Mains transformer

Aliases: step-down transformer, power transformer, center-tapped transformer, 12-0-12 transformer, trafo. It is the black-and-gray block with an iron core at the start of every linear power supply, before the rectifier, the smoothing capacitor and the regulator.

Two coils share one iron core. AC in the primary makes a changing magnetic field in the iron, and that field makes a voltage in the secondary in proportion to its turns. Fewer turns on the secondary give a lower voltage and more current. Because the two coils only meet through the iron, the secondary is isolated: it floats until the circuit gives it a ground.

Pins

Pin	What it does
P1	Primary, one end. Mains live goes here, through a fuse on a real build.
P2	Primary, the other end. Mains neutral.
S1	Secondary, one end.
CT	The middle of the secondary, the center tap.
S2	Secondary, the other end.

On a 12-0-12V model, S1 to CT and CT to S2 are 12 V each, and S1 to S2 is 24 V. Two diodes from the ends and CT as ground make a full-wave rectifier. On the single-secondary models, CT is still the middle of the winding, and you can leave it unconnected.

Properties

model picks the transformer by its rating:

Model	Primary	Secondary at full load	Rated current
230V to 12V 1A	230 V, 50 Hz	12 V	1 A
230V to 12-0-12V 1A	230 V, 50 Hz	12-0-12 V	1 A
230V to 9V 1A	230 V, 50 Hz	9 V	1 A
230V to 6V 500mA	230 V, 50 Hz	6 V	0.5 A
120V to 12V 1A	120 V, 60 Hz	12 V	1 A
120V to 12-0-12V 1A	120 V, 60 Hz	12-0-12 V	1 A
120V to 24V 1A	120 V, 60 Hz	24 V	1 A

The default is 230V to 12V 1A.

Feeding it mains

Use a generator set to sine at the mains frequency, with offset 0 and amplitude the peak voltage: 325 V for 230 V rms, 170 V for 120 V rms. Wire its OUT to P1 and its GND to P2.

While it runs

The readout is the secondary's rms voltage end to end (S1 to S2), over the last mains cycle. Before Run it shows the model.

What the model gets right

• The turns ratio, and the isolation between the two sides.
• Regulation: a small transformer is sold by its voltage at full load, and its open-circuit voltage is about 10% higher. So a 12 V 1 A model reads about 13.2 V with nothing on it and 12 V at 1 A.
• The magnetizing current: the primary draws about a tenth of its rated current with no load at all.
• DC passes nothing. A steady voltage on the primary only drives current through the copper, which on a real one blows a fuse or burns the winding.

What it does not do

• Saturation, core loss and heating, so a transformer overloaded for a long time does not get hot or fail.
• The inrush of the first half cycle when it is switched on.
• The thermal fuse some of them have inside.

A real mains transformer has lethal voltage on its primary. Simulate the mains side here, and build only the low-voltage side by hand.