1. A dc chopper converts directly from dc to dc. A. True.(Your Answer) B. False.
2. A chopper can be used to step up a dc voltage. A. True.(Your Answer) B. False.
3. Which of the following finds application in speed control of a dc motor? A. FET B. NPN transistor. C. SCR.(Your Answer) D. none of the above.
4. A silicon controlled rectifier (SCR) is a A. unijunction device. B. device with 3 junctions.(Your Answer) C. device with 4 junctions. D. none of the above.
5. A triac is a A. 2 terminal switch. B. 2 terminal bilateral switch. C. 3 terminal unilateral switch. D. 3 terminal bidirectional switch.(Your Answer)
6. A TRIAC can be considered as two SCRS connected in antiparallel with a gate connection. True False
7. Thyristor is a ___ layered semiconductor device. 2 3 4
8. An inverter converts dc to ac ac to dc ac to ac
9. A SCR is a ___ dimensional switch. 1 2 3
10. A TRIAC conducts when triggered in one direction conducts when triggered in both direction conducts when not triggered
11. UJT is commonly used for generating triggering signals for SCR's. True False
12. A ___ is a bidirectional device. TRIAC DIODE FET
13. Cyclo conveters are used to convert dc to ac dc to dc ac to ac
14. The value of current flowing through a semi-conductor switch below which it will return to its off state is called Leakage current Latching current Holding current
15. The ac component of current present on a dc supply, which has not been removed by filtering is Holding current Ripple current latching current
16. The power range of these converters is from some milliwatts (as in a mobile phone) to hundreds of megawatts (e.g in a ________transmission system). High-voltage direct current Super grid Grid energy storage Smart grid 17. In industry the most common application is the variable speed drive (VSD) that is used to control an ________. Electric motor Brushless DC electric motor AC motor
Induction motor
18. In power distribution networks AC/AC converters may be used to exchange power between ________ 50 Hz and 60 Hz power grids. Transformer Railway electrification system Utility frequency Electric power transmission
19. In modern systems the conversion is performed with semiconductor switching devices such as ________,thyristors and transistors. Semiconductor device Diode Electronic component Vacuum tube 20. The power range is typically from tens of________ to several hundred watts. Becquerel Joule Volt Watt 21. The power range of VSDs start from a few hundred watts and end at tens of ________.
Joule Becquerel Volt Watt 22. AC/DC converters (________) are used every time an electronic device is connected to the mains (computer, television,...). Diode High-voltage direct current Vacuum tube Rectifier
23. The first very high power electronic devices were________. Mercury arc valve Transformer Rectifier Vacuum tube
24. In mercury arc rectifiers the voltage drop in arc is n early (A) 0.1 volt per centimeter of arc length (B) 1 volt per centimeter of arc length (C) 10 volts per centimeter of arc length (D) 12 to 15 volts per centimeter of arc length.
25. Which of the following equipment needs direct current? (A) Relays (B) Telephones (C) Time switches (D) All of the above 26. Which of the following devices cannot convey from dc to ac ? (A) Motor generator (B) Motor converter (C) Mercury arc rectifier
(D) All of the above.
27. In large motor generator sets ac motor is usually (A) induction motor squirrel cage type (B) synchronous motor (C) induction motor wound rotor type (D) any of the above.
28. For single phase rotary converters, when the power factor is unity, the ac line to dc current ratio will be (A) 1.4 (B) 1.0 (C) 0.707 (D) 0.5.
29. A 100 kW motor generator set has induction motor of six poles and dc generator of eight poles. If the supply frequency is 50 Hz, the speed of the set will be (A) 214.3 rpm (B) 428.6 rpm (C) 536.4 rpm (D) 624.8 rpm.
30. The number of slip rings in a single phase synchronous converter will be (A) 1 (B) 2 (C) 3 (D) None.
Questions 31 to 33 are based on the following data: A 1 kW single phase rotary converter operates at full load from 230 volts ac source. Assume 100% efficiency and unity power factor. 31. The voltage will be (A) 230 V (B) 250 V (C)275 V (D) 325 V 32. The dc current will be approximately (A) 2.5 A (B) 3 A (C) 3.25 A (D) 3.75 A.
