Thyristor MCQ

Thyristor MCQ

Thyristor MCQ, Multiple Choice Questions on Thyristor, Power Electronics MCQ, SCR MCQ, Silicon Controlled Rectifier MCQ, Engineering MCQ, UJT MCQ, Thyristor MCQ, Multiple Choice Questions on Thyristor, Power Electronics MCQ, SCR MCQ, Silicon Controlled Rectifier MCQ, Engineering MCQ, UJT MCQ, Thyristor MCQ, MCQ on Thyristor, Thyristor MCQ

Multiple Choice Questions

Q.1. The number of p-n junctions in a thyristor is

  • 1
  • 2

Answer: 3

Q.2. When the anode is positive with respect to the cathode in an SCR, the number of blocked p-n junctions is

  • 1
  • 3
  • 4

Answer: 1

Q.3. When the cathode is positive with respect to the anode in an SCR, the number of blocked p-n junctions is

  • 1
  • 2
  • 3
  • 4

Answer:  2

Q.4. In a thyristor, anode current is made up of

  • electrons only
  • electrons or holes
  • electrons and holes
  • holes only  

Answer: electrons and holes

Q.5. A thyristor, when triggered, will change from forward blocking state to conduction state if its anode to cathode voltage is equal to

  • peak repetitive off-state forward voltage
  • peak working off-state forward voltage
  • peak working off-state reverse voltage
  • peak non-repetitive off-state forward voltage  

Answer: peak working off-state forward voltage

Q.6. An SCR can be brought to forward conducting state with gate-circuit open when the applied voltage exceeds

  • the forward breakover voltage
  • reverse breakdown voltage
  • 1.5 V
  • peak non-repetitive off-state voltage  

Answer: the forward breakover voltage

Q.7. In a thyristor, holding current is

  • more than latching current IL
  • less than IL
  • equal to IL
  • very small  

Answer: less than IL

Q.8. When a thyristor gets turned on, the gate drive

  • should not be removed as it will turn-off the SCR
  • may or may not be removed
  • should be removed
  • should be removed in order to avoid increased losses and higher junction temperature

Answer: should be removed in order to avoid increased losses and higher……

Q.9. For normal SCRs, turn-on time is

  • less than turn-off time, tq
  • more than tq
  • equal to tq
  • half of tq

Answer: less than turn-off time, tq

Q.10. The forward voltage drop during SCR on-state is 1.5 V. This voltage drop

  • remains constant and is independent of load current
  • increases slightly with load current
  • decreases slightly with load current
  • varies linearly with load current

Answer: increases slightly with load current

Q.11. During forward blocking state, a thyristor is associated with

  • large current, low voltage
  • low current, large voltage
  • medium current, large voltage
  • low current, medium voltage  

Answer: low current, large voltage

Q.12. In a thyristor, the ratio of latching current to holding current is

  • 0.4
  • 1.0
  • 2.5
  • 6.0  

Answer: 2.5

Q.13. The on-state voltage drop across a thyristor used in a 250-V supply system is of the order of

  • 100-110 V
  • 240-250 V
  • 1 to 1.5 V
  • 0.5 to 1 V    

Answer: 1 to 1.5 V

Q.14. Once SCR starts conducting a forward current, its gate loses control over

  • anode circuit voltage only
  • anode circuit current only
  • anode circuit voltage and current
  • anode circuit voltage, current and time 

Answer: anode circuit voltage and current

Q.15. In a thyristors

  • latching current IL is associated with turn-off process and holding current IH with turn-on process
  • both IL and IH are associated with turn-off process
  • IH is associated with turn-off process and IL with turn-on process
  • both IL and IH are associated with turn-on process  

Answer: IH is associated with turn-off process and IL with turn-on process

Q.16. The SCR ratings, di/dt in A/µsec and dv/dt in V/µsec, may vary, respectively, between

  • 20 to 500, 10 to 100
  • both 20 to 500
  • both 10 to 100
  • 50 to 300, 20 to 500

