Capacitors

What is a Capacitor?

A capacitor is a component that stores electrical energy in an electric field between two conductive plates separated by an insulating material called a dielectric. When voltage is applied, charge accumulates on the plates — positive on one side, negative on the other. When the supply is removed, the capacitor can release that stored energy back into the circuit.

Capacitance (C)

Capacitance measures how much charge a capacitor can store per volt of applied voltage. The SI unit is the Farad (F), named after Michael Faraday. One Farad is a very large amount of capacitance — in practice, most capacitors are measured in microfarads (μF = 10⁻⁶ F), nanofarads (nF = 10⁻⁹ F), or picofarads (pF = 10⁻¹² F).

What Affects Capacitance?

• •Plate area (A): Larger plates → more capacitance.
• •Plate separation (d): Closer plates → more capacitance.
• •Dielectric material (ε): Different insulators between the plates change capacitance. Air has ε ≈ 1; ceramic can be 1,000–10,000×.

Types of Capacitors

• •Ceramic capacitors — Small, cheap, non-polarised. Common values: 100 pF to 1 μF. Used for decoupling and high-frequency filtering.
• •Electrolytic capacitors — Larger capacitance (1 μF to 10,000 μF) but polarised — the + and − terminals must be connected correctly or the capacitor can explode. Used in power supplies.
• •Film capacitors — Good stability and precision. Used in audio and timing circuits.
• •Tantalum capacitors — Compact, stable, polarised. Used where space is limited and stable capacitance is needed.
• •Supercapacitors — Extremely high capacitance (1 F to 3,000 F), used for energy storage and backup power. Very slow charge/discharge compared to normal capacitors.

Charging & Discharging

When a capacitor charges through a resistor, the voltage across it rises exponentially — quickly at first, then slowly as it approaches the supply voltage. The time it takes is governed by the RC time constant (τ = R × C). After one time constant, the capacitor reaches about 63 % of the supply voltage. After five time constants it's considered fully charged (~99 %).

Capacitors in Series & Parallel

Capacitors combine in the opposite way to resistors. In parallel, capacitances add up (like resistors in series). In series, the reciprocals add up (like resistors in parallel). This is because connecting capacitors in parallel effectively increases the plate area.

• •Parallel: C_total = C₁ + C₂ + C₃ (capacitances add — more storage)
• •Series: 1/C_total = 1/C₁ + 1/C₂ + 1/C₃ (total is less than smallest — voltage rating increases)

Common Uses of Capacitors

• •Decoupling/bypass — Placed near IC power pins to absorb voltage spikes (usually 100 nF ceramic).
• •Filtering — In power supplies to smooth the DC output.
• •Timing — Paired with a resistor (RC circuit) to create time delays or oscillations.
• •Coupling — Blocks DC while allowing AC signals to pass (used in audio circuits).
• •Energy storage — Camera flash circuits, backup power for clocks/memory.