Designing Embedded Hardware, 2nd Edition

Transformers

Transformers are related to inductors. A transformer consists of two coils of wire, known as the primary and the secondary, that are closely coupled magnetically. The schematic symbol for a transformer is shown in Figure 4-45.

Figure 4-45. Schematic symbol for a transformer

An AC current flowing through the primary will generate an associated electro-magnetic field. The strength of the field is proportional to the number of turns in the coil of the primary. Because the secondary is within this field, the field will generate a current flow through (and therefore a voltage difference across) the secondary. Since the secondary has a different number of windings in the coil than the primary, the field generated by the primary will create a different voltage and current in the secondary (provided, of course, that the secondary is part of a circuit so that current can flow). Therefore, a transformer can be thought of as a voltage multiplier (or divider). The ratio of the number of turns in the primary and secondary will determine the voltage multiplication.

Since transformers are usually exceptionally efficient, most of the power in the primary is transferred across to the secondary. If the secondary increases the voltage of the primary, then the secondary's current will correspondingly be smaller than in the primary. Conversely, if the voltage across the secondary is less than ...