Capacitor

This article is about the electrical component. For the

physical phenomenon, see capacitance. For an overview
of various kinds of capacitors, see types of capacitor.
“Capacitive” redirects here. For the term used when re-
ferring to touchscreens, see capacitive sensing.
A capacitor (originally known as a condenser) is a

4 electrolytic capacitors of different voltages and capacitance

Miniature low-voltage capacitors (next to a cm ruler)

Solid electrolyte, resin-dipped 10 μF 35 V tantalum capacitors.

The + sign indicates the positive lead.

oxide layer etc. Capacitors are widely used as parts of

electrical circuits in many common electrical devices.
Unlike a resistor, an ideal capacitor does not dissipate en-
ergy. Instead, a capacitor stores energy in the form of an
electrostatic field between its plates.
A typical electrolytic capacitor
When there is a potential difference across the conductors
passive two-terminal electrical component used to store (e.g., when a capacitor is attached across a battery), an
electrical energy temporarily in an electric field. The electric field develops across the dielectric, causing posi-
forms of practical capacitors vary widely, but all con- tive charge +Q to collect on one plate and negative charge
tain at least two electrical conductors (plates) separated −Q to collect on the other plate. If a battery has been
by a dielectric (i.e. an insulator that can store energy attached to a capacitor for a sufficient amount of time,
by becoming polarized). The conductors can be thin no current can flow through the capacitor. However, if
films, foils or sintered beads of metal or conductive elec- a time-varying voltage is applied across the leads of the
trolyte, etc. The nonconducting dielectric acts to in- capacitor, a displacement current can flow.
crease the capacitor’s charge capacity. A dielectric can An ideal capacitor is characterized by a single constant
be glass, ceramic, plastic film, air, vacuum, paper, mica, value, its capacitance. Capacitance is defined as the ratio

1
2 1 HISTORY

of the electric charge Q on each conductor to the potentialing the wire resulted in a powerful spark, much more
difference V between them. The SI unit of capacitance is painful than that obtained from an electrostatic machine.
the farad (F), which is equal to one coulomb per volt (1 The following year, the Dutch physicist Pieter van Muss-
C/V). Typical capacitance values range from about 1 pF chenbroek invented a similar capacitor, which was named
(10−12 F) to about 1 mF (10−3 F). the Leyden jar, after the University of Leiden where he
[3]
The larger the surface area of the “plates” (conductors) worked. He also was impressed by the power of the
and the narrower the gap between them, the greater shock he received, writing, “I would [4]
not take a second
the capacitance is. In practice, the dielectric between shock for the kingdom of France.”
the plates passes a small amount of leakage current and Daniel Gralath was the first to combine several jars in
also has an electric field strength limit, known as the parallel into a “battery” to increase the charge storage ca-
breakdown voltage. The conductors and leads introduce pacity. Benjamin Franklin investigated the Leyden jar
an undesired inductance and resistance. and came to the conclusion that the charge was stored on
Capacitors are widely used in electronic circuits for the glass, not in the water as[5][6] others had assumed. He also
blocking direct current while allowing alternating current adopted the term “battery”, (denoting the increasing
to pass. In analog filter networks, they smooth the output of power with a row of similar units as in a battery of can-
of power supplies. In resonant circuits they tune radios non), subsequently applied to clusters of electrochemical
[7]
to particular frequencies. In electric power transmission cells. Leyden jars were later made by coating the inside
systems, they stabilize voltage and power flow. [1] and outside of jars with metal foil, leaving a space at the
mouth to prevent arcing between the foils. The earliest
unit of capacitance was the jar, equivalent to about 1.11
nanofarads.[8]
1 History Leyden jars or more powerful devices employing flat glass
plates alternating with foil conductors were used exclu-
sively 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. More compact construction meth-
ods began to be used, such as a flexible dielectric sheet
(like oiled paper) sandwiched between sheets of metal
foil, rolled or folded into a small package.
Early capacitors were also known as condensers, a term
that is still occasionally used today, particularly in high
power applications, like automotive systems. The term
was first used for this purpose by Alessandro Volta in
1782, with reference to the device’s ability to store a
higher density of electric charge than a normal isolated
conductor.[9]
Since the beginning the study of electricity non conduc-
tive materials like glass, porcelain, paper and mica have
been used as insulators. These materials some decades
later were also well-suited for further use as the dielectric
for the first capacitors. Paper capacitors made by sand-
wiching a strip of impregnated paper between strips of
metal, and rolling the result into a cylinder were com-
monly used in the late 19century; their manufacture
started in 1876,[10] and they were used from the early 20th
century as decoupling capacitors in telecommunications
Battery of four Leyden jars in Museum Boerhaave, Leiden, the (telephony). Porcelain was the precursor in case of all ca-
Netherlands pacitors now belonging to the family of ceramic capaci-
tors. Even in the early years of Marconi`s wireless trans-
In October 1745, Ewald Georg von Kleist of Pomerania, mitting apparatus porcelain capacitors were used for high
Germany, found that charge could be stored by connect- voltage and high frequency application in the transmitters.
ing a high-voltage electrostatic generator by a wire to a
On receiver side the smaller mica capacitors were used
volume of water in a hand-held glass jar.[2] Von Kleist’s
for resonant circuits. Mica dielectric capacitors were in-
hand and the water acted as conductors, and the jar as a
vented in 1909 by William Dubilier. Prior to World War
dielectric (although details of the mechanism were incor-
II, mica was the most common dielectric for capacitors
rectly identified at the time). Von Kleist found that touch-
2.1 Overview 3

in the United States,[10] see Ceramic capacitor#History

Charles Pollak (born Karol Pollak), the inventor of Charge -+
-+
-+
-+
-+ -+
Aluminum electrolytic capacitors, found out that that the
oxide layer on an aluminum anode remained stable in a +Q -+
-+
-+
-+
-Q
neutral or alkaline electrolyte, even when the power was -+ -+
switched off. In 1896 he filed a patent for an “Electric -+ -+
liquid capacitor with aluminum electrodes” based on his -+
-+
-+
-+
dielectric
idea of using the oxide layer in a polarized capacitor in -+ -+
combination with a neutral or slightly alkaline electrolyte, -+ -+
see Electrolytic capacitor#History. -+ -+
-+ -+
With the development of plastic materials by organic
chemists during the Second World War, the capacitor in- Electric -+
-+
-+
-+ Plate
-+ -+
dustry began to replace paper with thinner polymer films.
One very early development in film capacitors was de- field E -+
-+
-+
-+
area A
scribed in British Patent 587,953 in 1944,[10] see Film -+ -+
capacitor#History

Plate separation d
Solid electrolyte tantalum capacitors were invented by
Bell Laboratories in the early 1950s as a miniaturized
and more reliable low-voltage support capacitor to com-
plement their newly invented transistor, see Tantalum ca- Charge separation in a parallel-plate capacitor causes an internal
pacitor#History. electric field. A dielectric (orange) reduces the field and increases
the capacitance.
Last but not least the electric double-layer capacitor (now
Supercapacitors) were invented. In 1957 H. Becker de-
veloped a “Low voltage electrolytic capacitor with porous
carbon electrodes”.[10][11][12] He believed that the energy
was stored as a charge in the carbon pores used in his ca-
pacitor as in the pores of the etched foils of electrolytic
capacitors. Because the double layer mechanism was not
known by him at the time, he wrote in the patent: “It is
not known exactly what is taking place in the component
if it is used for energy storage, but it leads to an extremely
high capacity”, see Supercapacitor#History.

