21. A DC chopper feeds an RLE load. If the value of E is increased by 20%, the current ripple ________
A. increases by 20%
B. decreases by 20%
C. increases only 20%
D. remains the same
Answer: D
For a buck converter, the current ripple doesn’t depend on the value of E.
ΔIL = Vdc×D×(1-D.×T÷L.
The expression is independent of the value of E.
22. The conduction loss versus device current characteristic of a power MOSFET is best approximated by a ________
A. Straight line
B. Rectangular hyperbola
C. Parabola
D. Exponential decaying functions
Answer: C
A MOSFET in the ON state behaves as resistance so the conduction power loss is given by, P=I2R. Hence, the power vs current curve will be a parabola.
23. Which of the following device is NOT suitable for parallel operation?
A. MOSFET
B. BJT
C. IGBT
D. TRIAC
Answer: B
BJT has a negative temperature coefficient of resistance. If it is operated in parallel operation thermal run-away will take place and the device will damage.
24. SCR is uni-directional in nature.
A. True
B. False
Answer: A
SCR is uni-directional in nature. It only allows current to flow from anode to cathode. If the current is greater than the latching current then only it will work in forwarding conduction mode.
25. Which of the following device should be used as a switch in a low-power switched-mode power supply (SMPS)?
A. GTO
B. BJT
C. MOSFET
D. TRIAC
Answer: C
MOSFET has a low power rating and high-frequency rating and so it can be used in a low power switch mode power supply. MOSFET stands for metal oxide silicon field-effect transistor.
26. The slope of the V-I curve is 78°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. 4.732 Ω
B. 4.608 Ω
C. 4.543 Ω
D. 4.648 Ω
Answer: A
The slope of the V-I curve is resistance. The slope given is 78° so
R=tan(78°)=4.732 Ω.
The slope of the I-V curve is reciprocal to resistance.
27. Calculate mark to space ratio if the system is on for 17 sec and the time period is 30 sec.
A. 1.307
B. 1.457
C. 1.478
D. 1.146
Answer: A
Mark to space is Ton÷Toff. It is the ratio of the time for which the system is active and the time for which is inactive.
M = Ton÷Toff=17÷(30-17) = 1.307.
28. Turn-on and turn-off times of transistor depend on _________
A. Static Characteristic
B. Junction Capacitance
C. Current Gain
D. Voltage Gain
Answer: B
The depletion layer capacitance and diffusion capacitance affect the turn-on and turn-off behavior of transistors. Due to these internal capacitances, transistors do not turn on instantly.
29. The generated e.m.f from 45-pole armature having 400 turns driven at 70 rev/sec having flux per pole as 90 mWb, with 17 parallel paths is ___________
A. 13341.17 V
B. 12370.14 V
C. 14700.89 V
D. 15690.54 V
Answer: A
The generated e.m.f can be calculated using the formula
Eb = Φ×Z×N×P÷60×A
Where
Φ represents flux per pole
Z represents the total number of conductors
P represents the number of poles
A represents the number of parallel paths
N represents speed in rpm.
One turn is equal to two conductors.
Eb = .09×45×400×2×4200÷60×17 = 13341.17 V.
30. The unit of Magnetic flux density is Tesla.
A. True
B. False
Answer: A
Magnetic Flux density is defined as the number of magnetic lines passing through a certain point or a surface. It is generally expressed in terms of Tesla. Its C.G.S unit is Gauss.
