16. R.M.S value of the trapezoidal waveform V=Vmsin(Ωt+α).
A. Vm÷2½
B. Vm÷2¼
C. Vm÷2¾
D. Vm÷3½
Answer: D
R.M.S value of the sinusoidal waveform is Vm÷2½ and r.m.s value of the trapezoidal waveform is Vm÷3½. The peak value of the sinusoidal waveform is Vm.
17. What is the unit of the admittance?
A. ohm
B. ohm-1
C. ohm2
D. ohm.5
Answer: B
The admittance measures how easily the current can flow in the circuit. It is the ratio of current and voltage. It is given in ohm-1. It is reciprocal of impedance.
18. Calculate the value of the frequency if the inductive reactance is 45 Ω and the value of the inductor is 15 H.
A. 0.477 Hz
B. 0.544 Hz
C. 0.465 Hz
D. 0.412 Hz
Answer: A
The frequency is defined as the number of oscillations per second. The frequency can be calculated using the relation
XL = 2×3.14×f×L. F
= XL÷2×3.14×L
= 45÷2×3.14×15 = .477 Hz.
19. The slope of the V-I curve is 19°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. .3254 Ω
B. .3608 Ω
C. .3543 Ω
D. .3443 Ω
Answer: D
The slope of the V-I curve is resistance. The slope given is 19° so
R=tan(19°)=.3443 Ω.
The slope of the V-I curve is resistance.
20. Calculate the active power in a 41 H inductor.
A. 2 W
B. 1 W
C. 0 W
D. .5 W
Answer: C
The inductor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy.
The voltage and current are 90° in phase in the case of the inductor so the angle between V & I is 90°.
P = VIcos90 = 0 W.
Voltage leads the current in the case of the inductor.
21. Calculate the active power in a 19 F capacitor.
A. 7.8 W
B. 0 W
C. 5.4 W
D. 1.5 W
Answer: B
The capacitor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy. The voltage and current are 90° in phase in the case of the capacitor so the angle between V & I is 90°.
P = VIcos90 = 0 W.
Current leads the voltage in the case of the capacitor.
22. Calculate the active power in a 241 H inductor.
A. 21 W
B. 11 W
C. 0 W
D. .51 W
Answer: C
The inductor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy. The voltage and current are 90o in phase in the case of the inductor so the angle between V & I is 90°.
P = VIcos90 = 0 W.
23. Calculate the active power in a 5 Ω resistor with 5 A current flowing through it.
A. 125 W
B. 110 W
C. 115 W
D. 126 W
Answer: A
The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in the case of the resistor so the angle between V & I is 90°.
P=I2R=5×5×5=125 W.
24. In DC chopper, the waveform for input and output voltages is respectively __________
A. Discontinuous and Continuous
B. Continuous and Discontinuous
C. Both continuous
D. Both discontinuous
Answer: B
Chopper has a perfect DC at the input which is chopped into pulses which means the output voltage is discontinuous and by varying the duty cycle we can vary the average output voltage.
25. A chopper behaves as a __________
A. DC equivalent of AC switching device
B. DC equivalent of AC transformer
C. DC equivalent of AC relay
D. AC equivalent of circuit breaker
Answer: B
A chopper is used to step up or step down the DC voltage whereas the transformer is used to step up or step down AC voltage so the chopper is the DC equivalent of the AC transformer.
26. 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.
27. 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.
28. 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.
29. 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.
30. 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.
31. 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.
32. 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.
