100 Most Important MCQ Of Power electronics with answer & explanation

100 Most Important MCQ Of Power electronics with answer & explanation | Important MCQ Question of Power electronics for SSC JE (electrical) with explanation


Ques.1. A silicon controlled rectifier (SCR) is a

  1. Unijunction device
  2. Device with three junction
  3. Device with four junction
  4. None of the above

Answer.2. Device with three junction


Silicon-controlled rectifier or semiconductor-controlled rectifier is a four-layer solid-state current-controlling unidirectional device (i.e. can conduct current only in one direction).

silicon control rectifier

The silicon control rectifier (SCR) consists of four layers of semiconductors, which form NPNP or PNPN structures, having three P-N junctions labeled J1, J2, and J3, and three terminals. Silicon is used as the intrinsic semiconductor, to which the proper dopants are added. The junctions are either diffused or alloyed (an alloy is a mixed semiconductor or a mixed metal). The anode terminal of an SCR is connected to the p-type material of a PNPN structure, and the cathode terminal is connected to the n-type layer. SCR is connected to the p-type material nearest to the cathode.


Ques.2. A thyristor is basically

  1. PNPN device
  2. A combination of diac and triac
  3. A set of SCRs
  4. A set of SCR, diac and a triac

Answer.1. PNPN device


The thyristor is also called a silicon-controlled rectifier (SCR), is basically a four-layer three-junction p-n-p-n device. It has three terminals: anode, cathode, and gate. It is basically an electronic switching device that can remain in conducting (on) and nonconducting (OPI:) state. However, it is a unidirectional device and can conduct only in one direction like a diode. The switching state of the device can be controlled by one of its terminals.

silicon control rectifier

The thyristor has three terminals. The terminal connected to the end p-region is called the anode (A), the terminal connected to the end n-region is called cathode (K) and the terminal connected to the middle p-region is called gate (G). The thyristor is manufactured by diffusing four layers of impurities (p and n-type) into a silicon wafer of suitable thickness. The whole assembly of the thyristor is mounted on a heat sink with the help of a thread. The purpose of the heat sink is to dissipate the heat generated in the thyristor. 


Ques.3. Which semiconductor power device out of the following, is not a current triggering device?

  1. Thyristor
  2. Triac
  3. G.T.O

Answer.4. MOSFET


Current-driven Devices

A current-driven device is a device that requires a constant current drive for a period of time in order to initiate and/or remain in conduction. Two popular types of current-controlled devices are the bipolar power transistor and the thyristor (SCRs). The SCR is mainly used in AC-DC converters such as controlled rectifiers where the input AC voltage helps to commutate (turn off the devices by polarity reversal. In DC-AC inverters, the gate turn-off thyristors (GTOs) are generally used due to their ability to be both turned on and off by a gate control signal. This eliminates the need for forced commutation circuitry, needed to turn off the thyristor in these applications due to the absence of a reversing polarity AC input voltage. Thyristors and GTOs still dominate the high voltage and high current applications, like DC transmission, requiring converters up to the megawatt range. 

Voltage-controlled Devices

These devices are semiconductors that require a constant voltage drive on the gate control terminal in order to remain in conduction. The input drive requirements of these devices are substantially lower than their current-driven counterpart and are the preferred choice in modern power electronics. Two such devices are the power MOSFET and the IGBT which are forced commutated switching devices being fully controlled the gate terminal under normal operating conditions. These devices do not latch into conduction and therefore do not require special commutation circuits. The gate input junction of a MOSFET and IGBT is purely capacitive, so, no gate drive current is needed in the steady-state, unlike transistors. A minimum gate drive voltage, however, must be maintained (above the gate threshold voltage) at the device gate in order for it to remain in conduction. A high current low impedance drive circuit is needed to inject or remove current, from the gate high slew rates in order to switch the device rapidly. 


MOSFET abbreviated as Metal Oxide Semiconductor Field Effect Transistor is another semiconductor device like a BJT which can be used as an amplifier or switch. Like BJT, MOSFET is also a Four terminal device; however, the principle of operation of MOSFET is completely different from that of BJT.

The three terminals of MOSFET are named as Drain (D), Source (S) and Gate (G), as shown in the fig. Out of these, the gate terminal acts as a controlling terminal.

