gives a high (short circuit) current, but a tiny voltage.
Somewhere in between, there is an optimum impedance
(usually just a few Ω) that sucks a decent current out of
the converter without letting the voltage drop too far. For
these kinds of converters, the short circuit current can be
rather high, so you more or less want as low an
impedance as possible.
Constructing Your Mug
The mug in action. The propeller spins for around 20 minutes.
(just as for a photovoltaic cell). Using a very high impedance load measures the open circuit voltage, but admits
a very small current. On the other hand, a low impedance
~ ~ ~ ~ ~
~ ~ HOT ~ ~
~ ~ LIQUID ~ ~
~~~ ~ ~
NUTS & VOLTS
One difficulty is that a mug is cylindrical, whereas the
converter is flat. One possibility is to find or make a
square or hexagonal mug — or perhaps bash some flat
surfaces onto a tin mug. Another approach is to make
some adapter pieces to help mate the converter to the
mug; if you’re ambitious, you could machine these out of
metal. My quick-and-dirty approach is to wad up aluminum foil, which acts as a fairly conductive — but
moldable — material.
You could make the construction of the mug as
robust as you like — using machined parts or glue. On the
other hand, if — like me — you just want to try it for awhile
before using the converters in another project, you could
strap the converters on to the mug using some steel or
bare copper wire, twisting the wire tight on the mug
handle to hold everything in place.
I found that, with 40 mm heatsinks and a steel mug
with flat sides, I could typically get about 0.3 volts and
several tens of milliamps out of each converter. To get
higher voltages, the simplest approach is to
mount more than one converter (in thermal
parallel, but electrical series). I used five — about
as many as there was room for on a large mug —
to get about 1.5 V. If someone can come up with
a circuit that can multiply DC voltages of a few
hundred millivolts up to several volts, it would
make these converters much easier to use! Note
that thermoelectric devices are like solar cells or
batteries — they have a polarity, which depends
not only on their internal wiring, but also the
direction that heat is flowing through them. So
make sure that you wire them correctly to add
AIR So, what can you drive with this sort of
power? An LED works pretty well — 20 mA is
certainly enough, but you’ll need to make sure
you have enough voltage from the converters to
exceed the forward voltage drop of the diode.
Some LEDs can have forward voltage drops of 2
V or more, but some that are as low as 1.4 V will
work better. Some small DC electric motors will
run on a volt or less, but you’ll need to make sure
it runs smoothly enough at low current. Motors
sold in solar power kits may work well; one I use
was ripped from an old CD player. I glued a small
balsa wood propeller onto it to make it very
There is a cascade of temperature drops from the hot liquid to the air.
In order to maximize the power output, we want ∆Tconverter
as high as possible, so ∆Tmug must be low, as well as ∆Tcontact
(e.g., heatsink compound, represented by the long, thin ellipses)
and ∆Theatsink (large heatsink).