Most Important MCq on Power Electronics | Electrical Engineering

Question 1 A forward-biased PN junction acts as a/an

  1. Thyristor
  2. Closed switch
  3. Amplifier
  4. Chopper
Answer : Closed switch

Question 2 Leakage current flows through the thyristor in

  1. Forward conduction mode
  2. Reverse blocking mode
  3. Forward blocking mode
  4. Both forward and reverse blocking mode
Answer : Both forward and reverse blocking mode

Question 3 A step up chopper has input voltage 110 V and output voltage 150 V. The value of duty cycle is

  1. 0.32.
  2. 0.67.
  3. 0.45.
  4. 0.27.
Answer : 0.27

Question 4 A single phase full bridge inverter can operated in load commutation mode in case load consist of

  1. RL.
  2.  RLC underdamped.
  3.  RLC overdamped.
  4.  RLC critically damped.
Answer : RLC underdamped.

Question 5 A freewheeling diode is phase-controlled rectifiers.

  1. Stops rectifier operations
  2. Improves line power factor
  3. Is the reason for additional harmonics
  4. Is the reason for the sudden breakdown
Answer : Improves line power factor

Question 6 Find the average output of a semi-converter connected to a 220 V, 50 Hz power supply, and firing angle is  𝝅/𝟑

  1. 178.52
  2. 248.05
  3. 148.55
  4. 198.49
Answer : 148.55

Question 7 Which triggering is the most reliable?

  1. Forward voltage triggering.
  2. Gate triggering.
  3. dV / dt triggering.
  4. Thermal triggering.
Answer : Gate triggering.

Question 8 In a 3-∅ controlled bridge rectifier, the frequency of ripple in the output voltage depends on

  1. Power factor
  2. Supply frequency
  3. Voltage source
  4. Firing angle
Answer : Supply frequency

Question 9 Which semiconductor power device out of the following, is not a current triggering device?

  1. Thyristor
  2. Triac
  3. G.T.O
  4. MOSFET
Answer :MOSFET

Question 10 Which of the given device is the most suitable power device for a higher frequency (above 100 kHz) switching application

  1. SCR
  2. Power MOSFET
  3. GTO
  4. BJT
Answer : Power MOSFET

Question 11 A half-wave rectifier circuit using an ideal diode has an input voltage of 10 sin ?t V; find the average and RMS value of output voltage?

  1. 3.18 V, 5V
  2. 3.68 V, 8V
  3. 4.18 V, 5V
  4. 4.68 V, 8V
Answer : 3.18 V, 5V

Question 12 A three phase full controlled converter can operate as a

  1. Converter for α = 0 to 1200
  2. Converter for α = 0 to 900
  3. Converter for α = 0 to 1800
  4. Converter for α = 0 to 600
Answer : Converter for α = 0 to 180 degree

Question 13 The advantages of SCS over SCR is

  1. Slow switching time and large VH
  2. Slow switching time and smaller VH
  3. Faster switching time and smaller VH
  4. Faster switching time and large VH
Answer : Faster switching time and smaller VH

Question 14 Leakage current flows through the thyristor in

  1. forward blocking mode.
  2.  reverse blocking mode.
  3.  both forward and reverse blocking mode.
  4.  forward conduction mode.
Answer : both forward and reverse blocking mode.

Question 15 The minimum duration of the pulse in a pulse triggering system for thyristors should be at

  1. 10 μs
  2. 10 ms
  3. 30 ms
  4. 1 sec
Answer : 10 μs

Question 16 The triple frequency of a six-phase half wave rectifier for 220 V, 60 Hz input will be

  1. 2160 Hz
  2. 720 Hz
  3. 360 Hz
  4. 60 Hz
Answer : 360 Hz

Question 17 In a three-phase bridge rectifier, the maximum conduction of each thyristor is

  1. 1200
  2. 900
  3. 300
  4. 600
Answer : 1200

Question 18 Which of the given regulator provide output voltage polarity reversal without the involvement of a transformer.

