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Multiple choice question for engineering

Set 1

1. The usual way to accomplish higher gate current for improved di/dt rating is by using
a) varistors
b) pilot thyristors
c) twisted cables
d) op-amps

View Answer

Answer: b [Reason:] Pilot SCR is an SCR which is fired which activates the firing circuit and fires the main SCR.

2. Inter-digitating of gate-cathode regions in SCR devices improves the
a) I2t rating
b) di/dt rating
c) dv/dt rating
d) thermal resistance

View Answer

Answer: b [Reason:] Inter-digitating is the inter-mixing of the gate-cathode area to improve the di/dt ratings. di/dt rating is improved by providing more cathode conduction area during the delay and rise time.

3. The dv/dt rating of SCR can be improved by using
a) cathode-short structure
b) anode-short structure
c) gate-short structure
d) centre gate thyristor

View Answer

Answer: a [Reason:] Cathode shorts are realized by overlapping metal on cathode n+ layers with a narrow p-region in between.

4. The total thermal resistance between junction and ambient θjA is 10°C/W. θjc is 2°C/W. θcs is 4°C/W. θsA = ?
a) 4°C/W
b) 2°C/W
c) 10°C/W
d) 16°C/W

View Answer

Answer: a [Reason:] θjA = θjc+ θcs+ θsA.

5. Which of the following thermal resistance parameters depends on the size of the device and the clamping pressure?
a) θsA
b) θcs
c) θjc
d) None

View Answer

Answer: b [Reason:] The case-to-sink thermal resistance depends on the size of the device, pressure, grease between the interface, etc.

6. The sink to ambient thermal resistance of SCR θsA
a) depends on the flatness of the device
b) depends on the length of the device
c) depends on the current carrying capabilities
d) is independent on thyristor configuration

View Answer

Answer: d [Reason:] It does not depend on any of the device configurations.

7. Pav x (θjc + θcs + θsA) =
a) Maximum specified temperature
b) Energy lost
c) Difference in temperature between junction & ambient
d) Sum of junction & ambient temperature

View Answer

Answer: c [Reason:] Pav = (Tj – Ta)/θjA θjA = (θjc + θcs + θsA).

8. Heat dissipation from heat sink mainly takes place by
a) radiation
b) convection
c) absorption
d) none of the mentioned

View Answer

Answer: b [Reason:] Heat DISSIPATION (rejection of heat to the atmosphere) takes place through convention only.

9. For low power SCRs (about 1 Ampere current) _____________ type of mounting is used
a) lead
b) stud
c) bolt-down
d) press-fit

View Answer

Answer: a [Reason:] Lead mounting is a very simple time of mounting used for low power devices.

10. In the ___________ type of mounting the SCR is pressed between two heat sinks
a) bolt-down mounting
b) stud-mounting
c) press-pak mounting
d) cross-fit mounting

View Answer

Answer: c [Reason:] In the press-pak type the device is pressed or clamped between two heat-sinks & external pressure is applied from both the sides.

Set 2

1. di/dt protection is provided to the thryistor by
a) connecting an inductor in parallel across the load
b) connecting an inductor in series with the load
c) connecting an inductor in parallel across the gate terminal
d) connecting an inductor in series with the gate

View Answer

Answer: b [Reason:] By placing the di/dt inductor (L) in series with the load, the change in the anode current can be limited to a small value.

2. The local hot spot formation in the cross-section of the SCR is avoided by
a) reducing the junction temperature
b) applying gate current nearer to the maximum gate current
c) using only R loads
d) proper mounting of the SCR on heat sink

View Answer

Answer: b [Reason:] Applying the higher gate current spreads the ions quickly and avoids hotspot formation.

3. The dv/dt protection is provided in order to
a) limit the power loss
b) reduce the junction temperature
c) avoid accidental turn-on of the device
d) avoiding sudden large voltage across the load

View Answer

Answer: c [Reason:] Accidentally some voltage spike or noise may occur in the vicinity of the device, if the magnitude is large enough it may turn on the SCR.

4. dv/dt protection is provided to the SCR by
a) connecting a capacitor in parallel with the load
b) connecting an inductor in series with the load
c) connecting a capacitor & resister in parallel with the device
d) connecting an inductor & resister in parallel with the device

View Answer

Answer: c [Reason:] Snubber circuit R-C in parallel with SCR is connected for dv/dt protection.

