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Cork Institute of Technology
Bachelor of Engineering (Honours) in Mechanical Engineering- Stage 2
(NFQ Level 8)
Summer 2006
Electrical & Electronic Engineering
(Time: 3 Hours)
Instructions Answer SIX questions – three from each section. Use separate answer books for each Section.
Examiners: Dr. R. A. Guinee Dr. M. N. Barry Mr. J. E. Hegarty Prof. M. Gilchrist
SECTION A
Q.1 (a) Draw the ANSI symbol for a two input NAND, NOR and EXOR gates complete with truth tables in each case. (4.66%)
(b) State and prove De Morgan's laws for 3 input variable using truth tables. (4%)
(c) Analyze the logic circuit shown in Fig. Q1 by means of (i) a truth and (ii) a Boolean expression. (8%) A B L
M
N
C
P
Q
R
Fig.Q
Q.2 (a) Minimise the Boolean expression: F = A B C.. + A B C.. + A B C.. + A B C.. + A B C.. by means of (i) Boolean reduction and (ii) Karnaugh map. Which do you think is the easiest method to use? (6%) (b) Draw the logic circuit of a clocked SR flip flop using NAND gates. Examine the response of the flip flop via a truth table for the various input combinations and comment. What is the role of the clock input in such a circuit and point out any difficulties in circuit operation? (5%) (c) Draw the block schematic of a 4 stage serial in – parallel out shift register using D flip flops. Examine the operation of the shift register using a state table for each stage in the loading of the binary number 1101. (5.66%)
Q3 (a) List the ideal parameters of an operational amplifier. (2.66%) (b) Show that the voltage gain of the amplifier circuit in Fig. Q3 with feedback is given by AVf = − RRfs (5%)
(c) Explain how the circuit in Fig. Q3 can be modified to provide a difference amplifier such that
V o = RRsf ( V 2 − V 1 )
where V 2 is the additional input to the non-inverting terminal. (9%) Rf Rs V (^1) Vo Fig. Q
SECTION B
Q5 (a) Derive the expression P = VI cos θfor the power dissipated in an ac circuit.
[5 marks] (b) A circuit consists of three branches in parallel across a 100V, 50Hz supply. Branch 1 is a 100 Ω Branch 2 is a coil of resistance 4 Ω and inductance 0.02H Branch 3 consists of an inductor in series with a 200μF capacitor. Draw the phasor (vector) diagram of the circuit. [5 marks] Calculate (i) The branch currents [4 marks] (ii) The overall current [3 marks] (iii) The total power dissipated. [3 marks]
Q6 (a) Derive from first principles the expression v ( t )= V [ 1 − e − t /^ RC ], as the voltage on a capacitor, after a voltage V has been switched across the capacitor (C) and a resistor (R) in series with the supply. (b) A 0.05μF capacitor is in series with a 2M Ω resistance, is suddenly switched across a 100V direct supply. Find: (i) The initial charge current. (ii) The time constant, T (iii) The value of the capacitor charging current and voltage at a time t=T. (iv) The charge and energy stored on the capacitor at a time, t=T
Q7. (a) State Faradays Law of Electromagnetic induction and give an expression for it. [5 marks] (b) Show by means of the superposition of magnetic fields that a current carrying conductor in a magnetic field experiences a force acting on it. [5 marks] (c) What do we mean when we say something has inductance? Explain. [5 marks] (d) The twin starter cable of an automobile takes a starting current of 100A. If the conductor spacing is 2mm,between centres, calculate the force per metre length on the cable. [5 marks]
( μ 0 = 4 π × 10 −^7 H / m )
Q8. (a) State (i) Kirchhoff’s laws, and (ii) Thevenin’s theorem. [4 marks] (b) Using either Kirchhoff’s laws, or Thevenin’s theorem find the currents through each resistor in the circuit in Fig.Q(8), below. [16 marks]
12V
2 Ω
1Ω
3Ω
7V
Fig. Q(8)
