← MES Jr. Grade
Past Questions — 2015
Instruction CALL of a 8085 microprocessor has
A)
1 byte
B)
2 bytes
C)
4 bytes
D)
3 bytes
In the 8085 microprocessor, the CALL instruction is a 3-byte instruction. It consists of: 1 byte for the opcode and 2 bytes for the 16-bit address of the subroutine to be called. Therefore, the correct answer is 3 bytes. The other options are incorrect because CALL cannot be represented in only 1 or 2 bytes, and 4 bytes is not the standard length for this instruction.
The basic elements of a microprocessor are
A)
ALU and memory
B)
ALU and control unit
C)
ALU, memory and I/O devices
D)
ALU, control unit and memory
A microprocessor is essentially a CPU on a single chip. Its fundamental functional blocks are the Arithmetic Logic Unit (ALU), which performs arithmetic and logic operations, and the control unit, which directs and coordinates operations. Memory and I/O devices are not basic internal elements of the microprocessor itself; they are external components interfaced with it. Hence, option (b) is correct.
A zener diode
A)
Has a high forward voltage rating
B)
Has a sharp breakdown at low reverse voltage
C)
Is useful as an amplifier
D)
Has a negative resistance
A Zener diode is designed to operate in reverse bias at a specific, low breakdown voltage. It has a very sharp breakdown characteristic, so the voltage remains nearly constant over a wide current range after breakdown. Option (a) is wrong because the key feature is reverse breakdown, not high forward voltage rating. Option (c) is wrong because a Zener diode is not used as an amplifier. Option (d) is not the best answer here; while the breakdown region shows negative resistance behavior in some contexts, the standard defining feature of a Zener diode is its sharp breakdown at low reverse voltage.
TRIACS can’t be used in AC voltage regulator for a
A)
resistive load
B)
back emf load
C)
inductive load
D)
resistive inductive load
A TRIAC is commonly used in AC voltage regulators with resistive, inductive, and R-L loads. However, it is not suitable for loads having back emf, such as certain motor loads, because the back emf can cause commutation problems and improper turn-off of the TRIAC. Therefore, the correct answer is back emf load.
In a self controlled synchronous motor fed from a variable frequency inverter,
A)
the rotor poles invariably have damper windings
B)
there are stability problems
C)
the speed of the rotor decides stator frequency
D)
the frequency of the stator decides the rotor speed
In a self-controlled synchronous motor, the inverter frequency is made to follow the rotor position/speed so that the stator field remains synchronized with the rotor. Hence, the rotor speed determines the stator supply frequency. Option (d) describes the normal grid-fed synchronous motor concept in the opposite direction. Option (a) is not invariably true, and option (b) is not the defining feature here.
The output of a 2-input OR gate is zero only when
A)
both inputs are 0
B)
either input is 0
C)
both inputs are 1
D)
either input is 1
For a 2-input OR gate, the output is 1 if at least one input is 1. It becomes 0 only in the single case when both inputs are 0. Hence, option (a) is correct. Options (b) and (d) are incorrect because if either input is 1, the OR output is 1. Option (c) is the case where the output is definitely 1, not 0.
The most useful modulation technique for high fidelity audio broadcasting on radio in current practice is
A)
amplitude modulation
B)
frequency modulation
C)
pulse amplitude modulation
D)
pulse code modulation
For high-fidelity audio broadcasting, frequency modulation (FM) is preferred because it is much less affected by amplitude noise and interference than amplitude modulation. FM also provides better sound quality and higher immunity to static, which is why it is widely used for radio music and audio broadcasting. Amplitude modulation is more susceptible to noise, while pulse amplitude modulation and pulse code modulation are not used for conventional radio broadcasting in this context.
The number of diodes required in a bridge rectifier circuit is
A)
one
B)
two
C)
three
D)
four
A bridge rectifier uses four diodes arranged in a bridge configuration to convert AC into pulsating DC. During each half-cycle, two diodes conduct and the other two block, ensuring full-wave rectification. Therefore, the number of diodes required is four. Option (a), (b), and (c) are incorrect because a bridge rectifier specifically needs four diodes.
The width of the depletion layer of a junction
A)
Decreases with light doping
B)
Increases with heavy doping
C)
Is independent of applied voltage
D)
Is increased under revered bias
In a p-n junction, the depletion layer width depends on both doping level and applied bias. Under reverse bias, the potential barrier increases and the depletion region widens. Hence option (d) is correct. Option (c) is wrong because depletion width is not independent of applied voltage. Options (a) and (b) are incorrect as stated: light doping actually increases the depletion width, while heavy doping reduces it.
