Electronics - Transformers - Discussion

We have 1:5 turns ratio and 50V on the primary. The output voltage would be 5 times of 50V i.e. 250 V. Load is given 1K across the secondary this would produce a secondary current of 0.25 Amp. (Ohm's law).

Assuming the transformer is 100% efficient then the power out must equal the power in. This means the primary current will be 0. 25*5 times of the secondary current i.e.1.25 amps. So at the primary side we have 50V and a current of 1.25 amps.

If the transformer circuit was replaced by a single equivalent load then by Ohm's law this would be equal to 50V/1.25A = 40 ohms. This is the reflected impedance of the 1000 ohm load on the secondary.

Here is to find Source Resistance through load resistance.

Now load resistance is double of present valve.

Then R2 = 2R2.

The valve of R2 is 2000 ohm.

Now, use below formula for this question because here load resistance is given. If we convert load is into source are then we use below formula.

Rp =Rs * a^2.

Where, a= N1/N2.

Rp = 2000 * (1/5) ^2 => 80 ohm.

For an Ideal transformer Eff. is 100%, so power o/p & i/p is same

For that,

Ip^2 * R1 = Is^2 * R2 (Ip/Is=K).

R1 = R2/K^2.

And there was load being double so,secondary load resistance became double R2 = 1k * 2
R1 = 2000/25 = 80 Ohms.

Primary voltage = 50
Secondary voltage = (1/5) ==== 250.
Secondary resistance = 1000ohm.

Then we find secondary current = (250/1000) == 250amper,
And question ask if load(secondary resistance) dbl then what source(primary) resistance.
So we dbl the load mean secondary resistance = 2 * 1000ohm =2000ohm.
Then secondary current is (250/2000) == 0.125 amper.
And if secondary is current 0.125 then,
Primary current is (1/5)=====0.125 * 5 = 0.625amper.

So then we find source(primary) resistance.(primary voltage/primary current) = (50/0.625) == 80 ohm is the answer.

I agree @Jaskaran Singh.

It's so simple the equivalent resistance on primary side = r2*a*a,
So, 2000*(1/5)*1/5 = 80.

When referred to primary side, the secondary rest. R2 becomes R2'=R2/(K^2) where K=N2/N1.
i.e. Transformation ratio so using the equation, we get R2'=1000/(5/1)^2 =80 ohms.
For referring primary rest, to the secondary side, use formula R1'=(K^2)*R1.
Transformation ratio(K)= N2/N1,
Turns ratio(a)= N1/N2,
So, Transformation ratio= 1/Turns ratio.

R1/R2 = (a)^2.
R1/2(1000)=(1/5)^2.
R1 = 80 ohms.

Note: it is stated in the problem that the load is doubled.
Therefore, the correct answer is A.