| Electronics Demystified, 2nd edition |
| Stan Gibilisco |
| Explanations for Quiz Answers in Chapter 4 |
| 1. When we have a transformer with a primary-to-secondary turns ratio of exactly 2:1, the RMS primary voltage always equals twice the RMS secondary voltage (assuming our AC wave has no DC component). We therefore have a 2:1 step-down transformer. Standard 60-Hz, 120-V RMS utility electricity is, for most practical purposes, sinusoidal and has no DC along with it. If we apply this electricity to the primary of a 2:1 step-down transformer, we get 60 V RMS at the output. The correct choice is C. |
| 2. In a full-wave bridge rectifier circuit, we should use diodes with
peak-inverse-voltage (PIV) ratings of at least 2.1 times the RMS voltage that appears
across the transformer's secondary winding. In the situation of Question 1, we have 60 V
RMS across the transformer secondary, so the rectifier diodes should have at least PIV = 2.1 x 60 The correct choice is A. |
| 3. In a power-supply filter, the capacitor(s) should have value(s) large enough so that ripple is effectively eliminated in the circuit's DC output. The correct choice is B. The other three choices are completely irrelevant. Filter capacitors have nothing to do with the ability of a power supply to handle high current, avoid transformer damage from a surge, or avoid avalanche breakdown. |
| 4. We can connect an inductor in series, and/or connect a capacitor in parallel, to filter the pulsating DC from a rectifier circuit. The correct choice is B. In fact, we can use multiple series inductors and multiple parallel capacitors to get enhanced filtering. We'd never get good results by placing an inductor in parallel with the rectifier output, nor would we ever want to place a capacitor in series, so choices A and C are absolutely wrong! |
| 5. We can minimize the risk of power-supply diode failure from an initial current surge by taking any or all of the measures described in choices A, B, and C. The correct answer is therefore D, "Any of the above." |
| 6. If you install a quick-break fuse in a system designed for use with a slow-blow fuse, and if the quick-break fuse has the same current and voltage ratings as the old slow-blow fuse did, then the new fuse might work just fine. However, you shouldn't be surprised if the quick-break fuse burns out fairly often, even if nothing goes wrong with the rest of the system. If an unusually large (but not dangerous) current surge takes place at the moment you switch the power supply on, a quick-break fuse might burn out, even if a slow-blow fuse of the same size would "let the surge pass." The correct choice is A. |
| 7. Nickel-metal-hydride (NiMH), lead-acid, and secondary cells or batteries can be recharged. Zinc-carbon cells and batteries can't. The correct choice is C. |
| 8. We want to obtain 1500 Ah at 12 V DC, which represents 18 kWh, for our communications system's power supply. That's a lot of energy to expect from a battery! We'd do best by choosing a battery made from flooded cells in this scenario. The correct choice is D. |
| 9. Transistor batteries, which provide 9 V DC, generally contain six 1.5-V alkaline or zinc-carbon cells, connected in series and enclosed in a small housing. The correct choice is B. |
| 10. If we want to "go off the grid," live in a remote cabin far away from the commercial utility lines, and nevertheless have a supply of electricity, we would need to set up a stand-alone photovoltaic (PV) electrical system. The PV cells would use sunlight to keep a battery charged; we'd run our appliances from the battery, perhaps in conjunction with a power inverter. The correct choice is A. |