6. Capacitors and Capacitance

Introduction to Capacitors Summary

We have seen in this tutorial that the job of a capacitor is to store electrical charge onto its plates. The amount of electrical charge that a capacitor can store on its plates is known as its Capacitance value and depends upon three main factors.

  • Surface Area – the surface area, A of the two conductive plates which make up the capacitor, the larger the area the greater the capacitance.
  • Distance – the distance, d between the two plates, the smaller the distance the greater the capacitance.
  • Dielectric Material – the type of material which separates the two plates called the “dielectric”, the higher the permittivity of the dielectric the greater the capacitance.

We have also seen that a capacitor consists of metal plates that do not touch each other but are separated by a material called a dielectric. The dielectric of a capacitor can be air, or even a vacuum but is generally a non-conducting insulating material, such as waxed paper, glass, mica different types of plastics etc. The dielectric provides the following advantages:

  • The dielectric constant is the property of the dielectric material and varies from one material to another increasing the capacitance by a factor of k.
  • The dielectric provides mechanical support between the two plates allowing the plates to be closer together without touching.
  • Permittivity of the dielectric increases the capacitance.
  • The dielectric increases the maximum operating voltage compared to air.

Capacitors can be used in many different applications and circuits such as blocking DC current while passing audio signals, pulses, or alternating current, or other time varying wave forms. This ability to block DC currents enables capacitors to be used to smooth the output voltages of power supplies, to remove unwanted spikes from signals that would otherwise tend to cause damage or false triggering of semiconductors or digital components.

Capacitors can also be used to adjust the frequency response of an audio circuit, or to couple together separate amplifier stages that must be protected from the transmission of DC current.

When used on DC supplies a capacitor has infinite impedance (open-circuit), at very high frequencies a capacitor has zero impedance (short-circuit). All capacitors have a maximum working DC voltage rating, (WVDC) so it is advisable to select a capacitor with a voltage rating at least 50% more than the supply voltage.

There are a large variety of capacitor styles and types, each one having its own particular advantage, disadvantage and characteristics. To include all types would make this tutorial section very large so in the next tutorial about The Introduction to Capacitors I shall limit them to the most commonly used types.

Leave a Reply

Your email address will not be published. Required fields are marked *