A VDAELRUXIEATECST BENCH
BY ROBERT REED
For the last 10 years, I have needed to
use a variac for various bench testing.
Up until recently, my barebones test
setup consisted of a two amp variac, a
DMM, and a collection of test/jumper
leads. To perform tests in this manner,
it was awkward to say the least.
Common scenario: Measure the
variac’s set voltage, connect load,
disconnect DMM and change leads for current measurement, reconnect
everything, and hook up to load. Then, when any change was needed or
performed, it was swapping leads back and forth again. You can imagine that
after a while, a “rats nest” of twisted, jumbled meter leads and jumper leads
were all over the place — quite annoying!
Then, one evening when I inadvertently connected the
wrong end of this test lead to the wrong end of that
jumper lead, the whole project went up in smoke! It was
at that point I said “enough” and set out to forever correct
this messy situation. Since deluxe versions of these variacs
can run $500 and up, I just could not afford or justify
buying a new one. However, I was sure I could build one
a lot cheaper and that’s what this article is all about.
For starters, that pile of leads and even the DMM had
■ FIGURE 1.
to go. Also, I wanted more power than my little two amp
unit would put out. What I envisioned was the smallest
enclosure with the largest practical variac that I could
cram into it. I also wanted simultaneous voltage and
current metering. One NEMA standard three prong outlet,
along with a power on switch, and fused both on the
input and output would do the trick. In other words, a
moderately small unit that would sit out of the way in a
corner of my test bench and yet perform all the functions
of my “ 20 jumper lead” setup.
For some of you not familiar with these devices,
you may be asking ‘what is a variac and why would I
want one?’ A variac (as used in this article) is a single
winding transformer — more properly known as an auto-transformer — with a movable tap or slider to vary the
output voltage as opposed to a fixed input voltage. These
devices (which have been around for over 70 years) look
like something right out of Thomas Edison’s laboratory.
They have remained virtually unchanged in all that time
which gives testimony to their simplicity and ruggedness
of design. The unit is shown schematically in Figure 1 and
pictorially in Figure 2. The input (V1) at usually 120 VAC
impresses a voltage across the single total winding of L1.
The coil will reach a potential of 140 VAC at its upper
end. This is because the coil is wound on a common core