Halliwell P.S. Merry Christmas to all. Or Season's Greetings, etc. Choose one.