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Past Questions
The maximum torque of D.C. motor is limited by
A)
Commutation
B)
Heating
C)
Speed
D)
Armature current
In a D.C. motor, torque is proportional to armature current. In principle, the torque can be increased by increasing current, but in practice the maximum torque is limited by commutation. If the armature current becomes too high, sparking at the commutator and brushes worsens, causing poor commutation and possible damage. Heating also limits operation, but the standard limiting factor for maximum torque is commutation. Hence, option (a) is correct.
Thermal runaway of a thyristor occurs because
A)
Positive resistance coefficient of the junction
B)
Negative resistance coefficient of the junction
C)
If the latching current is more
D)
If the thyristor is loaded with wider current pulses
Thermal runaway in a thyristor happens when an increase in temperature causes the junction current to increase further, producing more heat and creating a self-accelerating effect. This is associated with the negative temperature coefficient (negative resistance coefficient) of the junction. Therefore, option (b) is correct. Option (a) is wrong because a positive coefficient would tend to stabilize the device. Options (c) and (d) may affect operation, but they are not the fundamental cause of thermal runaway.
In a UST, intrinsic stand-off ratio n is typically
A)
0.2
B)
0.4
C)
0.7
D)
0.99
In a UJT (written here as UST in the question), the intrinsic stand-off ratio \( \eta \) is typically around 0.5 to 0.8, and its common nominal value is about 0.7. This ratio is defined as \( \eta = \frac{R_{B1}}{R_{B1}+R_{B2}} \). Hence, the correct option is 0.7. The other values are not typical intrinsic stand-off ratios for a UJT.
If antenna directivity and antenna gain are equal, then antenna efficiency is %,
A)
20
B)
50
C)
75
D)
100
Antenna gain is given by G = ηD, where η is antenna efficiency and D is directivity. If gain and directivity are equal, then η = 1. Therefore, antenna efficiency = 100%. Option (d) is correct. The other options correspond to efficiencies less than 100%, which would make gain lower than directivity.
In a single phase full converter, if α and β are firing and excitation angles respectively, then the load current is
A)
Discontinuous if (β-α) < π
B)
Discontinuous if (β-α) > π
C)
Discontinuous if (β-α) = π
D)
Discontinuous if (β-α) < π/2
In a single-phase full converter, the load current becomes continuous only when the conduction interval is long enough for the current to persist until the next triggering. The current continuity depends on the relation between the firing angle α and the extinction (excitation) angle β. If the conduction interval (β − α) is less than π, the current falls to zero before the next cycle completes, so the load current is discontinuous. Hence, option (a) is correct. Options (b), (c), and (d) do not represent the general condition for discontinuous current.
What is the total reactance of a series RLC circuit at resonance?
A)
Equal to XL
B)
Equal to XC
C)
Equal to R
D)
Zero
In a series RLC circuit at resonance, the inductive reactance and capacitive reactance are equal in magnitude: XL = XC. Since they are opposite in nature, they cancel each other out. Therefore, the total reactance of the circuit becomes zero, and the circuit behaves as a purely resistive circuit. Hence, the correct option is (d) Zero.
___ wave remains unaffected during day and night
A)
Ground
B)
Tropospheric
C)
Sky
D)
None of these
Ground waves travel along the surface of the Earth and are not dependent on ionospheric reflection, so they remain essentially unaffected during day and night. Sky waves depend on the ionosphere, which changes between day and night, and tropospheric waves are influenced by atmospheric conditions. Therefore, the correct answer is Ground.
Superposition theorem can be applied only to circuits having ;
A)
Resistive elements
B)
Passive elements
C)
Non-linear elements
D)
Linear bilateral elements
The superposition theorem is valid only for linear circuits, because it relies on proportionality and additivity of responses. It is commonly applied to networks made of linear bilateral elements such as resistors, inductors, and capacitors. It cannot be applied directly to non-linear elements, since their response is not linear. Hence, the correct choice is linear bilateral elements.
The on-state voltage drop across the IGBT is ;
A)
Less than that across the MOSFET
B)
Greater than that across the MOSFET
C)
Equal to that of MOSFET
D)
None of these
An IGBT combines MOS gate control with bipolar conduction. In the ON state, an IGBT has a conduction characteristic similar to a transistor, giving a relatively fixed voltage drop (typically around 2 V or more). A power MOSFET, on the other hand, conducts through its channel resistance RDS(on), so its voltage drop is usually lower at moderate currents. Therefore, the on-state voltage drop across an IGBT is generally greater than that across a MOSFET. Hence option (b) is correct. Options (a) and (c) are incorrect, and therefore (d) is also incorrect.
A SCR has half cycle surge current rating of 3000A for 50Hz supply. One cycle surge current rating will be
A)
2121A
B)
3000A
C)
4242A
D)
6000A
For SCR surge current rating, the permissible current is inversely proportional to the square root of time for the same I²t capability. If the half-cycle rating is 3000 A, then for one full cycle the current reduces by a factor of \u221a2. So, one-cycle surge current rating = 3000 \u00d7 \u221a2 = 4242 A (approximately). Hence option (c) is correct.