5. Figure below shows SCR having dv/dt and di/dt protection, when the switch(shown in green) is closed the current through Rl =
power-electronics-questions-answers-thyristor-protection-q5
a) I(1 – e-t/τ)
b) I(1 + e-t/τ)
c) I(1 – et/τ)
d) I(1 + e-t/τ)

View Answer

Answer: a [Reason:] As soon as the switch is closed, C acts like a S.C & the voltage equation gives Vs = (Rs + Rl) I + L di/dt Solve the above D.E.

6. The effect of over-voltages on SCR are minimized by using
a) RL circuits
b) Circuit breakers
c) Varistors
d) di/dt inductor

View Answer

Answer: c [Reason:] Varistors are non-linear voltage clamping devices, RC circuits across the loads can also be used.

7. Over-current protection in SCRs is achieved through the use of
a) Varistors
b) Snubber Circuits
c) F.A.C.L.F & C.B.
d) Zener diodes

View Answer

Answer: c [Reason:] FACLF stands for Fast Acting Current Limiting Fuse.

8. False triggering of the SCRs by varying flux & noise is avoided by using
a) F.A.C.L.F & C.B
b) Shielded cables & twisted gate leads
c) Snubber circuits
d) di/dt inductor in series with the gate terminal

View Answer

Answer: b [Reason:] Shielded cables provide isolation from outside noise.

9. The thyristor has the following specifications
Vs = 400 V
(di/dt)max = 25 A/μsec
Find the value of L
power-electronics-questions-answers-thyristor-protection-q5
a) 8 μH
b) 80 μH
c) 16 μH
d) 160 μH

View Answer

Answer: c [Reason:] As soon as the switch is closed, C acts like a S.C & the voltage equation gives, Vs = (Rs + Rl) I + L di/dt Solve the above D.E. di/dt = Vs/L e-t/τ di/dt is maximum at t = 0, substitute the above given values & find L

10. Thyristors are used in electronic crowbar protection circuits because it possesses
a) high surge current capabilities
b) high amp 2-sec rating
c) less switching losses
d) voltage clamping properties

View Answer

Answer: a [Reason:] Crowbar protection circuits have high surge current capabilities.

Set 3

1. The voltage safety factor (VSF) for an SCR is the ratio of
a) peak working voltage & peak reverse repetitive voltage
b) dv/dt & di/dt
c) peak repetitive reverse voltage & maximum value of input voltage
d) peak repetitive reverse voltage & rms value of input voltage

View Answer

Answer: c [Reason:] It is peak repetitive reverse voltage/ the maximum value of input voltage(Vm).

2. The forward dv/dt rating of an SCR
a) increases with increase in the junction temperature
b) decreases with increase in the junction temperature
c) increases with decrease in the rms value of forward anode-cathode voltage
d) decreases with decrease in the rms value of forward anode-cathode voltage

View Answer

Answer: a [Reason:] If the temperature is high, lesser dv/dt is required to turn on the device as the higher temperature has already excited few of the holes & electrons.

3. The finger voltage of an SCR is
a) minimum value of Vak to turn on the device with gate triggering
b) maximum value of Vak to turn on the device with gate triggering
c) minimum value of Vak to turn on the device without gate triggering
d) maximum value of Vak to turn on the device without gate triggering

View Answer

Answer: a [Reason:] Finger voltage is the minimum value of Vak (anode to cathode voltage) to turn on the device with gate triggering, it is to be avoided for accidental turn-on of the device.

4. Which among the following anode current waveforms will have the minimum junction temperature?
a) 100 % DC
b) 25 % DC
c) 50 % DC
d) AC

View Answer

Answer: b [Reason:] N % DC is nothing but a wave with n % duty cycle. Lower the Duty cycle lesser is the current flowing & lesser is the temperature dissipation.

5. An SCR has half cycle surge current rating of 3000 A for 50 Hz. Calculate its one-cycle surge current rating
a) 3121.32 A
b) 2121.32 A
c) 3131.32 A
d) 2131.32 A

View Answer

Answer: b [Reason:] By equating the energies involved in one cycle & subcycle I2.T = Isb2.t Isb = 3000 A T = 1/50 t = T/2.

