An AC adapter for a telephone answering machine uses a transformer to reduce the

Question

An AC adapter for a telephone answering machine uses a transformer to reduce the line voltage of 120 V to a voltage of 9.00 V. The RMS current delivered to the answering machine is 530 mA. If the primary (input) coil of the transformer has 600 turns, then how many turns are there on the secondary (output) coil?

What is the power drawn from the electric outlet, if the transformer is assumed to be ideal?

Hints:
Are we transforming the voltage up or down?
What is the relationship between the ratio of the voltages and the ratio of the turns?

Explanation / Answer

Vp = 120V, Np = 600

Vs = 9V, Ns =?

to calculate the number of turns in teh secondary coil, we use the relation

Vs=Vp * (Ns/Np)

9 = 120 * (Ns/ 600)

so, Ns = 9*600/120 = 45

Power drawn from the electrical outlet if the transformer is assumed to be ideal; P = IpVp =IsVs

or P = 120*5.3*10-3 =63.6%

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