This is the Multiple Choice Questions (MCQs) in Chapter 18: Transformers and Impedance Matching from the book Teach Yourself Electricity and Electronics, 5th edition by Stan Gibilisco. If you are looking for a reviewer in Electronics Engineering this will definitely help you before taking the Board Exam.
Begin the Test
1. In a step-up transformer:
- A. The primary impedance is greater than the secondary impedance.
- B. The secondary winding is right on top of the primary.
- C. The primary voltage is less than the secondary voltage.
- D. All of the above.
2. The capacitance between the primary and the secondary windings of a transformer can be minimized by:
- A. Placing the windings on opposite sides of a toroidal core.
- B. Winding the secondary right on top of the primary.
- C. Using the highest possible frequency.
- D. Using a center tap on the balanced winding.
3. A transformer steps a voltage down from 117 V to 6.00 V. What is its primary-to-secondary turns ratio?
- A. 1:380.
- B. 380:1.
- C. 1:19.5.
- D. 19.5:1.
4. A step-up transformer has a primary-to-secondary turns ratio of 1:5.00. If 117V RMS appears at the primary, what is the RMS voltage across the secondary?
- A. 23.4 V.
- B. 585 V.
- C. 117 V.
- D. 2.93 kV.
5. A transformer has a secondary-to-primary turns ratio of 0.167. This transformer is:
- A. A step-up unit.
- B. A step-down unit.
- C. Neither step-up nor step-down.
- D. A reversible unit.
6. Which of the following is false, concerning air cores versus ferromagnetic cores?
- A. Air concentrates the magnetic lines of flux.
- B. Air works at higher frequencies than ferromagnetic.
- C. Ferromagnetic are lossier than air.
- D. A ferromagnetic-core unit needs fewer turns of wire than an equivalent air-core unit.
7. Eddy currents cause:
- A. An increase in efficiency.
- B. An increase in coupling between windings.
- C. An increase in core loss.
- D. An increase in usable frequency range.
8. A transformer has 117 V RMS across its primary and 234 V RMS across its secondary. If this unit is reversed, assuming it can be done without damaging the windings, what will be the voltage at the output?
- A. 234 V.
- B. 468 V.
- C. 117 V.
- D. 58.5 V.
9. The shell method of transformer winding:
- A. Provides maximum coupling.
- B. Minimizes capacitance between windings.
- C. Withstands more voltage than other winding methods.
- D. Has windings far apart but along a common axis.
10. Which of these core types, in general, is best if you need a winding inductance of 1.5 H?
- A. Air core.
- B. Ferromagnetic solenoid core.
- C. Ferromagnetic toroid core.
- D. Ferromagnetic pot core.
11. An advantage of a toroid core over a solenoid core is:
- A. The toroid works at higher frequencies.
- B. The toroid confines the magnetic flux.
- C. The toroid can work for dc as well as for ac.
- D. It’s easier to wind the turns on a toroid.
12. High voltage is used in long-distance power transmission because:
- A. It is easier to regulate than low voltage.
- B. The I2R losses are lower.
- C. The electromagnetic fields are stronger.
- D. Smaller transformers can be used.
13. In a household circuit, the 234-V power has:
- A. One phase.
- B. Two phases.
- C. Three phases.
- D. Four phases.
14. In a transformer, a center tap would probably be found in:
- A. The primary winding.
- B. The secondary winding.
- C. The unbalanced winding.
- D. The balanced winding.
15. An autotransformer:
- A. Works automatically.
- B. Has a center-tapped secondary.
- C. Has one tapped winding.
- D. Is useful only for impedance matching.
16. A transformer has a primary-to-secondary turns ratio of 2.00:1. The input impedance is 300 Ω resistive. What is the output impedance?
- A. 75 Ω
- B. 150 Ω
- C. 600 Ω
- D. 1200 Ω
17. A resistive input impedance of 50 Ω must be matched to a resistive output impedance of 450 Ω. The primary-to-secondary turns ratio of the transformer must be:
- A. 9.00:1.
- B. 3.00:1.
- C. 1:3.00.
- D. 1:9.00.
18. A quarter-wave matching section has a characteristic impedance of 75.0 Ω. The input impedance is 50.0 Ω resistive. What is the resistive output impedance?
- A. 150 Ω
- B. 125 Ω
- C. 100 Ω
- D. 113 Ω
19. A resistive impedance of 75 Ω must be matched to a resistive impedance of 300 Ω. A quarter-wave section would need:
- A. Zo = 188 Ω
- B. Zo = 150 Ω
- C. Zo = 225 Ω
- D. Zo = 375 Ω
20. If there is reactance at the output of an impedance transformer:
- A. The circuit will not work.
- B. There will be an impedance mismatch, no matter what the turns ratio of the transformer.
- C. A center tap must be used at the secondary.
- D. The turns ratio must be changed to obtain a match.