Introduction to Capacitors

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Category: Electrical Theory

Introduction to Capacitors

A capacitor is a passive electronic component that stores electrical energy in an electric field. Capacitors are widely used for filtering, timing, smoothing power supplies, and coupling signals in AC circuits.

How a Capacitor Works

A capacitor consists of two conductive plates separated by an insulating material (called the dielectric). When a voltage is applied, charge builds up on the plates — one becomes positively charged, the other negative.

Capacitance (C)

The ability of a capacitor to store charge is called capacitance. It's measured in farads (F), though typical values are in microfarads (µF), nanofarads (nF), or picofarads (pF).

Formula:

Q = C × V

  • Q = Charge in coulombs
  • C = Capacitance in farads
  • V = Voltage across the capacitor

Types of Capacitors

  • Ceramic: Small, non-polarised, used in high-frequency circuits
  • Electrolytic: Higher capacitance, polarised, used in power supplies
  • Tantalum: Stable, polarised, compact
  • Film and Supercapacitors: Used in specialised or high-capacity applications

Charging and Discharging

When connected to a voltage source, a capacitor charges up over time. When the voltage source is removed, it can discharge its stored energy through a load.

Charging in an RC Circuit:

The voltage across a charging capacitor follows this equation:

V(t) = Vmax × (1 - e-t/RC)

Discharging:

V(t) = Vmax × e-t/RC

  • R = Resistance in ohms
  • C = Capacitance in farads
  • t = Time in seconds

Time Constant (τ)

The product of resistance and capacitance gives the time constant:

τ = R × C

After 1 time constant, the capacitor charges to ~63% of the supply voltage or discharges to ~37%. After 5τ, charging/discharging is ~99% complete.

Common Uses of Capacitors

  • Filtering — smooth out voltage in power supplies
  • Timing — used in RC time delay circuits
  • Coupling/Decoupling — pass AC, block DC
  • Energy storage — short-term power backup

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