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Capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
s have many uses in electronic and electrical systems. They are so ubiquitous that it is rare that an electrical product does not include at least one for some purpose. Capacitors allow only AC signals to pass when they are charged blocking DC signals. The main components of filters are capacitors. Capacitors have the ability to connect one circuit segment to another. Capacitors are used by Dynamic Random Access Memory (DRAM) devices to represent binary information as bits.


Energy storage

A capacitor can store electric energy when it is connected to its charging circuit and when it is disconnected from its charging circuit, it can dissipate that stored energy, so it can be used as 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.) Conventional electrostatic capacitors provide less than 360
joule The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...
s per kilogram of energy density, while capacitors using developing technology can provide more than 2.52
kilo Kilo may refer to: *kilo- (k-), a metric prefix denoting a factor of 103 *Kilogram (kg), a metric unit of mass Music *Kilo, a funk/R&B band from Bloomington/Indianapolis/Indiana *KILO, a Colorado radio station *''El Kilo'', a 2005 album by th ...
joule The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...
s per kilogram. In
car audio Vehicle audio is equipment installed in a car or other vehicle to provide in-car entertainment and information for the occupants. Such systems are popularly known as car stereos. Until the 1950s, it consisted of a simple AM radio. Additions si ...
systems, large capacitors store energy for the
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...
to use on demand. An
uninterruptible power supply An uninterruptible power supply (UPS) or uninterruptible power source is a type of continual power system that provides automated backup electric power to a electrical load, load when the input power source or mains electricity, mains power fai ...
(UPS) can be equipped with maintenance-free capacitors to extend
service life A product's service life is its period of use in service. Several related terms describe more precisely a product's life, from the point of manufacture, storage, and distribution, and eventual use. Service life has been defined as "a product' ...
.industrial-europe.com
/ref>


Pulsed power and weapons

Groups of large, specially constructed, low-
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The electric current produces a magnetic field around the conductor. The magnetic field strength depends on the magnitude of the ...
high-voltage capacitors (''capacitor banks'') are used to supply huge pulses of current for many
pulsed power Pulsed power is the science and technology of accumulating energy over a relatively long period of time and releasing it instantly, thus increasing the instantaneous power. They can be used in some applications such as food processing, water treatme ...
applications. These include
electromagnetic forming Electromagnetic forming (EM forming or magneforming) is a type of high-velocity, cold forming process for electrically conductive metals, most commonly copper and aluminium. The workpiece is reshaped by high-intensity pulsed magnetic fields tha ...
,
Marx generator A Marx generator is an electrical circuit first described by Erwin Otto Marx in 1924. Its purpose is to generate a high-voltage pulse from a low-voltage DC supply. Marx generators are used in high-energy physics experiments, as well as to simul ...
s, pulsed
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
s (especially
TEA laser A TEA laser, or transversely excited atmospheric laser, is a gas laser energized by a high-voltage electrical discharge in a gas mixture generally at or above atmospheric pressure. The most common types are carbon dioxide lasers and excimer las ...
s), pulse forming networks, fusion research, and
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...
s. Large capacitor banks (reservoirs) are used as energy sources for the exploding-bridgewire detonators or
slapper detonator A slapper detonator, also called exploding foil initiator (EFI), is a detonator developed by Lawrence Livermore National Laboratory, US Patent No. 4,788,913 (Filed 1971, Granted 1988). It is an improvement over the earlier exploding-bridgewire de ...
s in
nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...
s and other specialty weapons. Experimental work is under way using banks of capacitors as power sources for
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...
armour Armour (Commonwealth English) or armor (American English; see American and British English spelling differences#-our, -or, spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, e ...
and electromagnetic
railgun A railgun or rail gun, sometimes referred to as a rail cannon, is a linear motor device, typically designed as a ranged weapon, that uses Electromagnet, electromagnetic force to launch high-velocity Projectile, projectiles. The projectile norma ...
s or
coilgun A coilgun is a type of mass driver consisting of one or more coils used as electromagnets in the configuration of a linear motor that accelerate a ferromagnetic or conducting projectile to high velocity. In almost all coilgun configurations, t ...
s.


Power conditioning

Reservoir 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 DC power circuits of most electronic devices to smooth current fluctuations 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

In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three-phase Electrical 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 devices like Induction motors, electric motors and transmission lines to make the load appear primarily resistive. Individual motor or lamp loads may have capacitors for power factor correction, or larger sets of capacitors (usually with automatic switching devices) may be installed at a load centre within a building or in a large utility
electrical substation A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station an ...
. In high-voltage direct current transmission systems, power factor correction capacitors may have tuning inductors to suppress harmonic currents that would otherwise be injected into the AC power system.


