capacitor wikipedia

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Capacitor 1 Capacitor Miniature low-voltage capacitors, by a cm ruler A typical electrolytic capacitor A capacitor (formerly known as condenser) is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric (insulator); for example, one common construction consists of metal foils separated by a thin layer of insulating film. Capacitors are widely used as parts of electrical circuits in many common electrical devices. When there is a potential difference (voltage) across the conductors, a static electric field develops across the dielectric, causing positive charge to collect on one plate and negative charge on the other plate. Energy is stored in the electrostatic field. An ideal capacitor is characterized by a single constant value, capacitance, measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. The capacitance is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called "plates," referring to an early means of construction. In practice, the dielectric between the plates passes a small amount of leakage current and also has an electric field strength limit, resulting in a breakdown voltage, while the conductors and leads introduce an undesired inductance and resistance. Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass, in filter networks, for smoothing the output of power supplies, in the resonant circuits that tune radios to particular frequencies, in electric power transmission systems for stabilizing voltage and power flow, and for many other purposes.

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Page 1: Capacitor Wikipedia

Capacitor 1

Capacitor

Miniature low-voltage capacitors, by a cm ruler

A typical electrolytic capacitor

A capacitor (formerly known as condenser) is a passive two-terminalelectrical component used to store energy in an electric field. Theforms of practical capacitors vary widely, but all contain at least twoelectrical conductors separated by a dielectric (insulator); for example,one common construction consists of metal foils separated by a thinlayer of insulating film. Capacitors are widely used as parts ofelectrical circuits in many common electrical devices.

When there is a potential difference (voltage) across the conductors, astatic electric field develops across the dielectric, causing positivecharge to collect on one plate and negative charge on the other plate.Energy is stored in the electrostatic field. An ideal capacitor is characterized by a single constant value, capacitance,measured in farads. This is the ratio of the electric charge on each conductor to the potential difference betweenthem.

The capacitance is greatest when there is a narrow separation between large areas of conductor, hence capacitorconductors are often called "plates," referring to an early means of construction. In practice, the dielectric betweenthe plates passes a small amount of leakage current and also has an electric field strength limit, resulting in abreakdown voltage, while the conductors and leads introduce an undesired inductance and resistance.Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current topass, in filter networks, for smoothing the output of power supplies, in the resonant circuits that tune radios toparticular frequencies, in electric power transmission systems for stabilizing voltage and power flow, and for manyother purposes.

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History

Battery of four Leyden jars in MuseumBoerhaave, Leiden, the Netherlands.

In October 1745, Ewald Georg von Kleist of Pomerania in Germanyfound that charge could be stored by connecting a high-voltageelectrostatic generator by a wire to a volume of water in a hand-heldglass jar.[1] Von Kleist's hand and the water acted as conductors, andthe jar as a dielectric (although details of the mechanism wereincorrectly identified at the time). Von Kleist found, after removing thegenerator, that touching the wire resulted in a painful spark. In a letterdescribing the experiment, he said "I would not take a second shock forthe kingdom of France."[2] The following year, the Dutch physicistPieter van Musschenbroek invented a similar capacitor, which wasnamed the Leyden jar, after the University of Leiden where heworked.[3]

Daniel Gralath was the first to combine several jars in parallel into a"battery" to increase the charge storage capacity. Benjamin Franklininvestigated the Leyden jar and "proved" that the charge was stored onthe glass, not in the water as others had assumed. He also adopted theterm "battery",[4][5] (denoting the increasing of power with a row of

similar units as in a battery of cannon), subsequently applied to clusters of electrochemical cells.[6] Leyden jars werelater made by coating the inside and outside of jars with metal foil, leaving a space at the mouth to prevent arcingbetween the foils. The earliest unit of capacitance was the 'jar', equivalent to about 1 nanofarad.

Leyden jars or more powerful devices employing flat glass plates alternating with foil conductors were usedexclusively up until about 1900, when the invention of wireless (radio) created a demand for standard capacitors, andthe steady move to higher frequencies required capacitors with lower inductance. A more compact constructionbegan to be used of a flexible dielectric sheet such as oiled paper sandwiched between sheets of metal foil, rolled orfolded into a small package.

Early capacitors were also known as condensers, a term that is still occasionally used today. The term was first usedfor this purpose by Alessandro Volta in 1782, with reference to the device's ability to store a higher density ofelectric charge than a normal isolated conductor.[7]

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Theory of operation

Charge separation in a parallel-platecapacitor causes an internal electricfield. A dielectric (orange) reduces

the field and increases thecapacitance.

A simple demonstration of a parallel-platecapacitor

A capacitor consists of two conductors separated by a non-conductiveregion.[8] The non-conductive region is called the dielectric. In simplerterms, the dielectric is just an electrical insulator. Examples ofdielectric media are glass, air, paper, vacuum, and even asemiconductor depletion region chemically identical to the conductors.A capacitor is assumed to be self-contained and isolated, with no netelectric charge and no influence from any external electric field. Theconductors thus hold equal and opposite charges on their facingsurfaces,[9] and the dielectric develops an electric field. In SI units, acapacitance of one farad means that one coulomb of charge on eachconductor causes a voltage of one volt across the device.[10]

The capacitor is a reasonably general model for electric fields withinelectric circuits. An ideal capacitor is wholly characterized by aconstant capacitance C, defined as the ratio of charge ±Q on eachconductor to the voltage V between them:[8]

Sometimes charge build-up affects the capacitor mechanically, causingits capacitance to vary. In this case, capacitance is defined in terms ofincremental changes:

Energy of electric fieldWork must be done by an external influence to "move" charge between the conductors in a capacitor. When theexternal influence is removed the charge separation persists in the electric field and energy is stored to be releasedwhen the charge is allowed to return to its equilibrium position. The work done in establishing the electric field, andhence the amount of energy stored, is given by:[11]

Current-voltage relationThe current i(t) through any component in an electric circuit is defined as the rate of flow of a charge q(t) passingthrough it, but actual charges, electrons, cannot pass through the dielectric layer of a capacitor, rather an electronaccumulates on the negative plate for each one that leaves the positive plate, resulting in an electron depletion andconsequent positive charge on one electrode that is equal and opposite to the accumulated negative charge on theother. Thus the charge on the electrodes is equal to the integral of the current as well as proportional to the voltage asdiscussed above. As with any antiderivative, a constant of integration is added to represent the initial voltage v (t0).This is the integral form of the capacitor equation,[12]

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.