  1. Linear voltage regulator
  2. Shunt voltage regulator
  3. Buck-Boost regulator
  4. Series voltage regulator
Answer : Buck-Boost regulator

Question 19 A chopper converts

  1. AC to DC
  2. AC to AC
  3. DC to AC
  4. DC to DC
Answer : DC to DC

Question 20 PWM switching is used in voltage source inverters for the purpose of

  1. Controlling output current
  2. Controlling input voltage
  3. Controlling input power
  4. Controlling output harmonics and output voltage
Answer : Controlling output harmonics and output voltage

Question 21 Which of the given configurations is used for both regenerative and motoring breaking?

  1. First quadrant chopper
  2. Fourth quadrant chopper
  3. Third quadrant chopper
  4. Two quadrant chopper
Answer : Two quadrant chopper

Question 22 Power diode is

  1. Two terminal semiconductor device
  2. Three terminal semiconductor device
  3. Four terminal semiconductor device
  4. None of these
Answer : Two terminal semiconductor device

Question 23 The V-I characteristics of diode lie in the

  1. First quadrant
  2. Fourth quadrant
  3. The third and second quadrant
  4. The first and third quadrant
Answer : The first and third quadrant

Question 24 If a firing angle α of a single phase fully controlled rectifier feeding a constant DC current into the load is 600, Find the displacement factor of the rectifier

  1. 0
  2. 0.5
  3. 1
  4. 1.5
Answer : 0.5

Question 25 For a specific transistor, if the value of beta is equal to 400 and the Base current is 8mA, Find the value of Emitter current?

  1. 4.308
  2. 3.208 A
  3. 7.808 A
  4. 9.276 A
Answer : 3.208 A

Power Electronics MCQ serires 1

Power electronics MCQ Series – 1

Top 20 questions of power electronics. For explanation of each answer you can watch my video

Question 1: A thyristor is basically

  • PNPN device
  • A combination of diac and triac
  • A set of SCRs
  • A set of SCR, diac and a triac

Answer: PNPN device

Question 2: A silicon controlled rectifier (SCR) is a

  • Unijunction device
  • Device with three junction
  • Device with four junction
  • None of the above

Answer: Device with three junction

Question 3: A thyristor equivalent of a thyratron tube is a

  • Diac
  • Triac
  • Silicon controlled rectifier
  • None of the above

Answer: Silicon controlled rectifier

Question 4: An SCR is made up of silicon because

  • Silicon has large leakage current than germanium
  • Silicon has small leakage current than germanium
  • Silicon has small leakage voltage than germanium
  • Silicon has large leakage voltage than germanium

Answer: Silicon has small leakage current than germanium

Question 5: An SCR is considered to be a semi controlled device because

  • It can be turned OFF but not ON with a gate pulse.
  • It conducts only during one half cycle of an alternating current wave.
  • It can be turned ON but not OFF with a gate pulse.
  • It can be turned ON only during one half cycle of an AC.

Answer: It can be turned ON but not OFF with a gate pulse.

Question 6: Which statement is true for latching current ?

  • It is related to turn off process of the device.
  • It is related to conduction process of device.
  • It is related to turn on process of the device.
  • Both C and D.

Answer: It is related to turn on process of the device.

Question 7: If holding current of a thyristor is 2 mA then latching current should be

  • 0.01 A.
  • 0.002 A.
  • 0.009 A.
  • 0.004 A

Answer: 0.004 A

Question 8: In reverse blocking mode of a thyristor

  • junction J2 is in reverse bias and J1, J3 is in forward bias.
  • junction J3 is in forward bias and J1, J2 in reverse bias.
  • junction J1, J3 is in reverse bias and J2 is in forward bias.
  • junction J1 and J2 is in forward bias and J3 is in reverse bias.

Answer: junction J1, J3 is in reverse bias and J2 is in forward bias.

Question 9: During the commutation period in 3 phase converter, overlap time is

  • Dependent on the load current
  • Dependent on the voltage
  • Dependent on both the load current and load voltage behind the short circuit current
  • Independent on both the load current and load voltage

Answer: Dependent on both the load current and load voltage behind the short circuit current

Question 10: A UJT has ___________.

  • Two pn junctions
  • One pn junction
  • Three pn junctions
  • None of the above

Answer: One pn junction

Question 11: The UJT may be used as ___________.

