capacitor - wikipedia, the free encyclopedia

Capacitor

Capacitor

Modern capacitors, by a cm rule.

Type Passive

Invented Ewald Georg von Kleist (October 1745)

Electronic symbol

A typical electrolytic capacitor

From Wikipedia, the free encyclopedia

A capacitor or condenser is a passive electronic componentconsisting of a pair of conductors separated by a dielectric.When a potential difference exists across the conductors, anelectric field is present in the dielectric. This field storesenergy and produces a mechanical force between the plates.The effect is greatest between wide, flat, parallel, narrowlyseparated conductors.

An ideal capacitor is characterized by a single constant value,capacitance, which is measured in farads. This is the ratio ofthe electric charge on each conductor to the potentialdifference between them. In practice, the dielectric betweenthe plates passes a small amount of leakage current. Theconductors and leads introduce an equivalent series resistanceand the dielectric has an electric field strength limit resulting ina breakdown voltage.

Capacitors are widely used in electronic circuits to block theflow of direct current while allowing alternating current topass, to filter out interference, to smooth the output of powersupplies, and for many other purposes. They are used in resonant circuits inradio frequency equipment to select particular frequencies from a signal withmany frequencies.

Contents1 History2 Theory of operation

2.1 Energy storage2.2 Current-voltage relation2.3 DC circuits2.4 AC circuits2.5 Parallel plate model2.6 Networks

3 Non-ideal behaviour3.1 Breakdown voltage3.2 Equivalent circuit3.3 Ripple current3.4 Instability of capacitance

12/9/2009 Capacitor - Wikipedia, the free encyclo…

http://en.wikipedia.org/wiki/Capacitor 1/17

Battery of four Leyden jars inMuseum Boerhave, Leiden, the

Netherlands.

4 Capacitor types4.1 Dielectric materials4.2 Structure

5 Capacitor markings6 Applications

6.1 Energy storage6.2 Pulsed power and weapons6.3 Power conditioning

6.3.1 Power factor correction

6.4 Supression and coupling6.4.1 Signal coupling6.4.2 Decoupling6.4.3 Noise filters and snubbers

6.5 Motor starters6.6 Signal processing

6.6.1 Tuned circuits

6.7 Sensing

7 Hazards and safety8 See also9 Notes10 References11 External links

HistoryIn October 1745, Ewald Georg von Kleist of Pomerania in Germany found thatcharge could be stored by connecting a high voltage electrostatic generator by awire to a volume of water in a hand-held glass jar.[1] Von Kleist's hand and thewater acted as conductors and the jar as a dielectric (although details of themechanism were incorrectly identified at the time). Von Kleist found that afterremoving the generator, touching the wire resulted in a painful spark. In a letterdescribing the experiment, he said "I would not take a second shock for thekingdom of France."[2] The following year, the Dutch physicist Pieter vanMusschenbroek invented a similar capacitor, which was named the Leyden jar,after the University of Leyden where he worked.[3] Daniel Gralath was the firstto combine several jars in parallel into a "battery" to increase the charge storagecapacity.

Benjamin Franklin investigated the Leyden jar and proved that the charge wasstored on the glass, not in the water as others had assumed. He also created theterm "battery",[4][5] (as in a battery of cannon), subsequently applied to clustersof electrochemical cells.[6] Leyden jars were later to be made by coating the

inside and outside of jars with metal foil, leaving a space at the mouth to prevent arcing between the foils. The



Charge separation in aparallel-plate capacitor

causes an internalelectric field. A

dielectric (orange)reduces the field and

increases thecapacitance.

A simple demonstration of aparallel-plate capacitor

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,and the 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 firstused for 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.

Theory of operationMain article: Capacitance

A capacitor consists of two conductors separatedby a non-conductive region.[7] The non-conductivesubstance is called the dielectric medium, althoughthis may also mean a vacuum or a semiconductordepletion region chemically identical to theconductors. A capacitor is assumed to be self-contained and isolated, with no net electric chargeand no influence from an external electric field. Theconductors thus contain equal and opposite chargeson their facing surfaces,[8] and the dielectriccontains an electric field. The capacitor is areasonably general model for electric fields within

electric circuits.

