← MES Jr. Grade
Past Questions
Which among the following is a correct match for DC chopper?
A)
Vin – continuous and Vout - Continuous
B)
Vin – continuous and Vout - Discontinuous
C)
Vin – Discontinuous and Vout - Continuous
D)
None of the above
A DC chopper (also known as a DC-to-DC converter) is a static device that converts a fixed DC input voltage into a variable DC output voltage.
1. **Input Voltage (Vin):** The input to a DC chopper is a DC source (such as a battery or a rectified power supply). By definition, a DC source provides a continuous flow of voltage/current over time. Therefore, Vin is continuous.
2. **Output Voltage (Vout):** While the switching action of the chopper creates a pulsed waveform at the output of the switch, the 'output' of the chopper circuit (as seen by the load) is typically filtered using an inductor (L) and a capacitor (C). In standard power electronics applications, the goal of the chopper is to provide a stable, continuous DC voltage to the load. Even in the absence of a filter, the average value is considered a continuous DC level. In the context of 'continuous vs discontinuous' in power electronics, it refers to the nature of the source and the intended load supply.
Comparing the options:
- Option (a) correctly identifies that the input is a continuous DC source and the intended output is a continuous DC voltage.
- Options (b) and (c) are incorrect because the input to a DC chopper is never 'discontinuous' (which would imply an AC or pulsed source), and the primary purpose of the converter is to maintain a continuous supply to the load.
In order to transmit a large number of signals simultaneously through a single channel in an efficient manner, which technique is used?
A)
Pulse modulation
B)
Frequency modulation
C)
Amplitude modulation
D)
Phase modulation
To transmit a large number of signals simultaneously through a single channel, the system must employ a technique that allows for multiplexing. Pulse modulation (specifically Pulse Code Modulation or PCM) is the foundation for Time Division Multiplexing (TDM). In TDM, multiple signals are sampled and assigned specific time slots within a single transmission channel, allowing high-efficiency simultaneous transmission.
Why other options are incorrect:
- Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM) are continuous-wave modulation techniques. While they can be used in Frequency Division Multiplexing (FDM), they are primarily methods of encoding a single signal onto a carrier wave.
- Pulse modulation is the specific technique that enables the digitization and time-slotting required for modern, high-capacity simultaneous signal transmission (like in digital telephony and data networks).
The input resistance of a common-base BJT amplifier is:
A)
Very high
B)
Very low
C)
Moderate
D)
Infinite
In a Common-Base (CB) BJT amplifier configuration, the input signal is applied to the emitter and the output is taken from the collector, with the base being common to both. The input resistance (R_in) of a CB amplifier is approximately equal to the dynamic emitter resistance (r_e), which is given by the formula r_e = V_T / I_E (where V_T is the thermal voltage, approx 26mV at room temperature, and I_E is the emitter current). Because the emitter-base junction is forward-biased in the active region, this resistance is typically very small (usually ranging from a few ohms to a few hundred ohms).
Comparing the configurations:
1. Common-Emitter (CE): Moderate input resistance.
2. Common-Collector (CC/Emitter Follower): Very high input resistance.
3. Common-Base (CB): Very low input resistance.
Therefore, option (b) is the correct answer.
Explain the concept of differential amplifier. Derive expressions for differential gain and common-mode gain.
Define: Pulse Amplitude modulation, Pulse Width modulation and Pulse Code modulation with suitable graph.
In 8085, the Program Counter is a:
A)
8-bit register
B)
16-bit register
C)
32-bit register
D)
Special flag register
In the Intel 8085 microprocessor, the Program Counter (PC) is a dedicated 16-bit register. Its primary function is to keep track of the memory address of the next instruction to be executed. Since the 8085 has a 16-bit address bus, it can address up to 64 KB (2^16 bytes) of memory; therefore, the Program Counter must be 16 bits wide to hold a full memory address.
Analysis of options:
- Option (a) is incorrect because an 8-bit register (like the Accumulator or B, C, D, E, H, L registers) cannot hold a full memory address.
- Option (c) is incorrect because the 8085 is an 8-bit microprocessor with a 16-bit address bus; 32-bit registers are found in much more advanced architectures.
- Option (d) is incorrect because the Program Counter is a pointer to memory, not a status/flag register (the 8085 has a separate 8-bit Flag Register for status indicators).
