Electronics for Building Things · Lesson 9 · Components in Depth

Capacitors in Depth

The tiny energy buffer — and the one-capacitor habit that fixes flaky projects.

Capacitors are the component beginners under-use and pros sprinkle everywhere. They don't "do" something dramatic on their own — they steady things. Understanding them turns "my project randomly resets" from a mystery into a known fix.

The one win You can identify a capacitor's type and value, respect its polarity, and place a decoupling cap to keep your circuits stable.

What a capacitor does

A capacitor stores a small amount of electric charge and releases it quickly. Back to the water analogy: it's a little surge tank on the pipe. When pressure dips for an instant, the tank pushes water back to smooth it; when pressure spikes, it absorbs some. Capacitance is measured in farads, but you'll only ever see tiny fractions: microfarads (µF), nanofarads (nF), picofarads (pF).

The two types you'll actually use

Type	Looks like	Polarity	Reading the value
Ceramic	Small disc / blob	None — fits either way	3-digit code: `104` = 10 + 4 zeros = 100,000 pF = 0.1 µF
Electrolytic	Little cylinder ("soda can")	Polarized! stripe = −, longer leg = +	Printed directly, e.g. `10µF 25V`

Polarity matters An electrolytic capacitor wired backwards can fail — even pop. The stripe marks the negative leg; the longer lead is positive. Ceramics have no polarity, so they're worry-free (SparkFun: Capacitors).

The job you'll use most: decoupling

Chips draw current in sudden bursts as they switch. Those bursts cause tiny dips in the supply voltage that can make a microcontroller misbehave or reset. A decoupling capacitor (also called a bypass cap) sits right next to a chip's power pin — between + and GND — acting as a local surge tank that supplies those bursts instantly.

The single most useful capacitor rule Put a 0.1 µF ceramic capacitor across power and ground, as close as possible to every chip. Add a larger electrolytic (1–10 µF) near where power enters the board. This one habit prevents a huge share of "randomly flaky" behavior.

Other roles (just so you recognize them)

Smoothing: a big electrolytic across a power supply flattens ripple into steadier DC.
Timing & filtering: paired with a resistor, a capacitor charges at a known rate — the basis of timers and simple filters. You'll meet this; no math needed today.
Debouncing: a small cap can smooth a button's bounce (from Lesson 5).

Check yourself

Read this next (primary source) SparkFun — Capacitors, plus the short Polarity: Electrolytic Capacitors page.

I'm your teacher — ask me anything. If a project resets randomly or acts glitchy, tell me — it's very often a missing decoupling cap, and I can help you spot it.