What does a transformer do to AC voltage?

Transformers either step up or step down the voltage of an alternating current.

Transformers only with alternating currents. At the most fundamental level a transformer consists of a primary coil, a secondary coil and an iron core that passes through each coil. The core ensures that the flux through the two coils is linked. The a.c. in the primary coil causes the flux to be continuously changing direction thus producing a changing flux linkage through the secondary coil which induces an alternating current in it.

If the number of turns in the secondary coil is more than that of the primary then the transformer steps up the voltage, i.e. the voltage output is larger than the voltage input (n.b. is the same if efficiency is 100% but less in real situations). The opposite is true if the number of turns in the secondary is less than that of the primary, i.e. step down, voltage output less than input.

As the flux through each coil is the same (at an instant in time) but the number of turns in each coil is different the flux linkage in each coil is different. The ratio of primary to secondary turns is equal to the ratio of primary to secondary voltages.

##N_p/N_s=V_p/V_s##