Transformer

A transformer moves electrical power from one coil of wire to another without them touching. Current changing in the first coil creates a changing magnetic field, which makes a voltage in the second. The ratio of turns in the two coils sets the ratio of the voltages, so it can step a high voltage down to a low one.

To build one, enameled copper wire is wound around a bobbin, a hollow spool molded from PBT plastic with pins for soldering. The primary winding goes on first. Layers of insulating tape made from PET film are wrapped over it, then the secondary winding is added, and more tape. This tape helps keep a safe barrier between the wall voltage and the parts you can touch. Finally two halves of a ferrite core are slid into the bobbin from each side and glued or clipped together, giving the magnetic field a closed loop to follow. The finished part is often dipped in varnish.

On the power supply board, the transformer is fed fast pulses from the switching transistors. Ferrite is used because it handles those high frequencies with little loss, unlike the laminated iron cores of old mains transformers.