Match List – I with List – 2 and select the correct answer using the coding given below for each listList – 1 (SCR Ratings)List – 2 (Protective Element)i. di/dt limit1. Snubber circuitii. dv/dt limit2. Heat Sinkiii. I2t3. Series reactoriv. Junction temperature limit4. To avoid runway speed on load

i – 3, ii – 4, iii – 2, iv – 1

i – 1, ii – 2, iii – 3, iv – 4

i – 3, ii – 1, iii – 4, iv – 2

i – 3, ii – 4, iii – 2, iv – 1

i – 4, ii – 3, iii – 1, iv – 2

During the transition time or turn-on time:

The forward anode voltage increases from 10% to 90% & the anode current also increase from 10% to 90% of the initial value

The forward anode voltage increases from 10% to 90% & the anode current decrease from 90% to 10% of the initial value

The forward anode voltage decreases from 90% to 10% & the anode current also decrease from 90 to 10% of the initial value

The forward anode voltage decreases from 90% to 10% & the anode current increases from 10% to 90% of the initial value

The forward anode voltage increases from 10% to 90% & the anode current also increase from 10% to 90% of the initial value

di/dt and dv/dt protections to thyristor are implemented using-

An inductor in parallel with the input and a capacitor in series with the load

An inductor in series with input and a capacitor in parallel with the switch

An inductor in parallel with the input and a capacitor in series with the load

An inductor in parallel with the input and a capacitor in parallel with the load

An inductor in series with the input and a capacitor in parallel with the load

An SCR triggered by current pulse applied to gate-cathode can be turned OFF

by applying a pulse to the anode

by applying another pulse of opposite polarity to the gate-cathode

by reversing the polarity of the anode and cathode voltage

A thyristor is connected to supply with load. Anode is positive with respect to cathode. What are the conditions of the junctions if the gate current is zero?

All the junctions are forward biased

All the junctions are reverse biased

P-n junctions near to the anode and the middle junction is forward biased.

p-n junctions near to the anode and cathode are forward biased and the middle junction is reverse biased.

Find the average load voltage of the rectifier circuit, if the device is switched to ON at α, angular angle.

$\frac{{{V}_{rms}}}{\pi }\left( 1+\cos \alpha \right)$

$\frac{{{V}_{rms}}}{\pi \sqrt{2}}\cos \alpha$

$\frac{{{V}_{rms}}}{\pi \sqrt{2}}(1+\cos \alpha )$

$\frac{{{V}_{rms}}}{\pi }\left( 1+\cos \alpha \right)$

$\frac{{{V}_{rms}}}{2\pi }\left( 1+\cos \alpha \right)$

An SCR is turned off when its turn-off time is

greater than the circuit turn-off time

less than the circuit time constant

greater than the circuit time constant

less than the circuit turn-off time

greater than the circuit turn-off time

dv/dt protection is provided to the SCR by:

Connecting a capacitor in parallel with the load

Connecting a capacitor & resistor in parallel with the device

Connecting a capacitor in parallel with the load

Connecting an inductor in series with the load

Connecting an inductor & resister in parallel with the device

A triac is a semi-conductor device which acts as a

5 terminal multi-directional switch

2 Terminal unidirectional switch

2 Terminal bidirectional switch

3 Terminal bidirectional switch

5 terminal multi-directional switch

3 SCRs are connected in series. The string efficiency is 91%. SCRs 1, 2 & 3 have leakage currents 4 mA, 8 mA & 12 mA. Which SCR will block more voltage?

