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+---- Thread: Capacitors, Inductors & Reactance — How Circuits Store and Release Energy (/showthread.php?tid=357)
Capacitors, Inductors & Reactance — How Circuits Store and Release Energy - Leejohnston - 11-17-2025
Thread 2 — Capacitors, Inductors & Reactance
Understanding Energy Storage in Electrical Systems
In electronic and electrical systems, not all components simply resist current.
Some store energy — temporarily — and release it when the circuit changes.
These components are essential in:
• power supplies
• radio systems
• filters
• microcontrollers
• motors
• communication electronics
This thread covers the three pillars of reactive components.
1. What Is a Capacitor?
A capacitor stores electrical energy in an electric field.
Simple structure:
• two metal plates
• separated by an insulator (dielectric)
Key behaviour:
• capacitors charge when connected to a voltage
• they release that stored energy when the voltage drops
Unit: farads (F)
Common uses:
• smoothing power supplies
• filtering signals
• timing circuits
• energy bursts (camera flash)
• memory (old DRAM tech)
2. Capacitor Equation — Q = C × V
Charge stored:
Q = C × V
Where:
• Q = charge (coulombs)
• C = capacitance (farads)
• V = voltage
Larger capacitance → more storage.
Higher voltage → more charge stored.
3. Capacitor Behaviour in DC & AC
DC circuits:
• a capacitor charges up
• once full → it blocks further current
(acts like an open circuit)
AC circuits:
• constantly charges/discharges
• allows AC to pass
• blocks DC
(acts like a frequency-dependent resistor)
This makes capacitors perfect for filters and signal processing.
4. What Is an Inductor?
An inductor stores energy in a magnetic field.
Structure:
• coil of wire
• sometimes wrapped around a magnetic core
Key behaviour:
• resists changes in current
• smooths current
• releases energy when current drops
Unit: henrys (H)
Used in:
• transformers
• motors
• filters
• DC–DC converters
• radio and communication circuits
5. Inductor Equation — V = L × (di/dt)
Voltage across an inductor is:
V = L × (di/dt)
Meaning:
• big change in current → large voltage spike
• slow change → small voltage
Inductors hate sudden changes.
6. Inductor Behaviour in DC & AC
DC circuits:
• resists current at first
• eventually acts like a short circuit (just a wire)
AC circuits:
• blocks high frequencies
• allows low frequencies through
Inductors and capacitors together make frequency filters.
7. Reactance — Frequency-Dependent Resistance
Reactance (X) is like resistance, but only for AC signals.
Capacitive reactance:
Xc = 1 / (2πfC)
Higher frequency → LOWER reactance
(capacitors pass high frequencies)
Inductive reactance:
Xl = 2πfL
Higher frequency → HIGHER reactance
(inductors block high frequencies)
8. Resonance — When Capacitors & Inductors Work Together
At one special frequency, their reactances cancel:
Xl = Xc
This creates a resonant circuit.
Used in:
• radio tuners
• oscillators
• filters
• wireless charging
Resonant circuits can:
• boost signals
• select specific frequencies
• generate stable oscillations
9. Capacitors & Inductors in Filters
Low-pass filter (LPF):
• lets low frequencies through
• blocks high frequencies
• often uses inductors
High-pass filter (HPF):
• passes high frequencies
• blocks low frequencies
• often uses capacitors
Band-pass filter:
• selects a specific frequency range
• essential in radios, Wi-Fi, communication systems
10. What Comes Next?
Next threads build on these concepts:
• Thread 3 — Semiconductors & Diodes
• Thread 4 — Transistors (BJTs, MOSFETs)
• Thread 5 — Logic Gates & Digital Systems
• Thread 6 — Microcontrollers & Embedded Systems
• Thread 7 — PCB Design Basics
• Thread 8 — Communication Systems
• Thread 9 — Power Electronics
• Thread 10 — Sensors & Instrumentation
Every major piece of electronics relies on capacitors and inductors somewhere.
End of Thread — Reactive Components & Circuit Behaviour