Electrical Engineering Multiple Choice Questions MCQs

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Which of the following is/are active elements?
A. Voltage source
B. Current source
C. Both
D. None of these.
 If P is the power of a star connected system then what will be power of an equivalent delta connected system?
A. P
B. 3P
C. P/3
D. None of the above
Magnetic flux has the unit of
A. Newton
B. Ampere-turn
C. Weber
D. Tesla
The voltage induced in an inductor is represented as,
A. product of its inductance and current through it.
B. ratio of its inductance to the current through it.
C. ratio of the current through it to its inductance.
D. product of its inductance and rate of change of current through it.
Instantaneous power in inductor is proportional to the
A. product of the instantaneous current and rate of change of current.
B. square of instantaneous current.
C. square of the rate of change of current.
D Temperature of the inductor.
The unit of resistivity is
A. Ω.
B. Ω – metre.
C. Ω / metre.
D. Ω / m².
How many coulombs of charge flow through a circuit carrying a current of 10 A in 1 minute?
A. 10
B. 60
C. 600
D. 1200
The resistance of a conductor of diameter d and length l is R Ω. If the diameter of the conductor is halved and its length is doubled, the resistance will be
A. R Ω
B. 2R Ω
C. 4R Ω
D. 8R Ω
The resistivity of the conductor depends on
A. area of the conductor.
B. length of the conductor.
C. type of material.
D. none of these.
If 1 A current flows in a circuit, the number of electrons flowing through this circuit is
A. 0.625 × 1019
B. 1.6 × 1019
C. 1.6 × 10-19
D. 0.625 × 10-19
Which of the following are the passive elements?
A. Resistor
B. Bulb
C. Both
D. None of these.
Power dissipation in ideal inductor is
A. Maximum
B. Minimum
C. Zero
D. A finite value
Inductor does not allow the sudden change of
A. current
B. voltage
C. power
D. None of the above
Capacitor does not allow the sudden change of ?
A. current
B. voltage
C. power
D. None of the above
Internal resistance of ideal voltage source is ?
A. zero
B. infinite
C. finite
D. 100 ohms
Internal resistance of ideal current source is ?
A. zero
B. infinite
C. finite
B. 100 ohms
Which quantity should be measured by the voltmeter ?
A. Current
B. Voltage
C. Power
D. Speed
Which quantity consists of a unit 1KWh ?
A. Energy
B. Time
C. Power
D. Charge
Which of the following has no units?
A. Permeability
B. Moment of a magnet
C. Magnetic susceptibility
D Permittivity
Which of the following quantities consists of SI unit as WATT ?
A. Force
B. Charge
C. Current
D. Power
A Triac is a ______________?
A. 2 terminal switch
B. 2 terminal bilateral switch
C. 3 terminal bilateral switch
D. 3 terminal bidirectional switch
A thyristor equivalent of a thyratron tube is a _________?
A. Diac
B. Triac
C. Silicon controlled-rectifier SCR
D. None of above
A silicon controlled racitifier (SCR) is a ____________?
A. Unijunction device
B. Device with three junction
C. Device with four junction
D. None of the above
A PNPN device having two gates is ___________?
A. Diac
B. Triac
A thyristor is basically________?
A. PNPN device
B. A combination of diac and triac
C. A set of SCRs
D. A set if SCR,diac and triac
A Triac has three terminals viz ___________?
A. Drain, source, gate
B. Two main terminal and a gate terminal
C. Cathode, anode, gate
D. None of the above
An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. One cycle surge current will be_____?
A. 1500 A.
B. 6000 A.
C. 2121.32 A.
D. 4242.64 A.
The typical value of SCR for modern alternator is____________?
A. 1.5.
B. 0.5.
C. 1.0.
D. 1.2.
An SCR is considered to be a semi controlled device because_______?
A. it can be turned OFF but not ON with a gate pulse.
B. it conducts only during one half cycle of an alternating current wave.
C. it can be turned ON but not OFF with a gate pulse.
D. it can be turned ON only during one half cycle of an AC.
A single phase full bridge inverter can operated in load commutation mode in case load consist of______?
A. RL.
B. RLC underdamped.
C. RLC overdamped.
D. RLC critically damped
A single phase one pulse controlled circuit has a resistance R and counter emf E load 400 sin(314 t) as the source voltage. For a load counter emf of 200 V, the range of firing angle control is _____?
A. 30° to 150°.
B. 30° to 180°.
C. 60° to 120°.
D. 60° to 180°.
A step up chopper has input voltage 110 V and output voltage 150 V. The value of duty cycle is_____?
