[geeks] Peltier devices....

[Mike Hebel]

GAW> [ On , September 16, 2001 at 10:17:46 (-0700), Gregory Leblanc wrote: ]
>> Subject: [geeks] Re: [rescue] This Just In: HP to buy Compaq
>>
>> I actually want to run it in reverse of what these guys are doing.  I've
>> got a couple of chunks of metal, one is a stand to hold things up off of
>> the wood stove, and the other is a set of heat-sinc fans.  The peltier
>> device gets sandwiched in between these two, and a small electric motor,
>> with a fan, gets hooked to the peltier device.  The heat from the wood
>> stove causes the peltier device to generate electrical power output,
>> which then runs the motor fan.  The peltier device that I had has a few
>> broken links, so I need to replace it.  

GAW> You can buy these things made commercially, though the models I've seen
GAW> are not terribly effective, and they're quite expensive ($160[cdn] for a
GAW> small one with a two-bladed fan and maybe 1" square Peltier device).

GAW> One major problem with these things is that you have to be careful not
GAW> to over-heat the Peltier device as you can easily destroy them at the
GAW> temperatures a modern air-tight wood stove can commonly get to.  The
GAW> patented commercial models have a little bi-metal spring under the base
GAW> which will tilt the whole thing up so that the base is not in full
GAW> contact once the temerature gets high enough.  Unfortunately this isn't
GAW> always enough as the whole device will be heated by radiant heat too.

GAW> IIRC the pamphlet that came with the model I've seen new gave 700F as
GAW> the maximum safe temperature, but other references suggest the reflow
GAW> temperature of the solder used is the critical factor in failure.  Some
GAW> manufactures claim much lower temperatures, such as 80C or 120C; and
GAW> as with anything related to temperature change, the size of the device
GAW> is also an important consideration.  This, from www.melcore.com, is the
GAW> best detailed explanation I've found of failures in these devices so far:

GAW>    Inadvertent overheating of the module.

GAW>    The direct soldering process does result in temperature restriction
GAW>    for operation or storage of the modules.  At temperatures above 80C
GAW>    two phenomena seriously reduce useful life:

GAW>    Above 80C copper diffusion into the thermoelements occurs due to
GAW>    increasing solid solubility in the thermoelectric material and
GAW>    increasing diffusion rate.  At 100 - 110C the combined solubility
GAW>    and diffusion rate could result in approximately 25% loss of device
GAW>    performance within 100 hours.

GAW>    Above 85C in the soldering process (using Bismuth-Tin Alloy) small
GAW>    amounts of selenium, tellurium, antimony and nickel are inherently
GAW>    dissolved into the bismuth-tin solder.  Although the melting point of
GAW>    the base solder is 136C, the combined mixture of all elements
GAW>    results in either a minute eutectic phase or a highly effective solid
GAW>    state reaction occurring at above 85C that starts to delaminate the
GAW>    ends of the thermoelements by physical penetration between cleavage
GAW>    planes in the thermoelectric material.  This results in a mechanical
GAW>    failure of the interface.


GAW> See also these sites:

GAW>         http://www.peltier-info.com/accessories.html
GAW>         http://www.electronickits.com/kit/complete/peltier/ck500.htm

You can also get smaller ones through surplus Mac places and even a
couple of 1 - 2" ones here: www.sciplus.com (American Science and
Surplus) although their stock varies greatly and regularly.


Mike Hebel                          mailto:drone8of9 at crosswinds.net

http://drone8of9.crosswinds.net/documents/Mike_Hebel_Current_3.html

http://drone8of9.crosswinds.net/documents/Mike_Hebel_Current_3.doc