When microwave propagate around the curvature of the Earth for long distance, it is called
A)
Skip distance
B)
Ducting
C)
Troposcatter
D)
Faraday effect
Microwaves can travel beyond the horizon over long distances when they are trapped and guided by layers of the atmosphere due to refraction. This phenomenon is called ducting.
Why others are incorrect:
- Skip distance: used mainly in ionospheric propagation of HF waves, not microwave propagation around Earth’s curvature.
- Troposcatter: microwave transmission by scattering from irregularities in the troposphere, but not specifically propagation around Earth’s curvature in a guided path.
- Faraday effect: rotation of polarization in a magnetic field, unrelated to long-distance microwave propagation.
The radiation pattern of a parabolic antenna is
A)
A figure of eight
B)
Highly directional
C)
Omni-directional
D)
None of these
A parabolic antenna reflects electromagnetic waves from its parabolic dish so that the energy is concentrated into a narrow beam. This gives it a highly directional radiation pattern with high gain. A figure of eight pattern is typical of dipole antennas, while omni-directional antennas radiate in all horizontal directions.
The Safe Operating Area (SOA) of IGBT is better than that of a power transistor because
A)
It is a majority carrier device
B)
It is a minority carrier device
C)
Second breakdown is absent due to its flat temperature coefficient of temperature
D)
None of these
An IGBT has a much better Safe Operating Area (SOA) than a conventional power transistor because it does not suffer from second breakdown in the same way. This is mainly due to its positive/flat temperature coefficient, which helps current sharing and prevents localized hot spots. Option (a) is wrong because an IGBT is not a majority carrier device alone; it has MOS-gated control with bipolar conduction. Option (b) is not the reason for better SOA. Therefore, the correct answer is (c).
If the pole flux of D.C. motor approaches zero, its speed will
A)
Approach zero
B)
Approach infinity
C)
No change due to corresponding change is back emf.
D)
Approach a stable value somewhere between zero and infinity
For a D.C. motor, speed is approximately given by N ∝ (V − IaRa) / Φ, where Φ is the pole flux. If the pole flux approaches zero, the denominator becomes very small, so the speed rises sharply and tends to become very large. Hence the motor speed approaches infinity in theory. In practice, the motor would be damaged before reaching such a condition. So option (b) is correct. Options (a), (c), and (d) are incorrect because reducing flux does not make speed zero or unchanged; it increases speed.
In performing the short circuit test of a transformer
A)
High voltage side is usually short circuited
B)
Low voltage side is usually short circuited
C)
Any side is short circuited with preference
D)
None of these
In the short-circuit test of a transformer, the low-voltage (LV) side is usually short-circuited and a reduced voltage is applied to the high-voltage side until rated current flows. This is done for safety and convenience: the LV winding generally has lower insulation requirements and can be safely shorted, while the applied test voltage remains low. Option (a) is incorrect because the high-voltage side is not usually short-circuited. Option (c) is incorrect because the choice is not arbitrary; the LV side is preferred. Option (d) is incorrect because option (b) is correct.
Pre-emphasis in FM systems involves
A)
Compression of the modulating signal
B)
Expansion of the modulating signals
C)
Amplification of lower frequency components of the modulating signals
D)
Amplification of higher frequency components of the modulating signals
In FM systems, pre-emphasis means boosting the higher-frequency components of the modulating signal before transmission. This improves the signal-to-noise ratio because higher frequencies are more affected by noise in FM. At the receiver, de-emphasis is used to restore the original balance. So, option (d) is correct. Options (a) and (b) refer to compression/expansion concepts used in companding, not pre-emphasis. Option (c) is incorrect because pre-emphasis does not boost lower frequencies.
Four people witnessed a mugging. Each gave a different description of the mugger. Which description is probably right?
A)
He was average height, thin, and middle-aged.
B)
He was tall, thin, and middle-aged.
C)
He was tall, thin, and young.
D)
He was tall, of average weight, and middle-aged.
In such aptitude questions, the most probable correct description is usually the one that appears most consistent and specific across differing witness accounts. Option (b) combines a clear physical build (thin), stature (tall), and age group (middle-aged), making it the most plausible reliable description. The other options either conflict more strongly with common witness variation or are less likely to be the best overall match.
120, 99, 80, 63, 48 ? ;
A)
35
B)
38
C)
39
D)
40
Look at the differences between consecutive terms: 120 → 99 = 21, 99 → 80 = 19, 80 → 63 = 17, 63 → 48 = 15. The differences are decreasing by 2 each time, so the next difference should be 13. Therefore, 48 − 13 = 35. But 35 is not in the sequence of given options? Let's check the pattern as products: 11×10+10=120, 10×9+9=99, 9×8+8=80, 8×7+7=63, 7×6+6=48. So the next term is 6×5+5 = 35. Hence the correct mathematical answer is 35, which corresponds to option (a).