Taking the derivative of this, and multiplying by C, yields the derivative form,[13]

.

The dual of the capacitor is the inductor, which stores energy in a magnetic field rather than an electric field. Itscurrent-voltage relation is obtained by exchanging current and voltage in the capacitor equations and replacing Cwith the inductance L.

DC circuits

A simple resistor-capacitor circuit demonstratescharging of a capacitor.

A series circuit containing only a resistor, a capacitor, a switch and aconstant DC source of voltage V

0 is known as a charging circuit.[14] If

the capacitor is initially uncharged while the switch is open, and theswitch is closed at t = 0, it follows from Kirchhoff's voltage law that

Taking the derivative and multiplying by C, gives a first-orderdifferential equation,

At t = 0, the voltage across the capacitor is zero and the voltage across the resistor is V0. The initial current is then i

(0) =V0

/R. With this assumption, the differential equation yields

where is the time constant of the system.As the capacitor reaches equilibrium with the source voltage, the voltages across the resistor and the current throughthe entire circuit decay exponentially. The case of discharging a charged capacitor likewise demonstratesexponential decay, but with the initial capacitor voltage replacing V

0 and the final voltage being zero.

AC circuitsImpedance, the vector sum of reactance and resistance, describes the phase difference and the ratio of amplitudesbetween sinusoidally varying voltage and sinusoidally varying current at a given frequency. Fourier analysis allowsany signal to be constructed from a spectrum of frequencies, whence the circuit's reaction to the various frequenciesmay be found. The reactance and impedance of a capacitor are respectively

where j is the imaginary unit and ω is the angular frequency of the sinusoidal signal. The - j phase indicates that theAC voltage V = Z I lags the AC current by 90°: the positive current phase corresponds to increasing voltage as thecapacitor charges; zero current corresponds to instantaneous constant voltage, etc.

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Impedance decreases with increasing capacitance and increasing frequency. This implies that a higher-frequencysignal or a larger capacitor results in a lower voltage amplitude per current amplitude—an AC "short circuit" or ACcoupling. Conversely, for very low frequencies, the reactance will be high, so that a capacitor is nearly an opencircuit in AC analysis—those frequencies have been "filtered out".Capacitors are different from resistors and inductors in that the impedance is inversely proportional to the definingcharacteristic, i.e. capacitance.

Parallel plate model

Dielectric is placed between two conductingplates, each of area A and with a separation of d

. The simplest capacitor consists of two parallel conductive platesseparated by a dielectric with permittivity ε (such as air). The modelmay also be used to make qualitative predictions for other devicegeometries. The plates are considered to extend uniformly over an areaA and a charge density ±ρ = ±Q/A exists on their surface. Assumingthat the width of the plates is much greater than their separation d, theelectric field near the centre of the device will be uniform with themagnitude E = ρ/ε. The voltage is defined as the line integral of theelectric field between the plates

Solving this for C = Q/V reveals that capacitance increases with area and decreases with separation

.

The capacitance is therefore greatest in devices made from materials with a high permittivity, large plate area, andsmall distance between plates. However solving for maximum energy storage using U

d as the dielectric strength per

distance and capacitor voltage at the capacitor's breakdown voltage limit V = Vbd = Udd.

we see that the maximum energy is a function of dielectric volume, permittivity, and dielectric strength per distance.So increasing the plate area while decreasing the separation between the plates while maintaining the same volumehas no change on the amount of energy the capacitor can store. Care must be taken when increasing the plateseparation so that the above assumption of the distance between plates being much smaller than the area of the platesis still valid for these equations to be accurate.

Several capacitors in parallel.

Networks

For capacitors in parallelCapacitors in a parallel configuration each have the same appliedvoltage. Their capacitances add up. Charge is apportionedamong them by size. Using the schematic diagram to visualizeparallel plates, it is apparent that each capacitor contributes tothe total surface area.

For capacitors in series

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Several capacitors in series.

Connected in series, the schematic diagram reveals that theseparation distance, not the plate area, adds up. The capacitorseach store instantaneous charge build-up equal to that of everyother capacitor in the series. The total voltage difference fromend to end is apportioned to each capacitor according to theinverse of its capacitance. The entire series acts as a capacitorsmaller than any of its components.

Capacitors are combined in series to achieve a higher working voltage, for example for smoothing a highvoltage power supply. The voltage ratings, which are based on plate separation, add up, if capacitance andleakage currents for each capacitor are identical. In such an application, on occasion series strings areconnected in parallel, forming a matrix. The goal is to maximize the energy storage of the network withoutoverloading any capacitor.Series connection is also sometimes used to adapt polarized electrolytic capacitors for bipolar AC use. Twopolarized electrolytic capacitors are connected back to back to form a bipolar capacitor with half thecapacitance. The anode film can only withstand a small reverse voltage however.[15] This arrangement canlead to premature failure as the anode film is broken down during the reverse-conduction phase and partiallyrebuilt during the forward phase.[16] A non-polarized electrolytic capacitor has both plates anodized so that itcan withstand rated voltage in both directions; such capacitors have about half the capacitance per unit volumeof polarized capacitors.

Non-ideal behaviourCapacitors deviate from the ideal capacitor equation in a number of ways. Some of these, such as leakage current andparasitic effects are linear, or can be assumed to be linear, and can be dealt with by adding virtual components to theequivalent circuit of the capacitor. The usual methods of network analysis can then be applied. In other cases, suchas with breakdown voltage, the effect is non-linear and normal (i.e., linear) network analysis cannot be used, theeffect must be dealt with separately. There is yet another group, which may be linear but invalidate the assumption inthe analysis that capacitance is a constant. Such an example is temperature dependence. Finally, combined parasiticeffects such as inherent inductance, resistance, or dielectric losses can exhibit non-uniform behavior at variablefrequencies of operation.