All the three will block equal voltage

The most practical way of obtaining a uniform distribution of series connected SCRs is to

connect a resistor of value R in series with one of the series connected SCRs

connect a resistor of value R in series with each of the series connected SCRs

connect a resistor of value R in parallel with each of the series connected SCRs

connect a resistor of value R in series with one of the series connected SCRs

connect a resistor of value R in parallel with one of the series connected SCRs

The measure of reliability of string is given by the factor

DRF = 1 ‚Äö√Ñ√∂‚àö√ë‚àö¬® String efficiency

DRF = String efficiency ‚Äö√Ñ√∂‚àö√ë‚àö¬® 1

To have maximum possible string efficiency

SCRs with the same dimensions must be used

SCRs of same rating must be used

SCRs with similar V-I characteristics must be used

SCRs with the same dimensions must be used

SCRs with similar thermal characteristics must be used

The term used to measure the degree of utilization of SCRs connected in series & parallel is

voltage/current utilization ratio

82 + Mcqs in Interconnecting Thyristors in Power Electronics Page 2 McqOptions

51.	Four power semiconductor devices are shown in the figure along with their relevant terminals. The device(s) that can carry dc current continuously in the direction shown when gated appropriately is (are)
A.	Triac only
B.	Triac and MOSFET
C.	Triac and GTO
D.	Thyristor and Triac
Answer» C. Triac and GTO

Discussion

52.	A Triac is equivalent to two SCRs
A.	in parallel
B.	in inverse parallel
C.	in series
D.	in inverse series
Answer» C. in series

Discussion

53.	An SCR is connected across 200 V battery through 0.2 H inductance. If the latching current in the circuit is 3 mA, what is the minimum width of the gating pulse required to properly turn on the SCR?
A.	1 μs
B.	2 μs
C.	3 μs
D.	4 μs
Answer» D. 4 μs

Discussion

54.	Match List – I with List – 2 and select the correct answer using the coding given below for each listList – 1 (SCR Ratings)List – 2 (Protective Element)i. di/dt limit1. Snubber circuitii. dv/dt limit2. Heat Sinkiii. I2t3. Series reactoriv. Junction temperature limit4. To avoid runway speed on load
A.	i – 1, ii – 2, iii – 3, iv – 4
B.	i – 3, ii – 1, iii – 4, iv – 2
C.	i – 3, ii – 4, iii – 2, iv – 1
D.	i – 4, ii – 3, iii – 1, iv – 2
Answer» C. i – 3, ii – 4, iii – 2, iv – 1

Discussion

55.	During the transition time or turn-on time:
A.	The forward anode voltage increases from 10% to 90% & the anode current decrease from 90% to 10% of the initial value
B.	The forward anode voltage decreases from 90% to 10% & the anode current also decrease from 90 to 10% of the initial value
C.	The forward anode voltage decreases from 90% to 10% & the anode current increases from 10% to 90% of the initial value
D.	The forward anode voltage increases from 10% to 90% & the anode current also increase from 10% to 90% of the initial value
Answer» D. The forward anode voltage increases from 10% to 90% & the anode current also increase from 10% to 90% of the initial value

Discussion

56.	If a TRIAC used to trigger using a negative gate voltage, in which of the following Quadrants would it be
A.	Quadrant 3 and Quadrant 4
B.	Quadrant 1 and Quadrant 2
C.	Quadrant 2 and Quadrant 3
D.	Quadrant 4 and Quadrant 1
Answer» D. Quadrant 4 and Quadrant 1

Discussion

57.	di/dt and dv/dt protections to thyristor are implemented using-
A.	An inductor in series with input and a capacitor in parallel with the switch
B.	An inductor in parallel with the input and a capacitor in series with the load
C.	An inductor in parallel with the input and a capacitor in parallel with the load
D.	An inductor in series with the input and a capacitor in parallel with the load
Answer» B. An inductor in parallel with the input and a capacitor in series with the load

Discussion

58.	An SCR triggered by current pulse applied to gate-cathode can be turned OFF
A.	by applying a pulse to cathode
B.	by applying a pulse to the anode
C.	by applying another pulse of opposite polarity to the gate-cathode
D.	by reversing the polarity of the anode and cathode voltage
Answer» E.

