for electronic technicians?

Question

A stepdown transformer with a 120-volt primary and a 24-volt secondary has a load impedance of 18 ohms across its secondary winding. What is the approximate reflected impedance "seen" at the primary

____a. 3.6 ohms

____b. 33 ohms

____c. 90 ohms

____d. 450 ohms



i am taking an electronics course and i am having trouble with this question. could you please explain how you came up with your answer and show any formulas that you used

Answer 1

I believe it is D. 450 ohms.

The equation I am using to get load impedance for primary is:
power = voltage squared / resistance (impedance)

P = V²/R

To get the reflected impedence, we need to compute the power for the secondary winding first:

24² / 18 = 32 watts

Now, knowing we have 32 watts with 120 volts on our primary winding, we can compute the reflected impedance: (Solve for R)

32 = 120²/R

32 = 14400/R

32R = 14400

R = 14400/32

R = 450 ohms

You just have to remember that the relationship for impedance is not linear, so a direct ratio won't work.

Answer 2

The relationship between voltage, current, and number of turns(or impedance) in a transformer coil is described by the following:

(V1/V2) = (I2/I1) = (N1/N2)

Where,

V1 = Primary Voltage
I1 = Primary Current
N1 = number of turns(or impedance) of wire at primary coil
V2 = Secondary Voltage
I2 = Secondary Current
N2 = number of turns(or impedance) of wire at secondary coil

The impedance seen at the primary coil will be found by:

N1 = N2 * (V1/V2) = 18ohms * (120V/24V) = 90ohms

Answer C is the correct answer.

Answer 3

I believe it's c. 90 ohms due to the step down ratio of 5