33. The ac input current at slip rings will be (A) 6.3 A (B) 5.8 A (C)4.1A (D) 3.8 A.
34. For 3 phase rotary converters, when power factor is unity. the ac line to dc current ratio is (A) 1.5 (B) 1.1 (C) 1.0 (D) 0.942.
35. If a synchronous converter is supplied with 12 phase ac supply, the number of slip rings will b e (A) 3 (B) 6
(C) 12 (D) 24. 36. In a synchronous converter, the no-load ratio of the voltage Eabetween successive slip-rings (i.e. the armature phase voltage) to the commutator voltage Ed for 3 phase "supply on ac side is (A) 1.0 (B) 0.7 (C) 0.6 (D) 0.16.
37. In a synchronous converter, ac and dc armature powers will be equal when (A) electrical losses are negligible (B) mechanical losses are negligible (C) electrical and mechanical losses are negligible (D) electrical and mechanical losses are negligible and power factor is unity 38. In a rotary converter (A) armature currents are dc only (B) armature currents are ac only (C) partly ac and partly dc. (D) All of the above. 2
39. In a rotary converter l R? losses as compared to a dc generator of the same size will be (A) four times more (B) double (C) same (D) less. 2
40. In a synchronous converter, the l R losses are less as compared to dc generator of the same size because (A) dc currents are negligible (B) ac components neutralize the dc (C) cyclic fluctuations have low frequency (D) operating speed is low.
41. A rotary converter can be started (A) from dc side as dc motor (B) from ac side as induction motor (C) by means of a small auxiliary motor (D) any of the above methods. 42. When a rotor converter is started by means of a small auxiliary motor, the power of motor must be (A) more than the dc output of converter (B) more than ac input of converter (C) half of dc output of converter (D) slightly more than the value of friction and windage losses at rated speed.
43. In a mercury arc rectifier positive ions are attracted towards (A) mercury pool (B) shell bottom (C) cathode (D) anode.
44. Maximum current rating of a glass bulb mercury arc rectifier is usually restricted to (A) 50 A (B) 100 A (C) 500 A (D) 1000 A.
45. Which of the following rectifiers can withstand maximum voltage on dc side (A) Mercury arc rectifier
(B) Glass bulb rectifier (C) Steel tank rectifier (D) None of the above.
46. In an arc rectifier the drop in voltage at the cathode is approximately (A) 1 volt (B) 6 to 7 volts (C) 60 to 70 volts (D) 1.2 V7 volts.
47. In arc rectifiers mercury is chosen as the liquid for rectifier because (A) it has low specific heat (B) it has low ionization potential (C) it has high atomic weight (D) all of the above.
48. Ionization potential of mercury is approximately (A) 1 V (B) 2 V (C) 10.4 V (D) 104 V. 49. Least undulating current will be delivered by which mercury arc rectifier (A) 1-phase (B) 2 phase (C) 3 phase (D) 6 phase. 50. In a mercury arc rectifier the cathode voltage drop is due to (A) surface resistance (B) expenditure of energy in liberating electrons from the mercury (C) expenditure of energy in ionization (D) expenditure of energy in overcoming the electrostatic field. 51. In mercury arc rectifier, voltage drop at anode is due to (A) energy spent in overcoming the electrostatic field (B) high temperature inside the rectifier (C) self-restoring property of mercury (D) high ionization potential.
52. The average life of the glass bulb rectifier is (A) 100 to 150 hours (B) 1000 to 1500 hours (C) 10,000 to 15,000 hours (D) 100,000 to 150,000 hours. 53. The vacuum inside the glass bulb of a mercury arc rectifier is of the order of -3 (A) 5 x 10 cm of Hg -4 (B) 5 x 10 cm of Hg -5 (C) 5 x 10 cm of Hg -8 (D) 5 x 10 cm of Hg. 54. For mercury arc rectifiers, the anode is usually made of (A) Aluminium (B) Copper (C) Tungsten (D) Graphite.