A. 0.32
B. 0.67
C. 0.45
D. 0.27
Which statement is true for latching current ___________?
A.It is related to turn off process of the device.
B. It is related to conduction process of device.
C. It is related to turn on process of the device.
D. Both C and D.
If holding current of a thyristor is 2 mA then latching current should be_____?
A. 0.01 A.
B. 0.002 A.
C. 0.009 A.
D. 0.004 A.
In reverse blocking mode of a thyristor______?
A. junction J2 is in reverse bias and J1, J3 is in forward bias.
B. junction J3 is in forward bias and J1, J2 in reverse bias.
C. junction J1, J3 is in reverse bias and J2 is in forward bias.
D. junction J1 and J2 is in forward bias and J3 is in reverse bias.
In forward blocking mode of a thyristor________?
A. junction J2 is in reverse bias and J1, J3 is in forward bias.
B. junction J3 is in forward bias and J1, J2 is in reverse bias.
C. Junction J1, J3 is in reverse bias and J2 is in forward bias.
D. Junction J1 and J2 is in forward bias and J3 is in reverse bias.
For an SCR gate – cathode characteristic is a straight line of 130. For triggered source volume of 15 V and allowable gate power dissipation of 0.5 W compute the gate source resistance_____?
A. 111.9 Ω.
B. 11.19 Ω.
C. 108 Ω.
D. 115 Ω
The function of snubber circuit connected across the SCR is to___________?
A. Suppress dV / dt.
B. Increase dV / dt.
C. Decrease dV / dt.
D. Decrease di / dt.
A single phase full bridge inverter can operate in load commutation mode in case load consists of____________?
A. RL.
B. RLC underdamped.
C. RLC overdamped.
D. RLC critically damped.
Thyristor can be protected from over voltages by using
A. 23 mA.
B. 40 mA.
C. 10 mA.
D. 60 mA.
Thyristor can be protected from over voltages by using____________?
A. voltage clamping device.
B. fuse.
C. heat sink.
D. snubber circuit.
Practical way of obtaining static voltage equalization in series connected SCRs is by the use of____________?
A. One resistor across the string
B. Resistors of the same value across each SCR
C. Resistors of different values across each SCR
D. One resistor in series with each SCR
Switching frequency of SITH is____________?
A. 5 KHz
B. 10 KHz
C. 60 KHz
D. 100 KHz
Example of a voltage clamping device______________?
A. fast acting fuse
B. snubber circuit
C. metal oxide varistor
D. aluminium block
The maximum di/dt in a SCR is____________?
A. Directly proportional to supply voltage
B. Directly proportional to inductance in the circuit
C. Inversely proportional to supply voltage
D. Both A and B
The latching current of GTO should be of order
A. 100 mA
B. 500 mA
C. 1 A
D. 2 A
Under over voltage condition impedance offered by the voltage clamping device is__________?
A. low
B. High
C. moderate
D. infinity
Which semiconductor device acts like a diode and two transistor?
B. Diac
C. Triac
Under normal operating condition voltage clamping device offers impedance of_________?
A. high value.
B. low value.
C. zero value.
D. moderate value.
What is used to protect the SCR from over current ________________?
A. CB and fuse.
B. Heat sink.
C. Snubber circuit.
D. Voltage clamping device.
If the anode current is 800 A, then the amount of current required to turn off the GTO is about____________?
A. 20 A
B. 200 A
C. 600 A
D. 400 A
CB used for over current protection of thyristor operates when the fault current is__________?
A. of long period
B. of short duration
C. both (A) and (B)
D. neither (A) nor (B)
A power semiconductor may undergo damage due to____________?
A. High di/dt
B. High dv/dt
C. Low di/dt
D. Low dv/dt
When a large surge current of very short duration flows through a thyristor then which one of the following device will operate to protect the thyristor ___________?
B. Snubber circuit
C. Voltage clamping device
D. Fast acting current limiting device (FACL fuse)
Which of the following is disadvantage of fast recovery diodes?
A. Recovery is only 5 µs
B. Recovery is only 50 µs
C. Doping is carried out
D. None of these
Leakage current flows through the thyristor in_________?
A. forward blocking mode
B. reverse blocking mode
C. both forward and reverse blocking mode
D. forward conduction mode
Which triggering is the most reliable____________?
A. Forward voltage triggering
B. Gate triggering
C. dV / dt triggering
D. Thermal triggering
Which of following devices has highest di/dt and dv/dt capability?
