Comprehensive Inductors Guide

Everything about inductors: types, formulas, applications, energy storage, calculations, and practical usage in circuits and electronics projects.

What is an Inductor?

An inductor is a passive electrical component that stores energy in a magnetic field when electric current passes through it. Inductors resist changes in current, making them essential in filtering, energy storage, timing, and signal conditioning circuits.

  • Unit: Henry (H)
  • Symbol in schematics: L
  • Voltage-current relationship: V = L × (di/dt)
  • Energy stored: E = ½ × L × I²

Inductor Construction

Inductors are usually made of coils of wire wound around a magnetic core or air core:

  • Air-Core Inductor: No magnetic core, used for high-frequency applications.
  • Iron-Core Inductor: Higher inductance, used in power applications.
  • Ferrite-Core Inductor: High-frequency, low-loss inductors.
  • Toroidal Inductor: Donut-shaped core, minimizes leakage flux.
  • Chokes: Special inductors for filtering AC in DC lines.

Inductor Parameters

  • Inductance (L) in Henrys
  • DC Resistance (DCR)
  • Current Rating (Imax)
  • Self-Resonant Frequency (SRF)
  • Q Factor (quality factor)

Inductor Formulas

// Inductor Voltage V = L × (di/dt) // Energy stored in magnetic field E = 0.5 × L × I² // Inductor in series with resistor (RL circuit) Time constant τ = L / R // Resonant frequency of LC circuit f = 1 / (2π × √(L × C))

Inductor Behavior in DC Circuits

At steady-state DC, an ideal inductor behaves like a short circuit, allowing constant current to pass. However, during current changes, it generates a voltage opposing the change.

// Example: RL Circuit with DC Supply R = 10Ω, L = 1H, V = 12V τ = L / R = 1 / 10 = 0.1 sec I(t) = (V/R) × (1 - e^(-t/τ))

Inductor Behavior in AC Circuits

Inductors oppose AC through reactance:

// Inductive reactance X_L = 2 × π × f × L // Impedance of inductor Z_L = j × X_L

Higher frequency → higher opposition → filter high-frequency signals.

Applications of Inductors

Inductors are widely used in electronics:

  • Chokes for AC line filtering
  • Transformers for voltage conversion
  • Energy storage in switching power supplies
  • Oscillators and tuned LC circuits
  • Inductive sensors and RFID circuits

Advanced Concepts

  • Mutual Inductance: Two inductors influence each other through magnetic fields, used in transformers.
  • Self-Inductance: Property of a coil resisting current changes through itself.
  • Core Saturation: Magnetic cores have limits; beyond saturation, inductance drops.
  • Q Factor: Ratio of reactance to resistance; higher Q → lower losses.
  • Resonance: LC circuits resonate at frequency f = 1/(2π√(LC)), used in tuning and filters.
// Energy stored example L = 0.5H, I = 2A E = 0.5 × 0.5 × 2² = 1 Joule

Inductor Circuits

Series: Total inductance is sum of individual inductances.

L_total = L1 + L2 + L3 + ...

Parallel: Reciprocal sum of inductances.

1 / L_total = 1 / L1 + 1 / L2 + 1 / L3 + ...

Resistor-inductor series circuits have time constants τ = L/R. Used in timing and filtering.

Used in radio frequency tuning, filters, and oscillators.

f_resonant = 1 / (2π√(L × C))

Inductors with mutual coupling create transformers for voltage step-up/down and isolation.

Used in switch-mode power supplies (SMPS), inductive charging, and energy harvesting circuits.

Inductor Types Quick Reference

Air-Core
High frequency
Iron-Core
Power applications
Ferrite-Core
Switching circuits
Toroidal
Minimized flux leakage
Chokes
Filtering AC lines
Variable
Adjustable inductance
RF Inductor
High-frequency tuning
Coupled Inductor
Transformer or filter

Choosing Inductors for Projects

DC-DC Converters
High current, low DCR
RF Circuits
Air-core, high Q
Power Filtering
Chokes or ferrite-core