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

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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

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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
B.300ht
C. 240/n
D.360/rc

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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?

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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

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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

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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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