A transformer consists of insulated wires wound around a magnetic flux conductor, such as an iron loop. It is used to alter or transform one AC voltage into another, and to provide electrical isolation from the power mains.

The transformer input winding is called the primary and the output winding is the secondary.

It is interesting to note that the ratio of winding voltage to number of turns is the same on both sides of the transformer. Thus by changing the number of turns, the input voltage can easily be stepped up or stepped down to the secondary.

This principle can be extended to any number of windings. In most cases, there is only one primary, but there may be several secondaries.

One of the most common transformers used in power supply design is the center-tapped transformer. Each of the secondary windings can generally be considered as a separate winding.

An ideal transformer has the following current and voltage relationships:

From these equations, we note that transformers can also be used to perform impedance matching. This is feature is used in audio, telephony, video, and RF applications.

All electronic components consume power. A typical power transformer has an overall efficiency of 85-95%. Several factors contribute to loss including: winding resistance, and eddy currents.

A more complete equivalent circuit for a transformer resembles:

A real transformer has series winding resistance and inductance, shut conductance and capacitance, and inter-winding capacitance. In many power supply applications, these additional complications can be ignored, however in signaling, telecommunications, and switch mode applications they must be considered.