Breakdown voltageAbove a particular electric field, known as the dielectric strength Eds, the dielectric in a capacitor becomesconductive. The voltage at which this occurs is called the breakdown voltage of the device, and is given by theproduct of the dielectric strength and the separation between the conductors,[17]

The maximum energy that can be stored safely in a capacitor is limited by the breakdown voltage. Due to the scalingof capacitance and breakdown voltage with dielectric thickness, all capacitors made with a particular dielectric haveapproximately equal maximum energy density, to the extent that the dielectric dominates their volume.[18]

For air dielectric capacitors the breakdown field strength is of the order 2 to 5 MV/m; for mica the breakdown is 100 to 300 MV/m, for oil 15 to 25 MV/m, and can be much less when other materials are used for the dielectric.[19] The dielectric is used in very thin layers and so absolute breakdown voltage of capacitors is limited. Typical ratings for capacitors used for general electronics applications range from a few volts to 1 kV. As the voltage increases, the dielectric must be thicker, making high-voltage capacitors larger per capacitance than those rated for lower voltages. The breakdown voltage is critically affected by factors such as the geometry of the capacitor conductive parts; sharp

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edges or points increase the electric field strength at that point and can lead to a local breakdown. Once this starts tohappen, the breakdown quickly tracks through the dielectric until it reaches the opposite plate, leaving carbon behindcausing a short circuit.[20]

The usual breakdown route is that the field strength becomes large enough to pull electrons in the dielectric fromtheir atoms thus causing conduction. Other scenarios are possible, such as impurities in the dielectric, and, if thedielectric is of a crystalline nature, imperfections in the crystal structure can result in an avalanche breakdown asseen in semi-conductor devices. Breakdown voltage is also affected by pressure, humidity and temperature.[21]

Equivalent circuit

Two different circuit models of a real capacitor

An ideal capacitor only stores and releaseselectrical energy, without dissipating any. Inreality, all capacitors have imperfections withinthe capacitor's material that create resistance.This is specified as the equivalent seriesresistance or ESR of a component. This adds areal component to the impedance:

As frequency approaches infinity, the capacitiveimpedance (or reactance) approaches zero and theESR becomes significant. As the reactancebecomes negligible, power dissipationapproaches PRMS = VRMS² /RESR.

Similarly to ESR, the capacitor's leads addequivalent series inductance or ESL to thecomponent. This is usually significant only at relatively high frequencies. As inductive reactance is positive andincreases with frequency, above a certain frequency capacitance will be canceled by inductance. High-frequencyengineering involves accounting for the inductance of all connections and components.

If the conductors are separated by a material with a small conductivity rather than a perfect dielectric, then a smallleakage current flows directly between them. The capacitor therefore has a finite parallel resistance,[10] and slowlydischarges over time (time may vary greatly depending on the capacitor material and quality).

Ripple currentRipple current is the AC component of an applied source (often a switched-mode power supply) (whose frequencymay be constant or varying). Some types of capacitors, primarily tantalum and aluminium electrolytic capacitors,usually have a rating for maximum ripple current. Ripple current causes heat to be generated within the capacitor dueto the current flow across the slightly resistive plates in the capacitor. The equivalent series resistance (ESR) is theamount of external series resistance one would add to a perfect capacitor to model this. ESR does not exactly equalthe actual resistance of the plates.•• Tantalum electrolytic capacitors are limited by ripple current and generally have the highest ESR ratings in the

capacitor family. Exceeding their ripple limits tends to result in explosive failure.•• Aluminium electrolytic capacitors, the most common type of electrolytic, suffer a large shortening of life

expectancy if rated ripple current is exceeded.•• Ceramic capacitors generally have no ripple current limitation and have some of the lowest ESR ratings.

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Capacitance instabilityThe capacitance of certain capacitors decreases as the component ages. In ceramic capacitors, this is caused bydegradation of the dielectric. The type of dielectric, ambient operating and storage temperatures are the mostsignificant aging factors, while the operating voltage has a smaller effect. The aging process may be reversed byheating the component above the Curie point. Aging is fastest near the beginning of life of the component, and thedevice stabilizes over time.[22] Electrolytic capacitors age as the electrolyte evaporates. In contrast with ceramiccapacitors, this occurs towards the end of life of the component.Temperature dependence of capacitance is usually expressed in parts per million (ppm) per °C. It can usually betaken as a broadly linear function but can be noticeably non-linear at the temperature extremes. The temperaturecoefficient can be either positive or negative, sometimes even amongst different samples of the same type. In otherwords, the spread in the range of temperature coefficients can encompass zero. See the data sheet in the leakagecurrent section above for an example.Capacitors, especially ceramic capacitors, and older designs such as paper capacitors, can absorb sound wavesresulting in a microphonic effect. Vibration moves the plates, causing the capacitance to vary, in turn inducing ACcurrent. Some dielectrics also generate piezoelectricity. The resulting interference is especially problematic in audioapplications, potentially causing feedback or unintended recording. In the reverse microphonic effect, the varyingelectric field between the capacitor plates exerts a physical force, moving them as a speaker. This can generateaudible sound, but drains energy and stresses the dielectric and the electrolyte, if any.