Answer: both 20 to 500

Q.17. A thyristor can be termed as

  • DC switch
  • AC switch
  • Either DC or AC switch 
  • square-wave switch   

Answer: DC switch

Q.18. In a thyristor, the magnitude of anode current will

  • increase if gate current is increased
  • decrease if gate current is decreased
  • increase if gate current is decreased
  • not change with any variation in gate current 

Answer: not change with any variation in gate current

Q.19. The Turn-on time of an SCR can be reduced by using a

  • rectangular pulse of high amplitude and narrow width
  • rectangular pulse of low amplitude and wide width
  • triangular pulse
  • trapezoidal pulse 

Answer: rectangular pulse of high amplitude and narrow width

Q.20. The Turn-on time of an SCR in series with RL circuit can be reduced by

  • increasing circuit resistance R
  • decreasing R
  • increasing circuit inductance L
  • decreasing L   

Answer: decreasing L

Q.21. For an SCR with a turn-on time of 5 microseconds, an ideal trigger pulse should have

  • short rise time with pulse width = 3 µsec
  • long rise time with pulse width = 6 µsec
  • short rise time with pulse width = 6 µsec
  • long rise time with pulse width = 3 µsec  

Answer: short rise time with pulse width = 6 µsec

Q.22. A forward voltage can be applied to an SCR after its

  • anode current reduces to zero
  • gate recovery time
  • reverse recovery time
  • anode voltage reduces to zero  

Answer: gate recovery time

Q.23. The turn-off time of an SCR is measured from the instant

  • anode current becomes zero
  • anode voltage becomes zero
  • anode voltage and anode current become zero at the same time
  • gate current becomes zero  

Answer: anode current becomes zero

Q.24. Turn-on time for an SCR is 10 µsec. If an inductance is inserted in the anode circuit, then the turn-on time will be

  • 10 µsec
  • less than 10 µsec
  • more than 10 µsec
  • about 10 µsec  

Answer: more than 10 µsec

Q.25. In an SCR, anode current flows over a narrow region near the gate during

  • delay time td
  • rise time tr and spread time tp
  • td and tp
  • td and tr

Answer: td and tr

Q.26. Gate characteristic of a thyristor

  • is a straight line passing through origin
  • is of the type Vg =a + b Ig
  • is a curve between Vg and Ig
  • has a spread between two curves of Vg – Ig

Answer: has a spread between two curves of Vg – Ig

Q.27. The average on-state current for an SCR is 20 A for a conduction angle of 120°. Its average on-state current for 60° conduction angle would be

  • 20 A
  • 10 A
  • less than 20 A
  • 40 A 

Answer: less than 20 A

Q.28. The average on-state current for an SCR is 20 A for a resistive load. If an inductance of 5 mH is included in the load, then the average on-state current would be

  • more than 20 A
  • less than 20 A
  • 15 A
  • 20 A 

Answer: more than 20 A

Q.29. Specification sheet for an SCR gives its maximum RMS on-state current as 35 A. This RMS rating for a conduction angle of 120° would be

  • more than 35 A
  • less than 35 A
  • 35 A
  • 52.5 A 

Answer: 35 A

Q.30. Surge current rating of an SCR specifies the maximum

  • repetitive current with sine wave
  • non-repetitive current with rectangular wave
  • non-repetitive current with sine wave
  • repetitive current with rectangular wave 

Answer: non-repetitive current with sine wave

Q.31. The di/dt rating of an SCR is specified for its

  • decaying anode current
  • decaying gate current
  • rising gate current
  • rising anode current 

Answer: rising anode current

Q.32. For an SCR, dv/dt protection is achieved through the use of

  • RL in series with SCR
  • RC across SCR
  • L in series with SCR
  • RC in series with SCR 

Answer: RC across SCR

Q.33. For an SCR, di/dt protection is achieved through the use of

  • R in series with SCR
  • RL in series with SCR
  • L in series with SCR
  • L across SCR 

Answer: L in series with SCR

Q.34. The function of snubber circuit connected across an SCR is to

  • suppress dv/ dt 
  • increase dv/dt
  • decrease dv/dt
  • keep transient overvoltage at a constant value 

Answer: suppress dv/ dt

Q.35. The object of connecting resistance and capacitance across the gate circuit is to protect the SCR gate against