An ideal capacitor is wholly characterized by a constant capacitance 'C', defined as theratio of charge ±'Q' on each conductor to the voltage 'V' between them:[7]

Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to vary. In this case,capacitance is defined in terms of incremental changes:

In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage ofone volt across the device.[9]

Energy storage

Work must be done by an external influence to move charge between the conductors in a capacitor. When the



A simple resistor-capacitorcircuit demonstrates charging

of a capacitor.

external influence is removed, the charge separation persists and energy is stored in the electric field. If charge islater allowed to return to its equilibrium position, the energy is released. The work done in establishing the electricfield, and hence the amount of energy stored, is given by:[10]

Current-voltage relation

The current i (t ) through a component in an electric circuit is defined as the rate of change of the charge q (t ) thathas passed through it. Physical charges cannot pass through the dielectric layer of a capacitor, but rather build up inequal and opposite quantities on the electrodes: as each electron accumulates on the negative plate, one leaves thepositive plate. Thus the accumulated charge on the electrodes is equal to the integral of the current, as well as beingproportional to the voltage (as discussed above). As with any antiderivative, a constant of integration is added torepresent the initial voltage v (t0). This is the integral form of the capacitor equation,[11]

.

Taking the derivative of this, and multiplying by C, yields the derivative form,[12]

.

The dual of the capacitor is the inductor, which stores energy in the magnetic field rather than the 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 series circuit containing only a resistor, a capacitor, a switch and a constantDC source of voltage V0 is known as a charging circuit.[13] If the capacitor isinitially uncharged while the switch is open, and the switch is closed at t = 0, itfollows from Kirchhoff's voltage law that

Taking the derivative and multiplying by C, gives a first-order differentialequation,

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 τ0 = RC is the time constant of the system.

As the capacitor reaches equilibrium with the source voltage, the voltage 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 V0 and the final voltage being zero.

AC circuits

See also: reactance (electronics) and electrical impedance#Deriving the device specific impedances

Impedance, 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 velocity 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.

Note that impedance decreases with increasing capacitance and increasing frequency. This implies that a higher-frequency signal or a larger capacitor results in a lower voltage amplitude per current amplitude—an AC "shortcircuit" or AC coupling. Conversely, for very low frequencies, the reactance will be high, so that a capacitor isnearly an open circuit 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

The simplest capacitor consists of two parallel conductive plates separated by adielectric with permittivity ε (such as air). The model may also be used to makequalitative predictions for other device geometries. The plates are considered toextend uniformly over an area A and a charge density ±ρ = ±Q/A exists on theirsurface. Assuming that the width of the plates is much greater than theirseparation d, the electric field near the centre of the device will be uniform withthe magnitude E = ρ/ε. The voltage is defined as the line integral of the electric



Dielectric is placed betweentwo conducting plates, each ofarea A and with a separation of

d.

Several capacitors in parallel.

Several capacitors in series.

field between the plates

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

.

The capacitance is therefore greatest in devices made from materials with a high permittivity.

Networks

See also: Series and parallel circuits

For capacitors in parallel

Capacitors in a parallel configuration each have the same applied voltage.Their capacitances add up. Charge is apportioned among them by size.Using the schematic diagram to visualize parallel plates, it is apparent thateach capacitor contributes to the total surface area.

For capacitors in series

Connected in series, the schematic diagram reveals that the separationdistance, not the plate area, adds up. The capacitors each storeinstantaneous charge build-up equal to that of every other capacitor in theseries. The total voltage difference from end to end is apportioned toeach capacitor according to the inverse of its capacitance. The entireseries acts as a capacitor smaller 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. In such anapplication, several series connections may in turn be connected in parallel, forming a matrix. The goal is tomaximize the energy storage utility of each capacitor without overloading it.

Series connection is also used to adapt electrolytic capacitors for AC use.

Non-ideal behaviour



Capacitors deviate from the ideal capacitor equation in a number of ways. Some of these, such as leakage currentand parasitic effects are linear, or can be assumed to be linear, and can be dealt with by adding virtual componentsto the equivalent circuit of the capacitor. The usual methods of network analysis can then be applied. In other cases,such as with breakdown voltage, the effect is non-linear and normal (i.e., linear) network analysis cannot be used,the effect must be dealt with separately. There is yet another group, which may be linear but invalidate theassumption in the analysis that capacitance is a constant. Such an example is temperature dependence.

