ELEN20005 FOUNDATIONS OF ELECTRICAL NETWORKS 2021
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Semester 1 Assessment June 2021
Department of Electrical and Electronic Engineering
ELEN20005 FOUNDATIONS OF ELECTRICAL NETWORKS
Question 1 (12 marks)
(a) (6 marks) Find the power generated or absorbed by each of the two current sources in
the circuit in Figure 1.
5 Ω 10 Ω
Figure 1: Circuit diagram for Question 1(a)
(b) (6 marks) The voltage across a 2 µF capacitor is shown in Figure 2. Sketch the graph
of power versus time, for 0 ms < t < 10 ms.
Figure 2: Voltage waveform for Question 1(b)
Question 2 (12 marks)
(a) (6 marks) For the circuit in Figure 3, use Node Voltage Analysis to ﬁnd the branch
current ix .
(b) (6 marks) Conﬁrm your answer to part (a) by using Mesh Current Analysis to ﬁnd ix .
Draw your mesh currents in a clockwise direction.
Figure 3: Circuit diagram for Question 2
Question 3 (10 marks)
(a) (6 marks) Find and draw the Th´evenin equivalent circuit at the output terminals a and
b for the circuit in Figure 4.
vl 4 Ω
Figure 4: Circuit diagram for Question 3(a).
(b) (4 marks) A load circuit consisting of a Zener diode in series with a 5 Ω resistor is now
attached to the output terminals of the circuit in part (a) as in Figure 5. The Zener diode has forward threshold of 700 mV and reverse threshold of -5.1 V . Find the current iL through the diode.
Figure 5: Circuit with load attached for Question 3(b).
Question 4 (12 marks)
In the circuit shown in Figure 6, the current source is (t) = 5 cos(1000t) A rms.
Figure 6: Circuit diagram for Question 4
(a) (4 marks) Find the branch currents il (t) and i2 (t), in rms units.
(b) (4 marks) Determine the real or reactive power consumed by each of the resistor, inductor
and two capacitors.
(c) (3 marks) Without referring to your power calculations in part (b), ﬁnd the real and reactive power generated by the current source.
(d) (1 marks) Show that your answers to parts (b) and (c) are consistent with the Law of Conservation of Electrical Energy.
Question 5 (8 marks)
Draw the circuit diagram for a voltage ampliﬁer model with the following parameters:
❼ Source resistance of 1 kΩ.
❼ Input resistance of 4 kΩ.
❼ Output resistance of 100 Ω.
❼ Load resistance of 500 Ω.
Assume the overall voltage gain is 20. Find the voltage gain, the open circuit voltage gain, the current gain and the power gain.
Question 6 (10 marks)
(a) (4 marks) Draw the circuit diagram for a balanced three-phase circuit with a Y-connected
source that is connected to a balanced Delta-connected load. The circuit has a negative phase sequence, and has no line impedances. Label all relevant nodes, currents, voltages and impedances.
(b) (3 marks) Assume that Van = VY 0。 , and that ZA = Zθ . Show that Vac = o3VY 30。 .
(c) (3 marks) Show that IAC = IA30。 - θ , where IA = .
Question 7 (16 marks)
(a) (7 marks) Draw the circuit diagram for a half-wave rectiﬁer power supply to deliver an
average voltage of 6 V with a peak-to-peak ripple of at most 0.6 V to a load. The average load current should be 30 mA. Assume that rectiﬁer diodes with forward threshold of 700 mV are available. Assume that the following range of resistors, capacitors and inductors are available :
❼ Resistors: 10 Ω , 12 Ω , 100 Ω , 120 Ω , 1 kΩ , 1.2kΩ
❼ Capacitors: 1 µF, 1.2µF, 10 µF, 12µF, 100 µF, 120µF, 1 mF, 1.2 mF ❼ Inductors: 10 µH, 12µH, 100 µH, 120 µH, 1 mH, 1.20 mH .
The ac voltage supply is of frequency 100 Hz .
Explain your choice of components, aiming to use the least number of components. Also determine the amplitude of the voltage source.
(b) (9 marks) Using the breadboard diagram in Figure 7, show how you would build your
rectiﬁer circuit using the AD2 kit. Include the probes required to measure the source voltage and the load voltage.
Explain how you would set the voltage supplied by the AD2 to obtain the required load voltage. Referring to your breadboard diagram, explain how you would take suitable mea- surements to determine whether the output voltage ripple meets the design requirement (Write about 80 words).
Figure 7: Breadboard Diagram for Question 7
Question 8 (10 marks)
In the ampliﬁer circuit in Figure 8, vs (t) = 5 sin(200t) V . Find vo (t).
Figure 8: Ampliﬁer circuit for Question 8
Question 9 (10 marks)
For the circuit in Figure 9, the switch has been open for a long period of time. At time t = 0 it is closed. Find the voltage vC (t) across the capacitor for t > 0. Assume the diode is ideal.
Figure 9: Circuit diagram for Question 9