As shown in Fig. in BJT the output current, IC is controlled by the base current IB. Hence BJT is a current controlled device. On the other hand, in MOSFET, the voltage applied between gate and source (VGS) controls the drain current ID. Therefore, MOSFET is a voltage-controlled device. The name Field effects are derived from the fact that the output current flow is controlled by an electric field set up in the device by an externally applied voltage between gate and source terminal.

bjt mosfet


Ques.4. Which of the following device incorporates a terminal for synchronizing purposes?

  1. Diac
  2. Triac
  3. SUS
  4. None of the above

Answer.3. SUS


The proper operation of power electronic converters with AC input voltage requires the use of synchronizing devices to accurately determine the moment when the supply voltage passes through zero. This group of power electronic converters includes thyristor and transistor-controlled rectifiers, regulators, converters for active power factor correction, matrix converters.

Silicon Unilateral Switch (SUS)

A silicon unilateral switch (SUS) is similar to a PUT (Programmable Unijunction Transistor), except for the fact that it has an internally built low-voltage avalanche diode between the gate and the cathode. The symbol for a SUS and its equivalent circuit is shown in Fig. Its anode-to cathode electrical characteristic is shown in Fig. b for no external connection to the gate terminal because of the presence of avalanche diode, SUS turns ON for a fixed anode gate voltage.

SUS Diode

The SUS is usually used in the basic relaxation oscillator circuit. The major difference in function between the SUS and UJT in relaxation oscillator circuitry is that the SUS switches at a fixed voltage, determined by its internal avalanche diode, rather than a fraction of a smaller voltage. Also, it should be noted that the switching current is much higher in the SUS than in the UJT, and is also very close to It. These factors restrict the upper and lower limits of frequency or time delay which are practical with the SUS. For synchronization, lock-out, or forced switching, bias or pulse signals may be applied to the gate terminal of the SUS. Silicon Unilateral Switch is used mainly in timing the logic and trigger circuits. The SUS rating is about 20 V, 0.5 A. Since, the device will turn on for a fixed anode gate voltage. The device can also be used as a relaxation oscillator. The output pulses are used for triggering SCRs.

 At the end of each half-cycle, each SCR will cease to conduct as the potential difference across it drops to zero and so a voltage pulse must be applied to its gate if it is required to conduct during the next half-cycle.

As an alternative to the SCR, a triac or SUS may be used. This device acts as two SCRs connected in inverse parallel and, if pulsed with an alternating supply, will conduct in both phases of the AC cycle. Like the SCR, the device will only conduct when the voltage is not at zero volts and the device has been pulsed.

During the exposure, the timer is simply required to apply a sequence of synchronized pulses to the gate of the device at a time slightly later than the mains zero to switch them back on and ensure their continued conduction. At the end of the exposure, these pulses stop, and conduction through the device stops at the end of the next half-cycle. The system allows the accuracy of one voltage pulse (i.e. an exposure time of 0.01 seconds in the case of a two-pulse unit, or 0.002 seconds in the case of a medium frequency unit.


Ques.5. The advantages of SCS over SCR is

  1. Slow switching time and large VH
  2. Slow switching time and smallerVH
  3. Faster switching time and smallerVH
  4. Faster switching time and large VH

Answer.3. Faster switching time and smallerVH


The silicon-controlled switch (SCS) is a tetrode thyristor. That is, it has four electrodes It has an anode gate (AG) like a PUT and a cathode gate (KG) like an SCR. A current of Efficient size on either gate will fire the SCS. A large reverse current through the anode gate may be used to turn the SCS off.  Its use is largely in voltage or current sensing circuits as signals on either gate fire it. It operates like an OR gate. its power capabilities are limited to timing, logic, and triggering applications. 

SCS Diode

An advantage of the SCS over a corresponding SCR is the reduced turn-off time, typically within the range 1µs to 10µs  for the SCS and 5µs to 30µs  for the SCR. Some of the remaining advantages of the SCS over an SCR include increased control and triggering sensitivity and a more predictable firing situation. At present. however, the SCS is limited to low power, current, and voltage ratings. Typical maximum anode currents range from 100 mA to 300 mA with dissipation (power) ratings of 100 mW to 500 mW.


The SCS is a low-power device compared to SCR. The power rating of SCS is very low compared to SCR. The SCS handles current in the mA range. But it has the advantage of faster turn OFF. The application areas of SCR and SCS are the same. The SCS is sometimes used in digital applications such as counters, registers, and various timing circuits.