Breakdown voltage

Main article: Breakdown voltage

Above 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,[14]

Vbd = Edsd

The maximum energy that can be stored safely in a capacitor is limited by the breakdown voltage. Due to thescaling of capacitance and breakdown voltage with dielectric thickness, all capacitors made with a particulardielectric have approximately equal maximum energy density, to the extent that the dielectric dominates theirvolume.[15]

For air dielectric capacitors the breakdown field strength is of the order 107 V/m and will be much less when othermaterials are used for the dielectric. The absolute breakdown voltage of most capacitors is nowhere near such ahigh number because of the very small distance between the plates. Typical ratings for capacitors used for generalelectronics applications range from a few volts to 100V or so. For high voltage applications physically much largercapacitors have to be used. In this field, there are a number of factors that can dramatically reduce the breakdownvoltage below that to be expected by considering the breakdown field strength of the dielectric alone. For one thing,the geometry of the capacitor conductive parts (plates and connecting wires) is important. In particular, sharp edgesor points hugely increase the electric field strength at that point and can lead to a local breakdown. Once this startsto happen, the breakdown will quickly "track" through the dielectric till it reaches the opposite plate and cause ashort circuit.[16]

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.[17]

Equivalent circuit

An ideal capacitor only stores and releases electrical energy,without dissipating any. In reality, all capacitors have imperfectionswithin the capacitor's material that create resistance. This isspecified as the equivalent series resistance or ESR of acomponent. This adds a real component to the impedance:



Two equivalent circuits of a real capacitor

As frequency approaches infinity, the capacitive impedance (orreactance) approaches zero and the ESR becomes significant. Asthe reactance becomes negligible, power dissipation approachesPRMS. = VRMS.² /RESR.

Similarly to ESR, the capacitor's leads add equivalent seriesinductance or ESL to the component. This is usually significant only at relatively high frequencies. As inductivereactance is positive and increases with frequency, above a certain frequency capacitance will be canceled byinductance. High frequency engineering 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,[9] and slowlydischarges over time (time may vary greatly depending on the capacitor material and quality).

Ripple current

Ripple current is the AC component of an applied source (often a switched-mode power supply) whose frequencymay be constant or varying. Certain types of capacitors, such as electrolytic tantalum capacitors, usually have arating for maximum ripple current (both in frequency and magnitude). This ripple current can cause damaging heat tobe generated within the capacitor due to the current flow across resistive imperfections in the materials used withinthe capacitor, more commonly referred to as equivalent series resistance (ESR). For example electrolytic tantalumcapacitors are limited by ripple current and generally have the highest ESR ratings in the capacitor family, whileceramic capacitors generally have no ripple current limitation and have some of the lowest ESR ratings.

Instability of capacitance

The capacitance of certain capacitors decreases as the component ages. In ceramic capacitors, this is caused bydegradation of the dielectric. The type of dielectric and the ambient operating and storage temperatures are themost significant aging factors, while the operating voltage has a smaller effect. The aging process may be reversedby heating the component above the Curie point. Aging is fastest near the beginning of life of the component, andthe device stabilizes over time.[18] 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 older components, can absorb sound waves resulting in a microphonic effect. Vibrationmoves the plates, causing the capacitance to vary, in turn inducing AC current. Some dielectrics also generatepiezoelectricity. The resulting interference is especially problematic in audio applications, potentially causingfeedback or unintended recording. In the reverse microphonic effect, the varying electric field between the capacitorplates exerts a physical force, moving them as a speaker. This can generate audible sound, but drains energy andstresses the dielectric and the electrolyte, if any.



Capacitor materials. From left:multilayer ceramic, ceramic

disc, multilayer polyester film,tubular ceramic, polystyrene,

metalized polyester film,aluminum electrolytic. Major

scale divisions are incentimetres.

Capacitor typesMain article: Types of capacitor

Practical 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.

