Capacitor

Stores electric charge and smooths voltage.

Schematic letter: C
Unit / key rating: Farad (F)

Pinout

+ (long lead): The positive terminal of an electrolytic capacitor.
− (striped side): The side with the stripe on the body is negative.

Real photos

Capacitor types: ceramic, film, tantalum and electrolytic

Electrolytic capacitors on a circuit board

Tantalum capacitors (through-hole and SMD)

Bulging (failed) electrolytic capacitors

What is it for?

A capacitor is made of two conducting plates with an insulator between them, and it stores electric charge. The unit is the farad, but in practice µF, nF and pF are used.

It is used to smooth ripple in power supplies (filtering), to block DC from signals (coupling) and in timing circuits.

Capacitor code reader

100 nF

The first two digits (10) are the value, the third (4) is the number of zeros, in picofarads: 100000 pF.

On ceramic and film capacitors the code is in picofarads : 104 = 10 × 10⁴ pF = 100 nF.
The letter after the code is the tolerance: J = 5%, K = 10%, M = 20%.
Electrolytics show the value and voltage in plain text (e.g. 470 µF 25 V); the striped side is negative.
On SMD tantalums the stripe marks the positive end; it's the opposite of an electrolytic, so don't mix them up.
SMD ceramic capacitors usually have no marking at all; they can only be measured with an LCR meter.

Things to watch out for

⚠ Never fit an electrolytic capacitor backwards: it heats up, bulges and can burst.
⚠ Large capacitors hold charge even when the device is off. Discharge them through a resistor before touching.
⚠ The voltage rating should be at least 1.5 times the voltage in the circuit.

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