VARIACS ARE NOT ISOLATION TRANSFORMERS!!!
VARIACs are wonderfup things and every electronics professional should own
one. But they are not transformers in the sense that you are most familiar
with. AC power transformers have two separate windings. This seperation
means no currents flow from one winding to the other. This is where the
isolation comes from. The primary creates a magnetic field that induces a
voltage in the secondary. But there is only a magnetic field coupling the
energy across the windings. There is no electrical path.
Variacs only have one winding which is connected directly across the 120V
line. The output comes from the sliding tap that goes from one end of this
winding to the other (and beyond). As the tap is slid along the primary
winding the voltage at the output is being tapped directly off the 120V
line. To prove this to yourself connect an ohmmeter from the tap to either
lead of the primary and measure the resistance (with the Variac
unplugged!!!). It is quite low (less than a few hundred ohms). Now do the
same thing with a conventional AC power transformer. The resistance measured
from the primary to the secondary should be infinite. Usually the Variac's
tapped off voltage can range from 0 to 145VAC if 120VAC is being applied to
it because the creators of this clever device (General Radio) included
additional windings beyond where the 120VAC high side connects to the
Variac.
An inexpensive isolation transformer can usually be found at a hamfest or
equivalent. It is not something that you should have hooked up permanently
since it can cause more problems that it solves. But there is occasionally a
situation where you need it.
An isolation transformer will not protect you from high voltage shocks. That
is not what they are designed to do. The isolation transformer will only be
as good as the insulation between the primary and secondary. Since both
windings are wound together (overlapping), as a rule they will not withstand
a large voltage difference and must not be counted on for HV isolation. If I
recall correctly,from my youth, HV transformers like you find in a TV have
primary and secondary windings that are seperated by physical distance on
separate parts of a non-conducting ferrite core. They also work at 15KHz
(which is OK for ferrites) rather than at 60Hz (which usually requires a
classic iron core).
A good oscilloscope is properly designed to work with the third prong
grounded through the wall socket. An inexpensive $7 devvice can be bought at
any hardware store to check all your outlets to insure they are properly
grounded. This tester will also detect other faults as well. Tektronix would
tell you otherwise if there were a better way to connect a scope than
through the third prong of the power cord.
Attaching your scope chassis directly to a water pipe is almost certainly
causing more problems that it solves. If the third prong of the power cord
is also connected in the wall socket then the water pipe ground will create
a ground loop that will be the cause of some very subtle problems (and
incorrect readings) you may never notice.
Variacs are not cheap. They are one of the very, very few things that is
worth more today than when they first came out in the 1920s or 1930s. A
typical 600 watt one in good shape will cost $75-$100.
Dennis