2 Theory of operation
Main article: Capacitance
A simple demonstration of a parallel-plate capacitor

2.1 Overview An ideal capacitor is wholly characterized by a constant

capacitance C, defined as the ratio of charge ±Q on each
A capacitor consists of two conductors separated by a conductor to the voltage V between them:[13]
non-conductive region.[13] The non-conductive region is
called the dielectric. In simpler terms, the dielectric is
just an electrical insulator. Examples of dielectric media Q
C=
are glass, air, paper, vacuum, and even a semiconductor V
depletion region chemically identical to the conductors.
A capacitor is assumed to be self-contained and isolated, Because the conductors (or plates) are close together, the
with no net electric charge and no influence from any ex- opposite charges on the conductors attract one another
ternal electric field. The conductors thus hold equal and due to their electric fields, allowing the capacitor to store
opposite charges on their facing surfaces,[14] and the di- more charge for a given voltage than if the conductors
electric develops an electric field. In SI units, a capaci- were separated, giving the capacitor a large capacitance.
tance of one farad means that one coulomb of charge on Sometimes charge build-up affects the capacitor mechan-
each conductor causes a voltage of one volt across the ically, causing its capacitance to vary. In this case, capac-
device.[15] itance is defined in terms of incremental changes:
4 2 THEORY OF OPERATION

2.3 Energy of electric field

dQ
C= Work must be done by an external influence to “move”
dV charge between the conductors in a capacitor. When the
external influence is removed, the charge separation per-
2.2 Hydraulic analogy sists in the electric field and energy is stored to be released
when the charge is allowed to return to its equilibrium
position. The work done in establishing the electric field,
and hence the amount of energy stored, is[16]

∫ Q ∫ Q
q 1 Q2 1 1
W = V (q)dq = dq = = CV 2 = V Q
0 0 C 2 C 2 2
In the hydraulic analogy, a capacitor is analogous to a rub-
ber membrane sealed inside a pipe. This animation illustrates Here Q is the charge stored in the capacitor, V is the volt-
a membrane being repeatedly stretched and un-stretched by the age across the capacitor, and C is the capacitance.
flow of water, which is analogous to a capacitor being repeatedly
charged and discharged by the flow of charge. In the case of a fluctuating voltage V(t), the stored energy
also fluctuates and hence power must flow into or out of
In the hydraulic analogy, charge carriers flowing through the capacitor. This power can be found by taking the time
a wire are analogous to water flowing through a pipe. A derivative of the stored energy:
capacitor is like a rubber membrane sealed inside a pipe.
Water molecules cannot pass through the membrane, but ( )
some water can move by stretching the membrane. The dW d 1 dV
P = = CV 2 = CV (t)
analogy clarifies a few aspects of capacitors: dt dt 2 dt

• The current alters the charge on a capacitor, just as 2.4 Current–voltage relation
the flow of water changes the position of the mem-
brane. More specifically, the effect of an electric The current I(t) through any component in an electric cir-
current is to increase the charge of one plate of the cuit is defined as the rate of flow of a charge Q(t) passing
capacitor, and decrease the charge of the other plate through it, but actual charges—electrons—cannot pass
by an equal amount. This is just as when water through the dielectric layer of a capacitor. Rather, one
flow moves the rubber membrane, it increases the electron accumulates on the negative plate for each one
amount of water on one side of the membrane, and that leaves the positive plate, resulting in an electron de-
decreases the amount of water on the other side. pletion and consequent positive charge on one electrode
• The more a capacitor is charged, the larger its voltage that is equal and opposite to the accumulated negative
drop; i.e., the more it “pushes back” against the charge on the other. Thus the charge on the electrodes
charging current. This is analogous to the fact is equal to the integral of the current as well as propor-
that the more a membrane is stretched, the more it tional to the voltage, as discussed above. As with any
pushes back on the water. antiderivative, a constant of integration is added to repre-
sent the initial voltage V(t 0 ). This is the integral form of
• Charge can flow “through” a capacitor even though the capacitor equation:[17]
no individual electron can get from one side to the
other. This is analogous to the fact that water can ∫
flow through the pipe even though no water molecule Q(t) 1 t
V (t) = = I(τ )dτ + V (t0 )
can pass through the rubber membrane. Of course, C C t0
the flow cannot continue in the same direction for-
ever; the capacitor will experience dielectric break- Taking the derivative of this and multiplying by C yields
[18]
down, and analogously the membrane will eventu- the derivative form:
ally break.

• The capacitance describes how much charge can dQ(t) dV (t)

be stored on one plate of a capacitor for a given I(t) = dt = C dt
“push” (voltage drop). A very stretchy, flexible
membrane corresponds to a higher capacitance than The dual of the capacitor is the inductor, which stores en-
a stiff membrane. ergy in a magnetic field rather than an electric field. Its
current-voltage relation is obtained by exchanging current
• A charged-up capacitor is storing potential energy, and voltage in the capacitor equations and replacing C
analogously to a stretched membrane. with the inductance L.
2.6 AC circuits 5

2.5 DC circuits Impedance, the vector sum of reactance and resistance,

describes the phase difference and the ratio of amplitudes
See also: RC circuit between sinusoidally varying voltage and sinusoidally
A series circuit containing only a resistor, a capacitor, a varying current at a given frequency. Fourier analysis
allows any signal to be constructed from a spectrum of
frequencies, whence the circuit’s reaction to the various
frequencies may be found. The reactance and impedance
R of a capacitor are respectively

