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o avoid commutation failure a) circuit turn-off time must be greater than the thyristor turn-off time b) circuit turn-off time must be lesser than the thyristor turn-off time c) circuit turn-off time must be equal to the thyristor turn-off time d) none of the above mentioned
a Explanation: If the thyristor turn off time is more than the circuit turn off time, the circuit will be turned off and the thyristor will keep conducting, which is not at all desirable.
a
See lessExplanation: If the thyristor turn off time is more than the circuit turn off time, the circuit
will be turned off and the thyristor will keep conducting, which is not at all desirable.
For effective turning off of the SCR after the anode current has reached zero value, ______________ a) chargers are injected by applying reverse anode-cathode voltage b) chargers are removed by applying reverse anode-cathode voltage c) chargers are injected by applying gate signal d) chargers are removed by applying gate signa
b Explanation: To enable the device to regain its reverse blocking capabilities, the stored charges in the junctions of the SCR must be removed.
b
See lessExplanation: To enable the device to regain its reverse blocking capabilities, the stored
charges in the junctions of the SCR must be removed.
For an SCR the total turn-on time consists of i) Delay time ii) Rise time and the iii) Spread time The spread time interval depends upon a) the value of gate current b) junction temperature c) area of the cathode d) area of the anode
c Explanation: During the spread time the conduction starts spreading all over the SCR cathode cross-section structure, which depends upon the structure of the gate & cathode. Higher the cathode area more is the time required for the charges to spread all over.
c
See lessExplanation: During the spread time the conduction starts spreading all over the SCR
cathode cross-section structure, which depends upon the structure of the gate &
cathode. Higher the cathode area more is the time required for the charges to spread all
over.
he latching current is _________ than the holding current a) lower b) higher c) same as d) negative of
b Explanation: The latching current is the value of current on which the device will remain in the on state even after removal of the gate signal. Whereas, the holding current is the threshold above which the device will work.
b
See lessExplanation: The latching current is the value of current on which the device will remain
in the on state even after removal of the gate signal. Whereas, the holding current is the
threshold above which the device will work.
For an SCR the total turn-on time consists of i) Delay time ii) Rise time and iii) Spread time During the rise time the a) anode current flows only near the gate b) anode current rises from zero to very high value c) losses are maximum d) anode to cathode voltage is zero
c Explanation: The losses are maximum during the rise time because both Ia & Va are high.
c
See lessExplanation: The losses are maximum during the rise time because both Ia & Va are
high.
The minimum value of anode current below which it must fall to completely turn-off the device is called as the a) holding current value b) latching current value c) switching current value d) peak anode current value
a Explanation: The device will remain in the conducting state unless the anode current falls below the holding current value.
a
See lessExplanation: The device will remain in the conducting state unless the anode current
falls below the holding current value.
For an SCR the total turn-on time consists of i) Delay time ii) Rise time and iii) Spread time During the delay time the a) anode current flows only near the gate b) anode current rises from zero to very high value c) losses are maximum d) anode to cathode voltage is zero
a Explanation: Initially for a fraction of a microsecond (delay time) after the gate signal is applied the anode current only flows near the gate terminal where the gate current density is maximum, as the gate current takes some time to spread all over the cross section of the device.
a
See lessExplanation: Initially for a fraction of a microsecond (delay time) after the gate signal is
applied the anode current only flows near the gate terminal where the gate current
density is maximum, as the gate current takes some time to spread all over the cross
section of the device.
During the transition time or turn-on time a) The forward anode voltage decreases from 90 % to 10 % & the anode current also decreases from 90 to 10 % of the initial value b) The forward anode voltage increases from 10 % to 90 % & the anode current also increases from 10 % to 90 % 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 decreases from 90% to 10% of the initial value
c Explanation: During the turn on time, the voltage across the SCR is going down and the current through it is slowly rising as it is going into the conduction mode.
c
See lessExplanation: During the turn on time, the voltage across the SCR is going down and the
current through it is slowly rising as it is going into the conduction mode.
________ are semiconductor thyristor devices which can be turned-on by light of appropriate wavelengths. a) LGTOs b) LASERs c) MASERs d) LASCRs
d Explanation: LASCR stands for light activated SCRs, which can be turned on in made to conduct by firing appropriate light pulses at its gate region.
d
See lessExplanation: LASCR stands for light activated SCRs, which can be turned on in made to
conduct by firing appropriate light pulses at its gate region.
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
b Explanation: It is like a capacitor, as the dv/dt voltage triggering turns on the device. The charging current is given by, IC = CjdVa/dt.
b
See lessExplanation: It is like a capacitor, as the dv/dt voltage triggering turns on the device. The
charging current is given by,
IC = CjdVa/dt.