a. External gate resistors b. Applying preset cathode
voltage levels c. An external capacitor d. Doped pn junctions
13-8 What is the peak output voltage in Fig. 13-42 if R3
is adjusted to 500 V?
13-9 If the SCR of Fig. 13-41 has a gate trigger voltage of 1.5 V, a gate trigger current of 15 mA, and a holding current of 10 mA, what is the input volt- age that triggers the SCR? The supply voltage that resets the SCR?
13-10 If the resistance is tripled in Fig. 13-41, what is the input voltage that triggers the SCR if VGT 5 2 V and IGT 5 8 mA?
13-11 In Fig. 13-42, R3 is adjusted to 750 V. What is the charging time constant for the capacitor? What is the Thevenin resistance facing the gate?
13-12 The resistor R2 in Fig. 13-43 is set to 4.6 kV. What are the approximate fi ring and conduction angles for this circuit? How much ac voltage is across C?
13-13 Using Fig. 13-43, when adjusting R2, what are the minimum and maximum fi ring angle values?
13-14 What are the minimum and maximum conduction angles of the SCR in Fig. 13-43?
SEC. 13-3 THE SCR CROWBAR
13-15 Calculate the supply voltage that triggers the crowbar of Fig. 13-44.
13-16 If the zener diode of Fig. 13-44 has a tolerance of 610 percent and the trigger voltage can be as high as 1.5 V, what is the maximum supply voltage where crowbarring takes place?
13-17 If the zener voltage in Fig. 13-44 is changed from 10 to 12 V, what is the voltage that triggers the SCR?
13-18 The zener diode of Fig. 13-44 is replaced by a 1N4741A. What is the supply voltage that triggers the SCR crowbar?
Figure 13-41
Vin
Vout RG
2.2 kΩ
RL 47 Ω VCC +12 V
– +
Figure 13-42
IGT = 200 mA VGT = 0.8 V
vout +90 V
R1 6.8 kΩ
R2 3.3 kΩ C1
4.7 mF R3
Figure 13-44
VZ = 10 V +9 V
VGT = 0.8 V IGT = 200 mA R1
100 Ω
RL 10 Ω
Figure 13-43
RL R1
1 kΩ R2 50 kΩ R3
1 kΩ C 0.47 mF
120 Vac SEC. 13-5 BIDIRECTIONAL THYRISTORS
13-19 The diac of Fig. 13-45 has a breakover voltage of 20 V, and the triac has a VGT of 2.5 V. What is the capacitor voltage that turns on the triac?
13-20 What is the load current in Fig. 13-45 when the triac is conducting?
13-21 All resistances are doubled in Fig. 13-45, and the capacitance is tripled. If the diac has a breakover
564 Chapter 13
Critical Thinking
13-25 Figure 13-47a shows an overvoltage indicator.
What is the voltage that turns on the lamp?
13-26 What is the peak output voltage in Fig. 13-47b?
13-27 If the period of the sawtooth is 20 percent of the time constant, what is the minimum frequency in Fig. 13-47b? What is the maximum frequency?
13-28 The circuit of Fig. 13-48 is in a dark room. What is the output voltage? When a bright light is turned on, the thyristor fi res. What is the approximate output voltage? What is the current through the 100 V?
Figure 13-47
9 V POWER SUPPLY
INCANDESCENT LAMP
VB = 10 V
LOAD
vout VB = 10 V C1
0.1 mF R2 1 kΩ
R1 50 kΩ VCC +50 V
(a) (b)
13-23 What will be the ideal peak voltage across R4 in Fig. 13-46, when the PUT fi res?
13-24 In Fig. 13-46, what will the voltage waveform across the capacitor look like? What will be the minimum and maximum voltage values of this waveform?
Figure 13-46
+VCC = 15 V
10 kΩ
0.47 mF
C 100 Ω
1 kΩ
2 kΩ R2
R3 R4
R1
+VCC = 15 V
voltage of 28 V and the triac has a gate trigger voltage of 2.5 V, what is the capacitor voltage that fi res the triac?