V0 C VC X=−
1
=−
1
ωC 2πf C
1 j j
Z= =− =−
jωC ωC 2πf C
where j is the imaginary unit and ω is the angular fre-
quency of the sinusoidal signal. The −j phase indicates
A simple resistor-capacitor circuit demonstrates charging of a ca-
pacitor. that the AC voltage V = ZI lags the AC current by 90°:
the positive current phase corresponds to increasing volt-
switch and a constant DC source of voltage V 0 is known age as the capacitor charges; zero current corresponds to
as a charging circuit.[19] If the capacitor is initially un- instantaneous constant voltage, etc.
charged while the switch is open, and the switch is closed
Impedance decreases with increasing capacitance and in-
at t0 , it follows from Kirchhoff’s voltage law that
creasing frequency. This implies that a higher-frequency
signal or a larger capacitor results in a lower voltage am-
∫ t plitude per current amplitude—an AC “short circuit” or
1 AC coupling. Conversely, for very low frequencies, the
V0 = vresistor (t) + vcapacitor (t) = i(t)R + i(τ )dτ
C t0 reactance will be high, so that a capacitor is nearly an
open circuit in AC analysis—those frequencies have been
Taking the derivative and multiplying by C, gives a first-
“filtered out”.
order differential equation:
Capacitors are different from resistors and inductors in
that the impedance is inversely proportional to the defin-
di(t) ing characteristic; i.e., capacitance.
RC + i(t) = 0
dt A capacitor connected to a sinusoidal voltage source will
At t = 0, the voltage across the capacitor is zero and the cause a displacement current to flow through it. In the
voltage across the resistor is V0 . The initial current is then case that the voltage source is V0 cos(ωt), the displace-
I(0) =V /R. With this assumption, solving the differential ment current can be expressed as:
0
equation yields
dV
I=C = −ωCV0 sin(ωt)
V0 − τt dt
I(t) = e 0
R( At sin(ωt) = −1, the capacitor has a maximum (or peak)
)
− τt current whereby I0 = ωCV0 . The ratio of peak voltage to
V (t) = V0 1 − e 0
peak current is due to capacitive reactance (denoted XC).
V0 V0 1
where τ0 = RC is the time constant of the system. As the XC = I0 = ωCV0 = ωC
capacitor reaches equilibrium with the source voltage, the XC approaches zero as ω approaches infinity. If XC
voltages across the resistor and the current through the en- approaches 0, the capacitor resembles a short wire that
tire circuit decay exponentially. The case of discharging a strongly passes current at high frequencies. XC ap-
charged capacitor likewise demonstrates exponential de- proaches infinity as ω approaches zero. If XC approaches
cay, but with the initial capacitor voltage replacing V 0 infinity, the capacitor resembles an open circuit that
and the final voltage being zero. poorly passes low frequencies.
The current of the capacitor may be expressed in the
2.6 AC circuits form of cosines to better compare with the voltage of the
source:
See also: reactance (electronics) and electrical impedance
§ Deriving the device-specific impedances
I = −I0 sin(ωt) = I0 cos(ωt + 90◦ )
6 2 THEORY OF OPERATION

In this situation, the current is out of phase with the volt-

age by +π/2 radians or +90 degrees (i.e., the current will
εA
lead the voltage by 90°). C=
d
The capacitance is therefore greatest in devices made
2.7 Laplace circuit analysis (s-domain) from materials with a high permittivity, large plate area,
and small distance between plates.
When using the Laplace transform in circuit analysis, the
impedance of an ideal capacitor with no initial charge is A parallel plate capacitor can only store a finite amount
represented in the s domain by: of energy before dielectric breakdown occurs. The ca-
pacitor’s dielectric material has a dielectric strength U
which sets the capacitor’s breakdown voltage at V = V
1 = U d. The maximum energy that the capacitor can store
Z(s) =
sC is therefore
where

• C is the capacitance, and 1 1 εA 1

E= CV 2 = (Ud d)2 = εAdUd2
2 2 d 2
• s is the complex frequency.
The maximum energy is a function of dielectric volume,
permittivity, and dielectric strength. Changing the plate
2.8 Parallel-plate model area and the separation between the plates while main-
taining the same volume causes no change of the max-
imum amount of energy that the capacitor can store, so
Conductive plates long as the distance between plates remains much smaller
than both the length and width of the plates. In addition,
these equations assume that the electric field is entirely
A concentrated in the dielectric between the plates. In real-
d ity there are fringing fields outside the dielectric, for ex-
ample between the sides of the capacitor plates, which
will increase the effective capacitance of the capacitor.
This is sometimes called parasitic capacitance. For some
simple capacitor geometries this additional capacitance
term can be calculated analytically.[20] It becomes negli-
gibly small when the ratios of plate width to separation
Dielectric and length to separation are large.

Dielectric is placed between two conducting plates, each of area

A and with a separation of d

The simplest model capacitor consists of two thin par-

C1 C2 Cn
allel conductive plates separated by a dielectric with
permittivity ε . This model may also be used to make
qualitative predictions for other device geometries. The
plates are considered to extend uniformly over an area A
and a charge density ±ρ = ±Q/A exists on their surface.
Assuming that the length and width of the plates are much
greater than their separation d, the electric field near the Several capacitors in parallel
centre of the device will be uniform with the magnitude
E = ρ/ε. The voltage is defined as the line integral of the
electric field between the plates 2.9 Networks
∫ d ∫ d See also: Series and parallel circuits
ρ ρd Qd
V = E dz = dz = =
0 0 ε ε εA
Solving this for C = Q/V reveals that capacitance in- For capacitors in parallel Capacitors in a parallel con-
creases with area of the plates, and decreases as separa- figuration each have the same applied voltage. Their
tion between plates increases. capacitances add up. Charge is apportioned among
 7

them by size. Using the schematic diagram to visu-

alize parallel plates, it is apparent that each capacitor ( )
1
contributes to the total surface area. (volts)Aeq = A 1 −
n+1
( )
A 1
(volts)B1..n = 1−
n n+1
Ceq = C1 + C2 + · · · + Cn
A−B =0
For capacitors in series
Note: This is only correct if all capacitance
values are equal.

The power transferred in this arrangement is:

C1 C2 Cn
1 1
Several capacitors in series P = · Avolts (Afarads + Bfarads )
R n+1

Connected in series, the schematic diagram re- 3 Non-ideal behavior

veals that the separation distance,not the plate
area, adds up. The capacitors each store instan-
Capacitors deviate from the ideal capacitor equation in a
taneous charge build-up equal to that of every
number of ways. Some of these, such as leakage current
other capacitor in the series. The total voltage
and parasitic effects are linear, or can be assumed to be
difference from end to end is apportioned to
linear, and can be dealt with by adding virtual compo-
each capacitor according to the inverse of its
nents to the equivalent circuit of the capacitor. The usual
capacitance. The entire series acts as a capac-
methods of network analysis can then be applied. In other
itor smaller than any of its components.
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
1 1 1 1 another group, which may be linear but invalidate the as-
= + + ··· +
Ceq C1 C2 Cn sumption in the analysis that capacitance is a constant.
Such an example is temperature dependence. Finally,
Capacitors are combined in series to achieve combined parasitic effects such as inherent inductance,
a higher working voltage, for example for resistance, or dielectric losses can exhibit non-uniform
smoothing a high voltage power supply. The behavior at variable frequencies of operation.
voltage ratings, which are based on plate sep-
aration, add up, if capacitance and leakage
currents for each capacitor are identical. In 3.1 Breakdown voltage
such an application, on occasion, series strings
are connected in parallel, forming a matrix. Main article: Breakdown voltage
The goal is to maximize the energy storage of
the network without overloading any capaci- Above a particular electric field, known as the dielectric
tor. For high-energy storage with capacitors in strength Eds, the dielectric in a capacitor becomes con-
series, some safety considerations must be ap- ductive. The voltage at which this occurs is called the
plied to ensure one capacitor failing and leak- breakdown voltage of the device, and is given by the prod-
ing current will not apply too much voltage to uct of the dielectric strength and the separation between
the other series capacitors. the conductors,[21]

Series connection is also sometimes used

to adapt polarized electrolytic capacitors for Vbd = Eds d
bipolar AC use. See electrolytic capaci-
tor#Designing for reverse bias. The maximum energy that can be stored safely in a capac-
itor is limited by the breakdown voltage. Due to the scal-
Voltage distribution in parallel-to-series networks. ing of capacitance and breakdown voltage with dielectric
To model the distribution of voltages from a single thickness, all capacitors made with a particular dielectric
charged capacitor (A) connected in parallel to a have approximately equal maximum energy density, to
chain of capacitors in series (Bn ) : the extent that the dielectric dominates their volume.[22]
8 3 NON-IDEAL BEHAVIOR

