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Air/vacuum = 1, Iron = 200-5000, Ferrite = 20-15000
L = μ₀ × μᵣ × N² × A / l
Ltotal = L₁ + L₂ + L₃ + ...
Inductors in series add directly (same as resistors in series).
1/Ltotal = 1/L₁ + 1/L₂ + 1/L₃ + ...
Inductors in parallel combine like resistors in parallel.
E = ½ × L × I²
Energy stored in an inductor's magnetic field is proportional to inductance and current squared.
| Type | Inductance Range | Application |
|---|---|---|
| Small Signal | 1 µH - 100 µH | RF circuits, filters |
| Power Inductor | 100 µH - 10 mH | DC-DC converters, power supplies |
| Choke | 1 mH - 100 mH | Noise filtering, EMI suppression |
| Audio Inductor | 0.1 mH - 10 mH | Crossover networks, audio filters |
| Toroidal | 10 µH - 1 H | High efficiency, low EMI |
| Air Core | 0.1 µH - 100 µH | High frequency, RF applications |
Inductance is a fundamental electrical property that quantifies a conductor's ability to store energy in a magnetic field when current flows through it. Measured in henries (H), inductance arises whenever a conductor is wound into a coil, creating a concentrated magnetic flux. When the current through an inductor changes, the magnetic field changes proportionally, inducing a voltage that opposes the current change—a phenomenon described by Faraday's law of electromagnetic induction and Lenz's law. Inductance depends on the coil geometry (number of turns, cross-sectional area, and length) and the permeability of the core material. Air-core inductors have low inductance values suitable for high-frequency RF applications, while ferrite and iron-core inductors achieve much higher inductance for power electronics and filtering. Inductors are essential components in power supplies, radio tuners, transformers, motors, and electromagnetic compatibility (EMC) filters. Understanding inductance calculations per IEEE and IEC standards is critical for designing reliable electronic circuits.
Determine the number of turns (N), cross-sectional area (A) in square meters, and the length (l) of the coil in meters. For toroidal inductors, measure the mean magnetic path length instead.
Look up the relative permeability (μr) of your core material. Air = 1, powdered iron = 10–100, ferrite = 100–10,000, and silicon steel = 1,500–10,000.
Calculate L = μ₀ × μr × N² × A ÷ l, where μ₀ = 4π × 10⁻⁷ H/m. For series combinations, add inductances directly. For parallel, use the reciprocal sum formula.
Use an LCR meter to measure the actual inductance at your operating frequency. Calculated values are theoretical—real-world factors like winding capacitance, core saturation, and fringing effects influence the result.
Inductors are the core energy-storage element in switch-mode power supplies. Proper inductance values ensure stable voltage regulation and efficient energy transfer in buck, boost, and flyback converters.
Inductors combined with capacitors form LC filters used in audio crossovers, RF tuning circuits, and EMI suppression. The inductance value determines the cutoff frequency and filter response.
Mutual inductance enables energy transfer in transformers, while self-inductance governs motor winding behavior. Accurate inductance calculations ensure optimal performance per IEEE standards.
| Core Material | Relative Permeability (μr) | Typical Frequency Range | Common Applications |
|---|---|---|---|
| Air | 1 | All frequencies | RF coils, high-frequency tuning |
| Powdered Iron | 10–100 | 1 kHz–100 MHz | EMI filters, RF chokes |
| Ferrite (MnZn) | 800–10,000 | 1 kHz–5 MHz | Power inductors, transformers |
| Ferrite (NiZn) | 10–1,500 | 500 kHz–500 MHz | High-frequency EMI suppression |
| Silicon Steel | 1,500–10,000 | 50/60 Hz | Power transformers, motors |
| Amorphous Metal | 10,000–100,000 | 50 Hz–100 kHz | High-efficiency transformers |
Permeability values vary by grade and manufacturer. Always consult the material datasheet for exact values.
Self-inductance describes how a coil opposes changes in its own current by generating a back-EMF. Mutual inductance describes the coupling between two coils, where a changing current in one induces a voltage in the other. Transformers rely on mutual inductance, while single inductors are characterized by self-inductance. The coupling coefficient (k) between 0 and 1 describes how well two coils are magnetically linked.
At higher frequencies, inductive reactance (XL = 2πfL) increases, meaning the inductor presents more opposition to current flow. However, real inductors also have parasitic capacitance from winding layers, creating a self-resonant frequency (SRF) above which the inductor behaves like a capacitor. Always choose inductors with an SRF well above your operating frequency.
Core saturation occurs when the magnetic flux density in the core reaches its maximum, causing the permeability to drop sharply and the inductance to collapse. When an inductor saturates, current increases rapidly and can damage components. Designers must ensure the peak current does not exceed the saturation current rating, especially in power supply applications.
Inductors in series add directly: Ltotal = L₁ + L₂ + L₃. Inductors in parallel follow the reciprocal formula: 1/Ltotal = 1/L₁ + 1/L₂ + 1/L₃. These formulas assume no magnetic coupling between the inductors. If there is mutual inductance, additional terms must be included depending on whether the fields are aiding or opposing.
The quality factor (Q) measures how efficiently an inductor stores energy relative to the energy it dissipates. Q = XL / R = 2πfL / R, where R is the DC resistance plus AC losses. Higher Q values indicate a more ideal inductor with less loss. Typical Q values range from 20–100 for power inductors and can exceed 200 for high-quality RF inductors.