[SML] hot wire foam carving transformer

[Duncan Mahoney]

Get the 20 amp Variac, it's a fair price and not much more $ than the 10 amp
one.  Get a supply of the appropriate fuses, especially if you are not going
to use an isolation transformer between the Variac and cutting wire...which
I really would not recommend doing.  Plug it into a good 20 amp circuit,
auto-transformers have big inrush currents when plugged in or switched on.

 I've done hot wire foam carving using just an auto-transformer many times
in years past, it works... although as has been pointed out, you can
electrocute yourself while burning yourself, or vise versa.  Generally for
lengths of wire under 4 feet I would run less than 20 volts and it was a
challenge keeping the amps to less than 20.  Apart from safety, the other
big advantages to an isolation transformer can be boosting amp flow into the
cutting wire and finer adjustment of the voltage/amperage to the cutting
wire, depending on the amp rating and step down ratio of the transformer.
The adjustment resolution on most autotransformers powered by 120 volts is
only 0.5 volts or so and that much of a change in voltage can make a big
change in current flow in the hot wire.  A 12 volt battery charger of
adequate size can be a good transformer donor, but the 16 volts max or so
that you get out is a bit limiting in terms of the size of the wires you can
heat, I suspect the "sweet spot" is somewhere around 36 or 48 volts, I've
set aside a couple big golf cart battery chargers with bad "electronics" but
good transformers to adapt into isolation transformers next time we have a
big foam show. 

 A few things I've learned over the years:

Hot wire gets longer, if the wire is mounted in a frame, a spring in the
system helps.  If you tighten the hot wire without a spring in the system,
something will break as the wire cools off...

If you use 18 or 16 ga copper wire from the power source to the hot wire,
the copper wire will get almost as hot as the cutting wire

Seven strand stainless steel fishing leader heats about as well as nichrome
wire, costs less, and has a bit more tensile strength

1/16" stainless steel TIG filler rod will work for "formed foam cutting
tools" in a pinch...not as well as nichrome strip, but it's not as expensive
either

Duncan Mahoney
Director of Technical Direction
Associate Professor of Theatre Practice
University of Southern California School of Dramatic Arts