A transformer connected to a 120 V (rms) ac line is to supply 13,000 V (rms) for a neon sign. To reduce the shock hazard, a fuse is to be inserted in the primary circuit and is to blow when the rms current in the secondary circuit exceeds 8.50 mA?

What power must be supplied to the transformer when the rms secondary current is 8.50 mA ?

917W
110W
102W
153W

1 Answer
A08
May 21, 2017

#110W#, rounded to units

Explanation:

By law of conservation of energy, for an ideal transformer the expression between input and pout power can be written in terms of primary and secondary variables as

#I_P V_P = I_S V_S#
As maximum current has been given for secondary, in terms of secondary voltage and current this becomes

#P_P=P_S=I_S V_S#

Inserting given values for secondary we get
#P_P=8.50xx10^-3xx 13000#
#P_P=110W#, rounded to units

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