Discussion

59.	A thyristor is connected to supply with load. Anode is positive with respect to cathode. What are the conditions of the junctions if the gate current is zero?
A.	All the junctions are forward biased
B.	All the junctions are reverse biased
C.	P-n junctions near to the anode and the middle junction is forward biased.
D.	p-n junctions near to the anode and cathode are forward biased and the middle junction is reverse biased.
Answer» E.

Discussion

60.	In case of silicon transistor the cutoff region voltage of base emitter is ____.
A.	0.5
B.	0.3
C.	0.4
D.	> 0.7
Answer» E.

Discussion

61.	If the latching current in the circuit shown in the figure below is 2 mA, then the minimum width of gate pulse required to property turn on the SCR is:
A.	6 μs
B.	3 μs
C.	2 μs
D.	4 μs
Answer» E.

Discussion

62.	Find the average load voltage of the rectifier circuit, if the device is switched to ON at α, angular angle.
A.	$\frac{{{V}_{rms}}}{\pi \sqrt{2}}\cos \alpha$
B.	$\frac{{{V}_{rms}}}{\pi \sqrt{2}}(1+\cos \alpha )$
C.	$\frac{{{V}_{rms}}}{\pi }\left( 1+\cos \alpha \right)$
D.	$\frac{{{V}_{rms}}}{2\pi }\left( 1+\cos \alpha \right)$
Answer» C. $\frac{{{V}_{rms}}}{\pi }\left( 1+\cos \alpha \right)$

Discussion

63.	An SCR has specified value of dv/dt as 157 V/μs. If a sinusoidal noise signal of frequency 500 kHz appears, the maximum allowed peak value of noise signal for not triggering the SCR is
A.	50 V
B.	20 V
C.	100 V
D.	150 V
Answer» B. 20 V

Discussion

64.	For an SCR, the gate-cathode characteristic has a slope of 130. The gate power dissipation is 0.5 Watts. Find Ig.
A.	6.2 mA
B.	0.62 A
C.	620 A
D.	62 mA
Answer» E.

Discussion

65.	An SCR is turned off when its turn-off time is
A.	less than the circuit time constant
B.	greater than the circuit time constant
C.	less than the circuit turn-off time
D.	greater than the circuit turn-off time
Answer» D. greater than the circuit turn-off time

Discussion

66.	For the V-I characteristics if an SCR, which of the following statements are correct?1. It will trigger when the applied voltage is more than the forward breakover voltage2. Holding current is greater than the latching current3. When reverse biased, a small of leakage current will flow.4. It can be triggered without gate current
A.	1, 2 and 3
B.	1, 3 and 4
C.	1, 2 and 4
D.	2, 3 and 4
Answer» C. 1, 2 and 4

Discussion

67.	dv/dt protection is provided to the SCR by:
A.	Connecting a capacitor & resistor in parallel with the device
B.	Connecting a capacitor in parallel with the load
C.	Connecting an inductor in series with the load
D.	Connecting an inductor & resister in parallel with the device
Answer» B. Connecting a capacitor in parallel with the load

Discussion

68.	A triac is a semi-conductor device which acts as a
A.	2 Terminal unidirectional switch
B.	2 Terminal bidirectional switch
C.	3 Terminal bidirectional switch
D.	5 terminal multi-directional switch
Answer» D. 5 terminal multi-directional switch

Discussion

69.	For the modern Alternators, the typical value of SCR will be:
A.	0.5
B.	0.8
C.	0
D.	1
Answer» B. 0.8

Discussion

70.	Once an SCR is turned on, it remains so until the anode current goes below:
A.	Trigger current
B.	Break over current
C.	Threshold current
D.	Holding current
Answer» E.

