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BEL Electronics Placement Paper
1. At room temperature the current in an intrinsic semiconductor is due to
A. holes
B. electrons
C. ions
D. holes and electrons
2. Work function is the maximum energy required by the fastest electron at 0 K to escape from
the metal surface.
A. True
B. False
3. The most commonly used semiconductor material is
A. silicon
B. germanium
C. mixture of silicon and germanium
D. none of the above
4. In which of these is reverse recovery time nearly zero?
A. Zener diode
B. Tunnel diode
C. Schottky diode
D. PIN diode
5. A transistor has a current gain of 0.99 in the CB mode. Its current gain in the CC mode is
A. 100
B. 99
C. 1.01
D. 0.99
6. In an n channel JFET, the gate is
A. n type
B. p type
C. either n or p
D. partially n & partially p
7. The amount of photoelectric emission current depends on
A. frequency of incident radiation
B. intensity of incident radiation
C. both frequency and intensity of incident radiation
D. none of the above
8. At very high temperatures the extrinsic semi conductors become intrinsic because
A. drive in diffusion of dopants and carriers
B. band to band transition dominants over impurity ionization
C. impurity ionization dominants over band to band transition
D. band to band transition is balanced by impurity ionization
9. Ferrite have
A. low copper loss
B. low eddy current loss
C. low resistivity
D. higher specific gravity compared to iron
10. A cycloconverter can be
A. step down
B. step up
C. step down or step up
D. none of the above
11. In a 3 phase fully controlled converter the firing frequency is
A. 3 times the line frequency
B. 6 times the line frequency
C. 9 times the line frequency
D. 12 times the line frequency
12. Two thyristor of same rating and same specifications
A. will have equal turn on and turn off periods
B. will have equal turn on but unequal turn off periods
C. may have equal or unequal turn on and turn off periods
D. will have unequal turn on and turn off periods
13. A semiconverter feeding an RLE load operates in
A. first quadrant only
B. first and second quadrants
C. first and fourth quadrants
D. first and third quadrants
14. The phenomenon of overlap in converter operation due to
A. source resistance
B. source inductance
C. both source resistance and source inductance
D. high value of firing angle
15. In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is
A. p - a
B. p
C. p + a
D. p - 2a
16. An SCR is triggered at 40° in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80°, the average anode current is likely to be
A. 50 A
B. 25 A
C. less than 50 A but more than 25 A
D. less than 25 A
17. Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.
A. True
B. False
18. The value of capacitor C for dynamic equalising circuit of series connected thyristors is
determined by
A. forward characteristics of thyristors
B. reverse recovery characteristics of thyristors
C. both forward and reverse recovery characteristics of thyristors
D. none of the above
19. A chopper
A. converts constant voltage dc into ac and then into variable voltage dc
B. converts constant voltage dc into variable voltage dc directly
C. converts ac of one frequency into ac of another frequency
D. converts ac to dc
20. In a single phase full wave regulator, the firing angles in the positive and negative half cycles
are generally
A. equal
B. different
C. equal or different
D. different but sometimes equal
