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 5.00 V. The RMS current delivered to the answering machine is 680 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? What is the power drawn by the transformer, if 13.5 percent of the input power is dissipated as heat in the coils and in the iron core of the transformer?

Explanation / Answer

a)

From Transformer turns ratio

Ns/NP = Vs/VP

Ns=600*(5/120)=25 turns

b)

If transformee is ideal then input power is equal to output power ,so power drawn from the outlet

Ps=VsIs=5*(680*10-3)=3.4 watta

c)

Power drawn by the transformer is

Ptransformer=0.135*3.4=0.46 W

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