Suppression and coupling

Capacitors used for suppressing undesirable frequencies are sometimes called filter capacitors. They are common in electrical and electronic equipment, and cover a number of applications, such as: * Glitch removal on
direct current Direct current (DC) is one-directional electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...
(DC) power rails * Radio frequency interference (RFI) removal for signal or power lines entering or leaving equipment * Capacitors used after a
voltage regulator A voltage regulator is a system designed to automatically maintain a constant voltage. It may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism or electronic components. Depending on the ...
to further smooth DC
power supplies A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a r ...
* Capacitors used in audio,
intermediate frequency In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in Transmission (telecommunications), transmission or reception. The intermediate frequency is ...
(IF) or
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
(RF)
filters Filtration is a physical process that separates solid matter and fluid from a mixture. Filter, filtering, filters or filtration may also refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Fil ...
(e.g. low pass, high pass, notch, etc.) *
Arc suppression Arc suppression is the reduction of the electric arc energy that occurs when current-carrying contacts are opened and closed. An electric arc is a man-made, continuous arc-discharge consisting of highly energized electrons and ions supported by an e ...
, such as across the
contact breaker A contact breaker (or "points") is a type of electrical switch, found in the ignition systems of spark-ignition internal combustion engines. The switch is automatically operated by a cam driven by the engine. The timing of operation of the switch ...
or 'points' in a
spark-ignition engine A spark-ignition engine (SI engine) is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug. This is in contrast to compression-ignition engines, ty ...


Signal coupling

Because capacitors pass AC but block DC
signals A signal is both the process and the result of Signal transmission, transmission of data over some transmission media, media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processin ...
(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 coupling'' or "capacitive 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.


Decoupling

A
decoupling capacitor In electronics, a decoupling capacitor is a capacitor used to decouple (i.e. prevent electrical energy from transferring to) one part of a circuit from another. Noise caused by other circuit elements is shunted through the capacitor, reduc ...
is a capacitor used to decouple one part of a circuit from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit. It is most commonly used between the power supply and ground. For higher frequencies an alternative name is ''
bypass capacitor In electronics, a decoupling capacitor is a capacitor used to decouple (i.e. prevent electrical energy from transferring to) one part of a circuit from another. Noise caused by other circuit elements is shunted through the capacitor, reduc ...
'' as it is used to bypass the power supply or other high impedance component of a circuit.


High-pass and low-pass filters

A
high-pass filter A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency ...
(HPF) is an electronic filter that passes signals with a
frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency depends on the filter design. A high-pass filter is usually modeled as a linear time-invariant system. It is sometimes called a low-cut filter or bass-cut filter. High-pass filters have many uses, such as blocking DC from circuitry sensitive to non-zero average voltages or radio frequency devices. They can also be used in conjunction with a low-pass filter to produce a
bandpass A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects ( attenuates) frequencies outside that range. It is the inverse of a '' band-stop filter''. Description In electronics and s ...
filter. A
low-pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
(LPF) is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter design. The filter is sometimes called a high-cut filter, or treble-cut filter in audio applications. A low-pass filter is the complement of a high-pass filter.


Noise filters 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 an
electric spark An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an Ionization, ionized, Electric current, electrically conductive channel through a normally-insulating medium, often air or other ga ...
, causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroy a solid-state switch. A
snubber A snubber is a device used to suppress ("wiktionary:snub, snub") a phenomenon such as voltage transients in electronics, electrical systems, pressure transients in fluid systems (caused by for example water hammer) or excess force or rapid moveme ...
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 A contact breaker (or "points") is a type of electrical switch, found in the ignition systems of spark-ignition internal combustion engines. The switch is automatically operated by a cam driven by the engine. The timing of operation of the switch ...
ignition system Ignition systems are used by heat engines to initiate combustion by igniting the fuel-air mixture. In a spark ignition versions of the internal combustion engine (such as petrol engines), the ignition system creates a spark to ignite the fuel-ai ...
s, 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. Capacitors are also used in parallel to interrupt units of a high-voltage
circuit breaker A circuit breaker is an electrical safety device designed to protect an Electrical network, electrical circuit from damage caused by current in excess of that which the equipment can safely carry (overcurrent). Its basic function is to interr ...
to distribute the voltage between these units equally. 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 An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...
).


DC motor suppression

Ceramic disc capacitors are usually used in
snubber A snubber is a device used to suppress ("wiktionary:snub, snub") a phenomenon such as voltage transients in electronics, electrical systems, pressure transients in fluid systems (caused by for example water hammer) or excess force or rapid moveme ...
circuits for low voltage motors for their low
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The electric current produces a magnetic field around the conductor. The magnetic field strength depends on the magnitude of the ...
and low cost.


Switched mode power supply filtering

Low ESR (equivalent series resistance) electrolytes are often required to handle the high ripple current.


Mains filtering

Mains filter capacitors are usually encapsulated wound-plastic-film types, since these deliver high voltage rating at low cost, and may be made self-healing and fusible. Mains filter capacitors are often ceramic RFI/EMI suppression capacitors. The additional safety requirements for mains filtering are: * Line to neutral capacitors are flame retardant, and in Europe are required to use class X dielectrics. * Line or neutral to earth: Must be flame retardant; also, the dielectric must be self healing and fusible. In Europe these are class Y capacitors.