Current and voltage reversalCurrent reversal occurs when the flow of current changes direction. Voltage reversal is the change of polarity in acircuit. Reversal is generally described as the percentage of the maximum rated voltage that reverses polarity. In DCcircuits this will usually be less than 100%, (often in the range of 0 to 90%), whereas AC circuits experience 100%reversal.In DC circuits and pulsed circuits, current and voltage reversal are affected by the damping of the system. Voltagereversal is encountered in RLC circuits that are under-damped. The current and voltage reverse direction, forming aharmonic oscillator between the inductance and capacitance. The current and voltage will tend to oscillate and mayreverse direction several times, with each peak being lower than the previous, until the system reaches anequilibrium. This is often referred to as ringing. In comparison, critically damped or over-damped systems usually donot experience a voltage reversal. Reversal is also encountered in AC circuits, where the peak current will be equalin each direction.For maximum life, capacitors usually need to be able to handle the maximum amount of reversal that a system willexperience. An AC circuit will experience 100% voltage reversal, while under-damped DC circuits will experienceless than 100%. Reversal creates excess electric fields in the dielectric, causes excess heating of both the dielectricand the conductors, and can dramatically shorten the life-expectancy of the capacitor. Reversal ratings will oftenaffect the design considerations for the capacitor, from the choice of dielectric materials and voltage ratings to thetypes of internal connections used.[23]

LeakageLeakage is equivalent to a resistor in parallel with the capacitor. Constant exposure to heat can cause dielectricbreakdown and excessive leakage, a problem often seen in older vacuum tube circuits, particularly where oiled paperand foil capacitors were used. In many vacuum tube circuits, interstage coupling capacitors are used to conduct avarying signal from the plate of one tube to the grid circuit of the next stage. A leaky capacitor can cause the gridcircuit voltage to be raised from its normal bias setting, causing excessive current or signal distortion in thedownstream tube. In power amplifiers this can cause the plates to glow red, or current limiting resistors to overheat,even fail. Similar considerations apply to component fabricated solid-state (transistor) amplifiers, but owing to lower

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heat production and the use of modern polyester dielectric barriers this once-common problem has become relativelyrare.

Electrolytic failure from disuseElectrolytic capacitors are conditioned when manufactured by applying a voltage sufficient to initiate the properinternal chemical state. This state is maintained by regular use of the equipment. If a system using electrolyticcapacitors is disused for a long period of time it can lose its conditioning, and will generally fail with a short circuitwhen next operated, permanently damaging the capacitor. To prevent this in tube equipment, the voltage can beslowly brought up using a variable transformer (variac) on the mains, over a twenty or thirty minute interval.Transistor equipment is more problementic as such equipment may be sensitive to low voltage ("brownout")conditions, with excessive currents due to improper bias in some circuits.

Capacitor typesPractical capacitors are available commercially in many different forms. The type of internal dielectric, the structureof the plates and the device packaging all strongly affect the characteristics of the capacitor, and its applications.Values available range from very low (picofarad range; while arbitrarily low values are in principle possible, stray(parasitic) capacitance in any circuit is the limiting factor) to about 5 kF supercapacitors.Above approximately 1 microfarad electrolytic capacitors are usually used because of their small size and low costcompared with other technologies, unless their relatively poor stability, life and polarised nature make themunsuitable. Very high capacity supercapacitors use a porous carbon-based electrode material.

Dielectric materials

Capacitor materials. From left: multilayerceramic, ceramic disc, multilayer polyester film,tubular ceramic, polystyrene, metalized polyesterfilm, aluminum electrolytic. Major scale divisions

are in centimetres.

Most types of capacitor include a dielectric spacer, which increasestheir capacitance. These dielectrics are most often insulators. However,low capacitance devices are available with a vacuum between theirplates, which allows extremely high voltage operation and low losses.Variable capacitors with their plates open to the atmosphere werecommonly used in radio tuning circuits. Later designs use polymer foildielectric between the moving and stationary plates, with no significantair space between them.

In order to maximise the charge that a capacitor can hold, the dielectricmaterial needs to have as high a permittivity as possible, while alsohaving as high a breakdown voltage as possible.

Several solid dielectrics are available, including paper, plastic, glass, mica and ceramic materials. Paper was usedextensively in older devices and offers relatively high voltage performance. However, it is susceptible to waterabsorption, and has been largely replaced by plastic film capacitors. Plastics offer better stability and agingperformance, which makes them useful in timer circuits, although they may be limited to low operating temperaturesand frequencies. Ceramic capacitors are generally small, cheap and useful for high frequency applications, althoughtheir capacitance varies strongly with voltage and they age poorly. They are broadly categorized as class 1dielectrics, which have predictable variation of capacitance with temperature or class 2 dielectrics, which can operateat higher voltage. Glass and mica capacitors are extremely reliable, stable and tolerant to high temperatures andvoltages, but are too expensive for most mainstream applications. Electrolytic capacitors and supercapacitors areused to store small and larger amounts of energy, respectively, ceramic capacitors are often used in resonators, andparasitic capacitance occurs in circuits wherever the simple conductor-insulator-conductor structure is formedunintentionally by the configuration of the circuit layout.

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Electrolytic capacitors use an aluminum or tantalum plate with an oxide dielectric layer. The second electrode is aliquid electrolyte, connected to the circuit by another foil plate. Electrolytic capacitors offer very high capacitancebut suffer from poor tolerances, high instability, gradual loss of capacitance especially when subjected to heat, andhigh leakage current. Poor quality capacitors may leak electrolyte, which is harmful to printed circuit boards. Theconductivity of the electrolyte drops at low temperatures, which increases equivalent series resistance. While widelyused for power-supply conditioning, poor high-frequency characteristics make them unsuitable for manyapplications. Electrolytic capacitors will self-degrade if unused for a period (around a year), and when full power isapplied may short circuit, permanently damaging the capacitor and usually blowing a fuse or causing arcing inrectifier tubes. They can be restored before use (and damage) by gradually applying the operating voltage, often doneon antique vacuum tube equipment over a period of 30 minutes by using a variable transformer to supply AC power.Unfortunately, the use of this technique may be less satisfactory for some solid state equipment, which may bedamaged by operation below its normal power range, requiring that the power supply first be isolated from theconsuming circuits. Such remedies may not be applicable to modern high-frequency power supplies as these producefull output voltage even with reduced input.Tantalum capacitors offer better frequency and temperature characteristics than aluminum, but higher dielectricabsorption and leakage.[24]

Polymer capacitors (OS-CON, OC-CON) capacitors use solid conductive polymer (or polymerized organicsemiconductor) as electrolyte and offer longer life and lower ESR at higher cost than standard electrolytic capacitors.A Feedthrough is a component that, while not serving as its main use, has capacitance and is used to conduct signalsthrough a circuit board.Several other types of capacitor are available for specialist applications. Supercapacitors store large amounts ofenergy. Supercapacitors made from carbon aerogel, carbon nanotubes, or highly porous electrode materials, offerextremely high capacitance (up to 5 kF as of 2010) and can be used in some applications instead of rechargeablebatteries. Alternating current capacitors are specifically designed to work on line (mains) voltage AC power circuits.They are commonly used in electric motor circuits and are often designed to handle large currents, so they tend to bephysically large. They are usually ruggedly packaged, often in metal cases that can be easily grounded/earthed. Theyalso are designed with direct current breakdown voltages of at least five times the maximum AC voltage.

