Electronics

Passive Components — resistors, capacitors, and inductors

"Passive" just means a component that can't amplify a signal or add energy of its own — it only resists, stores, or releases the energy already flowing through it. Almost every board you'll ever look at is mostly these three parts, plus the active semiconductors covered next.

Resistor

A resistor opposes current flow, converting electrical energy into heat in the process — see Fundamentals for Ohm's Law itself. Two ratings matter beyond its resistance value:

Power rating

How much heat it can dissipate continuously (commonly ⅛ W, ¼ W, ½ W for small through-hole parts) — exceed it and the resistor overheats, drifts in value, or fails outright.

Tolerance

How far the actual resistance can vary from the labeled value (±5%, ±1%, tighter for precision parts).

A potentiometer is a resistor with a movable wiper, giving a variable, user-adjustable resistance — the working part inside a volume knob or a trimmer used to calibrate a circuit.

The resistor color code

Small resistors are too tiny to print numbers on legibly, so value and tolerance are encoded as colored bands instead.

A 5-band resistor works the same way with one extra digit band, for tighter tolerances. When in doubt (or the bands are hard to see), a multimeter is faster and more reliable than reading tiny paint bands.

Capacitor

A capacitor is two conductive plates separated by an insulator (the dielectric). Applying a voltage across it pushes charge onto the plates, building up an electric field — that field is where the energy is actually stored.

Symbol

C

Unit

Farad (F) — practically always µF, nF, or pF; a full Farad is enormous

Key behavior

Blocks DC once fully charged (looks like an open circuit at steady state); passes AC (more easily at higher frequency)

Charging: the RC time constant

A capacitor doesn't charge instantly — through a resistor, it follows a predictable exponential curve, characterized by the time constant τ = R × C (in seconds, with R in ohms and C in farads).

This exact curve is why capacitors are used for timing delays, why a "power good" signal after startup takes a moment to settle, and why a debounce circuit's RC values are chosen deliberately — too small a τ and switch bounce sneaks through, too large and a real button press feels sluggish.

Capacitor types — and the polarity trap

Why 0.1 µF specifically for a decoupling capacitor, rather than some other round number? A real capacitor isn't a pure capacitance — its leads and construction add a small parasitic inductance, which means every capacitor has a self-resonant frequency above which it starts behaving inductively instead of capacitively and stops helping at all. A small ceramic capacitor's low parasitic inductance pushes that resonant point high enough to still be effective against the fast switching noise ICs actually generate — a much larger capacitor physically can't do the same job at those frequencies, which is exactly why boards use both a small ceramic right at the pin and a larger bulk capacitor nearby, rather than one capacitor doing everything.

Inductor

An inductor is a coil of wire. Current flowing through it builds a magnetic field, and that field is where the energy is stored — the mirror image of a capacitor storing energy electrically.

Symbol

L

Unit

Henry (H) — practically mH or µH

Key behavior

Passes DC at steady state (just the coil's wire resistance); opposes fast changes in current

Inductor types

Reactance: why frequency changes everything

"Blocks DC, passes AC" for a capacitor and "passes DC, blocks AC" for an inductor aren't just rules to memorize — they fall directly out of one idea: both components' opposition to current (their reactance, X) depends on frequency, in opposite directions.

Capacitive reactance

XC = 1 / (2πfC) — as frequency f rises, X_C falls toward zero, so a capacitor looks like less and less of an obstacle to higher-frequency signals (and like an open circuit, infinite reactance, at DC where f = 0).

Inductive reactance

XL = 2πfL — the opposite shape: as frequency rises, X_L grows, so an inductor looks like more and more of an obstacle to higher-frequency signals (and like a plain wire, zero reactance, at DC).

This is exactly why an LC filter works at all: put a capacitor to ground and an inductor in series on the same line, and low frequencies sail through the inductor and ignore the capacitor, while high frequencies do the opposite — sailing through the capacitor to ground and getting blocked by the inductor. Choosing L and C values is choosing exactly where that crossover happens.

Back-EMF and flyback

Because an inductor actively resists any change in its current, an inductor carrying current that gets suddenly interrupted (a transistor switching off a relay coil or motor) tries to keep that current flowing — and does so by generating a large voltage spike across itself as the magnetic field collapses. That spike, if left unmanaged, is easily large enough to destroy the switching transistor. The standard fix is a flyback diode wired backward across the inductive load, giving the collapsing current a safe path to circulate through instead of arcing through the switch.

R vs. C vs. L at a glance

Real-world applications

• Decoupling capacitors — a small ceramic capacitor next to almost every IC's power pin absorbs fast current spikes the power supply itself can't react to quickly enough.
• Power supply filtering — bulk electrolytic capacitors smooth out the ripple left over after AC is rectified to DC.
• Switching regulators (buck/boost converters) — an inductor is the core energy-storage element that makes efficient DC-DC voltage conversion possible.
• Motor and relay driver circuits — a flyback diode across the coil is close to mandatory any time a microcontroller switches an inductive load.
• Debounce circuits and RC delays — a resistor and capacitor together are the simplest way to filter switch bounce or generate a startup delay without any code.

Gotchas