6. For a SCR the maximum rms on-state current is 35 A. If the SCR is used in a resistive circuit for a rectangular wave with conduction angle of 90°. Calculate the average & rms currents respectively.
a) I/4, I/2
b) I/2, I/√2
c) I/4, I2/2
d) I/4, I/√2

View Answer

Answer: a [Reason:] First convert the conduction angle into N for rectangular wave. N = 360°/Conduction angle = 4 Therefore, Iavg = I/4 Irms = I/√4 = I/2.

7. For an SCR the average & rms values of current are I/4 & I/2 respectively. Calculate the average on-state current rating (ITAV). Take maximum RMS on-state current = 35 A.
a) 8.78 A
b) 10.10 A
c) 17.5 A
d) 24.74 A

View Answer

Answer: c [Reason:] Form factor = RMS/Average vaules of current Therefore, FF = 2 ITAV = 35/2 = 17.5.

8. The amp2-sec rating of the SCR specifies
a) The power dissipated by the device when fault occurs
b) The energy dissipated by the device when fault occurs
c) The energy that the device can absorb before the fault is cleared
d) The energy that the device can absorb while operating in the forward blocking mode.

View Answer

Answer: c [Reason:] The amp2-sec is the energy that the device can handle before the fault is cleared. Its value decides how fast the fault has to be cleared to avoid damage to the device.

9. The maximum rms current of an SCR is 50 A. For a 120° sine wave conduction the form factor (FF) = 1.878
Find the average on-state current rating (ITAV).
a) 93.9 A
b) 174 A
c) 26.62 A
d) 68.52 A

View Answer

Answer: c [Reason:] ITAV = 50/FF.

10. The thermal resistance between junction & the SCR (θjc) has the unit
a) Ω/°C
b) W/Ω
c) °C/W
d) ΩW/°C

View Answer

Answer: c [Reason:] Thermal resistance always has the unit degree temperature per watt.

Set 4

1. A thyristor (SCR) is a
a) P-N-P device
b) N-P-N device
c) P-N-P-N device
d) P-N device

View Answer

Answer: c [Reason:] An SCR (silicon controlled rectifier) is a four layer p-n-p-n type device.

2. Which terminal does not belong to the SCR?
a) Anode
b) Gate
c) Base
d) Cathode

View Answer

Answer: c [Reason:] The SCR is having three terminals viz. anode, cathode and the gate.

3. An SCR is a
a) four layer, four junction device
b) four layer, three junction device
c) four layer, two junction device
d) three layer, single junction device

View Answer

Answer: b [Reason:] SCR is a four layer p-n-p-n device which forms three p-n junctions.

4. Choose the false statement.
a) SCR is a bidirectional device
b) SCR is a controlled device
c) In SCR the gate is the controlling terminal
d) SCR are used for high-power applications

View Answer

Answer: a [Reason:] It is a unidirectional device, current only flows from anode to cathode.

5. In the SCR structure the gate terminal is located
a) near the anode terminal
b) near the cathode terminal
c) in between the anode & cathode terminal
d) none of the mentioned

View Answer

Answer: b [Reason:] The gate is located near the cathode, because it allows fast turning on of the device when the gate signal is applied by forward basing the second junction.

6. The static V-I curve for the SCR is plotted for
a) Ia (anode current) vs Ig (gate current), Va (anode – cathode voltage) as a parameter
b) Ia vs Va with Ig as a parameter
c) Va vs Ig with Ia as a parameter
d) Ig vs Vg with Ia as a parameter

View Answer

Answer: b [Reason:] The curve is plotted for Ia vs Va for different values of gate current Ig.

7. If the cathode of an SCR is made positive with respect to the anode & no gate current is applied then
a) all the junctions are reversed biased
b) all the junctions are forward biased
c) only the middle junction is forward biased
d) only the middle junction is reversed biased

View Answer

Answer: c [Reason:] The device is in the reverse blocking state (3rd quadrant) & only the middle junction is forward biased whereas other two are reversed biased.

8. For an SCR in the reverse blocking mode, (practically)
a) leakage current does not flow
b) leakage current flows from anode to cathode
c) leakage current flows from cathode to anode
d) leakage current flows from gate to anode

View Answer

Answer: c [Reason:] In the reverse blocking mode, the gate current is zero & a reverse voltage is applied at the cathode-anode.

