Question

The doorbell of a home requires 10 volts to operate. A transformer is used to connect a 120 v primary circuit within the home to the doorbell. The transformer in the doorbell has 500 turns on the primary coil and supplies the doorbell with 900 mA of current.

Required:
a. Determine whether the transformer features a step-up or step-down design.
b. Calculate the number of turns on the secondary coil.
c. Calculate the current that is drawn from the 120-V household circuit to operate the doorbell in mA.

Answer 1

See explanation

Step-by-step explanation:

a) The transformer must be a step down transformer because the secondary voltage which it supplies is less than the primary voltage.

b) From;

Ns/Np = Es/Ep

Where;

Ns= number of turns in secondary coil

Np= number of turns in primary coil

Es= voltage in secondary coil

Ep= voltage in primary coil

NsEp = EsNp

Ns = EsNp/Ep

Ns = 10 * 500/120

Ns = 42 turns

c) From;

Es/Ep = Ip/Is

EsIs = EpIp

Ip = EsIs/Ep

Ip = 10 * 900 * 10^-3/120

Ip = 75 mA