For air dielectric capacitors the breakdown field strength ries resistance or ESR of a component. This adds a real
is of the order 2 to 5 MV/m; for mica the breakdown is component to the impedance:
100 to 300 MV/m; for oil, 15 to 25 MV/m; it can be much
less when other materials are used for the dielectric.[23]
The dielectric is used in very thin layers and so abso- 1
RC = Z + RESR = + RESR
lute breakdown voltage of capacitors is limited. Typical jωC
ratings for capacitors used for general electronics appli-
cations range from a few volts to 1 kV. As the voltage As frequency approaches infinity, the capacitive
increases, the dielectric must be thicker, making high- impedance (or reactance) approaches zero and the ESR
voltage capacitors larger per capacitance than those rated becomes significant. As the reactance becomes negli-
for lower voltages. The breakdown voltage is critically gible, power dissipation approaches PRMS = VRMS²
affected by factors such as the geometry of the capaci- /RESR.
tor conductive parts; sharp edges or points increase the Similarly to ESR, the capacitor’s leads add equivalent se-
electric field strength at that point and can lead to a local ries inductance or ESL to the component. This is usually
breakdown. Once this starts to happen, the breakdown significant only at relatively high frequencies. As induc-
quickly tracks through the dielectric until it reaches the tive reactance is positive and increases with frequency,
opposite plate, leaving carbon behind and causing a short above a certain frequency capacitance will be canceled
(or relatively low resistance) circuit. The results can be by inductance. High-frequency engineering involves ac-
explosive as the short in the capacitor draws current from counting for the inductance of all connections and com-
the surrounding circuitry and dissipates the energy.[24] ponents.
The usual breakdown route is that the field strength be- If the conductors are separated by a material with a small
comes large enough to pull electrons in the dielectric from conductivity rather than a perfect dielectric, then a small
their atoms thus causing conduction. Other scenarios are leakage current flows directly between them. The capaci-
possible, such as impurities in the dielectric, and, if the tor therefore has a finite parallel resistance,[15] and slowly
dielectric is of a crystalline nature, imperfections in the discharges over time (time may vary greatly depending on
crystal structure can result in an avalanche breakdown as the capacitor material and quality).
seen in semi-conductor devices. Breakdown voltage is
also affected by pressure, humidity and temperature.[25]
3.3 Q factor

3.2 Equivalent circuit The quality factor (or Q) of a capacitor is the ratio of
its reactance to its resistance at a given frequency, and is
a measure of its efficiency. The higher the Q factor of
the capacitor, the closer it approaches the behavior of an
ideal, lossless, capacitor.
The Q factor of a capacitor can be found through the fol-
lowing formula:

XC 1
Q= = ,
RC ωCRC
where ω is angular frequency, C is the capacitance, XC
is the capacitive reactance, and RC is the series resistance
of the capacitor.

3.4 Ripple current

Ripple current is the AC component of an applied source
(often a switched-mode power supply) whose frequency
may be constant or varying. Ripple current causes heat
Two different circuit models of a real capacitor to be generated within the capacitor due to the dielec-
tric losses caused by the changing field strength together
An ideal capacitor only stores and releases electrical en- with the current flow across the slightly resistive supply
ergy, without dissipating any. In reality, all capacitors lines or the electrolyte in the capacitor. The equivalent
have imperfections within the capacitor’s material that series resistance (ESR) is the amount of internal series
create resistance. This is specified as the equivalent se- resistance one would add to a perfect capacitor to model
3.6 Current and voltage reversal 9

this. Some types of capacitors, primarily tantalum and 3.6 Current and voltage reversal
aluminum electrolytic capacitors, as well as some film ca-
pacitors have a specified rating value for maximum ripple Current reversal occurs when the current changes direc-
current. tion. Voltage reversal is the change of polarity in a cir-
cuit. Reversal is generally described as the percentage
• Tantalum electrolytic capacitors with solid man- of the maximum rated voltage that reverses polarity. In
ganese dioxide electrolyte are limited by ripple cur- DC circuits, this will usually be less than 100% (often in
rent and generally have the highest ESR ratings in the range of 0 to 90%), whereas AC circuits experience
the capacitor family. Exceeding their ripple limits 100% reversal.
can lead to shorts and burning parts. In DC circuits and pulsed circuits, current and voltage re-
• Aluminum electrolytic capacitors, the most com- versal are affected by the damping of the system. Voltage
mon type of electrolytic, suffer a shortening of life reversal is encountered in RLC circuits that are under-
expectancy at higher ripple currents. If ripple cur- damped. The current and voltage reverse direction, form-
rent exceeds the rated value of the capacitor, it tends ing a harmonic oscillator between the inductance and ca-
to result in explosive failure. pacitance. The current and voltage will tend to oscillate
and may reverse direction several times, with each peak
• Ceramic capacitors generally have no ripple current being lower than the previous, until the system reaches
limitation and have some of the lowest ESR ratings. an equilibrium. This is often referred to as ringing. In
comparison, critically damped or over-damped systems
• Film capacitors have very low ESR ratings but ex-
usually do not experience a voltage reversal. Reversal is
ceeding rated ripple current may cause degradation
also encountered in AC circuits, where the peak current
failures.
will be equal in each direction.
For maximum life, capacitors usually need to be able to
3.5 Capacitance instability handle the maximum amount of reversal that a system
will experience. An AC circuit will experience 100%
The capacitance of certain capacitors decreases as the voltage reversal, while under-damped DC circuits will ex-
component ages. In ceramic capacitors, this is caused perience less than 100%. Reversal creates excess elec-
by degradation of the dielectric. The type of dielectric, tric fields in the dielectric, causes excess heating of both
ambient operating and storage temperatures are the most the dielectric and the conductors, and can dramatically
significant aging factors, while the operating voltage has shorten the life expectancy of the capacitor. Reversal rat-
a smaller effect. The aging process may be reversed by ings will often affect the design considerations for the ca-
heating the component above the Curie point. Aging is pacitor, from the choice of dielectric materials and volt-
fastest near the beginning of life of the component, and age ratings to the types of internal connections used.[27]
the device stabilizes over time.[26] Electrolytic capacitors
age as the electrolyte evaporates. In contrast with ceramic
capacitors, this occurs towards the end of life of the com- 3.7 Dielectric absorption
ponent.
Capacitors made with any type of dielectric material will
Temperature dependence of capacitance is usually ex- show some level of "dielectric absorption" or “soakage”.
pressed in parts per million (ppm) per °C. It can usually On discharging a capacitor and disconnecting it, after a
be taken as a broadly linear function but can be noticeably short time it may develop a voltage due to hysteresis in the
non-linear at the temperature extremes. The temperature dielectric. This effect can be objectionable in applica-
coefficient can be either positive or negative, sometimes tions such as precision sample and hold circuits or timing
even amongst different samples of the same type. In other circuits. The level of absorption depends on many fac-
words, the spread in the range of temperature coefficients tors, from design considerations to charging time, since
can encompass zero. the absorption is a time-dependent process. However,
Capacitors, especially ceramic capacitors, and older de- the primary factor is the type of dielectric material. Ca-
signs such as paper capacitors, can absorb sound waves pacitors such as tantalum electrolytic or polysulfone film
resulting in a microphonic effect. Vibration moves exhibit very high absorption, while polystyrene or Teflon
the plates, causing the capacitance to vary, in turn in- allow very small levels of absorption.[28] In some capaci-
ducing AC current. Some dielectrics also generate tors where dangerous voltages and energies exist, such as
piezoelectricity. The resulting interference is especially in flashtubes, television sets, and defibrillators, the dielec-
problematic in audio applications, potentially causing tric absorption can recharge the capacitor to hazardous
feedback or unintended recording. In the reverse micro- voltages after it has been shorted or discharged. Any ca-
phonic effect, the varying electric field between the ca- pacitor containing over 10 joules of energy is generally
pacitor plates exerts a physical force, moving them as a considered hazardous, while 50 joules or higher is po-
speaker. This can generate audible sound, but drains en- tentially lethal. A capacitor may regain anywhere from
ergy and stresses the dielectric and the electrolyte, if any. 0.01 to 20% of its original charge over a period of several
10 4 CAPACITOR TYPES

