In France, the wall sockets provide an AC voltage with Vrms = 230 V. You want to

Question

In France, the wall sockets provide an AC voltage with Vrms = 230 V. You want to use an appliance designed to operate in the United States (Vrms = 120 V) and decide to build a transformer to convert the power line voltage in France to the value required by your appliance.
(a) Should you use a "step-down" transformer (to make Vrms smaller) or a "step-up" transformer (which makes Vrms larger)?

a "step-up" transformer
a "step-down" transformer
.
(b) If the input coil of your transformer has 2760 turns, how many turns should the output coil have?
_____ turns

Explanation / Answer

(a) Since the required Vrms value is less than the wall sockets voltage. So we must use step-down transformer to make Vrms smaller. (b) Number of turns in input coil is Ni = 2760      Voltage across the input coil is Vi = 230 V      Let  Number of turns in output coil is No = ?      Voltage across the output coil is Vo = 120 V For the tranformer ,                     Vi / Ni = Vo/ No                            No =  (Ni / Vi) Vo                                  = (2760 / 230V) 120V                                  =1440 turns                Voltage across the output coil is Vo = 120 V For the tranformer ,                     Vi / Ni = Vo/ No                            No =  (Ni / Vi) Vo                                  = (2760 / 230V) 120V                                  =1440 turns

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