Vincent has a paper route. Each morning, he delivers 37 newspapers to customers in his neighbourhood. It takes Vincent 50 minutes to deliver all the papers. If Vincent is sick or has other plans, his friend Thomas, who lives on the same street, will sometimes deliver the papers for him. Which statement must be true?
A)
Vincent and Thomas live in the same neighbourhood.
B)
It takes Thomas more than 50 minutes to deliver the papers.
C)
It is dark outside when Vincent begins his deliveries.
D)
Thomas would like to have his own paper route.
Thomas sometimes delivers the papers for Vincent, and the question says Thomas lives on the same street. This means they are in the same local area, so they must live in the same neighbourhood. The other options are not guaranteed by the information given: we do not know how long Thomas takes, whether it is dark when Vincent starts, or whether Thomas wants his own paper route.
The output of LVDT is in the form of :
A)
Pulse
B)
High frequency signals
C)
Rotary movement of core
D)
Linear displacement of core
An LVDT (Linear Variable Differential Transformer) is a transducer used to measure linear displacement. Its output voltage is proportional to the linear movement of the magnetic core. Therefore, the output is associated with linear displacement of the core. Option (a) pulse is incorrect because LVDT gives an AC voltage output, not pulses. Option (b) is incorrect because high-frequency signals describe the excitation, not the output form. Option (c) is incorrect because LVDT measures linear, not rotary, motion.
In case of a thyristor di/dt capability can be improved by
A)
Thermal triggering
B)
Voltage triggering
C)
dv/dt triggering
D)
Gate pulse triggering
A thyristor’s di/dt capability is improved by using a proper gate pulse trigger. A strong, fast-rising gate pulse turns on the device over a larger cathode area quickly, so current spreads more uniformly and localized current crowding is reduced. This improves the allowable rate of rise of anode current (di/dt).
Why others are incorrect:
- Thermal triggering: turn-on due to heat, not a method to improve di/dt capability.
- Voltage triggering: unwanted turn-on when breakover voltage is reached; it does not improve di/dt capability.
- dv/dt triggering: accidental turn-on due to rapid voltage rise across the thyristor; again not related to improving di/dt capability.
A single phase full bridge inverter can operate in load commutation mode in case load consists of
A)
RLC overdamped
B)
RLC underdamped
C)
RLC critically damped
D)
None of these
In a single-phase full-bridge inverter, load commutation occurs when the load current naturally goes to zero and reverses due to the resonant behavior of the load. This requires an RLC load with underdamped response, so that current oscillates and produces the necessary natural commutation. In an overdamped or critically damped RLC load, the current does not oscillate, so load commutation cannot occur effectively.
The turn-on time of an SCR with inductive load is 20μs. The pulse train frequency is 2.5kHz with a mark/space ratio of 1/10. Then the SCR will
A)
Turn on
B)
Not turn on
C)
Turn on if inductance is removed
D)
Turn-on if pulse frequency is increased to two times.
For an SCR to turn on, the gate pulse must remain applied for at least the turn-on time. Here, the turn-on time is 20 μs. The pulse train frequency is 2.5 kHz, so the period is 1/2500 = 400 μs. With mark/space ratio 1/10, the ON time (mark time) is 400 × 1/11 ≈ 36.4 μs if interpreted as mark:space = 1:10. Even then, the key issue is the inductive load, which slows current rise and can prevent the SCR from reaching latching current within the effective turn-on interval. Therefore, the SCR will not turn on reliably. Hence option (b) is correct.
Why others are wrong:
- (a) Turn on: not correct because the inductive load delays current rise.
- (c) Turn on if inductance is removed: this is a conditional statement, but the question asks what will happen with the given load.
- (d) Turn-on if pulse frequency is increased to two times: higher frequency alone does not guarantee turn-on; the pulse width matters more than frequency.
In a CRO time base signal is
A)
Square wave
B)
High frequency saw tooth wave
C)
Rectangular waveform
D)
High frequency sinusoidal wave
In a Cathode Ray Oscilloscope (CRO), the time-base generator produces a sweep signal that moves the electron beam horizontally at a constant rate. This sweep is a high-frequency sawtooth wave so that the beam moves steadily across the screen and then returns rapidly. A square wave or rectangular waveform is not suitable because it does not provide a linear sweep, and a sinusoidal wave would not give uniform horizontal motion.
The temperature coefficient of resistivity of an IGBT is
A)
Positive
B)
Negative
C)
Flat
D)
Exponential
An IGBT (Insulated Gate Bipolar Transistor) is a semiconductor power device whose conduction characteristics improve with temperature in a way similar to bipolar devices. Its on-state resistance/forward voltage behavior gives a **positive temperature coefficient of resistivity**. This means that as temperature increases, effective resistance also increases, helping current sharing in parallel operation. Hence, the correct option is **(a) Positive**. The other options are incorrect because the coefficient is not negative, flat, or exponential.