Page 310 - Op Amps Design, Applications, and Troubleshooting

288 POWER SUPPLY CIRCUITS

7. Substitute the load. Another method that can be helpful in isolating some
power supply defects is to remove the load. This eliminates the possibility
that a malfunction in the system is causing the supply to appear defective.
If the supply is shown to be defective, substituting an equivalent resistance
in place of the system circuitry can simplify troubleshooting of the actual
power supply. Additionally, it removes the possibility of causing damage to
the system circuitry if the output of the supply becomes excessively high
while troubleshooting the problem. (For example, a test probe may slip and
cause a momentary short circuit.)

REVIEW QUESTIONS

1. List the three basic classes of voltage regulator circuits.
2. If the DC output voltage of a shunt regulator circuit varies between 11 and 12,5 volts
as the input Une voltage varies from 110 to 130 volts, what is the percent of line
regulation for the regulator?
3. If a series regulator circuit provides 25 volts DC under no-load conditions, but drops
to 24.3 volts when a full load is applied, what is the percent of load regulation for the
regulator circuit?
4. If each of the following rectifier/regulator circuits requires 1.2 amps of current from the
120-VAC input line, which one will probably deliver the highest current to a 12-VDC
load: series, shunt, switching? Explain your choice.
5. Refer to Figure 6.8. If the input voltage is 16.8 VDC and the regulated output voltage
is 8.9 volts DC, what is the power dissipation of Qi with a 300-mA load connected?
6. Refer to Figure 6.8. If the reference voltage has a defect that causes it to go to +6 volts,
what is the effect on output voltage (increase, decrease, remains the same)? Explain
your answer.
7. Refer to Figure 6.8. If resistor R 2 increases in value, what relative effect does this have
on output voltage?
8. Refer to Figure 6.9. What is the effect on output voltage if resistor R 2' increases in value?
9. Refer to Figure 6.12. What is the effect on the current flow through Q t if resistor R 2 is
decreased in value? Explain your answer.
10. If resistor JR 3 in Figure 6.12 is changed to 40 ohms (and no components are damaged),
what will happen to the value of reference voltage? What will happen to the value of
output voltage (+V REG)7 What will happen to the value of current through QJ Will the
current through RI change?
11. Refer to Figure 6.19. What is the effect on the current flow through the load resistor
(assume a constant value of load resistance) if resistor R 2 is increased in value?
12. Refer to Figure 6.19. What happens to the value of voltage dropped across the emitter-
collector circuit of Qi if resistor R 3 is increased in value?
13. Refer to Figure 6.20. What is the effect on circuit operation if transistor Q 2 develops an
emitter-to-collector short?
14. Refer to Figure 6.21. What is the effect on circuit operation if capacitor C l develops a
short circuit?
15. Refer to Figure 6.21. What is the effect on circuit operation if resistor R 4 becomes open?