The main purpose of the isolation transformer is safety and protection of electronic components and the persons against electrical shock. It physically separates the power supplying from primary side and a secondary side circuit connected to electronic components and grounded metal parts which are in contact with the person. Basically, the transformer secondary side is isolated from the grounding.

Proper isolation transformers have ratio 1:1 and have a ground shield protecting the secondary winding from capacitive transference. Are also expensive and bulky items because most are designed to high power requirements.

I needed to take some measures on a low power led supply but risking a connection of my measuring equipment to mains voltage was not an option. To buy an expensive transformer was also not an option. But I had two salvaged halogen lamp transformers laying around, so I decided to use them to make a cheap isolation transformer for my low power and low money needs. This way I don't risk zapping my measuring equipment.

Putting two 230 -> 12 V transformers back to back is not ideal and I suppose that if I try to draw something near the 50 VA they are rated the losses will be high, but as I need at most 20W I think losses will be within reasonable limits.

Supplies

2 Salvaged transformers. In my case I used 2 50VA 230/12 toroidal transformers
1 ABS Box
4 Rubber feet 18X15X8 mm
4 Oval head M4 screws and nuts with washers
1 IEC C14 panel male plug (rated 10 A)
2 Panel fuse holders with rapid 0.5A fuses
1 Double pole switch stable ON - OFF with indicator light
2 High voltage sockets (Red for 230 V output)
2 High voltage sockets (Green for 12 V output) - optional but a nice to have to test 12V bulbs
4 Isolated Faston female terminals to connect to the switch
4 Isolated Faston female terminals to connect to the sockets
4 wago connectors for 2 cables
1 wago connector for 3 cables
1 wago connector for 5 cabes
12 cable ties 3x150 (black to match the box)
2 Nylon countersunk M3 screws and nuts to fasten the C14 plug
Heat shrink tube and Insulating tape

Step 1: Design

The main power arrives at an IEC C14 plug and both live and neutral are connected to the double pole ON - OFF switch trough two rapid fuses. The fuses are designed to protect from excess current but also to protect against a leak in the first transformer. The live and neutral then go to the primary of the first transformer.

The secondary of the first transformer is then directly connected to the secondary of the second transformer. As a bonus, the 12V is made available at the two green output poles. For protection against any leak one side of the secondary is connected to ground.

The primary of the second transformer is connected to the two red output poles. Those poles should never be grounded.

Step 2: Construction

The first step was making the necessary holes in the front and in the back of the box.

Picture 1 shows the front with 4 holes for the output poles and a large rectangular hole for the switch. The picture 2 shows the back has two round holes for the fuse holders, a rectangular hole for the plug and two small holes for the screws holding the IEC plug.

The large holes where started with a 3mm wood drill bit and then enlarged with a step drill bit. For the square holes I drilled a hole in each corner and then cut the rectangle with a hand jigsaw. Then with a round and a square file I corrected the shapes until everything fitted nicely. After that I marked and drilled the small holes to hold the plug with the 3mm bit for wood. Careful when fitting the posts, etc to the holes not to damage the threads.

On the base of the box I drilled 4 holes for the rubber feet and more 24 holes to hold the toroidal transformers with zip ties. The official way of holding the toroidal transformers is with a metal disc and a long screw, but that has two major problems. For one that kind of hardware it is increasingly expensive and hard to get and I could not salvage it. Another problem is that the bottom of the box is not metal but plastic and with only one screw holding the weight of the transformer it will easily deform or crack. So I devised an alternative way of holding the toroidal transformers with plastic zip ties. It is very important to be a non conducting material or it will create a loop, dangerously drawing current and power from the transformer.

The box completed and everything securely attached it is time to make connections. Just follow the schematic. The fuse holders and the IEC plug have their connections soldered and protected with heat shrink tube.I used Wago quick joiners mainly because the transformer's wires are a pain to solder and excess heat could burn the insulation. Ferrules are used for the other wires connected to the Wago joiners. The elements in the front panel have all male terminal connectors, so I crimped insulated female connectors to the wires and plugged them in.