Structure

Capacitor packages: SMD ceramic at top left;SMD tantalum at bottom left; through-hole

tantalum at top right; through-hole electrolytic atbottom right. Major scale divisions are cm.

The arrangement of plates and dielectric has many variationsdepending on the desired ratings of the capacitor. For small values ofcapacitance (microfarads and less), ceramic disks use metalliccoatings, with wire leads bonded to the coating. Larger values can bemade by multiple stacks of plates and disks. Larger value capacitorsusually use a metal foil or metal film layer deposited on the surface ofa dielectric film to make the plates, and a dielectric film ofimpregnated paper or plastic – these are rolled up to save space. Toreduce the series resistance and inductance for long plates, the platesand dielectric are staggered so that connection is made at the commonedge of the rolled-up plates, not at the ends of the foil or metalized filmstrips that comprise the plates.

The assembly is encased to prevent moisture entering the dielectric –early radio equipment used a cardboard tube sealed with wax. Modern paper or film dielectric capacitors are dippedin a hard thermoplastic. Large capacitors for high-voltage use may have the roll form compressed to fit into a

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rectangular metal case, with bolted terminals and bushings for connections. The dielectric in larger capacitors isoften impregnated with a liquid to improve its properties.

Several axial-lead electrolytic capacitors.

Capacitors may have their connecting leads arranged in manyconfigurations, for example axially or radially. "Axial" means that theleads are on a common axis, typically the axis of the capacitor'scylindrical body – the leads extend from opposite ends. Radial leadsmight more accurately be referred to as tandem; they are rarely actuallyaligned along radii of the body's circle, so the term is inexact, althoughuniversal. The leads (until bent) are usually in planes parallel to that ofthe flat body of the capacitor, and extend in the same direction; theyare often parallel as manufactured.

Small, cheap discoidal ceramic capacitors have existed since the 1930s,and remain in widespread use. Since the 1980s, surface mount packages for capacitors have been widely used. Thesepackages are extremely small and lack connecting leads, allowing them to be soldered directly onto the surface ofprinted circuit boards. Surface mount components avoid undesirable high-frequency effects due to the leads andsimplify automated assembly, although manual handling is made difficult due to their small size.

Mechanically controlled variable capacitors allow the plate spacing to be adjusted, for example by rotating or slidinga set of movable plates into alignment with a set of stationary plates. Low cost variable capacitors squeeze togetheralternating layers of aluminum and plastic with a screw. Electrical control of capacitance is achievable with varactors(or varicaps), which are reverse-biased semiconductor diodes whose depletion region width varies with appliedvoltage. They are used in phase-locked loops, amongst other applications.

Capacitor markingsMost capacitors have numbers printed on their bodies to indicate their electrical characteristics. Larger capacitorslike electrolytics usually display the actual capacitance together with the unit (for example, 220 μF). Smallercapacitors like ceramics, however, use a shorthand consisting of three numbers and a letter, where the numbers showthe capacitance in pF (calculated as XY × 10Z for the numbers XYZ) and the letter indicates the tolerance (J, K or Mfor ±5%, ±10% and ±20% respectively).Additionally, the capacitor may show its working voltage, temperature and other relevant characteristics.

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ExampleA capacitor with the text 473K 330V on its body has a capacitance of 47 × 103 pF = 47 nF (±10%) with a workingvoltage of 330 V.

Applications

This mylar-film, oil-filled capacitor has very lowinductance and low resistance, to provide the

high-power (70 megawatt) and high speed (1.2microsecond) discharge needed to operate a dye laser.

Energy storage

A capacitor can store electric energy when disconnected from itscharging circuit, so it can be used like a temporary battery.Capacitors are commonly used in electronic devices to maintainpower supply while batteries are being changed. (This preventsloss of information in volatile memory.)

Conventional capacitors provide less than 360 joules per kilogramof energy density, while capacitors using developing technologiescould provide more than 2.52 kilojoules per kilogram.[25]

However, a conventional alkaline battery has a density of 590kJ/kg.

In car audio systems, large capacitors store energy for theamplifier to use on demand. Also for a flash tube a capacitor isused to hold the high voltage.

Pulsed power and weapons

Groups of large, specially constructed, low-inductancehigh-voltage capacitors (capacitor banks) are used to supply hugepulses of current for many pulsed power applications. Theseinclude electromagnetic forming, Marx generators, pulsed lasers (especially TEA lasers), pulse forming networks,radar, fusion research, and particle accelerators.

Large capacitor banks (reservoir) are used as energy sources for the exploding-bridgewire detonators or slapperdetonators in nuclear weapons and other specialty weapons. Experimental work is under way using banks ofcapacitors as power sources for electromagnetic armour and electromagnetic railguns and coilguns.

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Power conditioning

A 10 millifarad capacitor in an amplifier power supply

Reservoir capacitors are used in power supplies wherethey smooth the output of a full or half wave rectifier.They can also be used in charge pump circuits as theenergy storage element in the generation of highervoltages than the input voltage.

