What Are Capacitors?

Capacitors 101 - Sizes & Shapes

Definition

In really simple terms, a capacitor is a passive two-terminal electrical component used to store energy electrostatically in an electric field. A capacitor holds a charge, similar to how a bucket holds water. The world’s first capacitor was in the shape of a jar and wasnamed the Leyden jar. Invented in the 18th century, it was a glass jar coated with metal on both the inside as well as the outside, with the glass acting as the dielectric. The lid topped off the jar. A hole in the lid had a metal rod passing through it, with its other end connected to the inner coat of metal. The exposed end of the rod culminated in a metal ball. The metal ball and rod was used to charge the inner electrode of the jar electrically.

The Worlds First Capacitor

Todays capacitors do not look anything like the jar. A capacitor has two conductive plates separated by a dielectric. This helps maintain an electric charge between its plates. Different materials are used for the dielectric, such as plastic, paper, air, tantalum, polyester, ceramic, etc. The main purpose of the dielectric is to prevent the plates from touching each other.

A capacitor can be used in a number of different ways. As an example, the telecommunication t industry uses variable capacitors to adjust the frequency and tuning of their communications equipment. You can measure a capacitor in terms of the voltage difference between its plates, as the two plates hold identical but opposite charge. Unlike the battery, a capacitor does not generate electrons, and therefore, there is no current flow if the two plates are electrically connected. The electrically connected plates rearrange the charge between them, effectively neutralizing each other.

As there is only a dielectric between the two plates, a capacitor will block direct current but will allow alternating current to flow within its design parameters. If you hook up a capacitor across the terminals of a battery, there will not be any current flow after the capacitor has charged. However, alternating current or AC signal will flow through, impeded only by the reactance of the capacitor, which depends on the frequency of the signal. As the alternating current fluctuates, it causes the capacitor to charge and discharge, making it appear as if a current is flowing.

Capacitors can dump their charge at high speed, unlike batteries. That makes capacitors eminently suitable for generating a flash for photography. This technique is also used in big lasers to get very bright and instantaneous flashes.

Lightning

Lightning, a naturally occurring phenomenon, works very similar to a capacitor. The cloud is one of the plates and the earth is the other. Charge slowly builds-up between the cloud and the earth. When this creates more voltage than the air (the dielectric) can bear, the insulation breakdown causes a flow of charges between the two plates in the form of a bolt of lightning.

The Many Sizes & Shapes Of Capacitors

You'll find one or more capacitors in almost every electronic circuit you build. And capacitors come in all sorts of shapes and sizes, influenced mostly by three things: the type of material used to create the plates, the type of material used for the dielectric, and the capacitance.

The most common types of capacitors are:

• Ceramic disk: The plates are made by coating both sides of a small ceramic or porcelain disk with silver solder. The ceramic or porcelain disk is the dielectric, and the silver solder forms the plates. Leads are soldered to the plates, and the entire thing is dipped in resin.

  Ceramic disk capacitors are small and usually have low capacitance values, ranging from 1 pF to a few microfarads. Because they're small, their values are usually printed using a three-digit shorthand notation.

  Ceramic disk capacitors aren't polarized, so you don't have to worry about polarity when you use them.

• Silver mica: The dielectric is made from mica, and this capacitor is sometimes referred to simply as a mica capacitor. As with ceramic capacitors, the plates in a silver mica capacitor are made from silver. Electrodes are joined to the plates, and then the capacitor is dipped in epoxy.

  Silver mica capacitors come in about the same capacitance range as ceramic disk capacitors. However, they can be made to much higher tolerances — as close as 1% in some cases. Like ceramic disk capacitors, silver mica capacitors aren't polarized.

  Although ceramic disk and mica capacitors are constructed in a similar way, they're easy to tell apart. Ceramic disk capacitors are thin, flat disks and are nearly always a dull, light-brown color. Silver mica capacitors are thicker, bulge at the ends where the leads are attached, and are shiny and sometimes colorful — red, blue, yellow, and green are common colors for silver mica capacitors.

• Film: The dielectric is made from a thin film-like sheet of insulating material, and the plates are made from film-like sheets of metal foil. In some cases, the plates and the dielectric are then tightly rolled together and enclosed in a metal or plastic can. In other cases, the layers are stacked and then dipped in epoxy.

  Depending on the materials used, capacitance for film capacitors can be as small as 1,000 pF or as large as 100 μF. Film capacitors aren't polarized.

• Electrolytic: One of the plates is made by coating a foil film with a highly conductive, semiliquid solution called electrolyte. The other plate is another foil film on which an extremely thin layer of oxide has been deposited; this thin layer serves as the dielectric. The two layers are then rolled up and enclosed in a metal can.

  Electrolytic capacitors are polarized, so you must be sure to connect voltage to it in the proper direction. If you apply voltage in the wrong direction, the capacitor may be damaged and might even explode.

  You find these two common types of electrolytic capacitors:

  • Aluminum: Can be quite large, with as much as a tenth of a farad or more (100,000 μF).

  • Tantalum: Are smaller, ranging up to about 1,000 μF.

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