Module 8: AC Circuit Theory

AC circuit theory is where electronics gets interesting. Mains power, audio, sensor signals, clocks and wireless signals are all alternating currents — voltages that swing positive and negative many times per second. To design, analyze, or repair real equipment, you must understand how AC behaves differently from DC and why components that are simple in DC circuits become frequency-dependent in AC circuits. This module takes you from the fundamentals of AC waveforms through reactance, impedance, resonance, Q factor, and filters — the building blocks of power supplies, audio circuits and signal processing.

Module Overview

The DC Circuit Theory module gave you a thorough understanding of DC circuit theory — how resistors behave, how current divides, how Kirchhoff's laws govern every loop and node. All of that still applies in AC circuits, but two new components — capacitors and inductors — introduce frequency-dependent behavior that transforms the analysis entirely. Where a resistor's opposition to current is constant regardless of frequency, a capacitor's opposition decreases as frequency rises and an inductor's opposition increases. This frequency dependence is the key to filters, tone controls, crossovers and every tuned circuit.

The module opens by reviewing AC waveform fundamentals before introducing the concept of RMS voltage — the value your multimeter reads and the one that determines actual power delivered to a load. You will learn to calculate capacitive reactance and inductive reactance, then combine them with resistance to find the total impedance of a circuit at any frequency. Phasor diagrams provide a geometric tool for visualizing how voltages and currents relate in phase across different components.

The centerpiece of the module is resonance. When a capacitor and inductor are combined in a circuit, there is one specific frequency — the resonant frequency — at which their opposing reactances cancel exactly. At resonance, an LC circuit behaves in a dramatically different way: a series circuit presents its minimum impedance, while a parallel circuit presents its maximum. This phenomenon is exploited in oscillators, wireless chargers, induction cookers and metal detectors, and in filters that select one band of frequencies and reject the rest.

The Q factor quantifies how sharply a resonant circuit discriminates between frequencies. High-Q circuits select a narrow bandwidth; low-Q circuits pass a wider range. Understanding Q lets you predict exactly how selective a filter or tuned circuit will be, and lets you calculate the 3 dB bandwidth directly from the resonant frequency and Q value.

The final lessons cover four filter types — low-pass, high-pass, band-pass, and band-stop — and show you how to design simple first-order filters using resistors, capacitors, and inductors. The module ends with Pi, L and T networks, the matching and filtering circuits found in power converters, audio equipment and high-frequency circuits.

Why This Module Matters in Practice

AC circuit theory is not abstract — it is the direct explanation of how everyday equipment works. The tone controls on an amplifier are filters. A loudspeaker crossover splits the music into bass and treble with inductors and capacitors. A wireless phone charger uses a resonant coil to transfer power across a gap. The input filter of a switching power supply keeps its high-frequency noise out of the mains. The anti-aliasing filter in front of an ADC removes frequencies the converter cannot handle.

Every one of these functions — tuning, filtering, impedance matching — is a direct application of the theory in this module. A student who masters this module can look at any schematic and immediately understand the purpose of each LC or RC combination they see.

Lessons