Capacitors are connected in parallel with the powercircuits of most electronic devices and larger systems(such as factories) to shunt away and conceal currentfluctuations from the primary power source to providea "clean" power supply for signal or control circuits.Audio equipment, for example, uses several capacitorsin this way, to shunt away power line hum before itgets into the signal circuitry. The capacitors act as alocal reserve for the DC power source, and bypass ACcurrents from the power supply. This is used in car audio applications, when a stiffening capacitor compensates forthe inductance and resistance of the leads to the lead-acid car battery.

Power factor correction

A high-voltage capacitor bank used for powerfactor correction on a power transmision system.

In electric power distribution, capacitors are used for power factorcorrection. Such capacitors often come as three capacitors connected asa three phase load. Usually, the values of these capacitors are given notin farads but rather as a reactive power in volt-amperes reactive (VAr).The purpose is to counteract inductive loading from devices likeelectric motors and transmission lines to make the load appear to bemostly resistive. Individual motor or lamp loads may have capacitorsfor power factor correction, or larger sets of capacitors (usually withautomatic switching devices) may be installed at a load center within abuilding or in a large utility substation.

Supression and coupling

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Signal coupling

Polyester film capacitors are frequently used ascoupling capacitors.

Because capacitors pass AC but block DC signals (when charged up tothe applied dc voltage), they are often used to separate the AC and DCcomponents of a signal. This method is known as AC coupling or"capacitive coupling". Here, a large value of capacitance, whose valueneed not be accurately controlled, but whose reactance is small at thesignal frequency, is employed.

Decoupling

A decoupling capacitor is a capacitor used to protect one part of acircuit from the effect of another, for instance to suppress noise ortransients. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have onthe rest of the circuit. It is most commonly used between the power supply and ground. An alternative name isbypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit.

Noise filters and snubbers

When an inductive circuit is opened, the current through the inductance collapses quickly, creating a large voltageacross the open circuit of the switch or relay. If the inductance is large enough, the energy will generate a spark,causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroying a solid-state switch. Asnubber capacitor across the newly opened circuit creates a path for this impulse to bypass the contact points, therebypreserving their life; these were commonly found in contact breaker ignition systems, for instance. Similarly, insmaller scale circuits, the spark may not be enough to damage the switch but will still radiate undesirable radiofrequency interference (RFI), which a filter capacitor absorbs. Snubber capacitors are usually employed with alow-value resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor combinations are availablein a single package.Capacitors are also used in parallel to interrupt units of a high-voltage circuit breaker in order to equally distributethe voltage between these units. In this case they are called grading capacitors.In schematic diagrams, a capacitor used primarily for DC charge storage is often drawn vertically in circuit diagramswith the lower, more negative, plate drawn as an arc. The straight plate indicates the positive terminal of the device,if it is polarized (see electrolytic capacitor).

Motor startersIn single phase squirrel cage motors, the primary winding within the motor housing is not capable of starting arotational motion on the rotor, but is capable of sustaining one. To start the motor, a secondary "Start" winding has aseries non-polarized starting capacitor to introduce a lead in the sinusoidal current. When the secondary(Start)winding is placed at an angle with respect to the primary(Run) winding, a rotating electric field is created. The forceof the rotational field is not constant, but is sufficient to start the rotor spinning. When the rotor comes close tooperating speed, a centrifugal switch (or current-sensitive relay in series with the main winding) disconnects thecapacitor. The start capacitor is typically mounted to the side of the motor housing. These are called capacitor-startmotors, that have relatively high starting torque. Typically they can have up-to 4 times as much starting torque than asplit-phase motor and are used on applications such as compressors, pressure washers and any small device requiringhigh starting torques.Capacitor-run induction motors have a permanently connected phase-shifting capacitor in series with a secondwinding. The motor is much like a two-phase induction motor.

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Capacitor 15

Motor-starting capacitors are typically non-polarized electrolytic types, while running capacitors are conventionalpaper or plastic film dielectric types.

Signal processingThe energy stored in a capacitor can be used to represent information, either in binary form, as in DRAMs, or inanalogue form, as in analog sampled filters and CCDs. Capacitors can be used in analog circuits as components ofintegrators or more complex filters and in negative feedback loop stabilization. Signal processing circuits also usecapacitors to integrate a current signal.

Tuned circuits

Capacitors and inductors are applied together in tuned circuits to select information in particular frequency bands.For example, radio receivers rely on variable capacitors to tune the station frequency. Speakers use passive analogcrossovers, and analog equalizers use capacitors to select different audio bands.The resonant frequency f of a tuned circuit is a function of the inductance (L) and capacitance (C) in series, and isgiven by:

where L is in henries and C is in farads.

SensingMost capacitors are designed to maintain a fixed physical structure. However, various factors can change thestructure of the capacitor, and the resulting change in capacitance can be used to sense those factors.Changing the dielectric:

The effects of varying the characteristics of the dielectric can be used for sensing purposes. Capacitors with anexposed and porous dielectric can be used to measure humidity in air. Capacitors are used to accuratelymeasure the fuel level in airplanes; as the fuel covers more of a pair of plates, the circuit capacitance increases.

Changing the distance between the plates:Capacitors with a flexible plate can be used to measure strain or pressure. Industrial pressure transmitters usedfor process control use pressure-sensing diaphragms, which form a capacitor plate of an oscillator circuit.Capacitors are used as the sensor in condenser microphones, where one plate is moved by air pressure, relativeto the fixed position of the other plate. Some accelerometers use MEMS capacitors etched on a chip tomeasure the magnitude and direction of the acceleration vector. They are used to detect changes inacceleration, e.g. as tilt sensors or to detect free fall, as sensors triggering airbag deployment, and in manyother applications. Some fingerprint sensors use capacitors. Additionally, a user can adjust the pitch of atheremin musical instrument by moving his hand since this changes the effective capacitance between theuser's hand and the antenna.

Changing the effective area of the plates:Capacitive touch switches are now used on many consumer electronic products.

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Hazards and safety

Swollen caps of electrolytic capacitors - specialdesign of semi-cut caps prevents capacitors from

bursting

This high-energy capacitor from a defibrillatorcan deliver over 500 joules of energy. A resistoris connected between the terminals for safety, to

allow the stored energy to be released.