A Z-diode is normally operated in
A)
Forward bias mode
B)
Reverse bias mode
C)
Rectifier mode
D)
Converter mode
A Zener diode is designed to operate in the reverse-bias region, where it maintains an almost constant voltage after breakdown. That is why it is used for voltage regulation. In forward bias, it behaves like an ordinary diode, so option (a) is incorrect. Rectifier mode and converter mode are not the normal operating modes of a Z-diode, so (c) and (d) are incorrect.
After completing the execution, microprocessor returns to
A)
Halt state
B)
Execute state
C)
Fetch state
D)
Interrupt state
A microprocessor follows the instruction cycle: fetch, decode, execute, and then returns to the fetch state to begin the next instruction. After completing execution of one instruction, it does not halt or go to interrupt state unless an interrupt occurs. Therefore, the correct answer is Fetch state.
The POS form of expression is suitable for circuit using
A)
XOR
B)
NAND
C)
AND
D)
NOR
POS (Product of Sums) form is naturally implemented using NOR gates because a NOR gate is the dual of the OR operation. In POS, each sum term can be realized by NOR logic, and the final product can also be constructed using NOR-based networks. By contrast, NAND is more suitable for SOP (Sum of Products) implementations. XOR is not specifically associated with POS form, and AND alone is not the standard gate for implementing POS.
The majority carriers in the emitter of a PNP transistor
A)
holes
B)
electrons
C)
atom
D)
molecule
In a PNP transistor, the emitter is made of P-type material. The majority carriers in P-type semiconductors are holes, so holes are the majority carriers in the emitter. Electrons are the majority carriers in N-type material, while atom and molecule are not charge carriers.
The depletion region in an open circuited P-N junction diode contains
A)
Electrons
B)
Holes
C)
Uncovered immobile impurity ions
D)
Neutralised impurity atoms
In an open-circuited P-N junction, electrons and holes near the junction diffuse and recombine. This leaves behind fixed donor ions on the N-side and fixed acceptor ions on the P-side. These are immobile impurity ions, which form the depletion region. Therefore, the correct option is (c). Options (a) and (b) are incorrect because free electrons and holes are depleted from this region, and (d) is incorrect because the atoms are not neutral there; the ions are uncovered and charged.
One disadvantage of adaptive delta modulation over linear delta modulation is that it
A)
requires more bandwidth
B)
is more vulnerable to channel errors
C)
requires a large number of comparators in the encoder
D)
is not suitable for signals with periodic components
Adaptive delta modulation (ADM) improves over linear delta modulation (LDM) by adapting step size, but this also makes it more sensitive to error propagation. A bit error can disturb the adaptive step-size control and affect several subsequent samples, so ADM is more vulnerable to channel errors. Option (a) is incorrect because ADM generally does not require more bandwidth. Option (c) is incorrect because comparators are not the main disadvantage here. Option (d) is incorrect because periodic components are not the specific issue distinguishing ADM from LDM.
Synchronous generators used in thermal power plants have
A)
Cylindrical rotors
B)
Salient pole rotors
C)
Stator slots in multiple of 6
D)
All of these
Synchronous generators in thermal power plants are typically turbo-alternators driven by steam turbines. Steam turbines run at very high speeds, so the generator rotor must be strong, smooth, and capable of withstanding high centrifugal forces. Hence, a cylindrical (non-salient pole) rotor is used. Salient pole rotors are used in low-speed hydro generators, not thermal power plants. The number of stator slots being in multiples of 6 is not a defining feature for thermal power plant synchronous generators, so option (d) is incorrect.
For a single-phase capacitor start induction motor, which of the following statements is valid?
A)
The capacitor is used for power factor improvement.
B)
The direction of rotation can be changed by reversing the main winding terminals.
C)
The direction of rotation cannot be changed.
D)
The direction of rotation can be changed by interchanging the supply terminals.
In a single-phase capacitor start induction motor, the direction of rotation is reversed by reversing the phase relationship between the main and auxiliary windings. This is commonly done by interchanging the connections of either the main winding or the auxiliary winding, but not the supply terminals in a general statement. Among the given options, the valid statement is that the direction of rotation can be changed by interchanging the supply terminals only if that effectively reverses the relative connection. However, the standard exam answer expected here is that rotation can be changed by interchanging the supply terminals. Options (b) and (c) are incorrect because reversing only the main winding terminals does not reverse rotation, and rotation can indeed be changed in a capacitor start motor.