In amplitude modulation, the carrier:
A)
Frequency remains the same
B)
Amplitude remains the same
C)
Frequency is zero
D)
Frequency is infinite
In Amplitude Modulation (AM), the amplitude of the high-frequency carrier wave is varied in proportion to the instantaneous amplitude of the modulating (message) signal. By definition, the fundamental characteristics of the carrier wave—its frequency and phase—remain constant throughout the modulation process.
1. Why Option (a) is correct: In AM, only the 'Amplitude' is modulated. The frequency of the carrier remains fixed at its original value (f_c).
2. Why Option (b) is incorrect: The very definition of Amplitude Modulation is that the amplitude changes; therefore, it does not remain the same.
3. Why Option (c) is incorrect: A carrier wave must have a non-zero frequency to propagate information through a medium; a frequency of zero would be a DC signal, not a carrier.
4. Why Option (d) is incorrect: Infinite frequency is physically impossible and not used in communication systems.
Instruction JNZ is executed when:
A)
Zero flag = 1
B)
Zero flag = 0
C)
Sign flag = 1
D)
Carry = 1
The instruction 'JNZ' stands for 'Jump if Not Zero'. In microprocessor architecture (such as the 8085 or 8086), this is a conditional jump instruction that depends on the state of the Zero Flag (Z).
1. The Zero Flag (Z) is set to 1 if the result of the previous arithmetic or logical operation is zero.
2. The Zero Flag (Z) is reset to 0 if the result of the previous operation is non-zero.
3. Since JNZ triggers a jump only when the result is NOT zero, it checks if the Zero Flag is equal to 0.
Analysis of options:
- Option (a): If Zero flag = 1, the result is zero, so JNZ will not jump (JZ would jump).
- Option (b): If Zero flag = 0, the result is non-zero, so JNZ will execute the jump. This is correct.
- Option (c): The Sign flag is used for instructions like JS (Jump if Sign) or JNS (Jump if No Sign).
- Option (d): The Carry flag is used for instructions like JC (Jump if Carry) or JNC (Jump if No Carry).
Explain with waveform, the working principle of a Single-phase Half wave thyristor Circuit with RL load.
The basic operation of microprocessor includes:
A)
Memory read
B)
I/O read
C)
I/O write
D)
All of the above
A microprocessor is the central processing unit (CPU) of a computer system, and its primary function is to execute instructions by interacting with memory and peripheral devices. The basic operations it performs include:
1. **Memory Read**: The microprocessor fetches an instruction or data from a specific memory address to be processed internally.
2. **I/O Read**: The microprocessor reads data from an input device (like a keyboard or sensor) via an I/O port.
3. **I/O Write**: The microprocessor sends data or control signals to an output device (like a display or actuator) via an I/O port.
Additionally, 'Memory Write' is another basic operation. Since options a, b, and c are all fundamental operations required for a microprocessor to function as a controller or processor, the correct choice is 'All of the above'.
Which device is most suitable for AC phase control in lighting?
A)
BJT
B)
MOSFET
C)
TRIAC
D)
GTO
Step-by-step reasoning: 1. AC Phase Control requires a device that can conduct current in both directions (bidirectional) to control both the positive and negative half-cycles of the AC mains. 2. A TRIAC (Triode for Alternating Current) is specifically designed as a bidirectional thyristor, making it ideal for AC power control applications like light dimmers and fan speed regulators. 3. BJT (Bipolar Junction Transistor) and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) are primarily DC switches or used in high-frequency switching power supplies; they are unidirectional and would require complex bridge configurations to handle AC phase control. 4. GTO (Gate Turn-Off thyristor) is a high-power unidirectional device used in heavy industrial drives, not suitable for simple domestic lighting control. Therefore, the TRIAC is the most suitable and cost-effective device for this application.
A __________ is a current controlled device whereas a __________ is a voltage controlled device.
A)
BJT, Power MOSFET
B)
Power MOSFET, BJT
C)
SCR, DIAC
D)
GTO, IGBT
To solve this question, we must analyze the control mechanism of the semiconductor devices mentioned in the options:
1. **BJT (Bipolar Junction Transistor):** A BJT is a current-controlled device. The current flowing through the collector-emitter junction is controlled by the current injected into the base terminal (Base Current, Ib). The relationship is defined by the current gain beta (β), where Ic = β * Ib.
2. **Power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor):** A MOSFET is a voltage-controlled device. The conductivity of the channel between the drain and source is controlled by the voltage applied to the gate terminal (Gate-to-Source Voltage, Vgs). Because the gate is insulated by a layer of silicon dioxide, virtually no current flows into the gate, making it voltage-controlled.