minutes, allowing a seemingly safe capacitor to become 4.1 Dielectric materials

surprisingly dangerous.[29][30][31][32][33]

3.8 Leakage

Leakage is equivalent to a resistor in parallel with the ca-

pacitor. Constant exposure to heat can cause dielectric
breakdown and excessive leakage, a problem often seen
in older vacuum tube circuits, particularly where oiled
Capacitor materials. From left: multilayer ceramic, ceramic disc,
paper and foil capacitors were used. In many vacuum multilayer polyester film, tubular ceramic, polystyrene, metalized
tube circuits, interstage coupling capacitors are used to polyester film, aluminum electrolytic. Major scale divisions are
conduct a varying signal from the plate of one tube to in centimetres.
the grid circuit of the next stage. A leaky capacitor can
cause the grid circuit voltage to be raised from its nor- Most types of capacitor include a dielectric spacer, which
mal bias setting, causing excessive current or signal dis- increases their capacitance. These dielectrics are most
tortion in the downstream tube. In power amplifiers this often insulators. However, low capacitance devices are
can cause the plates to glow red, or current limiting resis- available with a vacuum between their plates, which al-
tors to overheat, even fail. Similar considerations apply to lows extremely high voltage operation and low losses.
component fabricated solid-state (transistor) amplifiers, Variable capacitors with their plates open to the atmo-
but owing to lower heat production and the use of mod- sphere were commonly used in radio tuning circuits.
ern polyester dielectric barriers this once-common prob- Later designs use polymer foil dielectric between the
lem has become relatively rare. moving and stationary plates, with no significant air space
between them.
In order to maximise the charge that a capacitor can hold,
3.9 Electrolytic failure from disuse the dielectric material needs to have as high a permittivity
as possible, while also having as high a breakdown voltage
Aluminum electrolytic capacitors are conditioned when as possible.
manufactured by applying a voltage sufficient to initiate
the proper internal chemical state. This state is main- Several solid dielectrics are available, including paper,
tained by regular use of the equipment. In former times, plastic, glass, mica and ceramic materials. Paper was
roughly around the 1980s, if a system using electrolytic used extensively in older devices and offers relatively high
capacitors is unused for a long period of time it can lose voltage performance. However, it is susceptible to wa-
its conditioning. Sometimes they fail with a short circuit ter absorption, and has been largely replaced by plastic
when next operated. film capacitors. Plastics offer better stability and ageing
performance, which makes them useful in timer circuits,
although they may be limited to low operating temper-
atures and frequencies. Ceramic capacitors are gener-
4 Capacitor types ally small, cheap and useful for high frequency applica-
tions, although their capacitance varies strongly with volt-
Main article: Types of capacitor age and they age poorly. They are broadly categorized
as class 1 dielectrics, which have predictable variation
of capacitance with temperature or class 2 dielectrics,
Practical capacitors are available commercially in many
which can operate at higher voltage. Glass and mica
different forms. The type of internal dielectric, the struc- capacitors are extremely reliable, stable and tolerant to
ture of the plates and the device packaging all strongly high temperatures and voltages, but are too expensive
affect the characteristics of the capacitor, and its appli- for most mainstream applications. Electrolytic capaci-
cations. tors and supercapacitors are used to store small and larger
Values available range from very low (picofarad range; amounts of energy, respectively, ceramic capacitors are
while arbitrarily low values are in principle possible, stray often used in resonators, and parasitic capacitance oc-
(parasitic) capacitance in any circuit is the limiting factor) curs in circuits wherever the simple conductor-insulator-
to about 5 kF supercapacitors. conductor structure is formed unintentionally by the con-
Above approximately 1 microfarad electrolytic capacitors figuration of the circuit layout.
are usually used because of their small size and low cost Electrolytic capacitors use an aluminum or tantalum plate
compared with other types, unless their relatively poor with an oxide dielectric layer. The second electrode is a
stability, life and polarised nature make them unsuitable. liquid electrolyte, connected to the circuit by another foil
Very high capacity supercapacitors use a porous carbon- plate. Electrolytic capacitors offer very high capacitance
based electrode material. but suffer from poor tolerances, high instability, gradual
4.2 Structure 11

loss of capacitance especially when subjected to heat, and

high leakage current. Poor quality capacitors may leak
electrolyte, which is harmful to printed circuit boards.
The conductivity of the electrolyte drops at low tempera-
tures, which increases equivalent series resistance. While
widely used for power-supply conditioning, poor high-
frequency characteristics make them unsuitable for many
applications. Electrolytic capacitors will self-degrade if
unused for a period (around a year), and when full power
is applied may short circuit, permanently damaging the
capacitor and usually blowing a fuse or causing failure of
rectifier diodes (for instance, in older equipment, arcing
in rectifier tubes). They can be restored before use (and
damage) by gradually applying the operating voltage, of-
ten done on antique vacuum tube equipment over a period Capacitor packages: SMD ceramic at top left; SMD tantalum at
of 30 minutes by using a variable transformer to supply bottom left; through-hole tantalum at top right; through-hole elec-
AC power. Unfortunately, the use of this technique may trolytic at bottom right. Major scale divisions are cm.
be less satisfactory for some solid state equipment, which
may be damaged by operation below its normal power
range, requiring that the power supply first be isolated tiple stacks of plates and disks. Larger value capacitors
from the consuming circuits. Such remedies may not be usually use a metal foil or metal film layer deposited on
applicable to modern high-frequency power supplies as the surface of a dielectric film to make the plates, and a
these produce full output voltage even with reduced in- dielectric film of impregnated paper or plastic – these are
put. rolled up to save space. To reduce the series resistance
and inductance for long plates, the plates and dielectric
Tantalum capacitors offer better frequency and tempera- are staggered so that connection is made at the common
ture characteristics than aluminum, but higher dielectric edge of the rolled-up plates, not at the ends of the foil or
absorption and leakage.[34] metalized film strips that comprise the plates.
Polymer capacitors (OS-CON, OC-CON, KO, AO) use The assembly is encased to prevent moisture entering the
solid conductive polymer (or polymerized organic semi- dielectric – early radio equipment used a cardboard tube
conductor) as electrolyte and offer longer life and lower sealed with wax. Modern paper or film dielectric capaci-
ESR at higher cost than standard electrolytic capacitors. tors are dipped in a hard thermoplastic. Large capacitors
A feedthrough capacitor is a component that, while not for high-voltage use may have the roll form compressed
serving as its main use, has capacitance and is used to to fit into a rectangular metal case, with bolted terminals
conduct signals through a conductive sheet. and bushings for connections. The dielectric in larger ca-
pacitors is often impregnated with a liquid to improve its
Several other types of capacitor are available for specialist
properties.
applications. Supercapacitors store large amounts of en-
ergy. Supercapacitors made from carbon aerogel, carbon
nanotubes, or highly porous electrode materials, offer ex-
tremely high capacitance (up to 5 kF as of 2010) and can
be used in some applications instead of rechargeable bat-
teries. Alternating current capacitors are specifically de-
signed 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 be
physically large. They are usually ruggedly packaged, of-
ten in metal cases that can be easily grounded/earthed.
They also are designed with direct current breakdown
voltages of at least five times the maximum AC voltage.