55. Essential requirement of the anode material in mercury arc rectifier is that (A) it should not be wetted by mercury (B) it should be light (C) it should be cheap
(D) it should be black in color.
56. The advantage of mercury arc rectifier is (A) it is noiseless in operation (B) it responds quickly to varying load demands (C) it has high efficiency (D) all of the above. 0
57. A 3 anode mercury arc rectifier has an anode current of overlap 30 . Neglecting arc drop, the regulation will be approximately (A) 1% (B) 2% (C) 7% (D) 17%. 58. As compared to mercury arc rectifiers, metal rectifiers (A) can operate on high loads (B) can operate on high voltages (C) operate on low temperatures (D) give poor regulation. 59. The current carried by the cathode spot of the mercury arc rectifier is of the order of (A) 40 A/sq-cm (B) 400 A/sq-cm (C) 4000 A/sq-cm (D) 40,000 A/sq-cm.
60. Which of the following is the loss within the mercury arc rectifier chamber ? (A) Voltage drop at the anode (B) Voltage drop at the cathode (C) Voltage drop in arc (D) All of the above.
61. The voltage drop at the cathode is of the order of (A) 0.1 to 0.5 V (B) I lo 1.5 V (C) 7 to 9 V (D) 70 to 90 V.
62. The voltage drop in arc primarily depends on (A) arc length (B) arc temperature (C) purity of mercury (D) all of the above.
63. As the output voltage of a single anode mercury arc rectifier increases, the variation of internal efficiency is represented by
(A) Curve A (B) Curve B (C) Curve C (D) Curve D.
64. The voltage drop across the electrodes of a mercury pool rectifier (A) varies exponentially with the load current (B) is almost independent of load current (C) is directly proportional to load (D) is inversely proportional to load.
65. In a mercury arc rectifier (A) ion stream moves from cathode to anode (B) current flows from cathode to anode (C) electron stream moves from anode to cathode (D) ion stream moves from anode to cathode.
66. In a mercury arc rectifier if cathode and anode connections are interchanged (A) internal losses will be reduced (B) both ion and electron streams will move in the same direction (C) the rectifier will operate at reduced efficiency (D) the rectifier will not operate. 67. In mercury arc rectifier, mercury is used as (A) conducting medium (B) ionization medium (C) electron accelerator (D) a cathode.
68. In a mercury arc rectifier, mercury is used as cathode because (A) mercury is liquid metal (B) mercury readily vaporizes (C) mercury vapor is easily ionized (D) all of above.
69. In a mercury arc rectifier which of the following flows from anode to cathode ? (A) Electrons (B) Ions (C) Both electrons and ions (D) Electrons, ions and current.
70. In a mercury arc rectifier characteristic blue luminosity is due to (A) high temperature (B) electron streams (C) ionization (D) color of mercury. 71. For producing cathode spot in a mercury arc rectifier (A) An auxiliary electrode is used (B) Tube is evacuated (C) Low mercury vapor pressures are used (D) Anode is heated.
72. The internal efficiency of a mercury arc rectifier depends on (A) load current only (B) voltage only (C) current and voltage (D) rms value of current.
Questions 73 to 75 refer to data given below: A mercury arc rectifier has two anodes A and B as shown in figure. Anode A has potential difference of 500 V to earth and anode B a potential difference of 490 V both positive to the cathode.
73. If anode A is conducting the arc voltage drop will be (A) 500 V (B) 480 V (C) 280 V (D) 20 V.
74. When anode A is conducting (A) Anode B will conduct less efficiently (B) Anode B will conduct more efficiently (C) Conduction between anodes A and B will also start (D) Anode B will not conduct. 75. If the voltage of anode B is raised to 510 V (A) Anode B will conduct but anode A will not conduct (B) Anode B will conduct but anode A will also continue to conduct (C) Anode B will not conduct and anode A will continue to conduct (D) Both anodes will not conduct. 76. Which of the following mercury arc rectifiers will have the highest supply line power factor ? (A) 3 anode rectifier (B) 6 anode rectifier (C) 100 Hz (D) 200 Hz.
