Electrical and Electronics Questions

d) After the capacitor has discharged completely, switches S1 and S2 are closed while switch S3 remains closed. Taking this instant to be t = 0, derive expressions for the capacitor voltage and the capacitor current, in this particular circuit. e) At time t = 10s, switch S1 is opened while switches S2 and S3 remain closed. Derive expressions for the capacitor voltage and the capacitor current, in this particular circuit. f) Using the derived expressions for voltage and current, plot the capacitor voltage and current in d) and e) using a software package such as Excel.

___________________________________________________________________ Question 3 (P3.1, P3.2 and M2) a) Apply Kirchoff’s laws to the circuit shown in figure 3 below, for the case when RLOAD = 100 , and find the current through RLOAD, and the voltage on RLOAD. 47

Figure 3 56 39

12V

82

68

RLOAD

22

27

b) Repeat the exercise using Mesh current analysis and confirm both the results obtained in a) above. c) Repeat this exercise using Thevenin’s theorem and a Y – ∆ or ∆ – Υ transformation and reconfirm both the results obtained in a) above. d) Hence or otherwise, draw the Norton equivalent circuit. e) Hence or otherwise find the current through RLOAD and the voltage on RLOAD, when RLOAD = 820 .

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Question 4 (P3.3, D1) A 6VDC relay coil of inductance 500mH and resistance 5 needs to be switched on and off. The circuit used is shown in figure 4 below. (Throughout this entire question you can assume that the forward voltage drop of diode D1 is 0V and that the on-state voltage of transistor Q1 is 0V) a) Transistor Q1 is turned on at time t = 0. If Vs = 6V and Ron = 0 :i. ii. iii. iv. i. ii. iii. iv. v. vi. What is the steady-state current that flows through the coil when Q1 is on? What is the rate of increase of coil current at turn-on? What is the time constant of the charging circuit? After how long can the current be assumed to have reached steady state? What is the...