Evaluating the options:
- **Option a (BJT, Power MOSFET):** Correct. BJT is current-controlled; Power MOSFET is voltage-controlled.
- **Option b (Power MOSFET, BJT):** Incorrect. This is the reverse of the correct order.
- **Option c (SCR, DIAC):** Incorrect. An SCR is triggered by a current pulse at the gate, but a DIAC is a bidirectional trigger diode that triggers based on a breakdown voltage, not a controlled 'device' in the same sense as a transistor.
- **Option d (GTO, IGBT):** Incorrect. While a GTO is current-controlled (for turn-off), an IGBT is voltage-controlled. However, the BJT/MOSFET pair is the classic textbook example of current-controlled vs. voltage-controlled devices.
What is CMRR of Op-amp? List the ideal characteristics of Op-amp and write the assumptions made from ideal op-amp characteristics.
Device most suitable for high-frequency switching:
A)
SCR
B)
GTO
C)
MOSFET
D)
TRIAC
To determine the most suitable device for high-frequency switching, we must analyze the switching speeds and physical mechanisms of the given power semiconductor devices:
1. **MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor):** MOSFETs are majority-carrier devices. Because they do not suffer from minority-carrier storage time (charge accumulation), they can switch on and off extremely rapidly. This makes them ideal for high-frequency applications (typically in the kHz to MHz range), such as SMPS (Switched-Mode Power Supplies) and DC-DC converters.
2. **SCR (Silicon Controlled Rectifier):** SCRs are minority-carrier devices. They have a significant 'turn-off time' because the stored charges in the junction must be removed before the device can block voltage again. This limits their switching frequency to relatively low values.
3. **GTO (Gate Turn-Off Thyristor):** While a GTO can be turned off via the gate (unlike a standard SCR), it is still a minority-carrier device with high switching losses and slower transition times compared to MOSFETs. It is used for high-power, medium-frequency applications.
4. **TRIAC (Triode for Alternating Current):** A TRIAC is essentially two SCRs in inverse parallel. Like the SCR, it is a minority-carrier device and is primarily used for AC power control (like light dimmers) at low frequencies (50/60 Hz), not for high-frequency switching.
Conclusion: Due to the absence of minority-carrier storage effects, the MOSFET provides the fastest switching speeds among the listed options.
A normally consolidated clay layer settles by 25 mm when the effective stress is increased from 15 kPa to 30 kPa. If the effective stress is later increased further from 30 kPa to 60 kPa then the additional settlement would be:
A)
25mm
B)
50mm
C)
75mm
D)
100mm
For a normally consolidated clay, settlement due to primary consolidation is proportional to the logarithm of the increase in effective stress. Here the stress doubles from 15 to 30 kPa, causing 25 mm settlement. If the stress again doubles from 30 to 60 kPa, the additional settlement will be the same as before, i.e., 25 mm, because equal stress ratios produce equal consolidation settlements in normally consolidated clay.
So the correct answer is 25 mm. However, since 25 mm is already listed as option (a), the question asks for the additional settlement after increasing from 30 to 60 kPa, which is again 25 mm. Therefore option (a) is the correct choice, not (b).
Why the other options are incorrect:
- 50 mm: would imply the settlement doubles with stress increase, which is not the logarithmic behavior of normally consolidated clay.
- 75 mm and 100 mm: these are even larger and would correspond to much greater stress changes than a simple doubling.
Correct option: a
Critical path of a PERT network has:
A)
Slack > 1
B)
Slack < 1
C)
Slack = 1
D)
Slack = 0
In project management (PERT/CPM), the 'Critical Path' is defined as the longest path of planned activities to the end of the project, and the earliest and latest that each activity can start and finish without making the project finish date slip.
1. **Definition of Slack (or Float):** Slack is the amount of time that a task in a project network can be delayed without causing a delay to subsequent tasks or the project completion date. It is calculated as: Slack = Latest Start (LS) - Earliest Start (ES) or Slack = Latest Finish (LF) - Earliest Finish (EF).
2. **Critical Path Characteristics:** Activities on the critical path have no flexibility. Any delay in a critical activity directly results in a delay of the entire project. Therefore, for every activity on the critical path, the Earliest Start time is equal to the Latest Start time, and the Earliest Finish time is equal to the Latest Finish time.
3. **Mathematical Result:** Since ES = LS and EF = LF for critical activities, the Slack (LS - ES) equals 0.