Dielectric materials

Most types of capacitor include a dielectric spacer, which increases theircapacitance. These dielectrics are most often insulators. However, lowcapacitance devices are available with a vacuum between their plates, whichallows extremely high voltage operation and low losses. Variable capacitors withtheir plates open to the atmosphere were commonly used in radio tuning circuits.Later designs use polymer foil dielectric between the moving and stationaryplates, with no significant air space between them.

Several solid dielectrics are available, including paper, plastic, glass, mica andceramic materials. Paper was used extensively in older devices and offersrelatively high voltage performance. However, it is susceptible to waterabsorption, and has been largely replaced by plastic film capacitors. Plasticsoffer better stability and aging performance, which makes them useful in timercircuits, although they may be limited to low operating temperatures and frequencies. Ceramic capacitors aregenerally small, cheap and useful for high frequency applications, although their capacitance varies strongly withvoltage and they age poorly. They are broadly categorized as class 1 dielectrics, which have predictable variation ofcapacitance with temperature or class 2 dielectrics, which can operate at higher voltage. Glass and mica capacitorsare extremely reliable, stable and tolerant to high temperatures and voltages, but are too expensive for mostmainstream applications. Electrolytic capacitors and supercapacitors are used to store small and larger amounts ofenergy, respectively, ceramic capacitors are often used in resonators, and parasitic capacitance occurs in circuitswherever the simple conductor-insulator-conductor structure is formed unintentionally by the configuration of thecircuit layout.

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. The conductivity of the electrolyte drops at low temperatures, which increases equivalentseries resistance. While widely used for power-supply conditioning, poor high-frequency characteristics make themunsuitable for many applications. Tantalum capacitors offer better frequency and temperature characteristics thanaluminum, but higher dielectric absorption and leakage.[19] OS-CON (or OC-CON) capacitors are a polymerizedorganic semiconductor solid-electrolyte type that offer longer life at higher cost than standard electrolytic capacitors.

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 (as much as 3000 farads) 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 tobe physically large. They are usually ruggedly packaged, often in metal cases that can be easily grounded/earthed.



Capacitor packages: SMDceramic at top left; SMDtantalum at bottom left;

through-hole tantalum at topright; through-hole electrolytic

at bottom right. Major scaledivisions are cm.

They also are designed with direct current breakdown voltages of at least five times the maximum AC voltage.

Structure

The arrangement of plates and dielectric has many variations depending on thedesired ratings of the capacitor. For small values of capacitance (microfaradsand less), ceramic disks use metallic coatings, with wire leads bonded to thecoating. Larger values can be made by multiple stacks of plates and disks.Larger value capacitors usually use a metal foil or metal film layer deposited onthe surface of a dielectric film to make the plates, and a dielectric film ofimpregnated paper or plastic – these are rolled up to save space. To reduce theseries resistance and inductance for long plates, the plates and dielectric arestaggered so that connection is made at the common edge of the rolled-upplates, not at the ends of the foil or metalized film strips that comprise the plates.

The assembly is encased to prevent moisture entering the dielectric – early radioequipment used a cardboard tube sealed with wax. Modern paper or filmdielectric capacitors are dipped in a hard thermoplastic. Large capacitors forhigh-voltage use may have the roll form compressed to fit into a rectangularmetal case, with bolted terminals and bushings for connections. The dielectric in

larger capacitors is often impregnated with a liquid to improve its properties.

Capacitors may have their connecting leads arranged in many configurations, for example axially or radially. "Axial"means that the leads are on a common axis, typically the axis of the capacitor's cylindrical body – the leads extendfrom opposite ends. Radial leads might more accurately be referred to as tandem; they are rarely actually alignedalong radii of the body's circle, so the term is inexact, although universal. The leads (until bent) are usually in planesparallel to that of the flat body of the capacitor, and extend in the same direction; they are often parallel asmanufactured.

Small, cheap discoidal ceramic capacitors have existed since the 1930s, and remain in widespread use. Since the1980s, surface mount packages for capacitors have been widely used. These packages are extremely small andlack connecting leads, allowing them to be soldered directly onto the surface of printed circuit boards. Surfacemount components avoid undesirable high-frequency effects due to the leads and simplify 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 orsliding a set of movable plates into alignment with a set of stationary plates. Low cost variable capacitors squeezetogether alternating layers of aluminum and plastic with a screw. Electrical control of capacitance is achievable withvaractors (or varicaps), which are reverse-biased semiconductor diodes whose depletion region width varies withapplied voltage. They are used in phase-locked loops, amongst other applications.