Let of a thyristor Vc1, Vc2, Vc3 are forward break over voltage for gate current Ig1, Ig2, Ig3 respectively. Then________________?
A. Vc1 > Vc2 > Vc3 when Ig1 > Ig2 > Ig3.
B. Vc1 > Vc2 > Vc3 when Ig1 < Ig2 < Ig3.
C. Vc1 = Vc2 = Vc3 any value of Ig.
D. Vc1 > Vc2 > Vc3 when Ig1 ≥ Ig2 &Atil
BCT is used for____________?
A. High power phase control
B. High power current control
C. Low power current control
D. Low power phase control
The capacitance of reversed bised junction J2 in a thyristor is CJ2 = 20 pF and can be assumed to be independant of the off state voltage. The limiting value of the charging current to turn on the thyristor is 16 mA. What is the critical value of dv/dt?
A. 600 V/µs
B. 800 V/µs
C. 1200 V/µs
D. 1000 V/µs
SCR will be turned off when anode current is_______________?
A. < latching current but greater than holding current and gate signal is 0.
B. less than holding current.
C. < latching current but greater than holding current and gate signal is present.
D. both (A) and (B).
After proper turn on of thyristor____________?
A. gate signal is always present
B. gate signal must be removed
C. gate signal should present but can be removed
D. none of the above.
Gate circuit or triggering circuit of a thyristor is
A. lower power circuit.
B. high power circuit.
C. magnetic circuit.
D. may be low power or high power circuit
Thyristor is nothing but a____________?
A. Controlled transistor
B. Controlled switch
C. Amplifier with higher gain
D. Amplifier with large current gain
How can we protect SCR from thermal conditions _____________?
A. Use of snubber circuit.
B. Using heat sink.
C. Using CB and fuse.
D. Using equalizing circuit
Which of the following is used in heat sink_____________?
A. Iron
B. Aluminium
C. Carbon
D Silver
A thyristor can termed as__________?
A. AC switch
B. DC switch
C. Both a and B
D. Square wave switch
Materials used in heat sink should have_____________?
A. high thermal conductivity
B. large surface area
C. high melting point
D. All of these
Compared to transistor, _________ have lower on state conduction losses and higher power handling capability?
B. Semi conductor diodes
D. Thyristor
What is used to protect a thyristor from high di / dt conditions__________?
A. Fuse.
B. Snubber circuit
C. Inductor
D. Voltage clamping device
COOLMOS device can be used in application up to power range of___________?
A. 1 KVA
B. 2 KVA
C. 500 VA
D. 100 KVA
Which of the following is used in SCR to protect from high dV / dt__________________?
A. Snubber circuit
B. Fuse
C. Equalizing circuit
D. Circuit breaker
IGBT combines the advantages of_____________?
A. BJTs and SITs
D. None of these
What happen due to high di / dt____________?
A. Breakdown of junction
B. Local hot spot
C. Insulation failure
D. None of these
Thermal voltage VT can be given by____________?
A. Kq/T
B. KT/q
C. qT/K
D. (K2/q)(T + 1/T – 1)
What may happen high dV / dt____________?
A. Unwanted turn ON
B. Breakdown of J2 junction
C. Both A and B
D. Anyone of these
Why resistor is used in Snubber circuit_________________ ?
A. To minimize the loss
B. To minimize the charging current
C. To minimize the discharging current
D. All of these
Snubber circuit is used with SCR_________________?
A. in series
B. in parallel
C. either series or parallel
D. anti parallel
Which one is most suitable power device for high frequency (>100 KHz) switching application?
C. Schottky diode
D. Microwave transistor
Delay time is defined by the interval when_______________?
A. gate current increases from 90 % to 100 % of its final value
B. anode current reaches 10 % from forward leakage current
C. anode voltage drops from 100 % to 90 % of its actual value
D. all of these
A modern power semiconductor device that combines the characteristic of BJT and MOSFET is_____________?
Rise time is defined by the interval when____________?
A. gate current rises from 90 % to 100 % of it final value
B. anode voltage drops from 90 % to 10 % of its initial value
C. anode current rises 10 % to 90 % of its final value
D. both B and C
A power MOSFET has three terminals called___________?
A. Collector, emitter and gate
B. Drain, source and gate
C. Drain, source and base
D. Collector, emitter and base
The turn-on time of an SCR with inductive load is 20 µs. The puls train frequency is 2.5 KHz with a mark/space ratio of 1/10, then SCR will___________?
