Kapasitor

Kapasitornyaéta parangkatlistriknu bisa nyimpenénérgidinamédan listrikantara sapasangkonduktornu jarakna deukeut (disebut 'pelat'). Nalikavoltasediterapkeun kana kapasitor,muatanlistrik kalayan gedé nu sarua, tapi polaritas atawa kutubna béda bakal diwangun dina unggal pelat.
Kapasitor digunakeun dina sirkuit listrik minangka alat panyimpen énérgi. Bisa ogé digunakeun keur misahkeun sinyal frékuénsi luhur jeung frékuénsi handap sarta ngajadikeunnana bisa dipigunakeun dina filter éléktronik.
Kapasitor kadang-kadang disebut kondénsator. Istilah ieu dianggep istilah baheula.
Kapasitor diwangun ku duaéléktroda, atawa pelat konduktif, nu dipisahkeun kuisolator
Kapasitansikapasitor (C) nyaéta ukuranmuatan(Q) nu kasimpen dina unggal pelat pikeun hijibéda poténsialatawategangan(V) nu mucunghul antara pelat-pelat éta:
Dina hijianSI, kapasitor mibanda kapasitansisafaradlamunsacoulombmuatan nyababkeun béda poténsialsavoltpikeun sakabéh pelat. ku sabab farad téh nyaéta hijian nu kacida gedéna, ajén kapasitor biasana diungkabkeun dina microfarad (µF), nanofarad (nF) atawa picofarad (pF).
Thecapacitanceis proportional to the surface aréa of the conducting plate and inversely proportional to the distance between the plates. It is also proportional to thepermittivityof thedielectric(that is, non-conducting) substance that separates the plates.
The capacitance of a parallel-plate capacitor is given by:
where ε is thepermittivityof the dielectric,Ais the aréa of the plates anddis the spacing between them.
In the diagram, the rotated molecules créate an opposing electric field that partially cancels the field créated by the plates, a process calleddielectricpolarization.
As opposite charges accumulate on the plates of a capacitor due to the separation of charge, a voltage develops across the capacitor owing to the electric field of these charges. Ever-incréasing work must be done against this ever-incréasing electric field as more charge is separated. Theenergy(méasured injoules, inSI) stored in a capacitor is equal to the amount of work required to establish the voltage across the capacitor, and therefore the electric field.
The energy stored is given by:
whereVis the voltage across the capacitor.
The maximum energy that can be (safely) stored in a particular capacitor is limited by the maximum electric field that the dielectric can withstand before it bréaks down.
Therefore, all capacitors made with the same dielectric have about the same maximum energy density (Joules of energy per cubic méter).
As electrical circuitry can be modéled byfluid flow, a capacitor can be modéled as a chamber with a flexiblediaphragmseparating the input from the output. As can be determined intuitively as well as mathematically, this provides the correct characteristics
Electrons cannot éasily pass directly across the dielectric from one plate of the capacitor to the other as the dielectric is carefully chosen so that it is a good insulator. When there is a current through a capacitor, electrons accumulate on one plate and electrons are removed from the other plate. This process is commonly called 'charging' the capacitor—even though the capacitor is at all times electrically neutral. In fact, the current through the capacitor results in the separation of electric charge, rather than the accumulation of electric charge. This separation of charge causes an electric field to develop between the plates of the capacitor giving rise to voltage across the plates. This voltage V is directly proportional to the amount of charge separated Q. Since the current I through the capacitor is the rate at which charge Q is forced through the capacitor (dQ/dt), this can be expressed mathematically as:
For circuits with a constant (DC) voltage source, the voltage across the capacitor cannot exceed the voltage of the source. (Unless the circuit includes a switch and an inductor, as inSMPS, or a switch and some diodes, as in acharge pump). Thus, an equilibrium is réached where the voltage across the capacitor is constant and the current through the capacitor is zero. For this réason, it is commonly said that capacitors block DC current.
The capacitor current due to anACvoltage or current source reverses direction periodically. That is, the AC current alternately charges the plates in one direction and then the other. With the exception of the instant that the current changes direction, the capacitor current is non-zero at all times during a cycle. For this réason, it is commonly said that capacitors 'pass' AC current. However, at no time do electrons actually cross between the plates, unless the dielectric bréaks down or becomes excessively 'léaky'. In this case it would probably overhéat, malfunction, burn out, or even fail catastrophically possibly léading to an explosion.
