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Capacitors are electronic components that block the effects of DC voltage but allow the effects of AC voltage to pass. A capacitor that uses a plastic polymer, such as polystyrene or polyester, as part of its operational components is commonly called a poly capacitor. Since the introduction of poly capacitors in the late 1950s, improvements in plastics have allowed them to evolve alongside electronics. Once rarely used, poly capacitors became the standard general-purpose capacitor in almost all areas of electronics.
All capacitors function using a system of plates and dielectrics. Most capacitors have two plates, usually made of a metal such as aluminum or tantalum. The plates can be flat and parallel to one another, as in a poly capacitor, or rolled to form a spiral tube, as is the case in the tin can-looking electrolytic capacitors, also called condensers. Additionally, plates can be a segment of metal, a foil, or a film, depending on the capacitor and its intended use.
The space between the two plates of a capacitor is usually filled with a dielectric material. Dielectric materials are substances that are electrical insulators by nature, but are permeable by electromagnetic fields and can become polarized. Many different gases, liquids, and solids find use as dielectrics in capacitors. In a poly capacitor, the dielectric material is a solid polymer plastic. A number of different plastics find use as dielectrics, including polystyrene and polypropylene; however, polyester is by far the most common.
In operation, an electrical current enters one lead of the capacitor. Since there is a dielectric between the capacitor’s plates, it cannot pass directly from one plate to the other, which prevents a DC current from passing between them. The electrical potential of the charged plate causes a polarized electromagnetic field to build between the two plates through the dielectric. While DC currents are blocked, this field allows an AC current to pass between the two plates and through the capacitor. If the applied voltage is too high, however, it will exceed the insulation capability of the dielectric, damage it, and cause a phenomenon known as a breakdown to occur, which will allow any electrical signal to pass until it destroys the capacitor.
The properties of the field in a capacitor are determined by the properties of the dielectric. An ideal dielectric has the highest electrical insulation value possible, to prevent a breakdown, but is as easily penetrated by an electromagnetic field as possible. This description makes plastics the perfect materials for dielectrics. Additionally, if a breakdown does occur, the increased operational temperature it causes allows a poly capacitor to self-heal and continue to operate if the voltage is removed before it destroys the capacitor.
Other attributes of poly capacitors have added to their widespread use. Plastics can last for extremely long periods before breaking down, which, when combined with their self-healing abilities, makes poly capacitors very stable and long lived. They are also relatively immune to humidity and many caustic substances, which allows them to be used in a broad range of applications, though not all. Poly capacitors are adversely affected by high temperatures, which can melt or otherwise distort the plastic dielectrics. Additionally, due to the electrostatic nature of plastics in general, they are not suited to high-frequency applications.