Professional Electronics Engineer with a specialism in EMC.

Capacitors

Capacitors are used to “Store electrical energy”, along with Resistors they make up two of the fundamental components used in electronic circuits.

Capacitors – Use

Capacitors are used in several different ways in electronic circuits:

  • Used to store charge for high-speed use or as a back-up 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).

Current I of 1A represents the flow of a charge Q of 1C of electrons for 1 second, so I=Q/t, and Q=It.

A 1 Farad (F=As/V) capacitor has 1V at its terminals when charged with 1As.

The Farad is named after English physicist Michael Faraday.

To work out the capacitance of a particle capacitor you can use the following equation: C = ∈or A/d where C is the Capacitance, ∈o is the permittivity of free space (8.854187817 × 10−12 F/m). ∈r is the dielectric constants of the material between the plates, A is the area of the plates, and d is the distance between the plates.

Ceramic Capacitor code table
Ceramic Capacitor code table

Most capacitors have values that are very small parts of a Farad; pF, nF, and μF, are common units of measurements for capacitors, 1F is 1,000,000 μF, and as you can see from the table to the right n and p are even smaller.

Until fairly recently 1F capacitors were very uncommon outside specialist applications and Car audio systems.

There are a large number of different types of capacitors, each of which has a number of different performance criterium, fundamentally they are made up of two, or multiples of two sheets of metal in parallel with each other separated by a dielectric (I will cover different construction in a later post).

However, the most common ones are small SMD ceramic (look a lot like SMD resistors), Disk Capacitors, Metal film capacitors, Tantalum capacitors, and Super Capacitors.

Different types of capacitors (from application notes)
Different types of capacitors (from application notes)

Charging and Discharging

Simple RC circuit
Simple RC circuit

 

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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