77. A set of resonant shunts is connected across the output terminals of a multi-anode mercury rectifier to (A) reduce the effect of harmonics (B) reduce the noise level (C) improve the power factor of supply (D) reduce internal losses.
78. Ripple frequency of full wave rectifier working on 50 Hz supply will be (A) 25 Hz (B) 50 Hz (C) 6 anode rectifier with inter phase transformer (D) All wili have identical power factor.
79. In mercury arc rectifiers, mercury is selected as cathode because (A) its ionization potential is low (B) it has low specific heat (C) it has small latent heat of vaporization (D) all of the above.
80. A three anode mercury rectifier is connected to 440 V, 3 phase 50 Hz Supply. Each anode will conduct for (A) 50 seconds (B) 1/50 seconds (C) 1/3 of the period (D) 1/6 of the period.
81. A 6 anode mercury arc rectifier without inter-phase transformer has rims value o f anode current as 41 A. This rectifier with inter phase transformer will have rms value of anode current as (A) 66 A (B) 42.5 A (C) 41A (D) 22 A.
82. The rms value of half wave-rectified sine wave with im as peak value is (A) 0.707 i m (B) 0.66 im (C) 0.5 im (D) 0.318 im. 83. The mean value of half wave rectified sine wave i s (A) 0.707 i m (B) 0.66 im (C) 0.5 im
(D) 0.318 im. 84. The form factor for half wave rectified sine wave is (A) 1.0 (B) 1.11 (C) 1.44 (D) 1.57.
85. For full-wave rectified sine wave, rms value is
(A) 0.707 i m (B) 0.6036 i m (C)0.5 im (D) 0.318 im . 86. For full-wave rectified sine wave, mean value is (A) 0.70 im (B) 0.636 i m (C) 0.5 im (D) 0.318 lm. 87. For full-wave rectified sine wave, form factor is (A) 1.5 (B) 1.41 (C) 1.28 (D) 1.11.
88. A half-wave rectifier circuit with a capacitive filter is connected to a 200 volts, 50 Hz ac line. The output voltage across the capacitor should be approximately (A) 300 volts (B) 280 volts (C) 180 volts (D) 80 volts.
89. The ripple factor of a full-wave rectifier circuit compared to that of a half wave rectifier circuit without filter is (A) half of that for a half 'wave rectifier (B) less than half that for a half-wave rectifier circuit (C) equal to that of a half wave rectifier. (D) none of the above.
90. A thyratron is a (A) vacuum tube with four electrodes (B) gas-filled diode (C) gas-filled triode (D) none of the above.
91. A thyristor equivalent of a thyratron tube is a (A) Silicon controlled rectifier (SCR) (B)Triac (C) Diac (D) None of the above. 92. A silicon controlled rectifier is a (A) Unijunction device (B) Device with three junction (C) Device with four junctions (D) None of the above.
93. The RMS value of a half wave rectifier current is 10 A. Its value for full wave rectification would be (A) 10 A (B) 14.14 A (C) (20/π) A (D) 20 A.
94. For single phase supply frequency of 50 Hz, ripple frequency in full wave rectifier is (A) 25 (B) 50 (C)100 (D) 200. 95. The aim of introducing reactor in the ignition circuit of mercury arc rectifier is to limit (A) the current in the circuit (B) wave ripple (C) voltage of the circuit (D) the rate of change of flux.
96. A mercury vapor discharge tube used for domestic lighting (A) does not have a filament (B) has one filament (C) has two filaments (D) has two main and one auxiliary filament.
