Passives

Inductor

A coil of wire that stores energy in a magnetic field. It resists any change in current: it is slow to let current build up, and when current is cut it fights back with a voltage spike.

Think of it as: A heavy water wheel: hard to start, and hard to stop once spinning.

Pins

Either end: : An inductor has no polarity. The dot some symbols show only matters when two coils share a core (transformers).

Small inductors look like resistors or chips; power inductors are wound coils on a ferrite core, often in a shielded square box. The code works like a resistor’s, in µH: "4R7" = 4.7 µH, "100" = 10 µH.

What to look for inside

The coil is switched on for 4 seconds, then off for 4 seconds. A flyback diode gives the current somewhere to go when it switches off.
At switch-on the current starts at zero and builds up slowly, because the coil pushes back against the change (back-EMF).
As the current grows, the magnetic field around the coil (purple loops) grows with it. That field is where the energy is stored.
At switch-off the field collapses and keeps the current flowing for a while, through the diode, until the energy is used up.

The numbers that matter

Spec	Typical
Inductance How strongly it resists changes in current.	1 µH to 1 H
Saturation current I_sat Above it the core is "full" and the inductance collapses. Always stay below it.	0.1 A to 20 A
DC resistance (DCR) The plain resistance of the wire. It turns current into heat.	5 mΩ to 5 Ω
Rated current I_rms The current that warms it by about 40 °C.	amps

Where you’ll use it

The energy store inside buck and boost converters.
Filters that keep switching noise off a supply rail (with a capacitor: an LC filter).
Coils you meet as loads: relays, motors and solenoids are inductors too, which is why they need a flyback diode.

Common mistakes

Switching off an inductive load with no flyback diode. The spike can be hundreds of volts and kills the switch.
Choosing by inductance alone. A part whose saturation current is too low stops working at full load.

Lessons that use it

Go deeper

U = L\,\tfrac{dI}{dt}

The faster the current changes, the bigger the voltage it produces.

I(t) = \tfrac{U}{R}\left(1 - e^{-tR/L}\right),\quad E = \tfrac12 L I^2

Switching on through a resistor, and the energy held in the field.

X_L = 2\pi f L

Its opposition to AC rises with frequency, the opposite of a capacitor.

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