Test: Thyristor - 1
20 Questions MCQ Test Power Electronics | Test: Thyristor - 1
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An SCR (silicon controlled rectifier) is a four layer p-n-p-n type device.
The SCR is having three terminals viz. anode, cathode and the gate.
SCR is a four layer p-n-p-n device which forms three p-n junctions.
It is a unidirectional device, current only flows from anode to cathode.
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.
The curve is plotted for Ia vs Va for different values of gate current Ig.
If the cathode of an SCR is made positive with respect to the anode & no gate current is applied then
The device is in the reverse blocking state (3rd quadrant) & only the middle junction is forward biased whereas other two are reversed biased.
In the reverse blocking mode, the gate current is zero & a reverse voltage is applied at the cathode-anode.
With the anode positive with respect to the cathode & the gate circuit open, the SCR is said to be in the
The SCR is in the forward blocking mode with its top and bottom junctions forward biased and the middle junction reversed biased.
In the forward blocking mode, the gate current is zero & only the middle J2 junction is reversed biased.
It is the forward voltage at which the middle junction breaks down without any gate signal and pushes the device into the conducting state.
For a forward conducting SCR device, as the forward anode to cathode voltage is increased
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.
A thyristor can be bought from the forward conduction mode to forward blocking mode by
a) & c) are used to turn on the device, b) will damage the SCR.
Usually the forward voltage triggering method is not used to turn-on the SCR because
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.
Among the following, the most suitable method to turn on the SCR device is the
d) & b) are unreliable methods, c) can permanently damage the SCR
Gate triggering is simple, reliable & most efficient.
Higher the value of anode-cathode forward voltage, lower the gate requirements of the device.
Unlike the transistor devices, once the SCR is turned on by the gate terminal, the gate terminal losses its control over the device.
The value of anode current required to maintain the conduction of an SCR even though the gate signal is removed is called as the
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.
In the reverse blocking mode the middle junction (J2) has the characteristics of that of a
It is like a capacitor, as the dv/dt voltage triggering turns on the device. The charging current is given by,
IC = CjdVa/dt.
________ are semiconductor thyristor devices which can be turned-on by light of appropriate wavelengths.
LASCR stands for light activated SCRs, which can be turned on in made to conduct by firing appropriate light pulses at its gate region.
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