Capacitor markingsMost capacitors have numbers printed on their bodies to indicate their electrical characteristics. Many are indicatedwith numbers "XYZ", then letters "J or K or M" and finally "VOLTS V". XYZ represents the capacitance in picoFarads (calculated as XY x EXP Z), the letters J, K or M indicate the tolerance (±5%, ±10% and ±20%respectively) and VOLTS V represents the working voltage.



A 10,000 microfarad capacitor in a TRM-800 amplifier

Example: A capacitor has the following text on its body: "105K 330 V".

The capacitance is: 10 x EXP 5 pF = 1,000,000 pF = 1µF. The tolerance K = ±10%. The voltage = 330 V.

ApplicationsMain article: Applications of capacitors

Capacitors have many uses in electronic and electrical systems. They are so common that it is a rare electricalproduct that does not include at least one for some purpose.

Energy storage

A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporarybattery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are beingchanged. (This prevents loss of information in volatile memory.)

Conventional electrostatic capacitors provide less than 360 joules per kilogram of energy density, while capacitorsusing developing technologies can provide more than 2.52 kilojoules per kilogram[20].

In car audio systems, large capacitors store energy for the amplifier to use on demand. Also for a flash tube acapacitor is used to hold the high voltage.

Pulsed power and weapons

Groups of large, specially constructed, low-inductance high-voltage capacitors (capacitor banks) are used tosupply huge pulses of current for many pulsed power applications. These include electromagnetic forming, Marxgenerators, pulsed lasers (especially TEA lasers), pulse forming networks, radar, fusion research, and particleaccelerators.

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.

Power conditioning

Reservoir capacitors are used in power supplies where they smooththe output of a full or half wave rectifier. They can also be used incharge pump circuits as the energy storage element in the generationof higher voltages than the input voltage.

Capacitors are connected in parallel with the power circuits of mostelectronic devices and larger systems (such as factories) to shuntaway and conceal current fluctuations from the primary powersource to provide a "clean" power supply for signal or controlcircuits. Audio equipment, for example, uses several capacitors inthis way, to shunt away power line hum before it gets into the signalcircuitry. The capacitors act as a local reserve for the DC power



800 amplifiersource, and bypass AC currents from the power supply. This is usedin car audio applications, when a stiffening capacitor compensatesfor the inductance and resistance of the leads to the lead-acid car battery.

Power factor correction

In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as threecapacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but ratheras a reactive power in volt-amperes reactive (VAr). The purpose is to counteract inductive loading from deviceslike electric motors and transmission lines to make the load appear to be mostly resistive. Individual motor or lamploads may have capacitors for power factor correction, or larger sets of capacitors (usually with automatic switchingdevices) may be installed at a load center within a building or in a large utility substation.

Supression and coupling

Signal coupling

Main article: capacitive coupling

Because capacitors pass AC but block DC signals (when charged up to the applied dc voltage), they are oftenused to separate the AC and DC components of a signal. This method is known as AC coupling or "capacitivecoupling". Here, a large value of capacitance, whose value need not be accurately controlled, but whose reactanceis small at the signal frequency, is employed.

Decoupling

Main article: decoupling capacitor

A decoupling capacitor is a capacitor used to protect one part of a circuit from the effect of another, for instance tosuppress noise or transients. Noise caused by other circuit elements is shunted through the capacitor, reducing theeffect they have on the rest of the circuit. It is most commonly used between the power supply and ground. Analternative name is bypass capacitor as it is used to bypass the power supply or other high impedance componentof 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,thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance. Similarly,in smaller 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 areavailable in a single package.

