Introduction: Isolation Transformer

I normally work with electronic circuits at low voltages, but occasionally I need to work on things connected to mains (220V AC in my case) and then things become somewhat dangerous.

For years I have had two identical transformers lying around, so why not bring them to use in an isolation transformer? Basically the idea is, that the 220V is transformed down to 24V by one transformer and from 24V back up to (nearly) 220V by the other transformer. You can read more about the basic idea here:

Attention: you will be messing with mains voltage in this instructable, so be careful! I only have limited knowledge about working with mains voltage, so be sceptic!

Furthermore: do not think you are safe just because you have an isolation transformer. Touching the two wires at the same time will still chock you, and grounding one of the wires (e.g. while measuring) will make touching the other wire dangerous. I found this video by Todd Harrison very enlightening and I highly recommend you watch it before going ahead:

Step 1: Make a Test Setup

Wire up the transformers and verify that they are working as expected

  1. My transformer model provides 2 * 12V/6A, so I have connected the two outputs in series to form 1*24V/6A.
  2. The 24V of the two transformers are then connected 'back-to-back'
  3. Mount a mains socket (or a mains plug to the input transformer)
  4. Mount a mains wall-mount socket to the output tranformer
  5. Verify that your output socket is providing nearly 220V when you apply 220V to the input.

At this stage I did not add a fuse, so beware not to attach anything consuming too much power (in my case no more than 24V*6A=144W)

In the final setup you should provide a fuse somewhere to protect the two transformers. I have placed a fuse on the mains input side. I my case, I plan to use one that is rated at around 144W/220V=650mA.

Step 2: Construct a Suitable Box

As I have access to a laser cutter, I decided to go for a wooden box. It is designed using OpenSCAD:

The exported SVG file was post processed using InkScape to add the various holes. Finally everything was cut out using the laser cutter and glued together.

Once the gluing is done, it is time to also do a bit of painting.

Step 3: Mount the Electronics

Once you are happy with the box, it is time to mount the electronics.

So after adding an on/off switch and a fuse holder, everything is assembled onto the bottom plate.

The top part of the box is assembled with the bottom part, with some screws. The mains sockets on the fron and back are mounted, and thats about it (see the first picture in this instructable).

From now on hacking should be just a little bit safer.