Capacitors are manufactured in many styles, forms, dimensions, and from a large variety of materials. They all contain at least two electrical conductors, called plates, separated by an insulating layer (dielectric). Capacitors are widely used as parts of electrical circuits in many common electrical devices.
Capacitors, together with resistors and inductors, belong to the group of passive components in electronic equipment. Small capacitors are used in electronic devices to couple signals between stages of amplifiers, as components of electric filters and tuned circuits, or as parts of power supply systems to smooth rectified current. Larger capacitors are used for energy storage in such applications as strobe lights, as parts of some types of electric motors, or for power factor correction in AC power distribution systems. Standard capacitors have a fixed value of capacitance, but adjustable capacitors are frequently used in tuned circuits. Different types are used depending on required capacitance, working voltage, current handling capacity, and other properties.
While, in absolute figures, the most commonly manufactured capacitors are integrated into dynamic random-access memory, flash memory, and other device chips, this article covers the discrete components.
General characteristics Conventional construction
A conventional capacitor stores electric energy as static electricity by charge separation in an electric field between two electrode plates. The charge carriers are typically electrons. The amount of charge stored per unit voltage is essentially a function of the size of the plates, the plate material's properties, the properties of the dielectric material placed between the plates, and the separation distance (i.e. dielectric thickness). The potential difference between the plates is limited by the properties of the dielectric material and the separation distance.
Nearly all conventional industrial capacitors except some special types such as "feed-through capacitors", are constructed as "plate capacitors" even if their electrodes and the dielectric between are wound or rolled. The capacitance C of a plate capacitor is
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The capacitance increases with the area A of the plates and with the permittivity ε of the dielectric material, and decreases with the plate separation distance d. The capacitance is therefore greatest in devices made from materials with a high permittivity, large plate area, and small distance between plates.
Electrochemical construction
Another type – the electrochemical capacitor – makes use of two other storage principles to store electric energy. In contrast to ceramic, film, and electrolytic capacitors, supercapacitors (also known as electrical double-layer capacitors (EDLC) or ultracapacitors) do not have a conventional dielectric. The capacitance value of an electrochemical capacitor is determined by two high-capacity storage principles. These principles are: electrostatic storage within Helmholtz double layers achieved on the phase interface between the surface of the electrodes and the electrolyte (double-layer capacitance); and electrochemical storage achieved by a faradaic electron charge-transfer by specifically absorbed ions with redox reactions (pseudocapacitance). Unlike batteries, in these reactions, the ions simply cling to the atomic structure of an electrode without making or breaking chemical bonds, and no or negligibly small chemical modifications are involved in charge/discharge.
The ratio of the storage resulting from each principle can vary greatly, depending on electrode design and electrolyte composition. Pseudocapacitance can increase the capacitance value by as much as an order of magnitude over that of the double-layer by itself.
Classification Capacitors are divided into two mechanical groups: Fixed-capacitance devices with a constant capacitance and variable capacitors. Variable capacitors are made as trimmers, that are either adjusted only during circuit calibration, or a device tunable during operation of the electronic instrument.
The most common group is the fixed capacitors. Many are named based on the type of dielectric. For a systematic classification these characteristics cannot be used, because one of the oldest, the electrolytic capacitor, is named instead by its cathode construction. So the most-used names are simply historical.
The most common kinds of capacitors are: Ceramic capacitors have a ceramic dielectric. Film and paper capacitors are named for their dielectrics. Aluminum, tantalum and niobium electrolytic capacitors are named after the material used as the anode and the construction of the cathode (electrolyte). Polymer capacitors are aluminum, tantalum or niobium electrolytic capacitors with conductive polymer as electrolyte. Supercapacitor is the family name for: Double-layer capacitors were named for the physical phenomenon of the Helmholtz double-layer. Pseudocapacitors were named for their ability to store electric energy electro-chemically with reversible faradaic charge-transfer. Hybrid capacitors combine double-layer and pseudocapacitors to increase power density. Silver mica, glass, silicon, air-gap and vacuum capacitors are named for their dielectric.
In addition to the above shown capacitor types, which derived their name from historical development, there are many individual capacitors that have been named based on their application. They include: Power capacitors, motor capacitors, DC-link capacitors, suppression capacitors, audio crossover capacitors, lighting ballast capacitors, snubber capacitors, coupling, decoupling or bypassing capacitors.
Often, more than one capacitor family is employed for these applications, e.g. interference suppression can use ceramic capacitors or film capacitors.
Other kinds of capacitors are discussed in the #Special capacitors section.
Dielectrics
The most common dielectrics are: Ceramics Plastic films Oxide layer on metal (aluminum, tantalum, niobium) Natural materials like mica, glass, paper, air, SF6, vacuum
All of them store their electrical charge statically within an electric field between two (parallel) electrodes.
Beneath these conventional capacitors a family of electrochemical capacitors called supercapacitors was developed. Supercapacitors do not have a conventional dielectric. They store their electrical charge statically in Helmholtz double-layers and faradaically at the surface of electrodes with static double-layer capacitance in a double-layer capacitor and with pseudocapacitance (faradaic charge transfer) in a pseudocapacitor or with both storage principles together in hybrid capacitors.