4.2 Structure Several axial-lead electrolytic capacitors

The arrangement of plates and dielectric has many vari- Capacitors may have their connecting leads arranged
ations depending on the desired ratings of the capaci- in many configurations, for example axially or radially.
tor. For small values of capacitance (microfarads and “Axial” means that the leads are on a common axis, typ-
less), ceramic disks use metallic coatings, with wire leads ically the axis of the capacitor’s cylindrical body – the
bonded to the coating. Larger values can be made by mul- leads extend from opposite ends. Radial leads might more
12 6 APPLICATIONS

accurately be referred to as tandem; they are rarely actu- 6 Applications

ally aligned along radii of the body’s circle, so the term
is inexact, although universal. The leads (until bent) are Main article: Applications of capacitors
usually in planes parallel to that of the flat body of the ca-
pacitor, and extend in the same direction; they are 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. These packages are extremely small and lack
connecting leads, allowing them to be soldered directly
onto the surface of printed circuit boards. Surface mount
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
or sliding a set of movable plates into alignment with
a set of stationary plates. Low cost variable capac-
itors squeeze together alternating layers of aluminum
and plastic with a screw. Electrical control of capaci-
tance is achievable with varactors (or varicaps), which are
reverse-biased semiconductor diodes whose depletion re-
gion width varies with applied voltage. They are used in
phase-locked loops, amongst other applications.

5 Capacitor markings This mylar-film, oil-filled capacitor has very low inductance and
low resistance, to provide the high-power (70 megawatt) and high
speed (1.2 microsecond) discharge needed to operate a dye laser.
See also: Preferred number § E series

Most capacitors have numbers printed on their bodies to

indicate their electrical characteristics. Larger capacitors 6.1 Energy storage
like electrolytics usually display the actual capacitance to-
gether with the unit (for example, 220 μF). Smaller ca- A capacitor can store electric energy when disconnected
pacitors like ceramics, however, use a shorthand consist- from its charging circuit, so it can be used like a tem-
ing of three numeric digits and a letter, where the digits porary battery, or like other types of rechargeable energy
indicate the capacitance in pF (calculated as XY × 10Z storage system.[36] Capacitors are commonly used in elec-
for digits XYZ) and the letter indicates the tolerance (J, tronic devices to maintain power supply while batteries
K or M for ±5%, ±10% and ±20% respectively). are being changed. (This prevents loss of information in
volatile memory.)
Additionally, the capacitor may show its working voltage,
temperature and other relevant characteristics. Conventional capacitors provide less than 360 joules
per kilogram of energy density, whereas a conventional
For typographical reasons, some manufacturers print alkaline battery has a density of 590 kJ/kg.
“MF” on capacitors to indicate microfarads (μF).[35]
In car audio systems, large capacitors store energy for the
amplifier to use on demand. Also for a flash tube a ca-
pacitor is used to hold the high voltage.
5.1 Example
6.2 Pulsed power and weapons
A capacitor with the text 473K 330V on its body has a
capacitance of 47 × 103 pF = 47 nF (±10%) with a work- Groups of large, specially constructed, low-inductance
ing voltage of 330 V. The working voltage of a capacitor high-voltage capacitors (capacitor banks) are used to sup-
is the highest voltage that can be applied across it without ply huge pulses of current for many pulsed power appli-
undue risk of breaking down the dielectric layer. cations. These include electromagnetic forming, Marx
6.4 Suppression and coupling 13

generators, pulsed lasers (especially TEA lasers), pulse

forming networks, radar, fusion research, and particle ac-
celerators.
Large capacitor banks (reservoir) are used as en-
ergy 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 and coilguns.

6.3 Power conditioning

A high-voltage capacitor bank used for power factor correction

A 10,000 microfarad capacitor in an amplifier power supply on a power transmission system

Reservoir capacitors are used in power supplies where the load appear to be mostly resistive. Individual mo-
they smooth the output of a full or half wave rectifier. tor or lamp loads may have capacitors for power factor
They can also be used in charge pump circuits as the en- correction, or larger sets of capacitors (usually with auto-
ergy storage element in the generation of higher voltages matic switching devices) may be installed at a load center
than the input voltage. within a building or in a large utility substation.
Capacitors are connected in parallel with the power cir-
cuits of most electronic devices and larger systems (such
as factories) to shunt away and conceal current fluctua- 6.4 Suppression and coupling
tions from the primary power source to provide a “clean”
power supply for signal or control circuits. Audio equip- 6.4.1 Signal coupling
ment, for example, uses several capacitors in this way, to
shunt away power line hum before it gets into the signal Main article: capacitive coupling
circuitry. The capacitors act as a local reserve for the DC Because capacitors pass AC but block DC signals (when
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.

6.3.1 Power factor correction

In electric power distribution, capacitors are used for

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 de- Polyester film capacitors are frequently used as coupling capac-
vices like electric motors and transmission lines to make itors.
14 6 APPLICATIONS

