**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?

N_{2}/N_{1} = V_{2}/V_{1} ,
N_{2}/2000 = 6 / 110 , N_{2} = 109 turns. (number of
secondary turns)

Assuming 100 percent efficiency, input power = output power,

V_{2}I_{2} = V_{1} I_{1, }I_{1} 110 = 6 ( 0.4)
, I_{1} = 0.022 A (current in
the primary)

Power consumed by the bell is VI = 6 (0.4) = 2.4 Watts. (power requirement)