Question

When the turns ratio of a step-up-transformer is 20 and the primary ac voltage is 12V, the secondary voltage is
A. 12 V
B. 120 V
C. 240 V
D. 2400 V

Answer 1

The transformer used to get the voltage of the secondary coil greater than the voltage of the primary coil is known as the Step-up transformer. It has two coils that are secondary coil and primary coil. The ratio of voltages across both the coils i.e. primary and secondary coil depends upon the number of turns. So, the ratio of secondary voltage to primary voltage gives the ratio of turns. Substitute the value of turns ratio and primary voltage and calculate the secondary voltage.

Formula used:
$n= \dfrac {{V}_{S}}{{V}_{P}}$

Complete step by step answer:
Given: Turns ratio (n)= 20
Primary voltage ${V}_{P}$= 12 V
For a step-up transformer, turns ratio is given by,
$n= \dfrac {{V}_{S}}{{V}_{P}}$ …(1)
Where, ${V}_{S}$ is the secondary voltage of the transformer
${V}_{P}$ is the primary voltage of the transformer
Substituting values in the equation. (1) we get,
$20= \dfrac {{V}_{S}}{12}$
$\Rightarrow {V}_{S}= 20 \times 12$
$\therefore {V}_{S}= 240 V$
Hence, the secondary voltage is 240 V.

So, the correct answer is option C i.e. 240 V.

Note:
Students must remember that here by turns ratio of a step-up-transformer it is meant the ratio of number of turns in secondary coil to the number of turns in primary coil. In step-up transformers, the number of turns in primary coil is less than the number of turns in secondary coils. For both the step-up and step-down transformers, the relations are the same. The only difference is the number of turns which changes the working of the transformer. With the help of a step-up transformer, current can be decreased.