9. With the anode positive with respect to the cathode & the gate circuit open, the SCR is said to be in the
a) reverse blocking mode
b) reverse conduction mode
c) forward blocking mode
d) forward conduction mode

View Answer

Answer: c [Reason:] The SCR is in the forward blocking mode with its top and bottom junctions forward biased and the middle junction reversed biased.

10. For an SCR in the forward blocking mode (practically)
a) leakage current does not flow
b) leakage current flows from anode to cathode
c) leakage current flows from cathode to anode
d) leakage current flows from gate to anode

View Answer

Answer: b [Reason:] In the forward blocking mode, the gate current is zero & only the middle J2 junction is reversed biased.

Set 5

1. The forward break over voltage is the
a) anode-cathode voltage at which conduction starts with gate signal applied
b) anode-cathode voltage at which conduction starts with no gate signal applied
c) gate voltage at which conduction starts with no anode-cathode voltage
d) gate voltage at which conduction starts with anode-cathode voltage applied

View Answer

Answer: b [Reason:] It is the forward voltage at which the middle junction breaks down without any gate signal and pushes the device into the conducting state.

2. For a forward conducting SCR device, as the forward anode to cathode voltage is increased
a) the device turns on at higher values of gate current
b) the device turns on at lower values of gate current
c) the forward impedance of the device goes on increasing
d) the forward impedance of the device goes on decreasing

View Answer

Answer: b [Reason:] Higher the value of anode-cathode forward voltage, lower the gate requirements of the device. Also, the forward resistance of the device is always constant as long as the junction temperature is constant.

3. A thyristor can be bought from the forward conduction mode to forward blocking mode by
a) the dv/dt triggering method
b) applying a negative gate signal
c) applying a positive gate signal
d) applying a reverse voltage across anode-cathode terminals

View Answer

Answer: d [Reason:] a) & c) are used to turn on the device, b) will damage the SCR.

4. Usually the forward voltage triggering method is not used to turn-on the SCR because
a) it increases losses
b) it causes noise production
c) it may damage the junction & destroy the device
d) relatively it’s an inefficient method

View Answer

Answer: c [Reason:] In forward voltage triggering the middle junction breaks down without any gate signal and pushes the device into the conducting state. This method can permanently damage the J2 junction and make the device useless.

5. Among the following, the most suitable method to turn on the SCR device is the
a) gate triggering method
b) dv/dt triggering method
c) forward voltage triggering method
d) temperature triggering method

View Answer

Answer: a [Reason:] d) & b) are unreliable methods, c) can permanently damage the SCR Gate triggering is simple, reliable & most efficient.

6. The forward break over voltage is maximum when
a) Gate current = ∞
b) Gate current = 0
c) Gate current = -∞
d) It is independent of gate current

View Answer

Answer: b [Reason:] Higher the value of anode-cathode forward voltage, lower the gate requirements of the device.

7. For the SCR to remain in the ON (conducting) state
a) gate signal is continuously required
b) no continuous gate signal is required
c) no forward anode-cathode voltage is required
d) negative gate signal is continuously required

View Answer

Answer: b [Reason:] Unlike the transistor devices, once the SCR is turned on by the gate terminal, the gate terminal losses its control over the device.

8. The value of anode current required to maintain the conduction of an SCR even though the gate signal is removed is called as the
a) holding current
b) latching current
c) switching current
d) peak anode current

View Answer

Answer: b [Reason:] It is the minimum anode current value required to maintain the conduction of an SCR even though the gate signal is removed. It is a very important parameter when employing an SCR in any circuit.

9. In the reverse blocking mode the middle junction (J2) has the characteristics of that of a
a) transistor
b) capacitor
c) inductor
d) none of the mentioned

View Answer

Answer: b [Reason:] It is like a capacitor, as the dv/dt voltage triggering turns on the device. The charging current is given by, IC = CjdVa/dt.

10. ________ are semiconductor thyristor devices which can be turned-on by light of appropriate wavelengths.
a) LGTOs
b) LASERs
c) MASERs
d) LASCRs

View Answer

Answer: d [Reason:] LASCR stands for light activated SCRs, which can be turned on in made to conduct by firing appropriate light pulses at its gate region.