charged up to the applied dc voltage), they are often used Capacitors are also used in parallel to interrupt units of a
to separate the AC and DC components of a signal. This high-voltage circuit breaker in order to equally distribute
method is known as AC coupling or “capacitive coupling”. the voltage between these units. In this case they are
Here, a large value of capacitance, whose value need not called grading capacitors.
be accurately controlled, but whose reactance is small at In schematic diagrams, a capacitor used primarily for DC
the signal frequency, is employed. charge storage is often drawn vertically in circuit dia-
grams with the lower, more negative, plate drawn as an
6.4.2 Decoupling arc. The straight plate indicates the positive terminal of
the device, if it is polarized (see electrolytic capacitor).
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 to 6.5 Motor starters
suppress noise or transients. Noise caused by other cir-
cuit elements is shunted through the capacitor, reducing
the effect they have on the rest of the circuit. It is most Main article: motor capacitor
commonly used between the power supply and ground.
An alternative name is bypass capacitor as it is used to In single phase squirrel cage motors, the primary wind-
bypass the power supply or other high impedance com- ing within the motor housing is not capable of starting a
ponent of a circuit. rotational motion on the rotor, but is capable of sustain-
Decoupling capacitors need not always be discrete com- ing one. To start the motor, a secondary “start” wind-
ponents. Capacitors used in these applications may be ing has a series non-polarized starting capacitor to in-
built in to a printed circuit board, between the vari- troduce a lead in the sinusoidal current. When the sec-
ous layers. These are often referred to as embedded ondary (start) winding is placed at an angle with respect
capacitors.[37] The layers in the board contributing to the to the primary (run) winding, a rotating electric field is
capacitive properties also function as power and ground created. The force of the rotational field is not constant,
planes, and have a dielectric in between them, enabling but is sufficient to start the rotor spinning. When the ro-
them to operate as a parallel plate capacitor. tor comes close to operating speed, a centrifugal switch
(or current-sensitive relay in series with the main wind-
ing) disconnects the capacitor. The start capacitor is typ-
6.4.3 High-pass and low-pass filters ically mounted to the side of the motor housing. These
are called capacitor-start motors, that have relatively high
Further information: High-pass filter and Low-pass filter starting torque. Typically they can have up-to four times
as much starting torque than a split-phase motor and are
used on applications such as compressors, pressure wash-
ers and any small device requiring high starting torques.
6.4.4 Noise suppression, spikes, and snubbers
Capacitor-run induction motors have a permanently con-
nected phase-shifting capacitor in series with a second
Further information: High-pass filter and Low-pass filter
winding. The motor is much like a two-phase induction
motor.
When an inductive circuit is opened, the current through
Motor-starting capacitors are typically non-polarized
the inductance collapses quickly, creating a large volt-
electrolytic types, while running capacitors are conven-
age across the open circuit of the switch or relay. If the
tional paper or plastic film dielectric types.
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 con-
tact points, thereby preserving their life; these were com-
6.6 Signal processing
monly found in contact breaker ignition systems, for in-
stance. Similarly, in smaller scale circuits, the spark may The energy stored in a capacitor can be used to represent
not be enough to damage the switch but will still radiate information, either in binary form, as in DRAMs, or in
undesirable radio frequency interference (RFI), which a analogue form, as in analog sampled filters and CCDs.
filter capacitor absorbs. Snubber capacitors are usually Capacitors can be used in analog circuits as components
employed with a low-value resistor in series, to dissipate of integrators or more complex filters and in negative
energy and minimize RFI. Such resistor-capacitor com- feedback loop stabilization. Signal processing circuits
binations are available in a single package. also use capacitors to integrate a current signal.
6.8 Oscillators 15

6.6.1 Tuned circuits plate. Some accelerometers use MEMS capac-

itors etched on a chip to measure the magni-
Capacitors and inductors are applied together in tuned tude and direction of the acceleration vector.
circuits to select information in particular frequency They are used to detect changes in accelera-
bands. For example, radio receivers rely on variable ca- tion, in tilt sensors, or to detect free fall, as sen-
pacitors to tune the station frequency. Speakers use pas- sors triggering airbag deployment, and in many
sive analog crossovers, and analog equalizers use capaci- other applications. Some fingerprint sensors
tors to select different audio bands. use capacitors. Additionally, a user can adjust
The resonant frequency f of a tuned circuit is a function the pitch of a theremin musical instrument by
of the inductance (L) and capacitance (C) in series, and moving their hand since this changes the effec-
is given by: tive capacitance between the user’s hand and
the antenna.

1
f= √ Changing the effective area of the plates:
2π LC
where L is in henries and C is in farads.
Capacitive touch switches are now used on
6.7 Sensing many consumer electronic products.

Main article: capacitive sensing

6.8 Oscillators
Main article: Capacitive displacement sensor
Further information: Hartley oscillator
A capacitor can possess spring-like qualities in an oscil-
Most capacitors are designed to maintain a fixed physi-
cal structure. However, various factors can change the
structure of the capacitor, and the resulting change in ca-
pacitance 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 an exposed and porous dielec-
tric can be used to measure humidity in air. Ca-
pacitors are used to accurately measure the fuel
level in airplanes; as the fuel covers more of a
pair of plates, the circuit capacitance increases.
Squeezing the dielectric can change a capacitor
at a few tens of bar pressure sufficiently that it
can be used as a pressure sensor.[38] A selected,
but otherwise standard, polymer dielectric ca-
pacitor, when immersed in a compatible gas
or liquid, can work usefully as a very low cost Example of a simple oscillator that requires a capacitor to func-
pressure sensor up to many hundreds of bar. tion

Changing the distance between the plates: lator circuit. In the image example, a capacitor acts to
influence the biasing voltage at the npn transistor’s base.
The resistance values of the voltage-divider resistors and
Capacitors with a flexible plate can be used the capacitance value of the capacitor together control the
to measure strain or pressure. Industrial pres- oscillatory frequency.
sure transmitters used for process control use
pressure-sensing diaphragms, which form a ca-
pacitor plate of an oscillator circuit. Capaci- 6.9 Producing light
tors are used as the sensor in condenser micro-
phones, where one plate is moved by air pres- Main article: light emitting capacitor
sure, relative to the fixed position of the other
16 9 REFERENCES

A light-emitting capacitor is made from a dielectric that operation. Proper containment, fusing, and preventive
uses phosphorescence to produce light. If one of the con- maintenance can help to minimize these hazards.
ductive plates is made with a transparent material, the High-voltage capacitors can benefit from a pre-charge to
light will be visible. Light-emitting capacitors are used limit in-rush currents at power-up of high voltage direct
in the construction of electroluminescent panels, for ap- current (HVDC) circuits. This will extend the life of the
plications such as backlighting for laptop computers. In component and may mitigate high-voltage hazards.
this case, the entire panel is a capacitor used for the pur-
pose of generating light.
• Swollen caps of electrolytic capacitors – special
design of semi-cut caps prevents capacitors from
bursting
7 Hazards and safety • This high-energy capacitor from a defibrillator can
deliver over 500 joules of energy. A resistor is con-
The hazards posed by a capacitor are usually determined, nected between the terminals for safety, to allow the
foremost, by the amount of energy stored, which is the stored energy to be released.
cause of things like electrical burns or heart fibrillation.
Factors such as voltage and chassis material are of sec- • Catastrophic failure
ondary consideration, which are more related to how eas-
ily a shock can be initiated rather than how much damage
can occur.[33] 8 See also
Capacitors may retain a charge long after power is re-
moved from a circuit; this charge can cause dangerous or • Capacitance meter
even potentially fatal shocks or damage connected equip-
• Capacitor plague
ment. For example, even a seemingly innocuous device
such as a disposable-camera flash unit, powered by a 1.5 • Circuit design
volt AA battery, has a capacitor which may contain over
15 joules of energy and be charged to over 300 volts. This • Electric displacement field
is easily capable of delivering a shock. Service proce-
• Electroluminescence
dures for electronic devices usually include instructions
to discharge large or high-voltage capacitors, for instance • Electronic oscillator
using a Brinkley stick. Capacitors may also have built-
in discharge resistors to dissipate stored energy to a safe • Gimmick capacitor
level within a few seconds after power is removed. High-
• Vacuum variable capacitor
voltage capacitors are stored with the terminals shorted,
as protection from potentially dangerous voltages due to
dielectric absorption or from transient voltages the capac-
itor may pick up from static charges or passing weather 9 References
events.[33]
[1] Bird, John (2010). Electrical and Electronic Princi-
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as containing “Askarel” and several other trade names. II, Part VI: The Leyden Jar Discovered”. Retrieved 2013-
PCB-filled paper capacitors are found in very old (pre- 03-17.
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electric fluid, resulting in case bulging, rupture, or even P. F. Collier & Son. p. 71. Retrieved 2013-03-17.
an explosion. Capacitors used in RF or sustained high-
current applications can overheat, especially in the center [5] Isaacson, Walter (2003). Benjamin Franklin: An Amer-
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9780743260848. Retrieved 2013-03-17.
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 17

