What is the output voltage if an input signal is applied to the secondary winding of a 4:1 voltage step-down transformer instead of the primary winding?
Why A transformer's voltage ratio simply follows its turns ratio, Vp/Vs = Np/Ns, and it works in either direction. A 4:1 step-down transformer has four times as many turns on the primary as on the secondary, so feeding the signal into the low-turns secondary makes the high-turns winding the output and the voltage comes out four times larger. Running a step-down transformer backward turns it into a 1:4 step-up transformer.
Watch out Dividing by 4 is what happens in the normal direction, driving the high-turns primary and taking output from the low-turns secondary. The answers about adding resistance describe nothing the transformer ratio question asks about; turns ratio alone sets the voltage.
Feed a transformer backwards and the ratio flips: 4:1 down becomes 1:4 up, so voltage x4.