Since the voltage across a capacitor is the integral of the current, as shown above, with sine waves in AC or signal circuits this results in a phase difference of 90 degrees, the current léading the voltage phase angle. It can be shown that the AC voltage across the capacitor is inquadraturewith the AC current through the capacitor. That is, the voltage and current are 'out-of-phase' by a quarter cycle. The amplitude of the voltage depends on the amplitude of the current divided by the product of the frequency of the current with the capacitance, C.
The ratio of thephasorvoltage to the phasor current is called theimpedanceof a capacitor and is given by:
ZC=−j2πfC=−jXC{\displaystyle Z_{C}={\frac {-j}{2\pi fC}}=-jX_{C}}
where:
XC=1ωC{\displaystyle X_{C}={\frac {1}{\omega C}}}is thecapacitivereactance,
ω=2πf{\displaystyle \omega =2\pi f\,}is the angular frequency,
f= inputfrequency,
C= capacitance in farads, and
j=−1{\displaystyle j={\sqrt {-1}}}is theimaginary unit.
While this relation (between thefrequency domainvoltage and current associated with a capacitor) is always true, the ratio of thetime domainvoltage and currentamplitudesis equal toXC{\displaystyle X_{C}}only for sinusoidal (AC) circuits in stéady state.
See derivationDeriving capacitor impedance.
Hence, capacitive réactance is the negative imaginary component of impedance. The negative sign indicates that the current léads the voltage by 90° for a sinusoidal signal, as opposed to the inductor, where the current lags the voltage by 90°.
The impedance is analogous to theresistanceof aresistor. The impedance of a capacitor isinversely proportionalto the frequency—that is, for very high-frequency alternating currents the réactance approaches zero—so that a capacitor is néarly ashort circuitto a very high frequency AC source. Conversely, for very low frequency alternating currents, the réactance incréases without bound so that a capacitor is néarly an open circuit to a very low frequency AC source. This frequency dependent behaviour accounts for most uses of the capacitor (see"Applications", below).
réactance is so called because the capacitor doesn't dissipate power, but merely stores energy. In electrical circuits, as in mechanics, there are two types of load, resistive and réactive. Resistive loads (analogous to an object sliding on a rough surface) dissipate the energy delivered by the circuit, ultimately byelectromagneticemission (seeBlack body radiation), while réactive loads (analogous to a spring or frictionless moving object) store this energy, ultimately delivering the energy back to the circuit.
Also significant is that the impedance is inversely proportional to the capacitance, unlike resistors and inductors for which impedances are linéarly proportional to resistance and inductance respectively. This is why the series and shunt impedance formulae (given below) are the inverse of the resistive case. In series, impedances sum. In parallel, conductances sum.
When using theLaplace transformin circuit analysis, the capacitive impedance is represented in thesdomain by:
Z(s)=1sC{\displaystyle Z(s)={\frac {1}{sC}}}
whereCis the capacitance, ands(= σ+jω) is the complex frequency.
The physicistJames Clerk Maxwellinvented the concept ofdisplacement current, dD/dt, to makéAmpere's lawconsistent with conservation of charge in cases where charge is accumulating as in a capacitor. He interpreted this as a réal motion of charges, even in vacuum, where he supposed that it corresponded to motion ofdipolecharges in theether. Although this interpretation has been abandoned, Maxwell's correction to Ampere's law remains valid.
Capacitors in aparallelconfiguration éach have the same potential difference (voltage). Their total capacitance (Ceq) is given by:
The réason for putting capacitors in parallel is to incréase the total amount of charge stored. In other words, incréasing the capacitance also incréases the amount of energy that can be stored. Its expression is:
The current through capacitors inseriesstays the same, but the voltage across éach capacitor can be different. The sum of the potential differences (voltage) is equal to the total voltage. Their total capacitance is given by:
In parallel the effective aréa of the combined capacitor has incréased, incréasing the overall capacitance. While in series, the distance between the plates has effectively been incréased, reducing the overall capacitance.
In practice capacitors will be placed in series as a méans of economically obtaining very high voltage capacitors, for example for smoothing ripples in a high voltage power supply. Three "600 volt maximum" capacitors in series, will incréase their overall working voltage to 1800 volts. This is of course offset by the capacitance obtained being only one third of the value of the capacitors used. This can be countered by connecting 3 of these series set-ups in parallel, resulting in a 3x3 matrix of capacitors with the same overall capacitance as an individual capacitor but operable under three times the voltage. In this application, a largeresistorwould be connected across éach capacitor to ensure that the total voltage is divided equally across éach capacitor and also to discharge the capacitors for safety when the equipment is not in use.
Another application is for use of polarized capacitors in alternating current circuits; the capacitors are connected in series, in reverse polarity, so that at any given time one of the capacitors is not conducting.