A. Turn on
B. Not turn on
C. Turn on if inductance is removed
D. Turn on if pulse frequency us increased to two times
Spread time is defined as the interval during which_____________?
A. anode voltage drops from 10 % of its initial value to zero
B. anode current rises from 90 % to its final value
C. both (A) and (B)
D. anode current rises from 10 % to 90 % of its final value
Maximum power loss occurs during_________________?
A. delay time
B. rise time
C. spread time
D. all
The reverse recovery time of diode is trr = 3 μs and the rate off all of the diode current is di/dt = 30 A/μs. The storage charge current QRR is___________?
A. 130 μs
B. 135 μs
C. 140 μs
D. 145 μs
The typical time of rising time lies between______________?
A. 10 – 20 µs
B. 40 – 60 µs
C. 1 – 4 µs
D. 90 – 100 µs
SITH is also known as___________?
A. Filled controlled diode
B. Filled controlled rectifier
C. Silicon controlled rectifier
D. None of these
A GTO can be turned on by applying___________?
A. Positive gate signal
B. Positive drain signal
C. Positive source signal
D. None of these
During which time maximum conduction spreading take place in the thyristor during turn ON?
A. Delay time
B. Spread time
C. Rise time
D. Same for every case
Which of the following is true about SIT?
A. SIT is a high power, high frequency device
B. SIT is a high power, low frequency device
C. SIT is a high power, high voltage device
D. SIT is a low power, high frequency device
During reverse recovery time__________________?
A. charge carrier of junction J2 recombined
B. charge carrier of junction J1 is swept out
C. charge carrier of junction J3 is swept out
D. both B and C
During gate recovery time_____________?
A. charge carriers of J2 junction recombined
B. charge carriers of J2 junction is swept out
C. charge carrier of J1 junction removed
D. charge carriers of J3 junction is removed
Power transistor are type of___________?
A. B.JTs
D. All of above
Typical range of thyristor turn OFF time is______________?
A. 3 – 10 µs
B. 3 – 50 µs
C. 3 – 100 µs
D. 3 – 500 µs
Which of following is normally ON device?
Which statement is true ?
A. Reverse recovery time ( trr ) > gate recovery time (tgr)
B. Device turn OFF time ( tq ) > reverse recover time (trr)
C. Circuit turn OFF time > device turn OFF time ( tq )
D. All of these
Which of following is not a power transistor?
ON state voltage drop across SCR lie between the range_____________?
A. 0 – 0.5 V.
B. 0.5 – 1 V.
C. 1 – 1.5 V.
D. 1.5 – 2 V.
Which following is a two terminal three layer device?
B. Power dioed
D. None of above
SCRs are used in series to meet_______________?
A. high current demand
B. low voltage demand
C. low current demand
D. high voltage demand
Dynamic equalising circuit is useful____________?
A. To limit di / dt of SCR
B. To limit dV / dt of SCR
C. For voltage equalisation
D. Both B and C
To meet high current demand we use SCRs in______________?
A. series connection.
B. parallel connection.
C. anti parallel connection.
D. both B and C.
Anode current in an SCR consists of____________?
A. holes only
B. electrons only
C. either electron or holes
D. Both electron and holes
By which one of the following we can measure the reliability of a string_____________?
A. String efficient
B. Reliability factor
C. Factor of safety
D. Derating factor
For series connected SCR’s dynamic equalising circuit consists of___________?
A. R and C in series but with diode across C
B. R and C in series but with diode across R
C. Series R and diode with C across R
D. Series R and diode with C across R
A thyristor string is made of a no. of SCR connected in series and parallel. The string have volume and current of 11 KV and 4 KA. The voltage and current rating of available SCRs are 1800 V and 1000 A. For a string efficiency of 90 % let the number of SCRs in series and parallel are a and b respectively. Then the value of a and b will be_____________?
A. 5, 7
B. 4, 6
C. 7, 5
D. 6, 4
Maximum di / dt in a SCR is________?
A. Directly proportional to Vm of supply voltage
B. Inversely proportional to Vm of supply voltage
C. Inversely proportional to L in the circuit
D. Both A and C
60 thyrsistors are connected in series and parallel to form a 10 KV and 5.5 KA switch. Each thyristor is rated for 1.2 KV, 1 KA. The no. of parallel path are 6. The efficiency of the switch is______________________?
A. 76.3 %
B. 91.6 %
C. 83.3 %
D. 90.9 %
A string of n parallel SCRs is operated at 72 KA, the rating of each SCR is 1 KA. If derating factor of the string is 0.1. value of n will be___________?
