What is the maximum theoretical efficiency of a step-down chopper with duty cycle 0.5?
Answer: C
In an ideal chopper, theoretical efficiency is 100% as it transfers energy without dissipation. Practical efficiency is reduced by component losses
Q.142Easy
Which power semiconductor device is best suited for high-frequency switching applications above 100 kHz?
Answer: B
MOSFETs have switching frequencies up to several MHz, making them ideal for high-frequency applications. Thyristors are limited to lower frequencies (few kHz).
Q.143Easy
The holding current (Ih) specification of a thyristor is important to ensure:
Answer: B
Holding current is the minimum anode current required to keep the thyristor in ON state after the gate signal is removed. If anode current falls below Ih, the device turns OFF.
Q.144Easy
What is the purpose of a freewheeling diode in a converter circuit?
Answer: B
A freewheeling diode provides a discharge path for inductor current when the main switch opens, preventing voltage spikes and ensuring continuous current flow in inductive circuits.
Q.145Easy
What does the snubber circuit in a power converter primarily protect against?
Answer: B
Snubber circuits limit the rate of change of voltage (dv/dt) across semiconductor switches during switching transients, preventing false triggering and overvoltage damage.
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Q.146Easy
In a full-bridge DC-DC converter, when both upper switches conduct simultaneously, what occurs?
Answer: B
Simultaneous conduction of upper switches creates a direct short across the DC link, causing massive current spike and potential device failure. This is prevented using dead-time insertion.
Q.147Easy
In modern 2024-25 EV charging systems, what is the typical efficiency target for on-board DC-DC converters?
Answer: B
Modern automotive DC-DC converters target 90-95% efficiency using advanced topologies, SiC MOSFETs, and optimized gate drives to minimize losses in 12V auxiliary systems.
Q.148Easy
In a three-phase bridge rectifier, if the input AC voltage is 400V (line), what is the approximate average DC output voltage?
Answer: A
For a three-phase bridge rectifier: Vdc = (3√2/π) × Vline = 1.35 × 400 = 540V approximately.
Q.149Easy
Which of the following devices has the highest switching frequency capability among modern power semiconductors?
Answer: A
MOSFETs have switching frequencies up to several MHz, while IGBTs typically reach 20-50kHz, thyristors <10kHz, and GTOs ~5kHz.
Q.150Easy
For an industrial induction motor variable frequency drive (VFD) using vector control, what is the typical output frequency range in 2024-25 applications?
Answer: C
Modern VFDs support 0.1Hz to 500Hz+ output, enabling ultra-low speed precision applications and extended-speed motor operation.
Q.151Easy
Which of the following is the SI unit of electric current?
Answer: A
Ampere (A) is the SI base unit of electric current. It is defined as the flow of one coulomb of charge per second.
Q.152Easy
The accuracy of an electrical measuring instrument is expressed as a percentage of:
Answer: C
In electrical measurements, accuracy is typically expressed as a percentage of full scale deflection (FSD) or full scale reading (FSR) for analog instruments.
Q.153Easy
The error in an ammeter connected in series with a circuit is primarily due to:
Answer: B
An ideal ammeter should have zero internal resistance. Non-zero internal resistance causes voltage drop and measurement error.
Q.154Easy
The class of accuracy for a class 1.5 instrument means:
Answer: C
The accuracy class (e.g., 1.5, 2.0) indicates the maximum percentage error with respect to full scale deflection, as per IEC standards.
Q.155Easy
The input impedance of a digital multimeter is typically:
Answer: C
Modern digital multimeters have very high input impedance (typically 10 MΩ for voltage measurement) to minimize loading effects on the circuit.
Q.156Easy
In a CRO (Cathode Ray Oscilloscope), the Y-deflection sensitivity is expressed in:
Answer: A
Y-deflection sensitivity (or gain) in CRO is expressed in volts per division, indicating how many volts are represented by each vertical division.
Q.157Easy
The sensitivity of a galvanometer is defined as:
Answer: A
Sensitivity of a galvanometer is the deflection (in mm or degrees) produced per unit current (usually per microampere or milliampere).
Q.158Easy
Which of the following is NOT a source of error in analog voltmeters?
Answer: C
Analog voltmeters are primarily DC devices. Frequency changes affect AC measurements but not the instrument's basic construction errors.
Q.159Easy
A thermocouple generates 50 μV for a temperature difference of 1°C between its junctions. What is its sensitivity?
Answer: A
Sensitivity is the ratio of output to input. Here, sensitivity = 50 μV/1°C = 50 μV/°C.
Q.160Easy
In a CRO probe with 10:1 attenuation ratio, if the actual voltage is 100 V, what voltage appears across the CRO input terminals?
Answer: A
With 10:1 attenuation, the voltage at CRO terminals = Actual voltage/10 = 10100 = 10 V.