97. For a waveform more peaky than a sine wave, the form factor will be (A) more than 1.11 (B) less than 1.11 (C) 1.11 (D) 1. 98. Peak inverse voltage for a diode is the (A) voltage corresponding to rated maximum voltage (B) maximum voltage that can be applied across the diode in the conducting direction (C) maximum voltage that can be applied across the diode in the non-conducting direction (D) none of the above.
99. When voltage applied to a diode is more than PIV, it is likely to result in (A) More distortion on output side (B) Poor regulation (C) Conduction in both direction (D) Breakdown at the junction.
100. Which of the following is known as metal rectifier (A) Selenium disc rectifier (B) Copper oxide rectifier (C) Gas tube diode (D) All of the above. 101. PIV of a diode is usually (A) Half the dc voltage (B) Same as dc voltage (C) Twice the dc output voltage (D) Hundred times the dc output voltage.
102. The most commonly used connection for joining the six secondaries of a transformer used for 3 phase to 6 phase conversion is (A) Diametrical (B) Zig-Zag (C) Double delta
(D) Double star.
103. A rotary converter (A) Combines the functions of an induction motor and dc generator (B) has a set of slip rings at both ends (C) has an armature and two fields (D) is a synchronous motor and a dc generator combined. 104. The ac line current at slipping in a 6 phase, 6 ring rotary converter having 100% efficiency and unity power factor is........... times the dc current (A) 0.236 (B) 1.414 (C)0.472 (D) 0.943.
105. The advantage of motor generator set is (A) dc output voltage is practically constants (B) dc output can be controlled by adjusting shunt field regulator (C) unit is self-starting (D) all of the above. 106. A mercury arc rectifier works on the principle of (A) ionization by collision (B) directional movement of electrons (C) release of electrons by thermal excitation (D) none of the above. 107. A rectifier is a (A) Bilateral device (B) Linear device (C) Non-linear device (D) Passive device.
Answeres from 1 to 107 1.A ----- 2.D ----- 3.C ----- 4.B ----- 5.A ----- 6.B ----- 7.B -----8.D -----9.B -----10.C ----- 11.D -----12.C -----13.C ----14.D 15.C ----- 16.D ----- 17.B ----- 18.D ----- 19.D ----- 20.C ----- 21.C ----- 22.C -----23. 24.A ----- 25.D ----- 26.C ----- 27.B ----- 28.A ----- 29.B ----- 30.B -----31.D -----32.B -----33.C ----- 34.D -----35.C ----36.C ----- 37.D 38.C ----- 39.D ----- 40.B ----- 41.D ----- 42.D ----- 43.C ----- 44.C ----- 45.C -----46.B -----47.D -----48.C ----- 49.D ----50.B -----51.A ----- 52.C 53.C ----- 54.D ----- 55.A ----- 56.D ----- 57.C ----- 58.C ----- 59.C ----- 60.D -----61.C -----62.A -----63.D ----- 64.B ----65.D ----- 66.D ----- 67.D 68.D ----- 69.B ----- 70.C ----- 71.A ----- 72.B ----- 73.D ----- 74.D ----- 75.A ----- 76.C ----- 77.A -----78.A ----- 79.D ----80.D -----81.D ----- 82.C 83.D ----- 84.D ----- 85.B ----- 86.B ----- 87.D ----- 88.B ----- 89.B ----- 90.C ----- 91.A -----92.B -----93.B ----- 94.C ----95.A -----96.C ----- 97.A 98.C ----- 99.D ----- 100.B ----- 101.C ----- 102.A ----- 103.D ----- 104.C ----- 105.D -----106.A -----107.C
1. A thyristor can be used as a. A resistor b. An amplifier
c. A switch d. A power source c. A switch 2. Positive feedback means the returning signal a. Opposes the original change b. Aids the original change c. Is equivalent to negative feedback d. Is amplified b. Aids the original change 3. A latch always uses a. Transistors b. Feedback c. Current d. Positive feedback d. Positive feedback 4. To turn on a four-layer diode, you need a. A positive trigger b. low-current drop out c. Breakover d. Reverse-bias triggering c. Breakover 5. The minimum input current that can turn on a thyristor is called the a. Holding current b. Trigger current c. Breakover current d. Low-current drop out b. Trigger current 6. The only way to stop a four-layer diode that is conducting is by a. A positive trigger b. Low-current drop out c. Breakover d. Reverse-bias triggering b. Low-current drop out 7. The minimum anode current that keeps a thyristor turned on is called the a. Holding current
