Inductors

Inductors are the third fundamental passive component, after resistors and capacitors. They store energy in a magnetic field and resist changes in current. Where capacitors block DC and pass AC, inductors pass DC and block AC — which makes them natural partners for filtering. They also show up in switching power supplies and LC oscillators.

How They Work

An inductor is a coil of wire. Run current through it and it creates a magnetic field. Change the current and the magnetic field changes, inducing a voltage that opposes the change — this is Lenz's Law. The key relationship:

V = L × (dI/dt)

V — voltage across the inductor in volts
L — inductance in henrys (H)
dI/dt — rate of change of current in A/s

If current isn't changing, the voltage across an ideal inductor is zero — it looks like a short circuit to DC. Rapidly changing current produces a large voltage spike. This is why you can't just switch off an inductive load (motor, relay coil) without some form of protection — the collapsing magnetic field generates a voltage spike that can destroy the switching transistor.

Types

Air core — no magnetic core material. Low inductance values, low losses, good for high-frequency RF work. Literally just wire wound in a coil.
Ferrite core — ferrite material multiplies inductance without adding much loss. The workhorse for most electronics applications: switching supplies, EMI filters, RF chokes.
Iron core (laminated) — high inductance, used at low frequencies. Transformers and mains-frequency chokes use laminated iron cores.
Powdered iron toroid — good for switching supply inductors, handles high DC current without saturating.

Inductance Formula

The inductance of a coil depends on its geometry and core material:

L = (N² × μ × A) / l

N — number of turns
μ — permeability of the core material
A — cross-sectional area of the core
l — length of the core

More turns, larger core, and higher-permeability material all increase inductance. In practice, you buy inductors rather than wind your own for most applications.

Combining Inductors

Series:   L_total = L1 + L2 + ...          (same rules as resistors)
Parallel: 1/L_total = 1/L1 + 1/L2 + ...   (same rules as resistors)

This assumes no magnetic coupling between the inductors. If they're physically close, their fields can interact, which complicates things.

Common Uses

Switching power supplies — the inductor stores energy during the on-phase and releases it during the off-phase, allowing efficient voltage conversion. Buck and boost converters are built around this principle.
EMI filters — a choke (inductor in series with the power line) blocks high-frequency noise from getting into or out of a device. The common-mode choke on your PC power cord is doing this.
LC resonant circuits — an inductor and capacitor together form a resonant circuit at a frequency of f = 1 / (2π√LC). Used in radio tuning circuits and oscillators.
Inductive kickback protection — always put a flyback diode across relay coils, motor drivers, and solenoids. When the inductor's current is cut, the diode gives the stored energy somewhere to go instead of through your transistor.

Practical Notes

Inductors have a saturation current rating — exceed it and the core saturates, inductance drops dramatically, and the component stops working properly. For switching supply inductors, pick one rated for more than your peak current. They also have a DC resistance (DCR) which causes power loss in high-current applications.