Discussion

71.	In a thyristor, the minimum current required to maintain the device in the ‘ON’ state is called
A.	Latching current
B.	Ignition current
C.	Holding current
D.	Avalanche current
Answer» D. Avalanche current

Discussion

72.	For thyristor operation, following statements are given:(i) The supply voltage must be greater than the forward break overvoltage.(ii) The supply voltage must be less than the forward break overvoltage.(iii) ILatching < Iholding(iv) ILatching > IholdingWhich of the above statements are correct?Choose the correct answer from the code given below:
A.	(i) and (iii)
B.	(ii) and (iv)
C.	(ii) and (iii)
D.	(i) and (iv)
Answer» C. (ii) and (iii)

Discussion

73.	SCRS_WITH_A_RATING_OF_1000_V_&_200_A_ARE_AVAILABLE_TO_BE_USED_IN_A_STRING_TO_HANDLE_6_KV_&_1_KV._CALCULATE_THE_NUMBER_OF_SERIES_&_PARALLEL_UNITS_REQUIRED_IN_CASE_THE_DE-RATING_FACTOR_IS_0.1._(ROUND_OFF_THE_FRACTION_TO_THE_GREATEST_&_NEAREST_INTEGER)?$
A.	Series = 7, Parallel = 6
B.	Series = 6, Parallel = 7
C.	Series = 6, Parallel = 6
D.	Series = 7, Parallel = 7
Answer» B. Series = 6, Parallel = 7

Discussion

74.	Two parallel connect SCRs have same voltage drop (Vt) having rated current = 2I₁. SCR-1 carries a current of I₁=2.6 A whereas SCR-2 carries a current of I₂=1.4 A. Find the string efficiency?
A.	45 %
B.	77 %
C.	92 %
D.	84 %
Answer» C. 92 %

Discussion

75.	3 SCRs are connected in series. The string efficiency is 91%. SCRs 1, 2 & 3 have leakage currents 4 mA, 8 mA & 12 mA. Which SCR will block more voltage?
A.	SCR-1
B.	SCR-2
C.	SCR-3
D.	All the three will block equal voltage
Answer» B. SCR-2

Discussion

76.	The most practical way of obtaining a uniform distribution of series connected SCRs is to
A.	connect a resistor of value R in series with each of the series connected SCRs
B.	connect a resistor of value R in parallel with each of the series connected SCRs
C.	connect a resistor of value R in series with one of the series connected SCRs
D.	connect a resistor of value R in parallel with one of the series connected SCRs
Answer» C. connect a resistor of value R in series with one of the series connected SCRs

Discussion

77.	When an extra SCR is connected in series with a string
A.	DRF decreases
B.	DRF increases
C.	DRF remains constant
D.	None of the mentioned
Answer» C. DRF remains constant

Discussion

78.	The measure of reliability of string is given by the factor
A.	DRF = 1 ‚Äö√Ñ√∂‚àö√ë‚àö¬® String efficiency
B.	DRF = 1 + String efficiency
C.	DRF = String efficiency ‚Äö√Ñ√∂‚àö√ë‚àö¬® 1
D.	DRF = String efficiency x 2
Answer» B. DRF = 1 + String efficiency

Discussion

79.	For a string voltage of 3300 V, let there be six series connected SCRs each of voltage 600V. Then the string efficiency is
A.	99.36 %
B.	91.7 %
C.	98.54 %
D.	96 %
Answer» D. 96 %

Discussion

80.	To have maximum possible string efficiency
A.	SCRs of same rating must be used
B.	SCRs with similar V-I characteristics must be used
C.	SCRs with the same dimensions must be used
D.	SCRs with similar thermal characteristics must be used
Answer» C. SCRs with the same dimensions must be used

Discussion

81.	The term used to measure the degree of utilization of SCRs connected in series & parallel is
A.	tuf
B.	string efficiency
C.	voltage/current utilization ratio
D.	rectification efficiency
Answer» C. voltage/current utilization ratio

Discussion

82.	SCRs are connected in parallel to fulfill the ___________ demand
A.	high voltage
B.	high current
C.	size
D.	efficiency
Answer» C. size