Capacitors are also used in parallel to interrupt units of a high-voltage circuit breaker in order to equally distribute



the 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 starters

Main article: motor capacitor

In 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 winding is used inseries with a non-polarized starting capacitor to introduce a lag in the sinusoidal current through the startingwinding. When the secondary winding is placed at an angle with respect to the primary winding, a rotating electricfield is created. The force of the rotational field is not constant, but is sufficient to start the rotor spinning. When therotor comes close to operating speed, a centrifugal switch (or current-sensitive relay in series with the main winding)disconnects the capacitor. The start capacitor is typically mounted to the side of the motor housing. These are calledcapacitor-start motors, that have relatively high starting torque.

There are also capacitor-run induction motors which have a permanently-connected phase-shifting capacitor inseries with a second winding. The motor is much like a two-phase induction motor.

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

Signal processing

The 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.

Sensing



Most 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 physical and/or electrical characteristics of the dielectric can be used for sensingpurposes. Capacitors with an exposed and porous dielectric can be used to measure humidity in air.Capacitors are used to accurately measure the fuel level in airplanes; as the fuel covers more of a pair ofplates, 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 transmittersused for process control use pressure-sensing diaphragms, which form a capacitor plate of an oscillatorcircuit. Capacitors are used as the sensor in condenser microphones, where one plate is moved by airpressure, relative to the fixed position of the other plate. Some accelerometers use MEMS capacitors etchedon a chip to measure the magnitude and direction of the acceleration vector. They are used to detect changesin acceleration, 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.

Hazards and safetyCapacitors may retain a charge long after power is removed from a circuit; this charge can cause shocks or damageto connected equipment. For example, even a seemingly innocuous device such as a disposable camera flash unitpowered by a 1.5 volt AA battery contains a capacitor which may be charged to over 300 volts. This is easilycapable of delivering a shock. Service procedures for electronic devices usually include instructions to dischargelarge or high-voltage capacitors. Capacitors may also have built-in discharge resistors to dissipate stored energy toa safe level within a few seconds after power is removed. High-voltage capacitors are stored with the terminalsshorted, as protection from potentially dangerous voltages due to dielectric absorption.

Some old, large oil-filled capacitors contain polychlorinated biphenyls (PCBs). It is known that waste PCBs canleak into groundwater under landfills. Capacitors containing PCB were labelled as containing "Askarel" and severalother trade names. PCB-filled capacitors are found in very old (pre 1975) fluorescent lamp ballasts, and otherapplications.

High-voltage capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or asthey reach their normal end of life. Dielectric or metal interconnection failures may create arcing that vaporizesdielectric fluid, resulting in case bulging, rupture, or even an explosion. Capacitors used in RF or sustained high-current applications can overheat, especially in the center of the capacitor rolls. Capacitors used within high-energycapacitor banks can violently explode when a short in one capacitor causes sudden dumping of energy stored in therest of the bank into the failing unit. High voltage vacuum capacitors can generate soft X-rays even during normaloperation. Proper containment, fusing, and preventive maintenance can help to minimize these hazards.



High-voltage capacitors can benefit from a pre-charge to limit in-rush currents at power-up of high voltage directcurrent (HVDC) circuits. This will extend the life of the component and may mitigate high-voltage hazards.

See also

Capacitance meterCapacitor plague: capacitor failures on computermotherboardsCircuit designTypes of capacitorDecoupling capacitorElectric displacement field

Electronic oscillatorFilter capacitorLight emitting capacitorMemristorVacuum variable capacitor

Notes1. ^ 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) ".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/books?id=ko9BAAAAIAAJ&pg=PA71&dq=jar+%22von+Kleist%22&lr=&as_brr=1&ei=aniTR_uCJ5HwsgOQ5bU4#PPA71,M1) . P. F. Collier & Son. http://books.google.com/books?id=ko9BAAAAIAAJ&pg=PA71&dq=jar+%22von+Kleist%22&lr=&as_brr=1&ei=aniTR_uCJ5HwsgOQ5bU4#PPA71,M1.

3. ^ Dorf, p.2574. ^ Isaacson, Walter (2003). Benjamin Franklin (http://books.google.com/books?

id=oIW915dDMBwC&lpg=PA135&ots=6ZvKwVdZQe&dq=%22benjamin%20franklin%22%20leyden%20jar&pg=PA136#v=onepage&q=&f=false) . Simon and Schuster. p. 136. ISBN 0684807610, 9780684807614.http://books.google.com/books?id=oIW915dDMBwC&lpg=PA135&ots=6ZvKwVdZQe&dq=%22benjamin%20franklin%22%20leyden%20jar&pg=PA136#v=onepage&q=&f=false.

