Capacitor (component)

Practical capacitors are often classified according to the material used as the dielectric, with the dielectrics divided into two broad categories: bulk insulators and metal-oxide films (so-called "electrolytic capacitors").

Capacitor construction

Capacitors have thin conducting plates (usually made of metal), separated by a layer of dielectric, then stacked or rolled to form a compact device.

Many types of capacitors are available commercially, with capacitances ranging from the picofarad range to more than a farad, and voltage ratings up to hundreds of kilovolts. In general, the higher the capacitance and voltage rating, the larger the physical size of the capacitor and the higher the cost. Tolerances in capacitance value for discrete capacitors are usually specified as a percentage of the nominal value. Tolerances ranging from 50% (electrolytic types) to less than 1% are commonly available.

Another figure of merit for capacitors is stability with respect to time and temperature, sometimes called "drift". Variable capacitors are generally less stable than fixed types.

The electrodes need round edges to avoid field emission. Air has low breakdown voltage, so any air inside a capacitor - especially at plate edges - will reduce the voltage rating. Even closed air bubbles in the insulator or between the insulator and the electrode lead to gas discharge, particularly in AC or High Frequency applications. Groups of identically constructed capacitor elements are often connected in series for operation at higher voltage. High voltage capacitors need large, smooth, and round terminals to prevent corona discharge.

Types of dielectric

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Capacitor
Polarized Capacitor
Variable Capacitor
*Air-gap: An air-gap capacitor has a low dielectric loss. Large-valued, tunable capacitors that can be used for resonating HF antennas can be made this way.
*Ceramic: The main differences between ceramic dielectric types are the temperature coefficient of capacitance, and the dielectric loss. C0G and NP0 (negative-positive-zero, i.e. ±0) dielectrics have the lowest losses, and are used in filters, as timing elements, and for balancing crystal oscillators. Ceramic capacitors tend to have low inductance because of their small size. NP0 refers to the shape of the capacitor's temperature coefficient graph (how much the capacitance changes with temperature). NP0 means that the graph is flat and the device is not affected by temperature changes.
**C0G or NP0 — Typically 4.7 pF to 0.047 µF, 5%. High tolerance and good temperature performance. Larger and more expensive.
**X7R — Typical 3300 pF to 0.33 µF, 10%. Good for non-critical coupling, timing applications. Subject to microphonics.
**Z5U or 2E6 — Typical 0.01 µF to 2.2 µF, 20%. Good for bypass, coupling applications. Low price and small size. Subject to microphonics.
**Ceramic chip: 1% accurate, values up to about 1 µF, typically made from Lead zirconate titanate (PZT) ferroelectric ceramic
*Glass — used to form extremely stable, reliable capacitors.
*Paper — common in antique radio equipment, paper dielectric and aluminum foil layers rolled into a cylinder and sealed with wax. Low values up to a few μF, working voltage up to several hundred volts, oil-impregnated bathtub types to 5,000 V used for motor starting and high-voltage power supplies, and up to 25,000 V for large oil-impregnated energy discharge types.
*Polycarbonate good for filters, low tempco, good aging, expensive
*Polyester, (PET film): (from about 1 nF to 10 μF) signal capacitors, integrators.
*Polystyrene: (usually in the picofarad range) stable signal capacitors.
*Polypropylene: low-loss, high voltage, resistant to breakdown, signal capacitors.
*PTFE or Teflon : higher performing and more expensive than other plastic dielectrics.
*Silvered mica: These are fast and stable for HF and low VHF RF circuits, but expensive.
*Electrolytic capacitors have a larger capacitance per unit volume than other types, making them valuable in relatively high-current and low-frequency electrical circuits, e.g. in power-supply filters or as coupling capacitors in audio amplifiers. High-capacity electrolytics, also known as supercapacitors or ultracapacitors, have applications similar to those of rechargeable batteries, e.g. in electrically powered vehicles;
*Printed circuit board: Metal conductive areas in different layers of a multi-layer printed circuit board can act as a highly stable capacitor. It is common industry practice to fill unused areas of one PCB layer with the ground conductor and another layer with the power conductor, forming a large distributed capacitor between the layers, or to make power traces broader than signal traces.
*In integrated circuits, small capacitors can be formed through appropriate patterns of metallization on an isolating substrate.
*Vacuum: vacuum variable capacitors are generally expensive, housed in glass or ceramic body, typically rated for 5kV - 30kV. Typically used in high power RF transmitters because the dielectric has virtually no loss and is self-healing. May be fixed or adjustable.

Fixed capacitor comparisons

Colour coding

*Or ±0.5 pF, whichever is greater.

ee also

*Capacitor plague (premature failure of some electrolytic capacitors)
*Supercapacitor
*Electronic devices and circuits
*Electronic color code
*Inductor
*Flux capacitor (fictional component from the "Back to the Future" film series)

Notes

References

* Tre Clifford "Super Charged: A Tiny South Korean Company is Out to Make Capacitors Powerful enough to Propel the Next Generation of Hybrid-Electric Cars", IEEE Spectrum, January, 2005 Vol 42, No. 1, North American Edition.
* "The ARRL Handbook for Radio Amateurs, 68th ed", The Amateur Radio Relay League, Newington CT USA, 1991
* "Basic Circuit Theory with Digital Computations", Lawrence P. Huelsman, Prentice-Hall, 1972
* "Philosophical Transactions of the Royal Society LXXII", Appendix 8, 1782 (Volta coins the word "condenser")
* A. K. Maini "Electronic Projects for Beginners", "Pustak Mahal", 2nd Edition: March, 1998 (INDIA)

External links

* [http://www.sparkmuseum.com/BOOK_LEYDEN.HTM Spark Museum] (von Kleist and Musschenbroek)
* [http://www.acmi.net.au/AIC/VON_KLEIST_BIO.html Biography of von Kleist]
* [http://www.designers-guide.org/Modeling/da.pdf Modeling Dielectric Absorption in Capacitors]