Dick Cappels' project pages http://www.projects.cappels.org
Return to HOME (more projects)
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 circuit.

A fused isolation transformer and a variable auto transformer connected
 together in a grounded, fire safe box. What more could you ask?

Find updates at www.projects.cappels.org


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.

The Circuit

Very 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.

HOME (More Projects)
Contents ©2011 Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.org

First posted in January, 2011

You can send  email to me at projects(at)cappels.org. Replace "(at)" with "@" before mailing.

Use of information presented on this page is for personal, nonprofit educational and noncommercial use only. This material (including object files) is copyrighted by Richard Cappels and may not be republished or used directly for commercial purposes. For commercial license,
click here.

  Liability Disclaimer and intellectual property notice
(Summary: No warranties, use these pages at your own risk. You may use the information provided here for personal and educational purposes but you may not republish or use this information for any commercial purpose without explicit permission.) I neither express nor imply any warranty for the quality, fitness for any particular purpose or  user, or freedom from patents or other restrictions on the rights of use of any software, firmware, hardware, design, service,information, or advice provided, mentioned,or made reference to in these pages. By utilizing or relying on software, firmware, hardware, design, service,information, or advice provided, mentioned, or made reference to in these pages, the user takes responsibility to assume all risk and associated with said activity and hold Richard Cappels harmless in the event of any loss or expense associated with said activity. The contents of this web site, unless otherwise noted, is copyrighted by Richard Cappels. Use of information presented on this site for personal, nonprofit educational and noncommercial use is encouraged, but unless explicitly stated with respect to particular material, the material itself may not be republished or used directly for commercial purposes. For the purposes of this notice, copying binary data resulting from program files, including assembly source code and object (hex) files into semiconductor memories for personal, nonprofit educational or other noncommercial use is not considered republishing. Entities desiring to use any material published in this pages for commercial purposes should contact the respective copyright holder(s).