Capacitors may retain a charge long after power is removed from acircuit; this charge can cause dangerous or even potentially fatalshocks or damage connected equipment. For example, even aseemingly innocuous device such as a disposable camera flash unitpowered by a 1.5 volt AA battery contains a capacitor which may becharged to over 300 volts. This is easily capable of delivering a shock.Service procedures for electronic devices usually include instructionsto discharge large or high-voltage capacitors. Capacitors may also havebuilt-in discharge resistors to dissipate stored energy to a safe levelwithin a few seconds after power is removed. High-voltage capacitorsare stored with the terminals shorted, as protection from potentiallydangerous voltages due to dielectric absorption.

Some old, large oil-filled paper or plastic film capacitors containpolychlorinated biphenyls (PCBs). It is known that waste PCBs canleak into groundwater under landfills. Capacitors containing PCB werelabelled as containing "Askarel" and several other trade names.PCB-filled paper capacitors are found in very old (pre-1975)fluorescent lamp ballasts, and other applications.

Capacitors may catastrophically fail when subjected to voltages orcurrents beyond their rating, or as they reach their normal end of life.Dielectric or metal interconnection failures may create arcing thatvaporizes the dielectric fluid, resulting in case bulging, rupture, or evenan explosion. Capacitors used in RF or sustained high-currentapplications can overheat, especially in the center of the capacitor rolls.Capacitors used within high-energy capacitor banks can violentlyexplode when a short in one capacitor causes sudden dumping ofenergy stored in the rest of the bank into the failing unit. High voltagevacuum capacitors can generate soft X-rays even during normaloperation. Proper containment, fusing, and preventive maintenance canhelp to minimize these hazards.

High-voltage capacitors can benefit from a pre-charge to limit in-rushcurrents at power-up of high voltage direct current (HVDC) circuits. This will extend the life of the component andmay mitigate high-voltage hazards.

Notes[1] Henry Smith Williams. "A History of Science Volume II, Part VI: The Leyden Jar Discovered" (http:/ / www. worldwideschool. org/ library/

books/ sci/ history/ AHistoryofScienceVolumeII/ chap49. html). .[2] Houston, Edwin J. (1905). Electricity in Every-day Life (http:/ / books. google. com/ ?id=ko9BAAAAIAAJ& pg=PA71& dq=jar+ "von+

Kleist"). P. F. Collier & Son. .[3] Keithley, Joseph (1999). The Story of Electrical and Magnetic Measurements From 500 BC to the 1940s (http:/ / books. google. com/

?id=uwgNAtqSHuQC& printsec=frontcover& q). IEEE Press. p. 23. ISBN 0-7803-1193-0. .

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Catastrophic failure

[4] Isaacson, Walter (2003). Benjamin Franklin (http:/ / books. google. com/?id=oIW915dDMBwC& lpg=PA135& dq="benjamin franklin" leyden jar&pg=PA136#v=onepage& q=). Simon and Schuster. p. 136. ISBN 0684807610,9780684807614. .

[5] Franklin, Benjamin (1749-04-29). "Experiments & Observations on Electricity:Letter IV to Peter Collinson" (http:/ / www. chemteam. info/ Chem-History/Franklin-1749/ Franklin-1749-all. pdf) (PDF). pp. (page 28). . Retrieved 2009-08-09.

[6] Morse, Robert A., Ph.D. (September 2004). "Franklin and Electrostatics—Ben Franklin as my Lab Partner" (http:/ / www. tufts. edu/ as/wright_center/ personal_pages/ bob_m/ 04_Franklin_Lab_Part_IV. pdf) (PDF). Wright Center for Science Education. Tufts University.pp. (page 23). . Retrieved 2009-08-10. "After Volta’s discovery of the electrochemical cell in 1800, the term was then applied to a group ofelectrochemical cells"

[7] "Sketch of Alessandro Volta" (http:/ / books. google. com/ books?id=eCADAAAAMBAJ& pg=PA117& source=gbs_toc_r&cad=1#v=onepage& q& f=false). The Popular Science Monthly (New York): pp. 118–119. May–Oct 1892. .

[8][8] Ulaby, p.168[9][9] Ulaby, p.157[10][10] Ulaby, p.169[11] Hammond, P, Electromagnetism for Engineers, pp44-45, Pergamon Press, 1965.[12][12] Dorf, p.263[13][13] Dorf, p.260[14] "Capacitor charging and discharging : DC CIRCUITS" (http:/ / www. allaboutcircuits. com/ vol_6/ chpt_3/ 17. html). All About Circuits. .

Retrieved 2009-02-19.[15] F. F. Mazda, Discrete electronic components, Cambridge University Press,1981 ISBN 0521234700, page 71[16] http:/ / electrochem. cwru. edu/ encycl/ misc/ c04-appguide. pdf Retrieved 10/28/2011[17][17] Ulaby, p.170[18] S. T. Pai and Qi Zhang (1995). Introduction to High Power Pulse Technology (http:/ / books. google. com/ ?id=spZ_H4nwIN0C&

pg=PA47& dq=breakdown+ field+ energy-density+ dielectric). World Scientific. ISBN 9810217145. .[19] Stephen A. Dyer (ed) Survey of instrumentation and measurement ,Wiley-IEEE, 2001 ISBN 047139484X page 397[20] Scherz, P, Practical Electronics for Inventors, p100, McGraw-Hill Professional, 2006, ISBN 0071452818.[21] Bird, J, Electrical Circuit Theory and Technology, p501, Newnes, 2007, ISBN 075068139X.[22] Ceramic Capacitor Aging Made Simple (http:/ / www. johansondielectrics. com/ technicalnotes/ age/ )[23] http:/ / www. ga-esi. com/ support/ ep/ tech-bulletins/ voltage-reversal. pdf[24] Ask The Applications Engineer – 21 (http:/ / www. analog. com/ library/ analogDialogue/ Anniversary/ 21. html) , Steve Guinta, Analog

