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 I_1, 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)