Question

An AC adapter for a telephone answering machine uses a transformer to reduce the line voltage of 120 V to a voltage of 5.00 V. The RMS current delivered to the answering machine is 380 mA. If the primary (input) coil of the transformer has 960 turns, then how many turns are there on the secondary (output) coil? Submit Answer Tries 0/20 What is the power drawn from the electric outlet, if the transformer is assumed to be ideal? Submit Answer Tries 0/20 What is the power drawn by the transformer, if 17.5 percent of the input power is dissipated as heat in the coils and in the iron core of the transformer?

Answer 1

a) Transformer equation is

Ns/Np=Vs/Vp

Therefore number of turns in the secondary coil is

Ns = Np Vs/Vp = 960 x 5/120 = 40

b) Power drawn is RMS output current times RMS output voltage

P = 5 x 0.380 = 1.9 W

c) Suppose the input power is P. After the loss of 17.5 percent, the power that remains is 1.9 W

(1-0.175) P = 1.9

0.825 P = 1.9

or

P = 1.9/0.825 = 2.30 W