Discussion

62.	Find the average load voltage of the rectifier circuit, if the device is switched to ON at α, angular angle.
A.	\(\frac{{{V}_{rms}}}{\pi \sqrt{2}}\cos \alpha\)
B.	\(\frac{{{V}_{rms}}}{\pi \sqrt{2}}(1+\cos \alpha )\)
C.	\(\frac{{{V}_{rms}}}{\pi }\left( 1+\cos \alpha \right)\)
D.	\(\frac{{{V}_{rms}}}{2\pi }\left( 1+\cos \alpha \right)\)
Answer» C. \(\frac{{{V}_{rms}}}{\pi }\left( 1+\cos \alpha \right)\)

Explore topic-wise MCQs in Power Electronics.

Four power semiconductor devices are shown in the figure along with their relevant terminals. The device(s) that can carry dc current continuously in the direction shown when gated appropriately is (are)

A Triac is equivalent to two SCRs

An SCR is connected across 200 V battery through 0.2 H inductance. If the latching current in the circuit is 3 mA, what is the minimum width of the gating pulse required to properly turn on the SCR?

During the transition time or turn-on time:

If a TRIAC used to trigger using a negative gate voltage, in which of the following Quadrants would it be

di/dt and dv/dt protections to thyristor are implemented using-

An SCR triggered by current pulse applied to gate-cathode can be turned OFF

A thyristor is connected to supply with load. Anode is positive with respect to cathode. What are the conditions of the junctions if the gate current is zero?

In case of silicon transistor the cutoff region voltage of base emitter is ____.

If the latching current in the circuit shown in the figure below is 2 mA, then the minimum width of gate pulse required to property turn on the SCR is:

Find the average load voltage of the rectifier circuit, if the device is switched to ON at α, angular angle.

An SCR has specified value of dv/dt as 157 V/μs. If a sinusoidal noise signal of frequency 500 kHz appears, the maximum allowed peak value of noise signal for not triggering the SCR is

For an SCR, the gate-cathode characteristic has a slope of 130. The gate power dissipation is 0.5 Watts. Find Ig.

An SCR is turned off when its turn-off time is

dv/dt protection is provided to the SCR by:

A triac is a semi-conductor device which acts as a

For the modern Alternators, the typical value of SCR will be:

Once an SCR is turned on, it remains so until the anode current goes below:

In a thyristor, the minimum current required to maintain the device in the ‘ON’ state is called

SCRS_WITH_A_RATING_OF_1000_V_&_200_A_ARE_AVAILABLE_TO_BE_USED_IN_A_STRING_TO_HANDLE_6_KV_&_1_KV._CALCULATE_THE_NUMBER_OF_SERIES_&_PARALLEL_UNITS_REQUIRED_IN_CASE_THE_DE-RATING_FACTOR_IS_0.1._(ROUND_OFF_THE_FRACTION_TO_THE_GREATEST_&_NEAREST_INTEGER)?$

Two parallel connect SCRs have same voltage drop (Vt) having rated current = 2I1. SCR-1 carries a current of I1=2.6 A whereas SCR-2 carries a current of I2=1.4 A. Find the string efficiency?

3 SCRs are connected in series. The string efficiency is 91%. SCRs 1, 2 & 3 have leakage currents 4 mA, 8 mA & 12 mA. Which SCR will block more voltage?

The most practical way of obtaining a uniform distribution of series connected SCRs is to

When an extra SCR is connected in series with a string

The measure of reliability of string is given by the factor

For a string voltage of 3300 V, let there be six series connected SCRs each of voltage 600V. Then the string efficiency is

To have maximum possible string efficiency

The term used to measure the degree of utilization of SCRs connected in series & parallel is

SCRs are connected in parallel to fulfill the ___________ demand

Two parallel connect SCRs have same voltage drop (Vt) having rated current = 2I₁. SCR-1 carries a current of I₁=2.6 A whereas SCR-2 carries a current of I₂=1.4 A. Find the string efficiency?