— a hollow iron cylinder with a flatted area of bare
exposed wire on the top surface called the commutator.
The slider is a spring-loaded carbon brush attached to a
central shaft, and when rotated around this commutator,
picks off a variable output voltage (V2) of 0 to 140 volts.
This action is almost identical to the mechanics of a
The position of the slider now forms a transformer
which — for all practical purposes — follows all the rules of
an isolation transformer: volts x amps in equals volts x
amps out. In other words, the “secondary” step up/step
down voltage will always be rated at a current (up to the
maximum name plate rating) that produces the same
wattage as the rated incoming “primary” wattage.
The term “VARIAC” ( VARIable AC) was coined by
the General Radio Corp. back in the 1930s and although
they no longer manufacture these devices, the name stuck
and is synonymous even today with a variable auto-transformer. Today, Superior Electric and Staco, Inc.,
make most of these devices. Superior’s trademark is the
“PowerStat.” The PowerStat has become as popular as
the variac when identifying these devices and is used interchangeably when referring to variable auto-transformers.
Although you probably won’t use one of these as
much as more common test equipment (such as a DMM),
when the need arises, they are indispensable. I’ve used my
variac for transformer testing, initial power-up of new
power supply designs, regulator vs. line voltage tests, as
various AC voltage sources, and all the way up to power
tool testing. But that only touches the tip of the iceberg,
and I could go on and on about its uses. Initially, when
designing this unit, I decided that a 10 amp variac would
be the best compromise between power and manageable
size of the finished product.
This has proven to be a good choice over the last
couple years since I built it, as I have used it for testing
low milliamp devices all the way up to three horsepower
woodworking routers (speed control design). Even though
the name plate rating is 10 amps, it can put out
considerably more current for short time intervals
( 10-15 minutes). I have driven 15+ amp devices
for time periods such as that. It all comes down
to duty cycle and starting with a cold core,
then adequate cooling time between these high
with it sitting on its front face with the dial plate removed
is what you want to look for. You can always ‘Google’ for
exact model information, including detailed mechanical
drawings plus a ton of general information. Now that the
most expensive part is out of the way, the next step is to
locate an enclosure to house this unit, a couple of digital
panel meters, circuit board, power supply, and various
Fortunately, I had a metal box out in the garage
that — with the right shoehorn — I could cram all the
parts into (I have a habit of filling a 5 lb bag with 10 lbs
of parts). I needed to make a vented back panel, so I
used some scrap Bakelite sheeting I had and then I
fashioned a 1/8’ aluminum front panel that was adequate
to hold the weight of the variac. I screwed on a handle
and some bumper feet that I scrounged from my junk
box and I was all set. I gave it a fresh coat of paint then
picked up the rest of the hardware and associated panel
meters that I needed. Since I knew the internal circuit
board and its power supply board would be quite small,
I just allowed about 10-12 cubic inches of space to fit
those in later.
The box I used measures 9” wide x 6” high x 5”
deep. Do not go any smaller than this if you use a 10
amp variac. In fact, bigger is better. Of course, if you use
a smaller variac, just reduce the enclosure accordingly.
Once you have all the major components set out in front
of you on your workbench, you can then best determine
your layout and enclosure size. Remember, nothing is
engraved in stone here, so as I describe how I constructed
my unit, I’ll offer a few options along the way.
Referring to Figures 2 and 3, notice the four ears
slightly protruding at the front end (dial knob end). Lay
these holes out and transfer their locations to the front
panel. Machine them and then mount the variac with four
#10 x 1” bolts. In my unit, the variac terminal board was
very close to the bottom of the box when installed (1/4”).
I wasn’t comfortable with this small clearance, so I cut a
piece of Formica laminate and glued it to the bottom of
■ FIGURE 2. Bare
To start off this project, the first item to
purchase is a bare bones variac unit (no case or
accessories). Of course, eBay was the place to look
for one. The unit I chose was a Staco #1010. They
just seem to have a steady stream of these for sale.
The average selling price of these units ranges
between $35 to $55. As it was, I got mine for $25
and the seller even threw in a small 1.75 amp unit.
Figure 2 is typical of these 10 amp units. The type
June 2008 37