Question

A step down transformer (having windings of negligible resistance) is used to step down the mains voltage of 220V, to22V to operate a device of impedance 220V. Assuming an efficienty of 80%, the current flowing in the primary coil of two transformer is

(1) 8 nA

(2) 10.25 mA

(3) 12.5 mA

(4) 14.75 mA

Answer 1

(3) 12.5 mA

The secondary, supplying a voltage of 22V, operates a device of impedance 200 Ohms.

Hence current in the secondary 22/220 A = 0.1 A

∴ Power consumed in the secondary circuit = 22 x 0.1, W = 2.2 W 

Efficiency = 80%

Power available in the primary circuit = 2.2 x 100/80 W = 2.75 W

∴ If I is the current in the primary coil, we have