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How to Use a Solderless Breadboard
Written By: John R. Sewell

The Peltier Module, also known as a solid state refrigerator, thermoelectric cooler, TEC or just cooling module is a modern day use of an effect discovered back in the 1800’s. Any 2 dissimilar metals will produce this effect & it’s related Seebeck effect. The Seeback effect is that when heated, a junction of 2 metals will generate a small voltage. 12VDC Thermoelectric Cooling Module, 127 Couple This voltage is used to energize a meter and is used to display a temperature. In this application we call them Thermocouples and use them to monitor temperature. There are other uses of this effect to generate power to move vents or power deep space probes where they are sometimes called “thermopiles” but that’s another discussion. Our concern here is with the Peltier mode of operation. I will not go into the physics of this here but suffice to say this is the reverse of the Seebeck effect. Here the 2 metals are supplied a current and the junction will get a little cooler than ambient. Actually there is no production of cold but a removal of heat. This junction is acting as a “Pump” to remove the heat from point “A” to Point “B”. Peltier modules use pellets that are optimized for this heat pumping. Most pellets are Bismuth Telluride crystals with intentional impurities so to fabricate a “P” and “N” pellets (the 2 dissimilar “metal” junctions). To get any kind of useful “cooling”; we must get a bunch of these tiny pumps together. In a Peltier Module we pair up these P & N pellets into an array of couples between 2 thermally conductive ceramic plates with all the pellet pairs in parallel. This way all the “pumps” move heat in the same direction. Note here that by reversing the current flow the heat/cold sides will reverse. More pumps, more heat moved. Each plate has a circuit pattern, like a circuit board, to connect the pellets in series electrically. Theoretically you could have a very large array but the size is limited by several factors including thermal expansion differences and self heating. Most common modules are nearly square and contain 127 couples. This is an optimization for 12V operation with trade off in BTU pumped, Temperature differential and internal heat production (I2R). Remember the Peltier Module moves heat; that includes it’s own heat (power I2R or ExI). Everything here is to us a trade off; Max temperature difference is at “O” BTU heat pumped, so a spec that says 70”C” is under ideal conditions, no heat leakage, perfect Heatsinks and insulation etc. Any load will lower that figure significantly. In use we must consider minimizing any extraneous heat coming to the cold side. Peltier Cooling Assembly Always keep the Cold side as isolated from the Hot side through the use of a high R insulation. The module or modules should be Carefully so as not to crush, mounted between a “Cold Plate” of metal and a Hot side Heatsink. Thermal interface grease should be used. The cold side can be a small metal box well insulated from ambient; a plastic compartment with a heatsink & fan to circulate the air, even a liquid heat exchanger. The hot side is critical, as it must dissipate as much heat as possible. Remembering that the differential is from cold to hot so the lower the temperature rise on the hot side the lower the cold/ambient will be. In simplistic terms we use a 1:2+ ratio of Cold plate/Heatsink sizing along with a fan to move air across the heatsink.

To achieve lower temperatures, modules can be “Stacked” or cascaded. Here again we must consider efficiency so to “double” up we must use a sandwich of plates & modules employing our 1:2+ plate ratio. For 2 stages of the same size modules, we would use 2 modules on a larger heatsink then a intermediate Plate on the cold sides then a single module hot side to the intermediate plate and finally the final Cold plate. From a practical stand point, there is a limit to stacking; Cascaded modules become less efficient at colder temperatures, Watts pumped is limited to the single or smallest module in the system. Size and power also increases dramatically. Stacking is generally used in scientific instruments to achieve a very cold, small area.

If you are considering a project with TEC modules the size & scope needs to be thought out. One parameter is lack of efficiency as compared to other methods; The TEC module comes in at about 10% compared to compressor systems. The one great advantage is the Peltier module can be mounted in any orientation and be perfectly functional. The smaller the cooled space or object, the better. Cascaded or combinations of Compressor & Peltier Modules can attain cryogenic temperatures.
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