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)