Homemade coil

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Make your own coil from an old transformer
First, find a suitable transformer to be used as a donor for parts.
I have tried several, but the best i found was marked:

Sino-American Model A42455G
Input: 230V
Output: 24V 0,55A  13.2VA

I gueess others will work, too.


Take it apart.

Some are glued togheter, or use special screws, just break the case with a hammer.

Dissassemble the transformer.

The iron core plates can be a little hard to get out, use a knife to bend one at a time, and pull it out with a plier.

Throw away the "E" plates, you will only use the "I" plates.

If possible, separate the wire formers.
Mine was 0,15mm and 0,6mm
Assemble a pack of "I" plates to make a square core.
Mine took 11 plates, and was 6.6 x 6.6 x 41 mm.
Put some layers of tape on the core.

Make 2 end plates of balsa. 1,5mm balsa will do.

Fasten the end plates with CA.
Make a handle to put in a powerdrill.
I made it  from balsa block, and a wooden dowel.
Wind on 100 turns of the thick wire.
Draw an arrow on the former to mark the wiring direction.
Make holes in the former, and thread the wire ends through them.
Mark the starting end of the wire.
Put a layer of tape on top of the winding.
Put the assembly in a powerdrill.
Use a drill with adjustable speed.

Use the drill to wind the thin wire on the former.
Use all the thin wire from the transformer.
Make sure you wind in the same direction as the first layer.
Mark the starting end of the secound layer.
Put a layer of tape on top of the winding.
Trim the ends of the formers flush with the finished windings.
Use some coax as cable to the plug.
Make 2 short pieces of wire, one red and one black.
Solder toghether the screen of the coax, the black wire, and both start ends of the windings.
Solder the core of the coax to the end of the thin winding.
Solder the end of the thick winding to the red wire.
Put some thick shrink tube over the coil, an its done.

Homemade coil to the left, Modelectric coil to the right.

I got some comments from Jorge M. barrera;

- The wire I used for secondary winding is 0,2 mm diameter.  I have 200 volts in secondary with the slow test and more than 100 in the fast rate. Is possible to increase this voltage by reducing the value of the resistor for the transistor feedback (220 instead of 470).  Sometime ago I made modifications on the capacitor and resistor and the output of the toroid was very variable, easily over the 400 V, but when it increased too much the SCR beginned selftriggering and finally burnt.  I burnt 5 SCR first tu figure out that too much voltage was no good... :-)
    - One of the high voltage coils I made worked perfectly after submersing into polyurethane lacquer (wood paint), because insulation of regular transformer wire is often too weak to work with high voltage.  Some good glue (cyano or epoxy) can help.  In a very old book I read about putting some wax paper after every layer of copper wire, to help better insulation.  I'm convinced that insulation on the coil can be a very big issue, specially with the vibration of a plane.