Physics transformer question?

Question

A device operates at 6 volts and has a power rating of 4 watts. It is connected to a 240 volts a.c supply, using a suitable transformer.

-If the primary coil of the transformer has 4000 turns, how many turns are on the secondary coil?

-Calculate the current flowing in the primary coil?

Please explain, thanks!

Answer 1

The voltage ratio is the same as the turns ratio: the voltage ratio is 40:1 (240:6), therefore the primary has 40 times as many turns as the secondary. If the primary has 4000 turns, the secondary has 100 turns.

With no losses in the transformer, the wattage in equals the wattage out. The wattage is 4, the primary voltage is 240, therefore the primary current is 4/ 240 A = 16.7mA

Answer 2

The turns ratio is the same as the voltage ratio, so it will be 40::1. Since the primary has 4000 turns, the secondary will have 100 turns. If the load is using 4 watts, then (assuming a lossless transformer) the primary side must be reflecting this same 4 watt load and so the primary side current will be 4/240 = 16.67 mA.

Doug

Answer 3

The ratio of turns follows the ratio of voltage. This means that the volts per turn is the same in the primary as in the secondary. Watts are conserved in a lossless transformer. For the primary current you know the watts are 4 and the voltage is 240. I = W/V