b. Trigger current c. Breakover current d. Low-current drop out a. Holding current 8. A silicon controlled rectifier has
a. Two external leads b. Three external leads c. Four external leads d. Three doped regions b. Three external leads 9. A SCR is usually turned on by a. Breakover b. A gate trigger c. Breakdown d. Holding current b. A gate trigger
10. SCRs are a. Low-power devices b. Four-layer diodes c. High-current devices d. Bidirectional c. High-current devices 11. The usual way to protect a loa d from excessive supply voltage is with a a. Crowbar b. Zener diode c. Four-layer diode d. Thyristor a. Crowbar 12. An RC snubber protects an SCR against a. Supply overvoltages b. False triggering c. Breakover d. Crowbarring b. False triggering
13. When a crowbar is used with a power supply, the supply needs to have a fuse or a. Adequate trigger current b. Holding current c. Filtering d. Current limiting d. Current limiting 14. The photo-SCR responds to a. Current b. Voltage c. Humidity d. Light d. Light 15. The diac is a a. Transistor b. Unidirectional device c. Three-layer device d. Bidirectional device d. Light 16. The triac is equivalent to a. A four-layer diode b. Two diacs in parallel c. A thyristor with a gate lead d. Two SCRs in parallel d 17. The unijunction transistor acts as a a. Four-layer diode b. Diac c. Triac d. Latch d
18. Any thyristor can be turned on with a. Breakover
b. Forward-bias triggering c. Low-current dropout d. Reverse-bias triggering a 19. A Shockley diode is the same as a a. four-layer diode b. SCR c. diac d. triac a 20. The trigger voltage of an SCR is closest to a. 0 b. 0.7 V c. 4 V d. Breakover voltage b 21. Any thyristor can be turned off with a. Breakover b. Forward-bias triggering c. Low-current drop out d. Reverse-bias triggering c 22. Exceeding the critical rate of rise produces a. Excessive power dissipation b. False triggering c. Low-current drop out d. Reverse-bias triggering b 23. A four-layer diode is sometimes called a a. Unijunction transistor b. Diac c. pnpn diode d. Switch c 24. A latch is based on
a. Negative feedback b. Positive feedback c. The four-layer diode d. SCR action b
24.
What is the dominant form of distortion produced by the simple push-pull amplifier shown here?
Clipping. Crossover distortion. Harmonic distortion.
The efficiency of an electronic amplifier is important primarily because this determines the amount of waist heat produced within the amplifier. True False
What class of amplifier is shown here?
Class A. Class B. Class AB. Class C.
In Class C amplifiers, the active devices are switches and are either completely ON or completely OFF. True False
The symbol for which component is shown here?
A diac. A thyristor. A triac. A power diode.
When a thyristor has been fired by an appropriate gate pulse, when will it to turn off? When the applied voltage changes polarity. When the current through the device falls below t he holding current. When a gate pulse of the opposite polarity is applied. After a fixed period of time determined by circuit components.
In the simple lamp dimmer shown here, what method of power control is being used?
Burst firing. Phase control. Feedback control. Proportional control.
Determine the output voltage of the following regulator (assuming that the input voltage is sufficiently high to allow normal operation).
11 V. 12 V. 15 V. 16 V.
Which of the following statements is incorrect? A great advantage of switching regulators is that their power consumption is very low. The output voltage of a switching regulator is controlled by altering the switching frequency. Switching regulators use switching speeds of 20 kHz or more. Both bipolar transistors and FETs have very good switching characteristics.
A power supply that makes use of a switching regulator is referred to as a switch-mode power supply. True False