Devices[25] Next-gen car solution? Scientists expand uses for electrostatic capacitor (http:/ / cleantech. com/ news/ 4278/

next-gen-car-solution-capacitor)

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Capacitor 18

References• Dorf, Richard C.; Svoboda, James A. (2001). Introduction to Electric Circuits (5th ed.). New York: John Wiley

and Sons, Inc.. ISBN 0-471-38689-8.• Ulaby, Fawwaz T. (1999). Fundamentals of Applied Electromagnetics (1999 ed.). Upper Saddle River, New

Jersey: Prentice-Hall. ISBN 0-13-011554-1.• Zorpette, Glenn (2005). "Super Charged: A Tiny South Korean Company is Out to Make Capacitors Powerful

enough to Propel the Next Generation of Hybrid-Electric Cars" (http:/ / www. spectrum. ieee. org/ jan05/ 2777).IEEE Spectrum 42 (1): 32. doi:10.1109/MSPEC.2005.1377872. ISSN 0018-9235.

• The ARRL Handbook for Radio Amateurs (68th ed.). Newington CT USA: The Amateur Radio Relay League.1991.

• Huelsman, Lawrence P. (1972). Basic Circuit Theory with Digital Computations. Series in computer applicationsin electrical engineering. Englewood Cliffs: Prentice-Hall. ISBN 0-13-057430-9.

• 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)• Spark Museum (http:/ / www. sparkmuseum. com/ BOOK_LEYDEN. HTM) (von Kleist and Musschenbroek)• Biography of von Kleist (http:/ / www. acmi. net. au/ AIC/ VON_KLEIST_BIO. html)

External links• Howstuffworks.com: How Capacitors Work (http:/ / electronics. howstuffworks. com/ capacitor. htm/ printable)• How Capacitors are made (http:/ / www. engineersgarage. com/ insight/ how-capacitor-works)• CapSite 2009: Introduction to Capacitors (http:/ / my. execpc. com/ ~endlr/ )• Capacitor Tutorial (http:/ / www. sentex. ca/ ~mec1995/ gadgets/ caps/ caps. html) – Includes how to read

capacitor temperature codes• Introduction to Capacitor and Capacitor codes (http:/ / www. robotplatform. com/ electronics/ capacitor/

capacitor. html)• Low ESR Capacitor Manufacturers (http:/ / www. capacitorlab. com/ low-esr-capacitor-manufacturers/ )• How Capacitor Works - Capacitor Markings and Color Codes (http:/ / freecircuits. org/ 2012/ 01/

capacitors-basics-working/ )

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Article Sources and ContributorsCapacitor  Source: http://en.wikipedia.org/w/index.php?oldid=482973677  Contributors: 10v1walsha, 124Nick, 31.6, 4.35.185.xxx, A. B., A. 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Image Sources, Licenses and ContributorsImage:Photo-SMDcapacitors.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Photo-SMDcapacitors.jpg  License: Public Domain  Contributors: ShaddackFile:Condensador electrolitico 150 microF 400V.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Condensador_electrolitico_150_microF_400V.jpg  License: unknown Contributors: WilltronImage:Leidse flessen Museum Boerhave december 2003 2.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Leidse_flessen_Museum_Boerhave_december_2003_2.jpg  License:GNU Free Documentation License  Contributors: Original uploader was Alvinrune at en.wikipediaImage:Capacitor schematic with dielectric.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Capacitor_schematic_with_dielectric.svg  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: Papa NovemberFile:Plattenkondensator hg.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Plattenkondensator_hg.jpg  License: Creative Commons Attribution 3.0  Contributors: Hannes Grobe(talk)Image:RC switch.svg  Source: http://en.wikipedia.org/w/index.php?title=File:RC_switch.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: PureCoreImage:Parallel plate capacitor.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Parallel_plate_capacitor.svg  License: Public Domain  Contributors: inductiveloadImage:capacitors in parallel.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Capacitors_in_parallel.svg  License: GNU Free Documentation License  Contributors: OmegatronImage:capacitors in series.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Capacitors_in_series.svg  License: GNU Free Documentation License  Contributors: OmegatronImage:CircuitosEquivalentesCondensador.png  Source: http://en.wikipedia.org/w/index.php?title=File:CircuitosEquivalentesCondensador.png  License: Public Domain  Contributors: JoséLuis GálvezImage:Condensators.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Condensators.JPG  License: GNU Free Documentation License  Contributors: Duesentrieb, Joanjoc, Matijap,Vladsinger, 1 anonymous editsFile:Axial electrolytic capacitors.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Axial_electrolytic_capacitors.jpg  License: Creative Commons Attribution 3.0  Contributors:MataresephotosFile:Mylar-film oil-filled low-inductance capacitor 6.5 MFD @ 5000 VDC.jpg  Source:http://en.wikipedia.org/w/index.php?title=File:Mylar-film_oil-filled_low-inductance_capacitor_6.5_MFD_@_5000_VDC.jpg  License: Creative Commons Zero  Contributors: User:ZaerethFile:Capacitor.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Capacitor.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Daniel Christensen aten.wikipediaFile:Condensor bank 150kV - 75MVAR.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Condensor_bank_150kV_-_75MVAR.jpg  License: Public Domain  Contributors: PhilippeMertensFile:Polyester film capacitor.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Polyester_film_capacitor.jpg  License: GNU Free Documentation License  Contributors:MataresephotosFile:Defekte Kondensatoren.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Defekte_Kondensatoren.jpg  License: Creative Commons Attribution-Sharealike 2.0  Contributors: UserSmial on de.wikipediaFile:High-energy capacitor from a defibrillator 42 MFD @ 5000 VDC.jpg  Source:http://en.wikipedia.org/w/index.php?title=File:High-energy_capacitor_from_a_defibrillator_42_MFD_@_5000_VDC.jpg  License: Creative Commons Zero  Contributors: User:ZaerethFile:Exploded Electrolytic Capacitor.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Exploded_Electrolytic_Capacitor.jpg  License: Creative Commons Attribution-Sharealike 3.0 Contributors: User:Frizb99

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