🚀 go-pugleaf

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.