  • Am amplifier
  • A sawtooth generator
  • A rectifier
  • None of the above

Answer: A sawtooth generator

Question 12: The fig. below represents a

  • Triac thyristor
  • Diac trigger
  • Diode rectifier
  • None of the above

Answer: Diac trigger

Question 13: A diac has ______________ pn junctions.

  • One
  • Two
  • Three
  • Four

Answer: Two

Question 14: What is a TRIAC?

  • Two thyristors connected in series mode
  • Two thyristors connected in parallel mode
  • Two thyristors connected in anti parallel mode
  • Two transistors connected in anti parallel mode

Answer: Two transistors connected in anti parallel mode

Question 15: For an SCR, dv/dt protection is achieved through

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

Answer: RC across SCR

Question 16: Power diode is __________

  • a three terminal semiconductor device
  • a two terminal semiconductor device
  • a four terminal semiconductor device
  • a three terminal analog device

Answer: a two terminal semiconductor device

Question 17: An ideal power diode must have

  • low forward current carrying capacity
  • large reverse breakdown voltage
  • high ohmic junction resistance
  • high reverse recovery time

Answer:

Question 18: A diode is said to be forward biased when the

  • cathode is positive with respect to the anode
  • anode is positive with respect to the cathode
  • anode is negative with respect to the anode
  • both cathode & anode are positive

Answer: anode is positive with respect to the cathode

Question 19: To make a signal diode suitable for high current & high voltage carrying applications with minimum losses, ________

  • a lightly doped n layer is grown between the two p & n layers
  • a heavily doped n layer is grown between the two p & n layers
  • a lightly doped p layer is grown between the two p & n layers
  • a heavily doped p layer is grown between the two p & n layers

Answer: a lightly doped n layer is grown between the two p & n layers

Question 20: Which one of the following statements is TRUE for an ideal power diode?

  • Reverse recovery time is non zero and reverse saturation current is zero
  • Forward voltage drop is zero and reverse saturation current is non zero
  • Forward voltage drop is non zero and reverse recovery time is zero
  • Forward voltage drop is zero and reverse recovery time is zero

Answer: Forward voltage drop is non zero and reverse recovery time is zero


FAQ

What is a thyristor ?

Thyristor is also called a silicon-controlled rectifier (SCR), it is basically a four-layer three-junction pnpn device. It has three terminals: anode, cathode, and gate.
Thyristor is basically an electronic switching device which can remain in conducting (on) and nonconducting state. However, it is a unidirectional device and can conduct only in one direction like a diode. The switching state of the device can be controlled by one of its terminals.

What is silicon controlled rectifier (SCR)?

The thyristor is also called a silicon-controlled rectifier (SCR), it is basically a four-layer three-junction pnpn device. It has three terminals: anode, cathode, and gate.
The silicon control rectifier (SCR) consists of four layers of semiconductors, which form NPNP or PNPN structures, having three P-N junctions labeled J1, J2 and J3, and three terminals.

Which thyristor is equivalent of a thyratron tube?

SCR, The thyratron is a type of gas-filled tube which is used as a high voltage electrical switch and a controlled rectifier.

Why SCR is considered to be a semi controlled device ?

During positive half cycle SCR is in forward blocking mode. By applying gate pulse the SCR can be turned ON during forward blocking mode. But SCR can not be turned OFF by applying gate pulse. It automatically turns off when anode current is below the Holding current. That is why it is called semi controlled rectifier.

What is latching current?

Latching current is the minimum required anode current to turn on SCR. Latching current is 2 or 3 times of holding current

What is holding current?

Holding current is the minimum anode current below which SCR get turn off. Holding current is 2 or 3 times less than latching current.

What is reverse blocking mode of a thyristor?

When a negative voltage is applied to the anode and a positive voltage to the cathode, the SCR is in reverse blocking mode, making J1 and J3 reverse biased and J2 forward biased.
The device behaves as two reverse-biased diodes connected in series. A small leakage current flows. This is the reverse blocking mode.

What is unijunction transistor (UJT)?

A unijunction transistor (UJT) is a three-lead electronic semiconductor device with only one junction that acts exclusively as an electrically controlled switch. The UJT is not used as a linear amplifier.

What is power diode?

It has two terminals anode and cathode same as that of a ordinary diode. In fact, a power diode is nothing but a signal diode with a extra layer.