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EC 3251 Circuit Analysis question paper, April/May 2025

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Question Paper Code : 90978

B.E./B.Tech. DEGREE EXAMINATIONS, APRIL/MAY 2025.

Second Semester

Electronics and Communication Engineering

EC 3251 — CIRCUIT ANALYSIS

(Common to : Electronics and Telecommunication Engineering)

(Regulations 2021)

Time : Three hoursMaximum : 100 marks

Answer ALL questions.

PART A — (10 × 2 = 20 marks)

  1. 1.

    State Kirchhoff's Law.

  2. 2.

    Find the equivalent resistance of circuit with three resistors connected in parallel, each having a resistance value of 2 Ohms.

  3. 3.

    State the conditions for maximum power transfer in DC circuits.

  4. 4.

    State Norton theorem.

  5. 5.

    Define instantaneous power in an AC circuit.

  6. 6.

    A sinusoidal voltage is given by v(t) = 50 sin (100pi t + 30 deg). What are the amplitude, angular frequency, and phase angle of the signal?

  7. 7.

    A source-free RL circuit has R = 5 ohm and L = 2 H. Calculate the time constant.

  8. 8.

    What is the voltage across an inductor at t = 0+ in a source-free RL circuit?

  9. 9.

    Define a tree in network topology.

  10. 10.

    Two coils have L1 = 10 mH, L2 = 20 mH and mutual inductance M = 5 mH. Calculate the coefficient of coupling (k).

PART B — (5 × 13 = 65 marks)

  1. 11.
    (a)

    Using mesh analysis calculate [Figure 11(a): bridge A-B-C-D: 1 ohm from A to B (current I1), 2 ohm from B to C (current I1 - I3), 4 ohm from A to D (current I2), 3 ohm from D to C (current I2 + I3), 5 ohm from B to D (current I3); A is joined to F and C to E; a 10 V battery between F and the 1 ohm resistor leading to E; supply current I]

    • (i)equivalent resistance across the terminal of the supply(7)
    • (ii)total current supplied by the source(3)
    • (iii)power delivered to 5 ohm resistors in the circuit shown in Figure 11 (a)(3)
  2. Or
  3. (b)
    • (i)A 100 watt, 250 V lamp is connected in series with a 100 watt, 200 V lamp across 250 V supply. Calculate (1) circuit current and (2) voltage across each lamp. Assume the lamp resistances to remain unaltered.(9)
    • (ii)Explain the features and advantages of parallel circuits.(4)
  4. 12.
    (a)

    Using superposition theorem, Estimate the current in 23 ohm resistor in the circuit shown in Figure. 12 (a) [Figure 12(a): from left to right between top and bottom lines: 27 ohm; 47 ohm in series with a 200 V source (+ at top); then 4 ohm in the top line; 20 A current source (arrow up); 23 ohm]

  5. Or
  6. (b)

    Using Thevenin's theorem, find the current in 10 ohm resistor in the circuit shown in Figure. 12 (b) [Figure 12(b): 120 V battery in series with 40 ohm; 20 ohm shunt; then 60 ohm in series to the 10 ohm resistor]

  7. 13.
    (a)

    A coil having a resistance of 7W and an inductance of 31.8 mH is connected to 230 V, 50 Hz supply. Calculate

    • (i)the circuit current and phase angle(5)
    • (ii)power factor and power consumed and(4)
    • (iii)voltage drop across resistor and inductor(4)
  8. Or
  9. (b)

    A coil of resistance 50W and inductance 318 mH is connected in parallel with a circuit consisting of a 75W resistor in series with a 159 microF capacitor. The circuit is connected to a 230 V, 50 Hz supply. Determine the supply current and circuit power factor.

  10. 14.
    (a)

    Explain the concept of resonance in a series RLC circuit.

  11. Or
  12. (b)

    A parallel LC circuit has an inductance L = 100 microH which has a coil resistance of 12W. The capacitor C is adjustable over the range 200 pF to 300 pF. Find (i) the maximum and minimum resonance frequencies for the circuit (ii) the Q-factor and (iii) bandwidth of the circuit at the two resonance frequency extremes.

    (5+4+4)

  13. 15.
    (a)

    Coils A and B in a magnetic circuit have 600 and 500 turns respectively. A current of 8 A in coil A produces a flux of 0.04 Wb. If the coefficient of coupling is 02, calculate:

    • (i)Self-inductance of coil A, with B open-circuited.(5)
    • (ii)The average e.m.f. induced in coil B when the flux with it changes from zero to full value in 0.02 second.(4)
    • (iii)Mutual inductance.(4)
  14. Or
  15. (b)

    Explain the concept of magnetically coupled circuits with a detailed derivation of the voltage equations in terms of self-inductance (L1, L2) and mutual inductance (M).

PART C — (1 × 15 = 15 marks)

  1. 16.
    (a)

    Using nodal analysis, estimate the voltages at nodes A, B and C w.r.t. the reference node shown by the ground symbol in Figure . 16 (a) [Figure 16(a): 6 A current source (arrow down) on the left in series with 5 ohm to node A; 15 ohm from A to ground; 2.5 ohm from A to B; 20 ohm from B to ground; 6 ohm from B to C; 4 ohm from C to ground; 2.5 A current source (arrow up) on the right feeding C]

  2. Or
  3. (b)

    With the help of star/delta transformation, obtain the value of current supplied by the battery in the circuit shown in Figure. 16 (b). [Figure 16(b): 10 V battery in series with 7.6 ohm between the left node and the right node; 1 ohm from the left node to the top node; 8 ohm from the left node to the bottom node; 4 ohm from the right node to the bottom node; a delta of three 3 ohm resistors between the top node, the right node and an inner node; 1 ohm from the inner node to the bottom node]

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