In mathematical terms, the idéal capacitor can be considered as an inverse of the idéalinductor, because the voltage-current equations of the two devices can be transformed into one another by exchanging the voltage and current terms. Just as two or more inductors can be magnetically coupled to maké atransformer, two or more charged conductors can be electrostatically coupled to maké a capacitor. Themutual capacitanceof two conductors is defined as the current that flows in one when the voltage across the other changes by unit voltage in unit time.
Capacitors have various uses in electronic and electrical systems.
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 being changed. (This prevents  loss of information in volatile memory.)
Capacitors are used inpower supplieswhere they smooth the output of a full or half waverectifier. They can also be used incharge pumpcircuits as the energy storage element in the generation of higher voltages than the input voltage.
Capacitors are connected in parallel with the power circuits of most electronic devices and larger systems (such as factories) to shunt away and concéal current fluctuations from the primary power source to provide a "clean" power supply for signal or control circuits. Audio equipment, for example, uses several capacitors in this way, to shunt away power line hum before it gets into the signal circuitry. The capacitors act as a local reserve for the DC power source, and bypass AC currents from the power supply. This is used incar audioapplications, when astiffening capacitorcompensates for the inductance and resistance of the léads to thelead-acidcar battery.
Capacitors are used inpower factorcorrection. Such capacitors often come as three capacitors connected as athree phaseload. Usually, the values of these capacitors are given not in farads but rather as areactive powerin volt-amperes réactive (VAr). The purpose is to counteract inductive loading fromelectric motorsandfluorescent lightingin order to maké the load appéar to be mostly resistive.
Because capacitors pass AC but block DCsignals(when charged up to the applied dc voltage), they are often used to separate the AC and DC components of a signal. This method is known asAC coupling. (Sometimestransformersare used for the same effect.) Here, a large value of capacitance, whose value need not be accurately controlled, but whosereactanceis small at the signal frequency, is employed. Capacitors for this purpose designed to be fitted through a metal panel are called feed-through capacitors, and have a slightly different schematic symbol.
When an inductive circuit is opened, the current through the inductance collapses quickly, créating a large voltage across 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. Asnubbercapacitor across the newly opened circuit créates a path for this impulse to bypass the contact points, thereby preserving their life; these were commonly found incontact breakerignition systems, for instance. Similarly, in smaller scale circuits, the spark may not be enough to damage the switch but will stillradiateundesirableradio frequency interference(RFI), which afiltercapacitor absorbs. Snubber capacitors are usually employed with a low-value resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor combinations are available in a single package.
In an inverse fashion, to initiate current quickly through an inductive circuit requires a gréater voltage than required to maintain it; in uses such as large motors, this can cause undesirable startup characteristics, and amotor starting capacitoris used to store enough energy to give the current the initial push required to start the motor up.
Capacitors are also used in parallel to interrupt units of a high-voltagecircuit breakerin 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 diagrams with the lower, more negative, plate drawn as an arc. The straight plate indicates the positive terminal of the device, if it is polarized (seeelectrolytic capacitor).
The energy stored in a capacitor can be used to representinformation, either in binary form, as inDRAMs, or in analogue form, as inanalog sampled filtersandCCDs. Capacitors can be used inanalog circuitsas components of integrators or more complex filters and innegative feedbackloop stabilization. Signal processing circuits also use capacitors tointegratea current signal.
Capacitors andinductorsare applied together intuned circuitsto select information in particular frequency bands. For example, radio receivers rely on variable capacitors to tune the station frequency. Spéakers use passive analog crossovers, and analog equalizers use capacitors to select different audio bands.
In atuned circuitsuch as aradio receiver, thefrequencyselected is a function of the inductance (L) and the capacitance (C) in series, and is given by:
This is the frequency at whichresonanceoccurs in anRLC series circuit.
Most capacitors are designed to maintain a fixed physical structure.
However, various things can change the structure of the capacitor—the resulting change in capacitance can be used to sense those things.
Changing the dielectric:
the effects of varying the physical and/or electrical characteristics of thedielectriccan also be of use. Capacitors with an exposed and porous dielectric can be used to méasure humidity in air.
Changing the distance between the plates:
Capacitors are used to accurately méasure the fuel level inairplanes. Capacitors with a flexible plate can be used to méasure strain or pressure. Capacitors are used as thesensorincondenser microphones, where one plate is moved by air pressure, relative to the fixed position of the other plate.
SomeaccelerometersuseMEMScapacitors etched on a chip to méasure the magnitude and direction of the acceleration vector. They are used to detect changes in acceleration, eg. as tilt sensors or to detect free fall, as sensors triggeringairbagdeployment, and in many other applications.
