Question

A doorbell needs 0.5A and 8.0V and is therefore connected to a transformer secondary coil. The primary coil has 3000 turns and is connected to a 230V wall socket (a) how many turns does the secondary coil need? (b) How big is the current in the primary coil?

Answer 1

Transformation ratio of idealized transformer:

$$U_1/U_2=w_1/w2$$

Turns secondary coil:

$$w_2=w_1*U_2/U_1=3000*8/230=104 turns$$ $$U_1*I_1=/(U_2*I_2)=P$$

Current in the primary coil:

$$I_1=U_2*I_2/U_1=8*0.5/230=0.017A$$

Answer: 104 turns, 17mA