several tens of watts of heat.
In this project, we’ll supply heat to the converter and
draw a current from it. When heat is flowing through the
converter (which will have an efficiency of typically 1% if
the temperature difference across it is about 30° C),
negative charge appears at the heatsink side (the “cold
end”) of the converter in the N-type legs. Note that an
effective heatsink is crucial — it is no use making
the whole converter hot: one side must be hot and the
Thermal conduction is like electrical conduction —
the flow of heat is like a current and temperature is like
voltage. The thermal circuit is a chain of resistances
from the high voltage (the hot liquid) to “ground” — the
cool air. If the thermal resistance is low, then the “current”
or heat flow will be highest.
One of the resistances in the circuit is the thermal
converter itself. Ideally, this is the largest resistance in the
circuit, so that most of the power is dissipated in the
converter and not in useless resistances elsewhere.
So, the other resistances (thickness divided by thermal conductivity) should be minimized — this means a
thin-walled mug made of a thermally-conducting material
like metal. Similarly, the heatsink should be effective. The
better the heatsink, the cooler it will be and, thus, the
temperature difference across the converter will be
Poor contacts between surfaces can introduce very
high thermal resistances — unless surfaces are ground
exceptionally flat and perfectly aligned, a lot of airgaps fill
the contact area with contact being made only through
little bumps in the surfaces. The way to minimize these
resistances and the temperature drops they cause is to use
a thermal grease or heatsink compound that fills these
little gaps with a relatively conductive material.
The voltage produced by the converter is proportional
to the temperature across it — this is, after all, the way
thermocouples are used as temperature sensors. The
current produced by the converter is proportional to the
heat flow through it. The heat flow through the converter is
Circuit diagram. Five converters are wired in series (note polarity!).
They are in thermal parallel (i.e., the same temperature difference is
driving them all). Heatsinks are omitted for clarity.
proportional to the temperature difference across it. So,
the power produced by the converter (current times
voltage) is, therefore, proportional to the square of the
temperature difference. This is why it is vital to maximize
the temperature difference by hot liquid, conductive mug
and contacts, and a good heatsink.
To extract the most electrical power from the converter, it is important to choose the load impedance carefully
Thermoelectric Cooling Modules can now be found in many
electronic parts stores and catalogs. One manufacturer/vendor is
Magaland Technology, Inc. ( www.leadingtechnologysales.com).
I used several of their ICE-IT TEC 1-12705S cooling modules ( 40
x 40 x 3. 8 mm, rated max 15. 4 V at 5.2 A for cooling).
Another manufacturer is TE Technology ( www.tetech.com)
with a wide range of modules ($100.00 minimum order). I have also
used their TM-TB-127-1.4-1.05(P) modules ( 40 x 40 x 3. 8 mm, rated
15. 7 V 8. 6 A). TE Tech’s website also has a lot of technical papers you
Circle #117 on the Reader Service Card.