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).http://www.chemteam.info/Chem-History/Franklin-1749/Franklin-1749-all.pdf. Retrieved 2009-08-09.

6. ^ Morse, Robert A., Ph.D. (Sept. 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). WrightCenter for Science Education. Tufts University. pp. (page 23).http://www.tufts.edu/as/wright_center/personal_pages/bob_m/04_Franklin_Lab_Part_IV.pdf. 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. ^ a b Ulaby, p.1688. ^ Ulaby, p.1579. ^ a b Ulaby, p.169

10. ^ Hammond, P, Electromagnetism for Engineers, pp44-45, Pergamon Press, 1965.11. ^ Dorf, p.26312. ^ Dorf, p.26013. ^ "Capacitor charging and discharging : DC CIRCUITS (http://www.allaboutcircuits.com/vol_6/chpt_3/17.html) ".

All About Circuits. http://www.allaboutcircuits.com/vol_6/chpt_3/17.html. Retrieved 2009-02-19.14. ^ Ulaby, p.170



15. ^ S. T. Pai and Qi Zhang (1995). Introduction to High Power Pulse Technology (http://books.google.com/books?id=spZ_H4nwIN0C&pg=PA47&dq=breakdown+field+energy-density+dielectric&ei=bUGySNuEB4XWsgPf_4W8BA&sig=ACfU3U2ooCfDPKtlcl-yPee-vr_jMt6W3A) . WorldScientific. ISBN 9810217145. http://books.google.com/books?id=spZ_H4nwIN0C&pg=PA47&dq=breakdown+field+energy-density+dielectric&ei=bUGySNuEB4XWsgPf_4W8BA&sig=ACfU3U2ooCfDPKtlcl-yPee-vr_jMt6W3A.

16. ^ Scherz, P, Practical Electronics for Inventors, p100, McGraw-Hill Professional, 2006, ISBN 0071452818.17. ^ Bird, J, Electrical Circuit Theory and Technology, p501, Newnes, 2007, ISBN 075068139X.18. ^ Ceramic Capacitor Aging Made Simple (http://www.johansondielectrics.com/technicalnotes/age/)19. ^ Ask The Applications Engineer - 21 (http://www.analog.com/library/analogDialogue/Anniversary/21.html) , Steve

Guinta, Analog Devices20. ^ Next-gen car solution? Scientists expand uses for electrostatic capacitor (http://cleantech.com/news/4278/next-

gen-car-solution-capacitor)

ReferencesDorf, Richard C.; Svoboda, James A. (2001). Introduction to Electric Circuits (5th ed.). New York: JohnWiley 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 CapacitorsPowerful 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 (http://dx.doi.org/10.1109%2FMSPEC.2005.1377872) . ISSN0018-9235 (http://worldcat.org/issn/0018-9235) . http://www.spectrum.ieee.org/jan05/2777.The ARRL Handbook for Radio Amateurs (68th ed.). Newington CT USA: The Amateur Radio RelayLeague. 1991.Huelsman, Lawrence P. (1972). Basic Circuit Theory with Digital Computations. Series in computerapplications in electrical engineering. Englewood Cliffs: Prentice-Hall. ISBN 0-13-057430-9.Philosophical Transactions of the Royal Society LXXII, Appendix 8, 1782 (Volta coins the wordcondenser)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 linksCapacitance and Inductance (http://www.lightandmatter.com/html_books/4em/ch07/ch07.html) - a chapterfrom an online textbookHowstuffworks.com: How Capacitors Work (http://electronics.howstuffworks.com/capacitor.htm/printable)CapSite 2009: Introduction to Capacitors (http://my.execpc.com/~endlr/)Capacitor Tutorial (http://www.sentex.ca/~mec1995/gadgets/caps/caps.html) - Includes how to readcapacitor temperature codesCapacitor Converters and Calculators (http://www.csgnetwork.com/capacitorinfoconverters.html)

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