SEC. 13-7 OTHER THYRISTORS
13-22 In Fig. 13-46, what are the anode and gate volt- age values when the PUT fi res?
Figure 13-45
C1 2.2 mF
RL 15 Ω
Vin 100 V
R2 2.7 kΩ R1
68 kΩ
MPT32
TRIAC –
+
Troubleshooting
Use Fig. 13-49 for problems 13-29 and 13-30. This power supply has a bridge rectifi er working into a ca- pacitor-input fi lter. Therefore, the fi ltered dc voltage is approximately equal to the peak secondary voltage. All listed values are in volts, unless otherwise indicated.
Also, the measured voltages at points A, B, and C are given as rms values. The measured voltages at points D, E, and F are given as dc voltages. In this exercise,
you are troubleshooting at the system level; that is, you are to locate the most suspicious block for further test- ing. For instance, if the voltage is OK at point B but in- correct at point C, your answer should be transformer.
13-29 Find Troubles 1 to 4.
13-30 Find Troubles 4 to 8.
Figure 13-48
vin
vout RL
100 Ω VCC +15 V
Figure 13-49 Troubleshooting measurements.
Troubleshooting
(b) Trouble VA
115 115 0 115 115 130 115 115 115
115 115 0 115
0 130 115 115 0
12.7 12.7 0 0 0 14.4 12.7 12.7 0
18 18 0 0 0 20.5
0 18
0
18 0 0 0 0 20.5
0 18
0
18 0 0 0 0 20.5
0 0 0
100 Ω 100 Ω 100 Ω 100 Ω
0 100 Ω 100 Ω 100 Ω 100 Ω
Off Off Off Off Off Off Off Off Off
VB VC VD VE VF RL SCR
OK T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8
(a) POWER
OUTLET FUSE TRANSFORMER LOAD
BRIDGE RECTIFIER AND FILTER
SCR CROWBAR
A B C D E F
566 Chapter 13
Self-Test Answers
1. c 2. b 3. d 4. c 5. b 6. b 7. a 8. b 9. b 10. c 11. a
12. b 13. d 14. d 15. d 16. d 17. d 18. a 19. a 20. b 21. c 22. b
23. c 24. b 25. d 26. d 27. b 28. a 29. c 30. b 31. a
Job Interview Questions
1. Draw a two-transistor latch. Then, explain how the positive feedback can drive the transistors into satu- ration and into cutoff .
2. Draw a basic SCR crowbar. What is the theory of op- eration behind this circuit? In other words, tell me all the details of how it works.
3. Draw a phase-controlled SCR circuit. Include the waveforms for ac line voltage and gate voltage. Then explain the theory of operation.
4. In thyristor circuits, what is the purpose of snubber networks?
5. How might one employ an SCR in an alarm circuit?
Why would this device be preferable to one using a transistor trigger? Draw a simple schematic.
6. Where in the fi eld of electronics would a technician fi nd thyristors in use?
7. Compare a power BJT, a power FET, and an SCR for use in high-power amplifi cation.
8. Explain the diff erences in operation between the Schockley diode and an SCR.
9. Compare a power MOSFET and an IGBT used for high-power switching.
Multisim Troubleshooting Problems
The Multisim troubleshooting fi les are found on the Instructor Resources section of Connect for Electronic Principles, in a folder named Multisim Troubleshooting Circuits (MTC). See page XVI for more details. For this chapter, the fi les are labeled MTC13-31 through MTC13-35 and are based on the circuit of Figure 13-49.
Open up and troubleshoot each of the respec- tive fi les. Take measurements to determine if there is a fault and, if so, determine the circuit fault.
13-31 Open up and troubleshoot fi le MTC13-31.
13-32 Open up and troubleshoot fi le MTC13-32.
13-33 Open up and troubleshoot fi le MTC13-33.
13-34 Open up and troubleshoot fi le MTC13-34.
13-35 Open up and troubleshoot fi le MTC13-35.