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[22] Pai, S. T.; Qi Zhang (1995). Introduction to High Power
Pulse Technology. Advanced Series in Electrical and • Philosophical Transactions of the Royal Society
Computer Engineering 10. World Scientific. ISBN LXXII, Appendix 8, 1782 (Volta coins the word
9789810217143. Retrieved 2013-03-17. condenser)
[23] Dyer, Stephen A. (2004). Wiley Survey of Instrumentation • Ulaby, Fawwaz Tayssir (1999). Fundamentals of
and Measurement. John Wiley & Sons. p. 397. ISBN Applied Electromagnetics. Upper Saddle River, New
9780471221654. Retrieved 2013-03-17.
Jersey: Prentice Hall. ISBN 9780130115546.
[24] Scherz, Paul (2006). Practical Electronics for Inventors • Zorpette, Glenn (2005). “Super Charged: A
(2nd ed.). McGraw Hill Professional. p. 100. ISBN
Tiny South Korean Company is Out to Make
9780071776448. Retrieved 2013-03-17.
Capacitors Powerful enough to Propel the Next
[25] Bird, John (2007). Electrical Circuit Theory and Technol- Generation of Hybrid-Electric Cars”. IEEE
ogy. Routledge. p. 501. ISBN 9780750681391. Re- Spectrum (North American ed.) 42 (1): 32.
trieved 2013-03-17. doi:10.1109/MSPEC.2005.1377872.
18 11 EXTERNAL LINKS

• Deshpande, R.P. (2014). Capacitors. McGraw-Hill.

ISBN 9780071848565.

11 External links
• Capacitors: Interactive Tutorial National High Mag-
netic Field Laboratory

• Currier, Dean P. (2000). “Adventures in Cyber-

sound – Ewald Christian von Kleist”. Archived from
the original on 2008-06-25.
• “The First Condenser – A Beer Glass”. SparkMu-
seum.
• Howstuffworks.com: How Capacitors Work

• CapSite 2015: Introduction to Capacitors

• Capacitor Tutorial – Includes how to read capacitor
temperature codes
• Introduction to Capacitor and Capacitor codes

• Low ESR Capacitor Manufacturers

• How Capacitor Works – Capacitor Markings and
Color Codes
 19

12 Text and image sources, contributors, and licenses

12.1 Text

• Capacitor Source: https://en.wikipedia.org/wiki/Capacitor?oldid=683701004 Contributors: AxelBoldt, Sodium, Bryan Derksen, Zundark,

Ap, Andre Engels, Fredbauder, Aldie, PierreAbbat, Ray Van De Walker, Merphant, Waveguy, Heron, Patrick, Tim Starling, Chan siuman,
Modster, Dominus, Tjfulopp, Lousyd, Kku, Ixfd64, Ahoerstemeier, Mac, Stevenj, Muriel Gottrop~enwiki, Theresa knott, Darkwind, Glenn,
Bogdangiusca, Nikai, BAxelrod, Smack, Schneelocke, HolIgor, Timwi, Bemoeial, Wikiborg, Reddi, Denni, Sertrel, Maximus Rex, Fur-
rykef, Populus, Omegatron, Phoebe, Thue, Francs2000, Phil Boswell, Rogper~enwiki, Nufy8, Robbot, Ghouston, Hubertus~enwiki, Naddy,
Modulatum, Texture, Gidonb, Jondel, Intangir, Jleedev, Rik G., Wjbeaty, Giftlite, DavidCary, Wolfkeeper, Netoholic, Tom harrison, Tubu-
lar, Everyking, CyborgTosser, Niteowlneils, Leonard G., Starsong, Guanaco, Yekrats, Mboverload, Pascal666, Solipsist, Foobar, Edcolins,
StuartH, SebastianBreier~enwiki, Geni, Gzuckier, Mako098765, MisfitToys, Am088, ShakataGaNai, Jossi, Hutschi, Anythingyouwant,
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Adam1213, RussBot, Gokselgoksel, Crazytales, Red Slash, Hydrargyrum, Akamad, Stephenb, Yyy, Shaddack, Wiki alf, Spike Wilbury,
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Pinethicket, Jonesey95, Tom.Reding, RedBot, 124Nick, Foobarnix, Fumitol, Vin300, Abhishekchavan79, Hitachi-Train, LogAntiLog,
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paul, Vividvilla, Delusion23, 10v1walsha, ScottSteiner, Benfriesen12, Widr, Reify-tech, Vortex112, Helpful Pixie Bot, Doorknob747,
Lowercase sigmabot, Mataresephotos, BG19bot, IronOak, Vagobot, Vokesk, AntonioSajonia, Piguy101, Mark Arsten, AhsanAli408,
Rickey985, Isacp, Sleepsfortheweak, Frizb99, BattyBot, Clienthopeless, DarafshBot, Mahmud Halimi Wardag, HubabubbalubbahubbaYA-
BALICIOUS, SD5bot, JamesHaigh, Kshahinian, Dexbot, Aloysius314, Mogism, Salako1999, Bayezit.dirim, Isarra (HG), MZLauren,
Frosty, Paxmartian, FrostieFrost, Vahid alpha, Madhacker2000, Mark viking, Altered Walter, TREXJET, Fa.aref, Gomunkul51, Mur-
mur75, Gtrsentra, DavidLeighEllis, Glaisher, Jwratner1, Asadwarraich, Cricetone, Monkbot, JREling1, JaunJimenez, MadDoktor23, Ap-
plemusher123, NameloCmaS, Krelcoyne, Ruksakba, KasparBot, Goran Diklic, Delight dmix mutanda, AlwaysSomething and Anonymous:
1045
20 12 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

12.2 Images
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jpg License: CC BY-SA 2.0 Contributors: 12739s Original artist: Eric Schrader from San Francisco, CA, United States
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• File:Plattenkondensator_hg.jpg Source: https://upload.wikimedia.org/wikipedia/commons/d/d3/Plattenkondensator_hg.jpg License:
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12.3 Content license 21

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svg License: CC BY-SA 3.0 Contributors: Own work Original artist: User:Bastique, User:Ramac et al.
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