Winding a transformer for the ESR meter is not too hard to do. You should always use new enameled wire when doing so as recycled wire may have damage to the enamel which can cause problems.

This transformer was taken from a dead mobile phone power supply. It was disassembled by soaking it for a day in a dichloromethane based paint stripper to dissolve the lacquer holding the core halves in place before unwinding and washing it.

Dichloromethane is not your friend, always follow the instructions for its use.

The secondary is wound first and consists of twenty turns of 0.5mm enameled wire. The last one and a half turns were wound over the other turns as there was not quite enough room to fit all twenty turns on the one layer.

Once the ends had been soldered the secondary winding was covered with two turns of brown parcel tape.

The primary winding of 400 turns was then random wound on top of the tape with 0.15mm enameled wire. No real attempt was made to keep this tidy as there was plenty of room.

With the primary wound and its ends soldered it was covered with one and a half turns of brown parcel tape.

With the windings done the core was inserted and that was also held in place with a turn and a half of brown parcel tape. No glue is needed as when in use the magnetic field will also hold the two halves together.

Parcel tape can be quite hard to handle but this was made much easier by sticking a length of the tape to a glass mirror and using that as a cutting table to prepare strips of the required length and width.

Here you can see the newly complated transformer soldered onto the original prototype board which has also been updated with the components for the 400V input protection circuit.