Peltier devices
Peltier devices, also known as thermoelectric (TE) modules, are small solid-state devices that function as heat pumps. A "typical" unit is a few millimeters thick by a few millimeters to a few centimeters square. It is a sandwich formed by two ceramic plates with an array of small Bismuth Telluride cubes ("couples") in between. When a DC current is applied heat is moved from one side of the device to the other - where it must be removed with a heatsink. The "cold" side is commonly used to cool an electronic device such as a microprocessor or a photodetector. If the current is reversed the device makes an excellent heater.
As with any device, TE modules work best when applied properly. They are not meant to serve as room air conditioners. They are best suited to smaller cooling applications, although they are used in applications as large as portable picnic-type coolers. They can be stacked to achieve lower temperatures, although reaching cryogenic temperatures would require great care. They are not very "efficient" and can draw amps of power. This disadvantage is more than offset by the advantages of no moving parts, no Freon refrigerant, no noise, no vibration, very small size, long life, capability of precision temperature control, etc. The reader is directed to the following excellent sources of information on
Peltier devices from the manufacturers themselves:
Misc Thermoelectric Device Info & Tips...
Peltier & Seebeck Effects...
The cooling property of these devices is due to the Peltier Effect, while the electrical power generating property is due to the Seebeck Effect. A thermoelectric module can be used as either a cooler or a power generator, but not with the best efficiency. Peltier Effect coolers are almost always constructed with Bismuth Telluride (Bi2Te3) and used around room temperature and below. Seebeck Effect power generators are often constructed of PbTe or, SiGe as well as Bi2Te3 and are used at much higher temperatures.
Rate of Temperature Change...
Peltier device cooling & heating speed - they can change temperature extremely quickly, but to avoid damage from thermal expansion control the rate of change to about 1 degree C per second.
Heat AND Cool...
These things can heat as well as cool? Yes - they make great heaters. Just reverse the polarity of the power supply! But, be sure not to exceed the temperature rating of the module, usually 80C for standard models to 200C for high temperature models.
Power Supply Requirements...
What are the power supply requirements? A simple DC supply is fine if the AC ripple is not more than about 10% or 15%. Don't exceed the specified Vmax of
the module.
Temperature Control...
How can I control the amount of cooling? Marlow does not recommend ON/OFF control, while other manufacturers don't seem to object to this. Varying the
power supply voltage works. Pulse width modulation can be used, but a frequency above 1 KHz (Marlow) or 2 KHz (Tellurex) is recommended (watch out for EMI!) It's best to use some kind of temperature sensor feedback (thermistor or solid-state sensor) and a closed-loop control circuit. See Peltier Device Support & Accessory Manufacturers for commercially available controllers. If you would like to build your own controller then check out a Design Note on the Unitrode (now TI) UC3638 H-bridge motor controller chip: , and the Microsemi LX1810 TEC controller IC. This is a single supply PWM full-bridge (cool and heat) chip. An evaluation board is available -
Moisture Concerns...
If a TE module is to be used to cool anywhere near freezing one has to be very concerned about water condensation. Ever-present water vapor begins to drop out
of the air at the "Dew Point." This will result in the TE module, and what it is being used to cool, to get wet. Moisture inside of the TE module will cause corrosion and can result in a short-circuit. Solutions to this problem are to operate the TE module in a vacuum (best) or a dry nitrogen atmosphere, or to insulate or seal the module so well that no moisture can enter. One could seal the edges of the TE module with RTV silicone sealant, but some manufacturers recommend against this technique as water vapor can migrate through silicone sealant and become trapped inside of the module. As a solution Tellurex offers a polyamide epoxy sealant, TE Tech has a special potting material, and Nord has a special coating. If the TE module is being used to cool, for example, an energized electrical device to 0oC there is still the issue of preventing water condensing on that device and causing corrosion, a short circuit or electrical leakage.
Module Testing...
Can you test a TE module with an ohmmeter? No - the DC voltage that an standard ohmmeter applies will cause a temperature change (Peltier Effect) which will in
turn cause a voltage to be generated (Seebeck Effect) which will cause the ohmmeter to read goofy (drifting, and even a 'negative' resistance. So, then use the "diode test" position on the ohmmeter? No - even though a TE module is constructed of an array of N and P doped semiconductors there isn't an actual diode junction. A resistance test can be made with an LCR meter which measures resistance using an AC voltage. Using a HP 4274A LCR meter I have measured a few ohms for small modules and a fraction of an ohm for larger ones. While one does not usually see the resistance parameter specified on manufacturer's data sheets it is probably not a bad of a check of a module's health, especially if more sophisticated test methods are not available.
Heatsink Grease...
Heat must be transfered from the object being cooled (or heated) to the Peltier module, and heat must be transfered from the Peltier module to the heatsink. Realistically, the interface between the Peltier module surfaces to the object being cooled and to the heatsink will not be perfect. There will be peaks and valleys in the surfaces resulting in tiny air pockets which conduct heat poorly. It is common to place a "thermal interface material" (TIM) between the Peltier module surfaces and what it's mated to. One could write a book on TIMs alone. There are silicone-based greases, elastomeric pads, thermally conductive tapes, thermally conductive adhesives, and so on. Suffice it to say that old-fashioned zinc oxide silicone heatsink grease is still one of the most popular materials, albeit a bit messy.
There are better performing materials. Keep in mind that the object is to fill microscopic surface imperfections, which requires a thin film of heatsink grease. Don't apply this stuff with a trowel - it doesn't conduct heat as well as metal-to-metal (or metal-to-ceramic) contact. Think thinfilm.
TE modules can be purchased with one or both sides metallized, or metallized and pre-tinned. This allows soldering the heatsink and/or the object being cooled to the thermoelectric module for the ultimate in heat transfer. One needs to bear in mind that materials expand and contract with temperature change and not to mount the TE module in such a way that it can't move slightly with temperature change - otherwise it may crack under the stress.
More info on Thermal Interface Materials.
Heatsink Required!...
Finally, always remember that Peltier devices don't cool by making heat magically disappear! They move the heat from one side to the other – where you must remove it with a heatsink.
Misc Thermoelectric Device Links...
The International Thermoelectric Society, a nonprofit organization dedicated to the advancement of the thermoelectric industry, science & technology.
Information, consulting and analytic services for the thermoelectric and energy conversion communities:
Consulting, research, testing, publications:
Cardiff Thermoelectric Group (UK)
NanoThermal Partnership (Europe)
A trade magazine about thermal management - Electronics Cooling
TE Power Generator info:
Peltier module primer, Gateway Electronics
Optimizing ThermoelectricTemperatureControllers - an overview of how to piece together the load, TE, heatsink, and controller, from Wavelength Electronics: