Capacitors

Why does the indicator light on some devices keep glowing for a moment after you pull the plug? Somewhere inside sits a component that has stored charge and is now slowly releasing it: a capacitor. After the resistor and the diode, it is the third great basic component of electronics, and it sits in practically every device, often by the dozen.

A bucket for charge

A capacitor consists of two metal surfaces facing each other very closely, separated only by a thin insulating layer. It stops the charges from simply flowing together and cancelling out at once. If you apply a voltage, positive charge collects on one side and negative charge on the other, like water in a bucket. How much fits in is given by the capacitance, measured in farads. A whole farad is an enormous amount: most devices contain capacitors of a few millionths of a farad, that is microfarads. Only a supercapacitor reaches whole farads. The stored charge follows the formula $Q = C \cdot U$: the larger the capacitance and the higher the voltage, the more charge sits in the bucket.

Charging and discharging

When connected, a charging current flows at first and fills the capacitor. The fuller it gets, the smaller the current becomes, until it stops completely: a full capacitor blocks direct current. The special part is the discharge: the capacitor can release its charge much faster than it received it. A camera flash charges its capacitor for seconds with a small current and then discharges it in a tiny fraction of a second, briefly creating a huge current for the bright flash.

Smoothing voltage

The most common job of capacitors is less flashy: they smooth voltages. In a power supply, the voltage arrives after rectification as a bumpy up and down. A large capacitor behind it acts like a water reservoir: it fills up on the peaks and gives back in the valleys, keeping the voltage up. Small capacitors also sit next to sensitive chips, catching tiny dips in the supply before the chip even notices them.

Capacitor or battery?

Both store energy, but in very different ways. The battery stores it chemically: a lot of energy, but slow charging and discharging, and the chemistry wears out over time. The capacitor stores the charge directly on its surfaces: far less energy, but lightning fast charging and discharging, repeatable millions of times. That is why they complement each other: the battery provides the stamina, the capacitor the sprint.