Question

A step-down transformer converts 120 V to 5 V in a phone charger. If the primary coil has 600 turns, how many turns are in the secondary coil? Show all work with correct answer and unit as well.

Answer 1

25 turns

Step-by-step explanation:

Given the following data;

Input voltage = 120V

Output voltage = 5V

Number of turns in primary coil = 600 turns

`\frac {Ns}{Np} = \frac {Vs}{Vp}`

Where;

• Ns is the number of turns on the secondary coil.

• Np is the number of turns on the primary coil.

• Vs is the voltage across the secondary coil (output voltage).

• Vp is the voltage across the primary coil (input voltage).

Substituting into the equation, we have;

`\frac {Ns}{600} = \frac {5}{120}`

Cross-multiplying, we have;

`120*Ns = 600*5`

`120Ns = 3000`

`Ns = \frac {3000}{120}`

Ns = 25 turns

Therefore, the number of turns in the secondary coil is 25 turns.

Hence, it is a step-down transformer because the number of turns on the primary coil is greater than the number of turns on the secondary coil. Consequently, the output voltage (5V) will be lesser than the input voltage (120V).