Ques.6. A thyristor equivalent of a thyratron tube is a

  1. Diac
  2. Triac
  3. Silicon controlled rectifier
  4. None of the above

Answer.3. Silicon controlled rectifier


History of Power amplifier

Power electronics originated at the beginning of the 19th century with the development of mercury-arc rectifiers. A mercury-arc rectifier or mercury-vapor valve is a type of electrical rectifier which is used to convert high ac voltage into dc voltage. They were very useful to provide power for industrial motors, electric railways, and electric locomotives, as well as high voltage direct current (HVDC) power transmission. The mercury arc rectifiers with a glass envelope, grid controlled mercury-arc rectifiers and mercury-arc rectifiers with metal envelope were developed in 1900, 1903 and 1908 respectively. Thyratrons, i.e., hot cathode mercury arc rectifier with grid control was developed by Langmuir in 1914.

The thyratron is a type of gas-filled tube that is used as a high voltage electrical switch and a controlled rectifier. Usually, thyratrons are manufactured as triode, tetrode, and pentode. Due to the mercury vapor or neon or xenon gas fill, thyratrons can handle much greater currents. 

In 1925, the first solid-state power device, a selenium rectifier was developed without a glass tube. The selenium rectifier is also known as the metal rectifier. The selenium rectifier is an early type of semiconductor rectifier in which the semiconductor is a copper oxide or selenium. This device is used in phase-controlled converters, inverters, battery chargers, and cyclo-converters.

In 1930, cyclo-converters, i.e., a variable frequency output voltage from the fixed frequency input voltage, were developed by Rissik. In 1933, Lenz developed the ac voltage regulator or controller using solid-state power devices. In 1947, the point-contact transistor was developed by W. H. Brattain, J. Bardeen, and W. Shockley.

The bipolar junction transistor (BJT) using germanium was invented in 1948 and this was the beginning of the new age of semiconductor electronics. There has been much reduced in size, cost, and power consumption of solid-state power devices, and simultaneously the research is going on to develop equipment with more complexity and more power handling capability.

A 100A germanium power diode was developed in 1953. A new revolution began in 1956 with the development of thyristor, i.e., a four-layer silicon PNPN device. The most popular semiconductor device of the thyristor family is the SCR which was introduced by General Electric in 1957.

The thyratron is a gas-filled vacuum tube that acts as an on-off switch. Similar in action to a silicon-controlled rectifier, once fired by a small signal, it continues to conduct until the current flow is interrupted.

thyratron and SCR

Silicon Controlled Rectifier: a solid-state, controlled rectifier usually used as an electronic switch, like a super-quick relay. A small current “fires” or triggers the device, causing it to conduct freely. It is similar to a thyratron tube. 


Ques.7. A triac is a

  1. 2 terminal switch
  2. 2 terminal bilateral switch
  3. 3 terminal bilateral switch
  4. 3 terminal bidirectional switch

Answer.4. 3 terminal bidirectional switch


The major drawback of an SCR is that it can conduct current in one direction only. Therefore, an SCR can only control dc power or forward biased half-cycles of ac in a load. However, in an a.c system, it is often desirable and necessary to exercise control over both positive and negative half-cycles. For this purpose, a semiconductor device called triac is used.

The triac is a three-terminal ac switch that is triggered into conduction when a low-energy signal is applied to its gate terminal. Unlike the SCR, the triac conducts in either direction when turned on. The triac also differs from the SCR in that either a positive or negative gate signal trigger it into conduction. Thus the triac is a three-terminal, four layers, directional, semiconductor device that controls ac power whereas an SCR controls dc power or forward biased half cycles of ac in a load. Because of its bidirectional conduction property, the triac is widely used in the field of power electronics for control purposes.

“Triac” is an abbreviation for the three-terminal ac switch. ‘Tri’ indicates that the device has three terminals and ‘ac’ indicates the device controls alternating current or can conduct in either direction.



The triac is equivalent to two thyristors connected back to back with their gate terminals tied up. When MT2 is positive with respect to MT1, the SCR 1 is forward biased. If the gate is made positive with respect to MT2. the SCR 1 conducts and the device goes from high impedance state to low impedance state. When MT1 is made positive with respect to MT2, SCR 2 is forward biased and it conducts when the gate is made positive. Thus the triac can conduct in both directions. The SCR demands a positive voltage between the gate and cathode. But the triac can conduct with either positive or negative voltage at the gate.