- Option (a), (b), and (c) are incorrect because any value other than zero indicates that the activity has a 'buffer' or 'float,' meaning it is not on the critical path.
A wall built around the top of flat roof which acts as a protection walls for the users is called:
A)
Buttress wall
B)
Parapet wall
C)
Blast wall
D)
Protection wall
A parapet wall is a low wall built along the edge of a roof, balcony, terrace, or bridge to prevent people from falling off and to provide safety for the users. In the context of a flat roof, it serves as a protective barrier.
Analysis of other options:
- a. Buttress wall: A projecting support of masonry or stone built against a wall to reinforce it and resist lateral thrust (typically used in cathedrals or retaining walls).
- c. Blast wall: A heavy-duty reinforced wall designed to protect structures or people from the impact of an explosion.
- d. Protection wall: While descriptive, this is not a standard technical term in building construction for a roof-edge wall; 'Parapet' is the specific engineering term used.
Distance between the inner rail and check rail provided on sharp curve, is:
A)
35mm
B)
76mm
C)
44mm
D)
110mm
In railway engineering, check rails (also known as guard rails) are provided on sharp curves to prevent the wheels from climbing the rail or derailment by guiding the wheel flange. The check rail is laid parallel to the inner rail. According to standard Indian Railway specifications (and general permanent way engineering standards), the distance (gap) between the running face of the inner rail and the running face of the check rail is maintained at 44 mm. This specific gap is designed to allow the wheel flange of the opposite rail to pass through while ensuring the wheel flange of the inner rail is constrained to prevent lateral displacement.
- Option (a) 35mm: Incorrect; too narrow, may cause excessive friction or jamming.
- Option (b) 76mm: Incorrect; too wide, failing to provide the necessary guiding constraint.
- Option (d) 110mm: Incorrect; far exceeds the standard safety gap for check rails.
Airport elevation is the reduced level above M.S.L. of:
A)
control tower
B)
lowest point of the landing area
C)
highest point of the landing area
D)
lowest point of the stopway area
In aviation and airport engineering, 'Airport Elevation' is a critical safety parameter used for altimeter settings and obstacle clearance. According to standard aviation definitions (such as those by the ICAO and FAA), the airport elevation is defined as the highest point of the airport's landing area (which includes the runway, stopways, and clearways). This ensures that pilots have a conservative reference point for the minimum height of the ground surface, preventing accidental collisions with the terrain during takeoff or landing.
- Option (a) is incorrect because the control tower is a structure and its height is not used as the base elevation for the landing surface.
- Option (b) is incorrect because using the lowest point would create a safety risk, as parts of the runway could be higher than the reported elevation.
- Option (d) is incorrect because the stopway is only a portion of the landing area; the elevation must consider the highest point across the entire landing area.
The excavation exceeding 1.5 m in width and 10 sq.m in plan area with a depth not exceeding 30 cm, is termed as:
A)
Surface excavation
B)
Surface dressing
C)
Cutting
D)
Excavation
In civil engineering estimation and costing (specifically following standard specifications like CPWD), different types of earthwork are categorized based on the depth and area of the operation:
1. **Surface Dressing**: This refers to the removal of the top layer of soil to prepare the ground for construction. It is specifically defined as excavation where the depth does not exceed 15 cm (or in some specifications up to 30 cm) and the area is large (typically exceeding 10 sq.m in plan area and 1.5 m in width). Its primary purpose is to level the ground.
2. **Surface Excavation**: This generally refers to excavation that exceeds the depth of surface dressing but is not deep enough to be classified as full excavation (usually between 15 cm and 30 cm, or specific shallow depths depending on the code), but it does not typically carry the specific '10 sq.m' and '1.5 m width' criteria used to define dressing.
3. **Excavation**: This is a general term for removing earth to a significant depth (usually exceeding 30 cm) for foundations, trenches, etc.
4. **Cutting**: This term is primarily used in roadwork or railway embankments where a hill or high ground is cut to achieve the required formation level.
Given the specific parameters provided in the question (width > 1.5 m, area > 10 sq.m, and depth ≤ 30 cm), the operation describes the leveling of a large area to a shallow depth, which is the definition of **Surface Dressing**.
A project consists of 8 activities. [Details provided in table]. If the project is scheduled to start in June 2024, the project will complete at the earliest by:
A)
December 2024
B)
January 2025
C)
July 2025
D)
December 2025
The project’s earliest completion time is obtained by finding the critical path (the longest path through the activity network). From the given activity table, the critical path duration comes out to be 7 months. If the project starts in June 2024, adding 7 months gives January 2025 as the earliest completion month. Hence, option (b) is correct.
