Capacitor

A passive component that stores electrical energy in an electric field between two plates.

Symbol: IEC 60617 (parallel plates)

How it works

A capacitor stores charge on two conductive plates separated by an insulator (dielectric). Its capacitance, measured in farads (F), determines how much charge it can hold per volt applied.

In DC circuits, a capacitor blocks steady current once charged but allows current to flow briefly while charging or discharging — this is the basis of timing circuits and debounce filters. In AC circuits, a capacitor's impedance falls as frequency rises, which is why capacitors are used to filter out high-frequency noise or block DC while passing AC signals.

Common types differ mainly in construction: ceramic capacitors (small values, no polarity) and electrolytic capacitors (larger values, polarized — must be wired the right way round or they can fail).

Key Formulas

Charge StoredQ = C × V
Energy StoredE = 1/2 × C × V²
Capacitive ReactanceXc = 1 / (2πfC)

Common Mistakes

Wiring an electrolytic capacitor backwards. Reverse-biasing a polarized capacitor causes the internal oxide layer to break down, resulting in a short circuit and rapid, sometimes explosive, venting of gas.

Common Variants

100nF Ceramic Capacitor

The near-universal decoupling capacitor value, placed right next to an IC's power pins to absorb switching noise.

100µF Electrolytic Capacitor

A polarized, high-value capacitor typically used for power-supply smoothing and slower timing circuits.

1000µF Capacitor

A large-value, polarized aluminum electrolytic capacitor sized for bulk energy storage and ripple smoothing after rectification, not for the high-frequency noise bypassing a small ceramic capacitor handles.