Capacitor
A capacitor is an
electrical/electronic device that can store energy in the
electric field between a pair of conductors (called "plates").
The process of storing energy in the capacitor is known as
"charging", and involves electric charges of equal magnitude,
but opposite polarity, building up on each plate.
Capacitors are often used in electrical circuit and electronic
circuits as energy-storage devices. They can also be used to
differentiate between high-frequency and low-frequency signals.
This property makes them useful in electronic filters.
Capacitors are occasionally referred to as condensers. This is
considered an antiquated term in English, but most other
languages use an equivalent, like the German word "Kondensator"
History
In October 1745, Ewald Georg
von Kleist of Pomerania invented 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 the energy density could be increased dramatically
compared with the situation with no insulator.
In January 1746, before Kleist's discovery became widely known,
a Dutch physicist Pieter van Musschenbroek independently
invented a very similar capacitor. It was named the Leyden jar,
after the University of Leyden where van Musschenbroek worked.
Daniel Gralath was the first to combine several jars in parallel
into a "battery" to increase the total possible stored charge.
The earliest unit of capacitance was the 'jar', equivalent to
about 1 nF.
Early capacitors were also known as condensers, a term that is
still occasionally used today. It was coined by Alessandro Volta
in 1782 (derived from the Italian condensatore), 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", as the
'in other languages' links from this article testify.
Physics
Acapacitor consists of two
conductive electrodes, or plates, separated by a dielectric.
Capacitance
The capacitor's capacitance (C) is a measure of the amount of
charge (Q) stored on each plate for a given potential difference
or voltage (V) which appears between the plates:
In SI units, a capacitor has a capacitance of one farad when one
coulomb of charge is stored due to one volt applied potential
difference across the plates. Since the farad is a very large
unit, values of capacitors are usually expressed in microfarads
(µF), nanofarads (nF), or picofarads (pF).
When there is a difference in electric charge between the
plates, an electric field is created in the region between the
plates that is proportional to the amount of charge that has
been moved from one plate to the other. This electric field
creates a potential difference V = E·d between the plates of
this simple parallel-plate capacitor.
The capacitance is proportional to the surface area of the
conducting plate and inversely proportional to the distance
between the plates. It is also proportional to the permittivity
of the dielectric (that is, non-conducting) substance that
separates the plates
where ε is the permittivity of the dielectric (see Dielectric
constant), A is the area of the plates and d is the spacing
between them.
In the diagram, the rotated molecules create an opposing
electric field that partially cancels the field created by the
plates, a process called dielectric polarization.
Stored energy
As opposite charges accumulate on the plates of a capacitor due
to the separation of charge, a voltage develops across the
capacitor due to the electric field of these charges.
Ever-increasing work must be done against this ever-increasing
electric field as more charge is separated. The energy (measured
in joules, in SI) 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:
where V is 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 breaks down. Therefore, all
capacitors made with the same dielectric have about the same
maximum energy density (joules of energy per cubic meter).
Capacitor types
Listed by di-electric material.
A 12 pF 20 kV fixed vacuum capacitor
Vacuum : Two metal, usually copper, electrodes are separated by
a vacuum. The insulating envelope is usually glass or ceramic.
Typically of low capacitance - 10 - 1000 pF and high voltage, up
to tens of kilovolts, they are most often used in radio
transmitters and other high voltage power devices. Both fixed
and variable types are available. Vacuum variable capacitors can
have a minimum to maximum capacitance ratio of up to 100,
allowing any tuned circuit to cover a full decade of frequency.
Vacuum is the most perfect of dielectrics with a zero loss
tangent. This allows very high powers to be transmitted without
significant loss and consequent heating.
Air : Air dielectric capacitors consist of metal plates
separated by an air gap. The metal plates, of which there may be
many interleaved, are most often made of aluminium or
silver-plated brass. Nearly all air dielectric capacitors are
variable and are used in radio tuning circuits.
Metallized plastic film: Made from high quality polymer film
(usually polycarbonate, polystyrene, polypropylene, polyester
(Mylar), and for high quality capacitors polysulfone), and metal
foil or a layer of metal deposited on surface. They have good
quality and stability, and are suitable for timer circuits.
Suitable for high frequencies.
Mica: Similar to metal film. Often high voltage. Suitable
for high frequencies. Expensive. Excellent tolerance.
Paper: Used for relatively high voltages. Now obsolete.
Glass: Used for high voltages. Expensive. Stable temperature
coefficient in a wide range of temperatures.
Ceramic: Chips of alternating layers of metal and
ceramic. Depending on their dielectric, whether Class 1 or Class
2, their degree of temperature/capacity dependence varies. They
often have (especially the class 2) high dissipation factor,
high frequency coefficient of dissipation, their capacity
depends on applied voltage, and their capacity changes with
aging. However they find massive use in common low-precision
coupling and filtering applications. Suitable for high
frequencies.
