An example problem with transformer:
A doorbell requires 0.4 A at 6V. It is connected to a transformer whose primary contains 2000 turns and is connected to 110-V household outlet. How many turns should there be in the secondary? What is the current in the primary? How many Watts does the bell require from the transformer?
N2/N1 = V2/V1 , N2/2000 = 6 / 110 , N2 = 109 turns. (number of secondary turns)
Assuming 100 percent efficiency, input power = output power,
V2I2 = V1 I1, I1 110 = 6 ( 0.4) , I1 = 0.022 A (current in the primary)
Power consumed by the bell is VI = 6 (0.4) = 2.4 Watts. (power requirement)