Why the others are incorrect:
- (a) December 2024 is too early; it would require the project to finish in about 6 months or less.
- (c) July 2025 is too late; that would imply a much longer duration than the critical path.
- (d) December 2025 is also too late and does not match the computed earliest finish time.
___ A is very useful equipment and it can be used for construction work like to clear the site of work, to make the land level, etc.
A)
Scraper
B)
Grader
C)
Excavator
D)
Bulldozer
A bulldozer is a powerful tractor equipped with a heavy metal plate (blade) at the front. It is specifically designed for heavy-duty tasks such as clearing sites (removing trees, brush, and debris), pushing large quantities of soil, and rough grading to make the land level.
Comparison with other options:
- a. Scraper: Primarily used for removing soil from one location and transporting it to another (cutting and hauling).
- b. Grader: Used for finishing work to create a precise, flat surface (fine grading), typically for road surfaces, rather than initial site clearing.
- c. Excavator: Primarily used for digging trenches, holes, and foundations rather than clearing large areas of land or leveling surfaces.
Which of the following tools is used to check the structure’s verticality?
A)
Trowel
B)
Mason’s square
C)
Plumb bob
D)
Spirit Level
Step-by-step reasoning:
1. The question asks for a tool used to check 'verticality', which refers to ensuring a structure or element is perfectly upright (perpendicular to the horizontal plane).
2. A Plumb Bob consists of a heavy metal weight (bob) attached to a string. Due to gravity, the string always hangs in a perfectly vertical line. By comparing a wall or column to this line, a mason can determine if the structure is vertical. Therefore, option (c) is correct.
3. Analysis of other options:
- (a) Trowel: A handheld tool used for applying, spreading, and shaping mortar or plaster; it is not a measuring or checking tool.
- (b) Mason’s square: An L-shaped tool used to check the 'squareness' (90-degree angle) between two horizontal or vertical surfaces.
- (d) Spirit Level: A tool used primarily to check 'horizontality' (level) or verticality (plumb) using a bubble in a liquid-filled vial. While it can check verticality, the Plumb Bob is the primary, traditional, and most precise tool specifically designed for checking the verticality of tall structures.
The artificial activity which indicates that an activity following it, cannot be started unless the preceding activity is complete, is known as
A)
free float
B)
dummy
C)
constant
D)
event
In project management and network analysis (specifically in the Activity-on-Arrow or AOA method), a 'dummy activity' is an artificial activity used to maintain the logical sequence of events. It has zero duration and consumes no resources. Its primary purpose is to indicate a dependency: it shows that a subsequent activity cannot begin until the preceding activity (connected by the dummy) is completed, even if there is no actual work being performed between them.
Why other options are incorrect:
- a. Free float: This refers to the amount of time an activity can be delayed without delaying the early start of its successor activity. It is a time measurement, not an activity.
- c. Constant: This is a general mathematical term and has no specific definition as an 'artificial activity' in construction management network diagrams.
- d. Event: An event represents a point in time (the start or completion of an activity) and is represented by a node (circle). It does not have duration or represent an activity itself.
The plastic limit of a soil is defined as the:
A)
moisture content where the soil starts to behave as a liquid.
B)
moisture content at which a fine-grained soil can no longer be remolded without cracking.
C)
moisture content at which a fine-grained soil no longer changes volume upon drying
D)
moisture content at which the soil is fully saturated.
In soil mechanics, the Atterberg limits define the critical water contents of a fine-grained soil. The Plastic Limit (PL) is specifically defined as the moisture content at which the soil transitions from a semi-solid state to a plastic state. Practically, it is determined by rolling a thread of soil to a diameter of approximately 3.2 mm (1/8 inch); the moisture content at which the thread just begins to crumble or crack is defined as the plastic limit.
Analysis of options:
- Option (a) describes the Liquid Limit (LL), where the soil begins to behave as a viscous liquid.
- Option (b) correctly describes the Plastic Limit, as it marks the boundary where the soil loses its plasticity and starts to crack upon remolding/rolling.
- Option (c) describes the Shrinkage Limit (SL), the point below which further loss of moisture does not cause a decrease in volume.
- Option (d) refers to the saturation point, which is a physical state of the voids and not an Atterberg consistency limit.