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Cork Institute of Technology
Bachelor of Engineering (Honours) in Mechanical Engineering – Stage 2
(Bachelor of Engineering in Mechanical Engineering – Stage 2)
(NFQ – Level 8)
Summer 2005
Electrical & Electronic Engineering
(Time: 3 Hours)
Answer six Questions, three from each section.
Use separate answer books for each section. μo = 4π x 10 -7^ H/m. εo = 8.85 x 10 -12^ F/m.
g = 9.81 m/s^2 c (water) = 4182 J/kg/K
Examiners: Mr. T. A. Barry Dr. R. A. Guinee Mr. J. E. Hegarty Prof. J. Monaghan
Section A
Q1. (a) State (i) Kirchhoff's laws, and (ii) Thevenin's theorem. (4 marks) (b) Use either Kirchhoff's laws, or Thevenin's theorem, to find the currents in the circuit of Fig 1. (8 marks) (c) Verify your answer using Millman's theorem. (4.7 marks)
Q2. (a) For a circuit consisting of two impedances in series show that the equivalent impedance Z is given by:- Z = Z 1 + Z 2 (4.7 marks) (b) A coil of resistance 24 Ω and inductance 5.73 mH is connected in series with a capacitance of 8.84 μF across a 90-V, 500-Hz supply. Calculate the overall impedance, and supply current. (6 marks)
(c) In a series RLC circuit, R = 0.5 Ω, L = 20 μH, and C = 8 nF. Calculate:- (i) the resonant frequency, (ii) the dynamic impedance, (iii) the Q-factor at resonance, and (v) the 3 db Bandwidth. (6 marks)
Q3. (a) Derive the expression v = V(1 - e - t/T^ ) for the instantaneous voltage across a capacitance C when charged through a resistance R from a direct supply V. (4.7 marks) (b) Explain what is meant by the time constant T of such a circuit, and show that T = RC. (4 marks) (c) A capacitance of 10 μF is connected in series with a resistance of 100 Ω across a 100-V direct supply. Calculate:- (i) the time constant, (ii) the initial charging current, (iii) the voltage across the plates after 1 ms, (iv) the time taken for the voltage to reach 95% of its final vaue. (8 marks)
Q4. (a) Define power factor (p.f.) and explain why in power circuits it is normally desirable to have it as close to unity as possible. (4 marks) (b) A 200-V, 50-Hz motor-pump set lifts 600 kg of water per minute through a height of 20m. If the overall efficiency of the set is 61.3%, and the current taken 20A, calculate (i) the p.f of the motor, and (ii) the value of a parallel capacitance required to improve the p.f to unity. (8.7 marks) (c) Estimate the hourly cost of operating the motor if the tariff of electrical energy is 20 cents per unit kWh. (4 marks)
Q7. (a) 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? (6%) (b) Draw the logic circuit diagram of a JK Flip Flop and complete its truth table. What operational feature does the JK flip flop have over that of the SR device? (4.67%) (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 1011. (6%) [16.67 Marks]
Q8. (a) List the parameters of an ideal operational amplifier and show that the voltage gain of the amplifier in Fig Q3 is given by V V
R R
o 1
2 = − 1 (6.67 %)
R 1
R 2
V (^1) Vo
Fig. Q
(b) Explain how the circuit in Fig. Q3 can be modified to provide a difference amplifier and show, as a result of the modification, that V (^) o = RR 12 ( V 2 − V 1 ) where V 2 is the additional input to the non-inverting terminal.^ (6%)
(c) Explain how the circuit in Fig. Q3 can be modified as a unity gain buffer amplifier. (4%)