  • overvoltages
  • dv/ dt
  • noise signals
  • overcurrents 

Answer: noise signals

Q.36. A practical way of obtaining static voltage equalization in series-connected SCRs is by the use of

  • one resistor across the string
  • resistors of different values across each SCR
  • resistors of the same value across each SCR
  • one resistor in series with the string

Answer: resistors of the same value across each SCR

Q.37. For series connected SCRs, the dynamic equalizing circuit consists of

  • resistor R and capacitor C in series but with a diode D across C
  • series R and D circuit but with C across R
  • series R and C circuit but with D across R
  • series C and D circuit but with R across C

Answer: series R and C circuit but with D across R

Q.38. For dynamic equalizing circuit used for series-connected SCRs, the choice of C is based on

  • reverse recovery characteristic
  • turn-on characteristics
  • turn-off characteristics
  • rise-time characteristics 

Answer: reverse recovery characteristic

Q.39. In a UJT, with VBB as the voltage across two base terminals, the emitter potential at the peak point is given by

  • ηVBB
  • ηVD
  • ηVBB + VD
  • η VD  + VBB

Answer: ηVBB + VD

Q.40. A UJT exhibits a negative resistance region

  • before the peak point
  • between peak and valley points
  • after the valley point
  • none

Answer: between peak and valley points

Q.41. In a UJT, the maximum value of charging resistance is associated with

  • peak point
  • valley point
  • any point between peak and valley points
  • after the valley point  

Answer: peak point

Q.42. When a UJT is used for triggering an SCR, the waveshape of the voltage obtained from UJT circuit is a

  • sine wave
  • saw-tooth wave
  • trapezoidal wave
  • square wave

Answer: saw-tooth wave

Q.43. For an UJT employed for the triggering of an SCR, stand-off ratio η = 0.64 and DC source voltage VBB is 20 V. The UJT would trigger when the emitter voltage is

  • 12.8 V
  • 13.1 V
  • 10 V
  • 5 V

Answer: 13.1 V

Q.44. An SCR can withstand a maximum junction temperature of 120°C with an ambient temperature of 75°C. If this SCR has thermal resistance from junction to ambient as 1.5°C/W, the maximum internal power dissipation allowed is

  • 30 W
  • 60 W
  • 80 W
  • 50 W

Answer: 30 W

Q.45. In synchronized UJT triggering of an SCR, voltage VC across the capacitor reaches UJT thresh-hold voltage thrice in each half cycle so that there are three firing pulses during each half cycle. The firing angle of the SCR can be controlled

  • once in each half cycle
  • thrice in each half cycle
  • twice in each half cycle
  • four times in each half cycle 

Answer: once in each half cycle

Q.46. The function of connecting a Zener diode in a UJT circuit, used for the triggering of SCRs, is to

  • expedite the generation of triggering pulses
  • delay the generation of triggering pulses
  • provide a constant voltage to UJT to prevent erratic firing
  • provide a variable voltage to UJT as the source voltage changes 

Answer: provide a constant voltage to UJT to prevent erratic firing

Q.47. A metal oxide varistor (MOV) is used for protecting

  • gate circuit against overcurrents
  • gate circuit against overvoltages
  • anode circuit against overcurrents
  • anode circuit against overvoltages 

Answer: anode circuit against overvoltages

Q.48. The functions of connecting a resistor in series with gate-cathode circuit and a zener-diode across gate-cathode circuit are, respectively, to protect the gate circuit from

  • overvoltages, overcurrents
  • overcurrents, overvoltages
  • overcurrents, noise signals
  • noise signals, overvoltages 

Answer: overcurrents, overvoltages

Q.49. In a GTO, anode current begins to fall when gate current

  • is negative peak at time t = 0
  • is negative peak at t = storage period ts
  • just begins to become negative at t = 0
  • is negative peak at t = (ts + fall time) 

Answer: is negative peak at t = storage period ts

Q.50. For a pulse transformer, the material used for its core and the possible turn-ratio from primary to secondary are, respectively,

  • ferrite; 20 : 1
  • laminated iron; 1 : 1
  • ferrite; 1 : 1
  • powdered iron; 1 : 1 

Answer: ferrite; 1 : 1

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