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An Isolated Adjustable Auto transformer
This is a means of
testing AC mains operated circuits at variable AC voltages,
and it also provides isolation to allow safe measurements of that
A fused isolation transformer and a
variable auto transformer connected
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together in a grounded,
fire safe box. What more could you ask?
I need to be able to test and measure the operation of circuits
operating directly from the AC mains, this includes being able to
slowly raise the voltage while monitoring circuit performance. This was
solved with a variable auto transformer and an isolation transformer.
Now, I can choose what part of the AC operated circuit to ground, and
connect my oscilloscope directly to the relevant part of the circuit;
no more using two probes and doing the old Channel 1 - Channel 2 bit,
along with its inherent inaccuracies. The variable transformer allows
me to slowly increase the voltage to power supplies under test, so that
I can back off on the voltage if signs of malfunction appear.
few components are needed to create a test tool that provides
a measure of safety for both the
user and the equipment under test.
This is pretty simple: An isolation transformer and a variable auto
transformer, very much like a Variac. Fuse 1 protects against gross
overload. Lamp 1, across the Fuse 1, is a red incandescent pilot lamp
and it lights up in the event of a blown fuse. Imagine how much
time this would save when bringing up a stubborn power supply. Take
note: Everything that can possibly come in contact with the user is
connected to earth ground. in practice, I also have a earth leakage
breaker dongle in series with this device.
The earth leakage dongle (GFIC dongle) also passes the earth ground
through to the output connector.
The circuit was carefully assembled using crimp connectors for all of
the screw-down connections, with proper insulation via heat shrink
tubing. All of the pieces of the steel and aluminum box were connected
by bonding wire. All of the conductors were taken from 0.5 square mm
lamp cord to assure the required dialectic strength.
Contents ©2011 Richard Cappels All Rights Reserved. Find updates
First posted in January, 2011
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