The most important material parameters of the different dielectrics used and the approximate Helmholtz-layer thickness are given in the table below.
+ Key parametersWIMA, Characteristics of Metallized Film Capacitors in Comparison with Other Dielectrics Tomáš Kárník, AVX, NIOBIUM OXIDE FOR CAPACITOR MANUFACTURING, METAL 2008, 13. –15. 5. 2008, Hradec nad Moravicí PDF. .P. Bettacchi, D. Montanari, D. Zanarini, D. Orioli, G. Rondelli, A. Sanua, KEMET Electronics Power Film Capacitors for Industrial Applications Capacitor style Dielectric RelativePermittivityat 1 kHz Maximum/realizeddielectric strength(Volt/μm) Minimum thicknessof the dielectric(μm) Ceramic capacitors, Class 1 paraelectric 12 to 40 < 100 (?) 1 Ceramic capacitors, Class 2 ferroelectric 200 to 14,000 < 35 0.5 Film capacitors Polypropylene ( PP) 2.2 650 / 450 1.9 to 3.0 Film capacitors Polyethylene terephthalate, Polyester (PET) 3.3 580 / 280 0.7 to 0.9 Film capacitors Polyphenylene sulfide (PPS) 3.0 470 / 220 1.2 Film capacitors Polyethylene naphthalate (PEN) 3.0 500 / 300 0.9 to 1.4 Film capacitors Polytetrafluoroethylene (PTFE) 2.0 450 (?) / 250 5.5 Paper capacitors Paper 3.5 to 5.5 60 5 to 10 Aluminum electrolytic capacitors Aluminium oxide Al2O3 9.6 710 < 0.01 (6.3 V) < 0.8 (450 V) Tantalum electrolytic capacitors Tantalum pentoxide Ta2O5 26 625 < 0.01 (6.3 V) < 0.08 (40 V) Niobium electrolytic capacitors Niobium pentoxide, Nb2O5 42 455 < 0.01 (6.3 V) < 0.10 (40 V) Supercapacitors Double-layer capacitors Helmholtz double-layer – 5000 < 0.001 (2.7 V) Vacuum capacitors Vacuum 1 40 – Air gap capacitors Air 1 3.3 – Glass capacitors Glass 5 to 10 450 – Mica capacitors Mica 5 to 8 118 4 to 50
The capacitor's plate area can be adapted to the wanted capacitance value. The permittivity and the dielectric thickness are the determining parameters for capacitors. Ease of processing is also crucial. Thin, mechanically flexible sheets can be wrapped or stacked easily, yielding large designs with high capacitance values. Razor-thin metallized sintered ceramic layers covered with metallized electrodes however, offer the best conditions for the miniaturization of circuits with SMD styles.
The figures in the table above gives the explanation for some simple facts: Supercapacitors have the highest capacitance density because of their special charge storage principles. Electrolytic capacitors have lesser capacitance density than supercapacitors but the highest capacitance density of conventional capacitors due to the thin dielectric. Ceramic capacitors class 2 have higher capacitance values in a given case than class 1 capacitors because of their higher permittivity. Film capacitors with their different plastic film material have a small spread in the dimensions for a given capacitance/voltage value of a film capacitor because the minimum dielectric film thickness differs between the different film materials.
Capacitance and voltage range
Capacitance ranges from picofarads to more than hundreds of farads. Voltage ratings can reach 100 kilovolts. In general, capacitance and voltage correlate with physical size and cost.
Miniaturization
As in other areas of electronics, volumetric efficiency measures the performance of electronic function per unit volume. For capacitors, the volumetric efficiency is measured with the "CV product", calculated by multiplying the capacitance (C) by the maximum voltage rating (V), divided by the volume. From 1970 to 2005, volumetric efficiencies improved dramatically.
Overlapping range of the applications
These individual capacitors can perform their application independent of their affiliation to an above shown capacitor type, so that an overlapping range of applications between the different capacitor types exists.
Types and styles
Ceramic capacitors
A ceramic capacitor is a non-polarized fixed capacitor made out of two or more alternating layers of ceramic and metal in which the ceramic material acts as the dielectric and the metal acts as the electrodes. The ceramic material is a mixture of finely ground granules of paraelectric or ferroelectric materials, modified by mixed oxides that are necessary to achieve the capacitor's desired characteristics. The electrical behavior of the ceramic material is divided into two stability classes: Class 1 ceramic capacitors with high stability and low losses compensating the influence of temperature in resonant circuit application. Common EIA/IEC code abbreviations are C0G/NP0, P2G/N150, R2G/N220, U2J/N750 etc. Class 2 ceramic capacitors with high volumetric efficiency for buffer, by-pass and coupling applications Common EIA/IEC code abbreviations are: X7R/2XI, Z5U/E26, Y5V/2F4, X7S/2C1, etc.
The plasticity of ceramic raw material works well for many special applications and enables a diversity of styles, shapes and dimensional spread of ceramic capacitors. The smallest discrete capacitor, for instance, is a "01005" chip capacitor with the dimension of only 0.4 mm × 0.2 mm.