Transformer

Steps AC voltage up or down and isolates the two sides.

Schematic letter: T / TR
Unit / key rating: Turns ratio, power (VA)

Pinout

Primary: The winding connected to the mains or the switching circuit.
Secondary: The winding that gives the output voltage; there may be several.

Real photos

What is it for?

A transformer is two (or more) windings on the same core. AC current in the primary creates a changing magnetic field in the core, which induces a voltage in the secondary. The voltages are in proportion to the numbers of turns.

There's no electrical connection between the windings: the transformer isolates the mains from the safe side. Old adapters use heavy 50 Hz iron-core transformers; switch-mode supplies use small ferrite-core ones.

Things to watch out for

⚠ Primary to secondary should read OL (infinite) on resistance; if not, the insulation has failed and it's dangerous.
⚠ Check continuity on each winding; shorted turns don't show on a multimeter, but the transformer heats up and the device trips.

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