Electrical circuit theory supports electronics, communication and hardware interfacing. PSC Computer Engineer candidates should know Ohm/Kirchhoff laws, AC phasors, impedance, power factor, resonance, transformers, filters and RC/RL transient behavior.

Concept Teaching

Circuit questions are easiest when you identify domain: DC steady state, AC sinusoidal steady state or transient switching. Resistors dissipate energy, inductors store magnetic energy and oppose current change, capacitors store electric energy and oppose voltage change.

DC Circuit Laws

Basic circuit analysis starts with Ohm law and Kirchhoff laws.

• Ohm law: V = IR.
• KCL: algebraic sum of currents at a node is zero.
• KVL: algebraic sum of voltages around a closed loop is zero.
• Series resistances add directly.
• Parallel resistance uses reciprocal addition.
• Voltage divider and current divider are frequent shortcuts.

AC Phasor and Impedance

Phasors convert sinusoidal differential equations into algebraic complex-number calculations.

AC Power and Power Factor

In AC circuits, not all apparent power becomes real work.

• Real power P is measured in watts.
• Reactive power Q is measured in VAR.
• Apparent power S is measured in VA.
• Power factor = cos phi = real power / apparent power.
• Low power factor increases current for same real power.
• Capacitor banks can improve lagging power factor in inductive loads.

Resonance and Filters

Frequency response decides which signals pass or attenuate.

• Series RLC resonance occurs when inductive and capacitive reactance cancel.
• At resonance, impedance is minimum in series RLC.
• Low-pass filter passes low frequencies and attenuates high frequencies.
• High-pass filter passes high frequencies and attenuates low frequencies.
• Band-pass filter passes a band around center frequency.
• Cutoff frequency marks the boundary where response drops by about 3 dB.

Transformers

Transformers work only with changing flux, so practical transformers require AC.

• Voltage ratio approximately equals turns ratio.
• Step-up transformer increases voltage and reduces current ideally.
• Step-down transformer reduces voltage and increases current ideally.
• Ideal transformer conserves power: VpIp approximately equals VsIs.
• Losses include copper loss, hysteresis loss, eddy current loss and leakage flux.
• Transformer isolation can improve safety and noise separation.

Transient Response

Transient analysis is about how capacitors and inductors respond to sudden switching.

• Capacitor voltage cannot change instantaneously.
• Inductor current cannot change instantaneously.
• RC time constant tau = RC.
• RL time constant tau = L/R.
• After one time constant, first-order response reaches about 63.2 percent of final change.
• After about five time constants, response is practically settled.

Engineering Mechanism

• Identify circuit type: DC, AC steady state or transient.
• Apply KCL/KVL and element laws.
• For AC, convert components to impedance and use phasor algebra.
• For filters, analyze how impedance changes with frequency.
• For transformer, apply turns ratio and power conservation approximation.
• For transients, use initial/final values and time constant.

Diagrams / Models To Draw

• Draw series and parallel RLC circuit.
• Draw phasor diagram for R, L and C.
• Draw transformer primary/secondary windings and core flux.
• Draw low-pass and high-pass RC filter response.
• Draw RC charging exponential curve.

Formulas, Tables and Algorithms

• Ohm law: V = IR.
• Inductive reactance: XL = 2 pi f L.
• Capacitive reactance: XC = 1/(2 pi f C).
• Resonant frequency: f0 = 1/(2 pi sqrt(LC)).
• Transformer ratio: Vp/Vs = Np/Ns.
• RC time constant: tau = RC; RL time constant: tau = L/R.

Worked Example

For an RC circuit with R = 10 k ohm and C = 10 microfarad, time constant tau = RC = 10000 x 10 x 10^-6 = 0.1 s. After about 0.5 s, the capacitor is practically near final value.

Subjective Answer Pattern

• Define circuit domain and key components.
• State relevant laws and formulas.
• Explain AC impedance/phasor if sinusoidal.
• Discuss transformer/filter/transient depending on question.
• Add diagram and one numerical relation.

Common Engineering Mistakes

• Using DC resistance formula alone for AC RLC circuit.
• Confusing leading and lagging current.
• Assuming transformer works with pure DC steady state.
• Forgetting units in time constant and reactance.
• Confusing low-pass and high-pass filter behavior.

MCQ Revision

• What is impedance of capacitor?
• Current leads in which component?
• What is transformer voltage ratio?
• What is RC time constant?
• What happens at series resonance?
• What does power factor measure?

Final Summary

• Electrical circuit analysis uses laws, impedance and energy-storage behavior.
• AC circuits require magnitude and phase reasoning.
• Transformers operate by mutual induction and turns ratio.
• Filters select frequency bands.
• Transients are controlled by time constants and initial conditions.