Test Gear: the Isolation Transformer From Salvage

Introduction: Test Gear: the Isolation Transformer From Salvage

About: For now see me at: http://www.opencircuits.com/User:Russ_hensel

If you have worked much with 120 or 220 AC mains power you may know what an isolation transformer is, if not you should because it is important piece of safety equipment.  As its name implies it is a transformer, normally a 1:1 transformer, 120 volts in 120 volts out.  So what is the point?  There is no direct connection between the primary side and the secondary, so there is no ground connection between the two sides and a high dc resistance ( normally good for 100's of volts ) between the two sides.  This means that if you are grounded ( touch a ground, stand on a damp floor, use grounded test equipment ) that you can connect to either side of the isolation transformer without risk of a shock.  You can also touch either side of  any device plugged into it. Isolation transformer
From Wikipedia, the free encyclopedia says:  http://en.wikipedia.org/wiki/Isolation_transformer.   Google for more information.

So maybe you should have one, but do not want to lay out the money for one (  they are surprisingly expensive ).  This instructable describes one I built and gives you tips to build your own.  With a bit of luck you can find a salvage transformer ( or 2 ) and put it together for cheap.

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Step 1: Materials and Tools

The most important components:
  • I used 2 identical old filament ( 6v ac ) transformers
  • Case in my case a plastic card file.
  • Wire
  • 120 volt wire, male and female plugs.
  • Soldering iron
  • Usual hand tools.
If your salvage box does not have the required parts here are a couple of transformer sources:

Step 2: Theory

There are a couple of ways of using more or less random transformers to make an isolation transformer. One way is to have 2 transformers with the same output voltage ( in my case 6 volts ) You can use one transformer to step the voltage down to 6 volts and the other transformer “backwards” to step the voltage up from 6 volts to 120. The transformers need not be identical they just need one voltage in common. Total power ( or really volt amps, VA ) should be limited to the lowest rating of any of the windings. This is how I first set up my isolation transformer.

A second way is to use a transformer rated for either 120 or 220 volts. Normally these have 2 completely separate primaries, you are expected to use them in parallel for 120 volts and in series for 220 volts. Instead use one as an input the other as an output. s makes a 1:1 transformer or an isolation transformer.

You might want to use a couple of microwave transformers to make you pair of transformers, just rewire to have equal low voltage sides. ( for higher power use as many secondary windings as will fit ).   I would not keep the high voltage secondaries, I think this is too dangerous.  The following instructable may help:

Sorry about the low quality schematics.

Step 3: The Build

Basically put the parts in/on a box and wire it up.  The picture ( and the schematics above ) should be enough.  Be careful any time you plug this guy in.  An on off switch and pilot light would be nice.

Step 4: Use

Plug in the transformer, then plug in the device to its output, test away in relative safety, but only relatively, still be careful.  Do not exceed the Volt Amps rating of the transformers.  Go out and fix something.

photo from:  http://tesladownunder.com/

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    2 Discussions


    3 years ago

    Go to a place where they recycle electronic equipment and look for two UPS. They contain transformers you can connect back to back, with lot of power if you find big ones.

    One of the main reasons to use an isolation transformer is that some radio & electronic equipment use a "hot chassis" in which the chassis is connected directly to one side of the power cord & some power cords don't have a polarized or 3 prong grounded cord. I made an experimental isolation transformer from two identical single/three phase transformers which fortunately works,salvaged from two junk elevator controllers with each transformer rated at 500 KVA.

    I fed 120 volts into the two single phase 115 volt taps of one transformer(the taps are identical on the second identical transformer) & measured 440 volts with my multimeter at the three three phase 480 volt taps. I connected the 480 volt taps to the same three phase taps on the second transformer & measured 120 volts on the two 115 volt taps on the second transformer.