Review of Transformer Action

Author: N.H. Crowhurst

Audio transformers are used to match one circuit to another. For an ideal transformer, the ratio of the primary and secondary voltages varies directly with the ratio of the number of turns in the two windings, the ratio of primary and secondary current varies inversely with the turns ratio, and the primary and secondary impedance varies directly with the square of the turns ratio.

Summary of transformer action

These relationships may be explained as follows: the first two arise from the fact that (in an ideal transformer) the magnetization (measured in ampere-turns) of the core is zero at all times. If current is drawn from the secondary, the product of this current and the number of secondary turns must be cancelled by the product of an equal and opposite product of primary current and the number of primary turns. Since the ideal transformer neither adds nor absorbs power, the product of the primary voltage and current must equal the product of the secondary voltage and current. Thus current varies inversely with the turns ratio and voltage varies inversely with current or directly with the turns ratio. Since the secondary impedance is measured by the ratio of secondary voltage to secondary current, and the voltage has Increased by the turns ratio while the current has decreased by the turns ratio, the impedance must change by the square of turns ratio.