A transformer is connected to a 120V(rms) ac line. The primary circuit has 100 w

Question

A transformer is connected to a 120V(rms) ac line. The primary circuit has 100 windings. The secondary circuit has 10000 windings. To reduce shock hazard, a fuse is to be inserted in the primary circuit; the fuse is to blow when the rms current in the secondary circuit exceeds 10mA.

a) What is the induced emf of the secondary circuit?

b) What power must be supplied to the transformer when the rms secondary current is 10mA? How large is the rms current in the primary circuit for this case.

c) A resistor is also part of the secondary circuit. Determine the minimum resistance R to not blow the fuse.

Explanation / Answer

Vp =120 V , Np = 100 , Ns =1000 , Is = 10 mA

(a) Vs/Vp = Ns/Np

Vs/120 = 1000/100

Vs = 1200 V

(b) P = Vs*Is =1200*10*0.001 =12 W

Ip/Is = Ns/Np = 1000/100 =10

Current in Primery circuit is 10 times greater tha current in secondary circuit

(c) P = I2^2R

12 = (10*10^-3)^2*R
R = 1.2*10^5 ohm

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