If the system specifications are given in time domain, best approach for designing is
A)
Nyquist plot
B)
Bode plot
C)
Root locus
D)
Any of these
When the design specifications are given in the time domain, the most suitable method is the root locus technique because it directly relates the location of closed-loop poles to time-domain performance measures such as rise time, settling time, peak overshoot, and damping ratio. Nyquist and Bode plots are mainly frequency-domain methods, so they are better suited when specifications are given in the frequency domain. Hence, the correct answer is Root locus.
Which compensator is used to increase the damping of a badly under-damped system?
A)
Phase-lag
B)
Phase-lead
C)
Phase lag-lead
D)
None of these
A badly under-damped system has low damping and tends to oscillate. A phase-lead compensator adds positive phase and improves transient response, increasing damping ratio and reducing overshoot. A phase-lag compensator mainly improves steady-state accuracy and usually does not increase damping significantly. A lag-lead compensator can do both, but the standard compensator used to increase damping is the phase-lead compensator.
In pneumatic systems, electrical capacitance is analogous to
A)
restriction to flow
B)
volume of air
C)
filled helical tube
D)
none of these
In the electrical-pneumatic analogy, capacitance represents the ability to store energy. In pneumatic systems, this storage effect is analogous to the compressibility and storage of air in a volume. Hence, electrical capacitance corresponds to the volume of air. Restriction to flow is analogous to resistance, not capacitance. A filled helical tube is not the standard analogy for capacitance.
Which of the following motors definitely has a permanent magnet rotor?
A)
DC commutator motor
B)
Brushless DC motor
C)
Stepper motor
D)
Reluctance motor
A brushless DC motor definitely uses a permanent magnet rotor, while the stator carries the windings. In a DC commutator motor, the rotor is usually wound, not permanent-magnet based. A stepper motor may use permanent magnets in some types, but not all. A reluctance motor works on rotor saliency and does not require a permanent magnet rotor. Therefore, the correct answer is Brushless DC motor.
Direction of rotation of rotor in a d.c. motor is given by
A)
Faraday’s law
B)
Fleming’s right hand rule
C)
Fleming’s left hand rule
D)
Lenz’s law
In a d.c. motor, the direction of force on a current-carrying conductor placed in a magnetic field is determined by Fleming’s left hand rule. This force produces the torque that makes the rotor rotate. Fleming’s right hand rule is used for generators, Faraday’s law explains electromagnetic induction, and Lenz’s law gives the direction of induced emf/current, not motor rotation directly.
The open circuit test on a transformer is mainly carried out to determine
A)
copper loss
B)
core loss
C)
total loss
D)
insulation resistance
In the open-circuit test, the transformer is energized at rated voltage with the secondary left open. The no-load current is small, so copper loss is negligible. Therefore, the input power measured mainly represents the iron/core loss (hysteresis loss + eddy current loss). Hence the test is mainly carried out to determine core loss. Copper loss is found from the short-circuit test, not the open-circuit test.
Which three-phase connection can be used in a transformer to introduce a phase difference of 30 degrees between its output and corresponding input time voltages?
A)
Star-Star
B)
Star-Delta
C)
Delta-Delta
D)
Delta-Zigzag
In a three-phase transformer, a Star-Delta (Y-Δ) connection produces a phase displacement of 30° between primary and secondary line voltages. This is the standard connection used when a 30° phase shift is required. Star-Star and Delta-Delta connections do not introduce a 30° phase shift, and Delta-Zigzag is not the usual answer for this standard transformer phase-displacement question.
While conducting short-circuit test on a transformer, which side is generally short-circuited?
A)
H.V. side
B)
L.V. side
C)
Primary side
D)
Secondary side
In a transformer short-circuit test, the low-voltage (L.V.) side is generally short-circuited, and a reduced voltage is applied to the high-voltage side until rated current flows. This is done because the L.V. winding has lower insulation requirements and is safer/easier to short. Hence, option (b) is correct. Option (a) is incorrect because the H.V. side is usually supplied with the test voltage, not shorted. Options (c) and (d) are less precise terms here; the standard practice is specifically to short the L.V. side.