Electricity Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 8

1. The circuit shown in Fig. 8-14 is a full-wave center-tap rectifier. In such a system, the output ripple frequency equals twice the AC input frequency. The correct choice is A.

2. Notice how the diodes are connected in the rectifier circuit of Fig. 8-14. When a diode conducts in the forward direction, electrons always move "against the arrow." As this circuit is drawn, electrons should emerge from both diodes "against the arrows" and toward the output terminal, producing a negative pulsating DC voltage there. The correct choice is C.

3. Figure 8-15 is a schematic diagram of a complete power supply using a bridge-rectifier design. The inductor and capacitor in the lower part of the diagram filter the pulsating DC from the diodes, so we'll obtain pure DC at the output terminal (assuming that the inductance and capacitance are large enough, of course!). When we examine the diode polarities, we can see that electrons can pass through them only in the direction going away from the output terminal; therefore, we'll get a positive voltage at that point. The correct choice is A.

4. In the circuit of Fig. 8-15, the components marked X should have relatively small capacitances, on the order of 0.01 µF. They go from both sides of the AC input to electrical ground, offering some protection against transients (voltage "spikes") that might appear at the AC input. The correct choice is B.

5. In the circuit of Fig. 8-15, the component marked Y is a fuse. It protects the transformer and diodes against damage from excessive current in case of a malfunction somewhere further on down the line (a short circuit in the filter capacitor, for example, or a load whose resistance drops too low). If one of the rectifier diodes happens to short out, the fuse will protect the transformer. The correct choice is C.

6. In the circuit of Fig. 8-15, the component marked Z is a filter choke. It helps to minimize or "smooth out" the ripple in the pulsating DC that comes from the rectifier diodes. The correct choice is B.

7. The effective (root-mean-square) output voltage from a half-wave rectifier equals approximately 35% of the positive or negative peak voltage, assuming that the AC input waveform constitutes a pure sinusoid with no DC component. The answer is D.

8. If we expect a rectifier diode to encounter instantaneous reverse-bias voltage levels of up to 100 peak inverse volts (100 PIV), then we should choose a component rated for at least 150 PIV. That gives us a "standard" 50% "safety margin." The correct answer is A.

9. When we design and build a full-wave bridge circuit, we should choose rectifier diodes whose PIV ratings equal at least 2.1 times the RMS AC voltage across the transformer secondary winding. If the AC secondary voltage equals 100 V RMS, then we should use diodes rated for at least 2.1 x 100, or 210 PIV. The correct response is D.

10. If a filter capacitor shorts out in a power supply of any kind, we'll get the equivalent of a direct short circuit across the output. In that situation, the fuse or circuit breaker should blow or trip, removing power from the entire system. The correct choice is D.