Triac characterstics


Ques.8. The fig. below represents a


  1. Triac thyristor
  2. Diac trigger
  3. Diode rectifier
  4. None of the above

Answer.2. Diac trigger



Diac is usually employed for triggering triacs. A diac is a two-electrode bidirectional avalanche diode that can be switched from the off-state to the on-state for either polarity of the applied voltage. This is just like a triac without a gate terminal, as shown in Figure(a). Its equivalent circuit is a pair of inverted four-layer diodes. Two schematic symbols are shown in Figure(b). Again the terminal designations are arbitrary since the diac, like triac, is also a bilateral device. The switching from off-state to on-state is achieved by simply exceeding the avalanche breakdown voltage in either direction.

 Diac Construction

A diac is p-n-p-n structured four-layer, two-terminal semiconductor device, as shown in Figure(a). MT2 and MT1 are the two main terminals of the device. From the diagram, a diac unlike a diode resembles a bipolar junction transistor (BJT) but with the following exceptions.

(i) there is no terminal attached to the middle layer (ii) the three regions are nearly identical in size

(iii) the doping level at the two end p-layers is the same so that the device gives symmetrical switching characteristics for either polarity of the applied voltage.

(iii) the doping level at the two end p-layers is the same so that the device gives symmetries switching characteristics for either polarity of the applied voltage.

Diac is a five-layer four junction device.  The word diac can be split into DI and AC. Dl stands for two electrodes namely MT1 and MT2. AC indicates its ability to conduct in both directions. From the structure, the equivalent circuit can be drawn with two PNPN devices connected back to back. When MT2 is made positive with respect to MT1, device 1 is forward biased. When MT1 is made positive with respect to MT2, device 2 is forward biased. 

diac characterstics


Ques.9. The triple frequency of a six-phase half-wave rectifier for 220 V, 60 Hz input will be

  1. 2160 Hz
  2. 720 Hz
  3. 360 Hz
  4. 60 Hz

Answer.3. 360 Hz


Six phases half-wave rectifier or 3 Phase Full Wave Rectifier


In three-phase full-wave rectifier, six diodes are used. It is also called 6-diode half-wave rectifier. In this, each diode conducts for 1/6th part of the AC cycle. The three-phase full-wave circuit is often used instead of the three-phase halfwave when less ripple is required and the static magnetization of the transformer core is to be avoided.

The six-phase voltages can be obtained in the secondary by using a center-tapped arrangement on a Star-connected three-phase winding and the vector diagram of six-phase voltages is shown in Fig. There are six diodes in a six-phase rectifier. When a particular phase voltage is higher than other phases, diodes on the particular phase conducts. This rectifier circuit is also called a three-phase midpoint six-pulse rectifier.

3 phase rectifier

Each diode conducts for π/3 or 60° duration. Current flows through one diode at a time. Therefore, the average current is low but the ratio between maximum current to the average current in the diodes is high. Hence the utilization of transformer secondary is poor. The dc currents in the secondary of the six-phase Star rectifier can be canceled in the secondary windings and core saturation is not encountered.

vector diagram six phase

The ripple frequency of a six-phase half-wave rectifier is a six-time supply frequency.

Since the Supply frequency is 60 Hz, therefore, the ripple frequency is 6 × 60 Hz = 360 Hz



Ques.10. The minimum duration of the pulse in a pulse triggering system for thyristors should be at

  1. 10 μs
  2. 10 ms
  3. 30 ms
  4. 1 sec

Answer.1. 10 µs


Thyristors and triacs can be triggered by a single pulse, a train of pulses, or a steady d.c voltage on the gate. Pulse triggering is most frequently used; single-pulse triggering is used in specific applications and low-cost systems, and d.c. triggering only in cases of difficulty in reaching latching current. A thyristor or triac with the anode positive with respect to the cathode, and with adequate gate drive, will turn on within 10μs. Conduction will continue irrespective of load current for as long as the gate drive is present. Conduction will continue after the gate drive ceases only if the load current has reached the latching level.

Pulse triggering is preferred to d.c. triggering for general use because the gate dissipation is lower and the trigger system is simpler. Both pulse and d.c. trigger systems must be synchronized to the mains supply, and both use a pulse generator operating through a variable delay to provide control over the trigger angle required for phase control. Whereas in a pulse trigger system the output from the pulse generator can be passed to the thyristor gate through an isolating transformer, a d.c. trigger system requires either rectification of the pulse transformer output or gating of a further supply by the pulse transformer to provide the d.c. gate drive. 

A simple trigger circuit for thyristors and triacs can be constructed with a diac. The diac is a three-layer device and therefore, strictly speaking, not a thyristor. Its function, however, is as a trigger device for thyristors, and it is generally considered with these devices.

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