Question

In a transformer the output current and voltage are 4 A and 20 V respectively. If the ratio of number of turns in the primary and secondary coil is 2 : 1 respectively, what is the input current and voltage?

• A. 2 A and 40 V
• B. 1 A and 20 V
• C. 4 A and 10 V
• D. 8 A and 40 V

Answer 1

Answer: (A)

Answer is A.

A transformer takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit. A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
If we change the number of turns in the coils we change the induced emf. This allows us to change (transform) the voltage from the primary to the secondary coil.
The Turns Rule is:
Ns/Np=Vs/Vp
Where,
Ns = number of turns on the secondary coil
Np = number of turns on the primary coil
Vs = voltage across the secondary coil
Vp = voltage across the primary coil
So if number of turns on the primary coil is more than on the secondary coil, the output voltage will be lesser than the input voltage. This is called a step down transformer.
In this case, the turns ratio is 2:1.
Np/Ns=Vp/Vs;
That is, 2/1 = x/20.
$x=\frac{20\times 2}{1}=40V$. So, the ratio of the voltages is 2:1.
It is a known fact that voltage and current are inversely proportional and so current will decrease as voltage increases.
Hence, the input current and voltage is 2 A and 40 V.