# Disagree with answer to simple Transformer problem



## BamaBino (Jul 22, 2011)

This is from sample college test. I disagree this answer but agree with the all the other problems answers.

I see the problem as the load across the secondary being 600 ohms.

The secondary voltage would be 600V and current 1 A.

And the circuit in the primary being 5 Amps, answer B.

Q.33 A 1:5 step-up transformer has 120V across the primary and 600 ohms resistance

across the secondary. Assuming 100% efficiency, the primary current equals

(A) 0.2 Amp. (B) 5 Amps.

© 10 Amps. (D) 20 Amps.

Ans: A

I1= V1 /R1 = 120/600 = 0.2 (n = 100%, losses are zero \V1 = VR = I1R1)


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## Flyer_PE (Jul 22, 2011)

The way I read that problem, the 120 volts is on the low voltage side of the transformer and the 600 ohm resistor is on the secondary (High Side). There are two ways to find the answer to that problem.

The first is to determine the current in the secondary:

VSecondary=120*(5/1)=600V

ISecondary=600V/600Ohm= 1 Amp

APrimary=ASecondary*5 = 5 Amps

The second way is to reflect the impedance across the transformer using the square of the turns ratio:

ZPrimary=600*(1/5)2=600/25=24

IPrimary = 120V/24ohms = 5 Amps


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## Wildsoldier PE (Jul 27, 2011)

There is also a third way on how to solve the problem.

First we need to know the current on the secondary side:

Voltage on secondary side since it is a step up transformer the voltage in the secondary side have to be higher so, Vs=120 x 5=600V

Current on secondary side have to be smaller than primary side since the voltage is higher so Is = 600V/600 ohms = 1Amp

Now Power on primary side = Power on secondary side

Ps = Pp = (600V)(1amp) = 600 VA

Ip = Pp/Vp = (600VA)/120V = 5 Amps


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