Aluminum electrolytic: Polarized. Constructionally
similar to metal film, but the electrodes are made of etched
aluminium to acquire much larger surfaces. The dielectric is
soaked with liquid electrolyte. They can achieve high capacities
but suffer from poor tolerances, high instability, gradual loss
of capacity especially when subjected to heat, and high leakage.
Tend to lose capacity in low temperatures. Bad frequency
characteristics make them unsuited for high-frequency
applications. Special types with low equivalent series
resistance are available.
Tantalum electrolytic: Similar to the aluminum electrolytic
capacitor but with better frequency and temperature
characteristics. High dielectric absorption. High leakage. Has
much better performance at low temperatures.
OS-CON (or OS-CON) capacitors are a polymerized organic
semiconductor solid-electrolyte type that offer longer life at
higher cost than standard electrolytics.
Supercapacitors: Made from carbon aerogel, carbon
nanotubes, or highly porous electrode materials. Extremely high
capacity. Can be used in some applications instead of
rechargeable batteries.
Gimmick capacitors are capacitors made from two insulated wires
that have been twisted together. Each wire forms a capacitor
plate. Gimmick capacitors are also a form of variable capacitor.
Small changes in capacitance (20 percent or less) are obtained
by twisting and untwisting the two wires.
Varactors or varicap capacitors are specialized, reverse-biased
diodes whose capacitance varies with voltage. Used in
phase-locked loops, amongst other applications.
ApplicationsCapacitors
have
various uses in electronic and electrical systems.
Energy storage
A capacitor can store electric energy when disconnected from its
charging circuit, so it can be used like a temporary battery.
Capacitors are commonly used in electronic devices to maintain
power supply while batteries are being changed. (This prevents
loss of information in volatile memory.)
Power conditioning
Capacitors are used in power supplies where they smooth the
output of a full or half wave rectifier. They can also be used
in charge pump circuits 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 conceal 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 in car audio applications, when a
stiffening capacitor compensates for the inductance and
resistance of the leads to the lead-acid car battery.
Power factor correction
Capacitors are used in power factor correction. Such capacitors
often come as three capacitors connected as a three phase load.
Usually, the values of these capacitors are given not in farads
but rather as a reactive power in volt-amperes reactive (VAr).
The purpose is to counteract inductive loading from electric
motors and fluorescent lighting in order to make the load appear
to be mostly resistive.
Filtering
Signal de-coupling
Because capacitors pass AC but block DC signals (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 as AC
de-coupling. Here, a large value of capacitance, whose value
need not be accurately controlled, but whose reactance is small
at the signal frequency, is employed.
Noise filters, motor starters, and snubbers
When an inductive circuit is opened, the current through the
inductance collapses quickly, creating 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. A snubber
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 radio frequency interference (RFI),
which a filter capacitor 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 greater voltage than required to
maintain it; in uses such as large motors, this can cause
undesirable startup characteristics, and a motor starting
capacitor is used to increase the coil current to help start the
motor.
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 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 (see electrolytic capacitor).
Signal processing
The energy stored in a capacitor can be used to represent
information, either in binary form, as in DRAMs, or in analogue
form, as in analog sampled filters and CCDs. Capacitors can be
used in analog circuits as components of integrators or more
complex filters and in negative feedback loop stabilization.
Signal processing circuits also use capacitors 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 analog crossovers, and
analog equalizers use capacitors to select different audio
bands.
In a tuned circuit such as a radio receiver, the frequency
selected is a function of the inductance (L) and the capacitance
(C) in series, and is given by:
This is the frequency at which resonance occurs in an LC
circuit.
Other applications
Sensing
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 the dielectric can also be
of use. Capacitors with an exposed and porous dielectric can be
used to measure humidity in air.
Changing the distance between the plates: Capacitors are used to
accurately measure the fuel level in airplanes. Capacitors with
a flexible plate can be used to measure strain or pressure.
Capacitors are used as the sensor in condenser microphones,
where one plate is moved by air pressure, relative to the fixed
position of the other plate. Some accelerometers use MEMS
capacitors etched on a chip to measure 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 triggering airbag deployment, and in many other
applications. Also some fingerprint sensors. Additionally, a
user can adjust the pitch of a theremin musical instrument by
moving his hand since this changes the effective capacitance
between the users hand and the antenna.
Changing the effective area of the plates: capacitive touch
switches [1] [2] [3].
Pulsed power and weapons
Groups of large, specially constructed, low-inductance
high-voltage capacitors (capacitor banks) are used to supply
huge pulses of current for many pulsed power applications. These
include 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 slapper detonators in
nuclear weapons and other specialty weapons. Experimental work
is under way using banks of capacitors as power sources for
electromagnetic armour and electromagnetic railguns or coilguns.
See also Explosively pumped flux compression generator.