Draw a labelled diagram of a device you would use to transform 220 V a.c. to 15 V a.c. Name the device and explain how does it work. Give it's two uses.

The labelled diagram of a device used to transform 220 V a.c. to 15 V a.c. is given below:

Name of this device is Step Down Transformer. It is used to change a high alternating voltage to a low alternating voltage (of same frequency).

When the terminals of primary coil are connected to the source of alternating e.m.f. a varying current flows through the primary coil. This varying current produces a varying magnetic field in the core of the transformer. Thus the magnetic flux linked with the secondary coil vary due to which an e.m.f. is induced in it. The induced e.m.f. varies in the same manner as the applied e.m.f. in the primary coil varies and thus the induced e.m.f. has the same frequency as that of the applied e.m.f.

In a step down transformer the number of turns in the secondary coil are less than the number of turns in the primary coil (N_s < N_p) i.e., turns ratio n < 1 as shown in figure. Since current in secondary coil is more than in the primary coil (i.e., I_s > I_p ) so the wire in secondary coil is kept thicker than in the primary coil.

Uses of a step down transformer —

1. With electric bells, night electric bulbs, mobile phones, computers etc.
2. At the power sub-stations to step down the voltage before it's distribution to the consumers.