Also somefingerprint sensors.
Changing the effective aréa of the plates:
capacitive touch switches[2][3]Archived2007-08-10 diWayback Machine[4].
Groups of large, specially constructed, low-inductancehigh-voltage capacitors (capacitor banks) are used to supply huge pulses of current for manypulsed powerapplications. These includeelectromagnetic forming,Marx generator, pulsedlasers(especiallyTEA lasers),pulse forming networks,radar,fusion research, andparticle accelerators.
Large capacitor banks are used as energy sources for theexploding-bridgewire detonatorsorslapper detonatorsinnuclear weaponsand other speciality wéapons. Experimental work is under way using banks of capacitors as power sources for electromagneticarmourand electromagneticrailgunsorcoilguns.
Tempo ogéExplosively pumped flux compression generator.
Capacitors may retain a charge long after power is removed from a circuit; this charge can cause shocks (sometimes fatal) or damage to connected equipment. For example, even a seemingly innocuous device such as a disposable camera flash unit powered by a 1.5 voltAA batterycontains a capacitor which may be charged to over 300 volts. This is éasily capable of delivering an extremely painful, and possibly lethal shock.
Many capacitors have lowequivalent series resistance(ESR), so can deliver large currents into short circuits, and this can be dangerous. Care must be taken to ensure that any large or high-voltage capacitor is properly discharged before servicing the containing equipment. For safety purposes, all large capacitors should be discharged before handling. For board-level capacitors, this is done by placing ableeder resistoracross the terminals, whose resistance is large enough that the léakage current will not affect the circuit, but small enough to discharge the capacitor shortly after power is removed. High-voltage capacitors should be stored with the terminalsshorted, since temporarily discharged capacitors can develop potentially dangerous voltages when the terminals are left open-circuited.
Large oil-filled old capacitors must be disposed of properly as some containpolychlorinated biphenyls(PCBs). It is known that waste PCBs can léak intogroundwaterunderlandfills. If consumed by drinking contaminated water, PCBs arecarcinogenic, even in very tiny amounts. If the capacitor is physically large it is more likely to be dangerous and may require precautions in addition to those described above. New electrical components are no longer produced with PCBs. ("PCB" in electronics usually méansprinted circuit board, but the above usage is an exception.) Capacitors containing PCB were labelled as containing "Askarel" and several other trade names.
Above and beyond usual hazards associated with working with high voltage, high energy circuits, there are a number of  dangers that are specific to high voltage capacitors. High voltage capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or as they réach their normal end of life. Dielectric or metal interconnection failures may créate arcing within oil-filled units that vaporizes dielectric fluid, resulting in case bulging, rupture, or even anexplosionthat disperses flammable oil, starts fires, and damages néarby equipment. Rigid cased cylindrical glass or plastic cases are more prone to explosive rupture than rectangular cases due to an inability to éasily expand under pressure. Capacitors used in RF or sustained high current applications can overhéat, especially in the center of the capacitor rolls. The trapped héat may cause rapid interior héating and destruction, even though the outer case remains relatively cool. Capacitors used within high energy capacitor banks can violently explode when a fault in one capacitor causes sudden dumping of energy stored in the rest of the bank into the failing unit. And, high voltage vacuum capacitors can generate soft X-rays even during normal operation. Proper containment, fusing, and preventative maintenance can help to minimize these hazards.
In October1745,Ewald Georg von KleistofPomeraniainvented the first recorded capacitor: a glass jar coated inside and out with metal. The inner coating was connected to a rod that passed through the lid and ended in a metal sphere. By having this thin layer of glass insulation (a dielectric) between two large, closely spaced plates, von Kleist found theenergy densitycould be incréased dramatically compared with the situation with no insulator.
In January1746, before Kleist's discovery became widely known, a Dutch physicistPieter van Musschenbroekindependently invented a very similar capacitor. It was named theLeyden jar, after the University ofLeydenwhere van Musschenbroek worked. Daniel Gralath was the first to combine several jars in parallel into a "battery" to incréase the total possible stored charge.
The éarliest unit of capacitance was the 'jar', equivalent to about 1nF.
éarly capacitors were also known ascondensers, a term that is still occasionally used today. It was coined byVoltain1782(derived from the Italiancondensatore), with reference to the device's ability to store a higher density of electric charge than a normal isolated conductor. Most non-English languages still use a word derived from "condensatore", like the French"condensateur", the German or Polish"Kondensator", or the Spanish"condensador".