Ed's Radio (AA6ED) – Transformers

Schematic Symbols
Component Symbol
Iron Core InductorIron Core Inductor

Mutual Inductance

When two inductors are in close proximity to one another, the magnetic field of each coil can interact with the other coil. This magnetic interaction can cause a linkage between the two inductors even if they are not physically connected. This coupling is called Mutual Induction. It is measured in henrys and the symbol is LM.


A transformer consists of two or more coils in mutual inductance to one another. They often share the same iron core. If an AC voltage is applied to one coil of a transformer (called the primary coil), a voltage will be induced in the other coil (called the secondary coil). There can be multiple primary and secondary coils in a transformer.

Turns Ratio

A key feature of transformers is their ability to increase or decrease AC voltage amplitudes. This occurs by means of the turns ratio of the transformer. Turns ratio is the ratio of the number of turns on the primary coil compared to the number on the secondary coil. A ratio of 1:1 means the primary and secondary have the same number of turns. A 3:1 ratio means the primary has three times more turns then the secondary. The reverse is true for a 1:3 ratio.

The turns ratio has a directly proportional relationship to the voltage amplitudes of the transformer. If the secondary has fewer turns then the primary, the secondary will have a lower voltage amplitude then the primary. This is called a step down transformer. If the secondary has more turns, then it will have a higher voltage amplitude and is called a step up transformer.

Step Up Step Down

Take a 1:3 ratio transformer for example. This is a step up transformer and the secondary will have three times the voltage amplitude as the primary. But remember, you don't get money for nothin', nor can you get energy for nothin' either. The secondary gets three times the voltage but only one third the current. Thus the power transfer maintains equilibrium.

Next: Capacitance