Capacitor
A capacitor stores energy in an electric field. In a series signal path it blocks steady DC after the charging transient while coupling changing signals, with an impedance that depends on frequency. Real capacitors also have leakage, resistance and inductance.
Two conductive plates sit a sliver apart, separated by an insulator. Apply voltage and electrons pile up on one plate while the other plate gives them up — building an electric field that holds charge until something asks for it back.
In plain terms
Think of a springy water tank. When the supply pressure dips, the tank pushes a little water out to fill the gap; when pressure rises, the tank drinks the surplus back in.
Also called: cap, C.
Why designers use it
- Decouple a chip's power pin so sudden current spikes don't sag the rail.
- Smooth the DC rail coming out of a rectifier or switching regulator.
- Pass an audio or signal voltage from one stage to the next while blocking DC offset.
- Set timing in an oscillator or filter — bigger cap, slower behaviour.
- Form a snubber across a switch to soak up nasty voltage spikes.
Best for
- Power rail decoupling
- Bulk smoothing
- Audio coupling
- Timing networks
- EMI filtering
Key specifications
- Capacitance: Check the effective value in your circuit (Nominal value, tolerance, temperature and bias all matter.)
- Voltage: Use the exact series rating and derating rules
- Dielectric: Choose for stability, size and application
- ESR and impedance: Check at the operating frequency
- Ripple current: Check heating and the series limit
- Lifetime: Depends on technology and operating stress (Ceramic capacitors are not exempt from ageing or reliability limits.)
When not to use it
- When the required stored energy, retention time or leakage budget exceeds the selected capacitor's capability.
- When the circuit requires a resistor's current-voltage relationship rather than frequency-dependent impedance.
- As a complete voltage regulator: a smoothing capacitor reduces variations but does not maintain an exact output voltage on its own.
Common mistakes
- Reversing a polarized capacitor or using it outside its permitted voltage and temperature conditions.
- For Class 2 ceramic capacitors, choosing only by the printed capacitance and ignoring changes under bias, temperature and ageing.
- Ignoring ripple-current heating, ESR, placement and the circuit's stability requirements.
Where you will find it
- In a microcontroller board, local capacitors help supply brief current demands and reduce noise on the power rail.
- In a power supply, a smoothing capacitor reduces ripple. It works with the rest of the supply; it does not turn a varying input into perfectly constant DC on its own.
- In an analog filter, capacitance and the surrounding resistance set how strongly different frequencies pass through.
A short history
The Leyden jar was an early capacitor: Ewald Georg von Kleist developed it in 1745, and Pieter van Musschenbroek independently investigated it in 1746. Early versions used water and a wire inside a glass vessel; later versions used metal foil on its inner and outer surfaces. Modern capacitor families retain the idea of storing energy in an electric field while using different materials and structures to suit different circuits.
Good to know
- A high-capacitance ceramic can lose some of its effective capacitance when DC voltage is applied. The printed value is only the start of the story.
- Some ceramic capacitance changes with elapsed time as well as temperature. The useful question is what value remains under your operating conditions.
- The early Leyden jar used glass as its dielectric. Modern capacitors keep the same electric-field idea but use very different materials and structures.