Capacitor Basics

Capacitors are used to “Store electrical energy” along with Resistors they make up two of the fundamental components used in electronic circuits. An ideal capacitor should have neither inductance nor resistance.

Capacitors – Use

Ceramic Capacitor code table
Ceramic Capacitor code table

Capacitors are used in several different ways in electronic circuits:

  • Used to store charge for high-speed use or as a backup supply in case of short power-cuts
  • Capacitors can also help to eliminate supply voltage ripples.
  • A capacitor can block DC voltage. Any alternating current (AC) signal flows through a capacitor unimpeded.
  • To filter out fast voltage changes and high-frequency signals.
  • To link high-frequency signals between two circuits at different D.C. voltages.
  • With a resistor to provide a time delay.
  • In oscillator circuits as a timing element.

Charge and Voltage

A capacitor is an electrical component used to store energy electrostatically in an electric field between its plates. The capacitance is dictated by the surface area of the plates and the distance (and the material) between them, the closer and larger the plates the higher the capacitance.

The higher the density of charge the higher the potential difference the capacitor has across the plates.

Unlike a resistor, an ideal capacitor does not dissipate energy.

Charge Q is measured in Coulombs. 1C (=As) is the amount of charge held by 6.25×1018 electrons (electron charge: 1.6×10-19C).

Capacitor Charge and Discharge curves
Capacitor Charge and Discharge curves

To charge a capacitor the Voltage curve can be resolved using the equation

VC = V0 (1 – e((-t)/RC))

The Current curve can be resolved using the equation

I = V0 / R e((-t)/RC)

τ=RC is the time constant of an RC-circuit

The discharge curve is the inverse.

I will cover this topic in more detail in a later post, as I suspect it is important to also cover Inductors before we go much further.

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