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ECE Objective Questions { Power Semiconductor Devices and Their Applications }

78. Which of the following conditions is necessary for a triggering system for thyristors?
A. It must use separate power supply.
B. It should provide a train of pulses.
C. It should be synchronised with the main supply.
D. None of these

79. For thyristers, pulse triggering is preferred to de triggering because
A. gate dissipation is low
B. pulse system is simpler
C. triggering system is required for a very short duration
D. all of these

80. A single phase halfwave controlled rectifier has 400 sin 314 t volts as the input voltage and resistor R as the load. For firing angle of 600 for the SCR, the average output voltage in volts is
A. 400/it
C. 240/n

81.  A single-phase one-pulse controlled circuit has resistance and counter emf load and 400 sin 314,t volt as the source voltage. For a load counter emf of 200 V, the range of firing angle control is
A. 30? to 1500
B. 60? to 180?
C. 60? to 120?
D. 30? to 180?

82. A single-phase fullwave midpoint thyristor converter uses a 230/200 V transformer with centre tap on the secondary skle. The P.I.V per thyristor is
A. 100 V
B. 141.4 V
C. 200 V
D. 282.8 V

83. For continuous conduction, in a single phase full converter each pair of SCRs conducts for
A. a radians
B. it radians
C. a radians
D. (it + a) radians

84. For discontinuous load current and extinction angle 13 > it radians, in a single phase full converter, each SCR conducts for
A. a radians
B. (13 ? a) radians
C. 13 radians
D. (a + 13) radians

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