An AC adapter for a telephone-answering unit uses a transformer toreduce the lin

Question

An AC adapter for a telephone-answering unit uses a transformer toreduce the line voltage of 120 V (rms) to a voltage of 8.0 V. The rms current delivered to the answeringsystem is 440 mA. (a) If the primary (input) coil in thetransformer in the adapter has 240 turns, how many turns are thereon the secondary (output) coil?
1 turns
(b) What is the rms power delivered to the transformer? Assume anideal transformer.
(avg)input = 2W (a) If the primary (input) coil in thetransformer in the adapter has 240 turns, how many turns are thereon the secondary (output) coil?
1 turns
(b) What is the rms power delivered to the transformer? Assume anideal transformer.
(avg)input = 2W

Explanation / Answer

Given V = 120 V

           V ‘ = 8 V

           I ‘= 440 mA

           N = 240

          N’ = ?

We know   V ‘ / V = N ‘ / N

From this N‘ = V’ N / V

                     = 16

(b). we know I ‘ / I = N / N ‘

From this current in primary I = I ‘ N ‘ /N

                                                 = 29.333 mA = 0.029333 A

Rms power delivered to the transformer P = VI / 2

                                                                    = 1.76W

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