hardware//electronics//component//capacitor

2026-02-15

A capacitor stores electrical charge on two conductors kept apart by an insulator, and circuits use it to hold a voltage steady, to filter noise and to pass a changing signal from one part to another without letting a steady current through. It is on every PCB, beside every chip, smoothing the power it is fed.

The idea is two plates and a gap. Put positive charge on one plate and the charges of the other plate rearrange to face it, pulled by the field across the insulator, while no charge crosses the gap. The charge stored is proportional to the voltage between the plates, and the constant is the capacitance:

Q=CV,C=εAdQ = CV, \qquad C = \frac{\varepsilon A}{d}Q=CV,C=dεA

A larger plate area AAA, a thinner gap ddd or an insulator of higher permittivity ε\varepsilonε stores more charge for the same voltage.

A capacitor passes change, never a steady current.

Because charge on one side induces charge on the other through the field, a signal that varies crosses it while direct current is blocked, which is what its impedance, 1/(2πfC)1/(2\pi f C)1/(2πfC), says in one line.

Capacitive coupling is that crossing used on purpose. A movement of charge on one side produces a matching movement on the other without any charge passing, so a stage can receive a signal and ignore the steady voltage under it; an electrode in a salt solution reads the solution's voltage this way, through the thin double layer of ions and electrons at its surface.

Decoupling is the same part used as a reservoir. Placed next to a chip's supply pin, it delivers the bursts of current the chip draws faster than the supply lines can, so the voltage at the pin does not dip.

Its insulator stops charge entirely, where a semiconductor lets a controlled amount through, and the two meet in the transistor: its gate is a tiny capacitor whose charge decides whether the channel under it conducts.

Every two conductors near each other form one. Tracks on a board, metal layers in a chip and the membrane of a cell are capacitors whether anyone drew them or not, and at high speed that unintended capacitance shapes the signal.