A. 60
B. 70
C. 80
D. 90
Dynamic equalizing circuit is used for________________?
A. equal division of voltage across each thyristor
B. equal division of current through each thyristor in parallel
C. equal division of voltage across each thyristor in parallel
D. equal division of current through each thyristor in series
Unipolar modulation is generally used in____________?
A. AC – AC converters
B. AC – DC converters
C. DC – AC converters
D. DC – DC converters
4 thyristors rated 200 V in series. The operating voltage of the string is 600 V. Derating factor of the string is_____________________?
A. 0.75
B. 0.7
C. 0.2
D. 0.25
The power demand can be estimated approximately by___________?
A. Load survey method
B. Mathematical method
C. Statistical method
D. Economic parameters
What is the advantage of HRC fuses over Rewirable fuses?
A. High speed operation
B. High rupturing capacity
C. No ageing effect
D. All of the above
A rectifier with an external low pass filter is an example of______________?
A. Indirect switch matrix circuits
B. Direct switch matrix circuits
C. Embedded converters
D. All of these
If a shunt motor is started with its field winding open then?
A. It will rotate at the same speed as that with its field winding closed
B. It will rotate at less speed as that with its field winding closed
C. It will rotate at dangerously high speed
D. None of these
In ac – dc conversion, when the switch is closed then the sum of voltages around the loop is____________?
A. Zero
B. Non zero
C. Equal to the sum of voltage when switch is open
D. Twice of the voltage when switch is open
An ideal switch is___________?
A. Lossless
B. Carry current in any direction when it is on
C. Does not carry any current in any direction when it is off
D. All of these
Electrical power output in a d.c. generator is equal to____________?
A. Electrical power developed in armature – copper losses
B. Mechanical power input – iron and friction losses
C. Electrical power developed in armature – iron and copper losses
D. Mechanical power input – iron and friction losses – copper losses
IGBT stands for___________?
A. Insulated gate bipolar transistor
B. Insulated gate bidirectional transistor
C. Inductive gate bipolar transistor
D. Inductive gate bidirectional transistor
The switching function of semiconductor devices can be characterized with___________?
A. Duty ratio only
B. Frequency only
C. Duty ratio and frequency
D. Duty ratio, frequency and time delay
Number of PN junction in an SCR is____________?
A. Two
B. Three
C. Four
D. Five
AC power in a load can be controlled by using____________?
A. Two SCR’s in parallel opposition
B. Two SCR’s in series
C. Three SCR’s in series
D. Four SCR’s in series
In a silicon controlled rectifier, the load is connected___________?
A. Across anode
B. In series with anode
C. Across cathode
D. In series with cathode
An SCR is made up of silicon because?
A. silicon has large leakage current than germanium
B. silicon has small leakage current than germanium
C. silicon has small leakage voltage than germanium
D. silicon has large leakage voltage than germanium
Anode of an operational SCR is _______________?
A. Always positive w.r.t cathode
B. Always negative w.r.t anode
C. Always positive w.r.t anode
D. Always negative w.r.t cathode
If the gate voltage of an SCR is removed, then the_______________?
A. anode current decreases
B. anode current does not decrease at all
C. anode current increases
D. cathode current increases
The output power of the cascaded amplifier / attenuator system can be determined using___________?
A. Actual gain of amplifier
B. Actual gain of amplifier and attenuator
C. Gain in dB of amplifier and attenuator
D. Actual gain of attenuator
A single phase ac – dc converter is also known as____________?
A. rectifier
B. inverter
C. chopper
D. regulator
In a single phase full wave rectifier, during blocking state the pea inverse voltage of diode is______________?
A. V m
B. 2 V m
C. V m / 2
D. 4 V m
LISN is a device used to measure conducted emissions. LISN stands for_____________?
A. Line integrated stabilization network
B. Line impedance stabilization network
C. Line integrated stored network
D. Laser integrated stabilization networking
In a full wave rectifier, the rectification ratio is approximately equal to____________?
A. 61%
B. 71%
C. 81%
D. 91%
For power output higher than 15 kW, the suitable rectifier is________________?
A. Single phase
B. 3 phase
C. Polyphase
D. Only (b) and (c)
In current commutated DC-DC choppers, the voltage spike appears across the load when_____________?
A. Voltage across the commutating inductances collapses
B. The capacitance-voltage adds to the supply voltage
C. Both A. and B.
D. None of these


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