Power rail filtering

Electrolytic capacitor An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...
s are usually used due to high capacity at low cost and low size. Smaller non-electrolytics may be paralleled with these to compensate for electrolytics' poor performance at high frequencies.
Computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...
s use large numbers of filter capacitors, making size an important factor. Solid tantalum and wet tantalum capacitors offer some of the best CV (capacitance/voltage) performance in some of the most volumetrically efficient packaging available. High currents and low voltages also make low
equivalent series resistance Capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated, to a very good degree of approximation, as being ideal capacitors and inductors in series with a re ...
(ESR) important. Solid tantalum capacitors offer low ESR versions that can often meet ESR requirements but they are not the lowest ESR option among all capacitors. Solid tantalums have an additional issue which must be addressed during the design stage. Solid tantalum capacitors must be voltage derated in all applications. A 50% voltage derating is recommended and generally accepted as the industry standard; e.g. a 50V solid tantalum capacitor should never be exposed to an actual application voltage above 25V. Solid tantalum capacitors are very reliable components if the proper care is taken and all design guidelines are carefully followed. Unfortunately, the failure mechanism for a solid tantalum capacitor is a short which will result in a violent flaring up and smoking on a PCB capable of damaging other components in close proximity as well as completely destroying the capacitor. Fortunately, most solid tantalum capacitor failures will be immediate and very evident. Once in application solid tantalum capacitor performance will improve over time and the chances of a failure due to component mis-manufacturing decrease. Wet tantalums are a type of the electrolytic capacitor, using a tantalum pellet in an electrolytic material sealed in a hermetic package. This type of tantalum capacitor does not require the same derating that a solid tantalum does and its failure mechanism is open. A 10% to 20% voltage derating curve is recommended for wet tantalums when operating from 85C to 125C. Wet tantalums are not commonly referred to as just 'electrolytics' because usually 'electrolytic' refers to aluminium electrolytics.


Motor starters

In single phase
squirrel cage Squirrel-cage may refer to: *a squirrel-cage rotor *a squirrel-cage fan, another name for a centrifugal fan *a hamster wheel A hamster wheel or running wheel is an exercise device used primarily by hamsters and other rodents, but also by othe ...
motors, the primary winding within the motor housing is not capable of starting a rotational motion on the rotor, but is capable of sustaining one. To start the motor, a secondary winding is used in series with a non-polarized ''
starting capacitor A motor capacitor is an electrical capacitor that alters the current to one or more windings of a single-phase AC motor, alternating-current induction motor to create a rotating magnetic field. There are two common types of motor capacitors, sta ...
'' to introduce a lag in the sinusoidal current through the starting winding. When the secondary winding is placed at an angle with respect to the primary winding, a rotating electric field is created. The force of the rotational field is not constant, but is sufficient to start the rotor spinning. When the rotor comes close to operating speed, a centrifugal switch (or current-sensitive relay in series with the main winding) disconnects the capacitor. The start capacitor is typically mounted to the side of the motor housing. These are called capacitor-start motors, and have relatively high starting torque. There are also capacitor-run induction motors which have a permanently connected phase-shifting capacitor in series with a second winding. The motor is much like a two-phase induction motor. Motor-starting capacitors are typically non-polarized electrolytic types, while running capacitors are conventional paper or plastic film
dielectric In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...
types.


Signal processing

The energy stored in capacitor can be used to represent information, either in binary form, as in
DRAM Dram, DRAM, or drams may refer to: Technology and engineering * Dram (unit), a unit of mass and volume, and an informal name for a small amount of liquor, especially whisky or whiskey * Dynamic random-access memory, a type of electronic semicondu ...
s, or in analogue form, as in
analog sampled filter An analog sampled filter an electronic filter that is a hybrid between an analog and a digital filter. The input is an analog signal, and usually stored in capacitors. The time domain is discrete, however. Distinct analog samples are shift ...
s and
Charge-coupled device A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
CCDs. Capacitors can be used in
analog circuit Analogue electronics () are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term ''analogue'' describes the proportional relationship between a signal ...
s as components of integrators or more complex filters and in
negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused ...
loop stabilization. Signal processing circuits also use capacitors to integrate a current signal.


Tuned circuits

Capacitors and
inductor An inductor, also called a coil, choke, or reactor, is a Passivity (engineering), passive two-terminal electronic component, electrical component that stores energy in a magnetic field when an electric current flows through it. An inductor typic ...
s are applied together in tuned circuits to select information in particular frequency bands. For example,
radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. ...
s rely on variable capacitors to tune the station frequency. Speakers use passive analog crossovers, and analog equalizers use capacitors to select different audio bands.


Sensing

Most capacitors are designed to maintain a fixed physical structure. However, various factors can change the structure of the capacitor; the resulting change in capacitance can be used to
sense A sense is a biological system used by an organism for sensation, the process of gathering information about the surroundings through the detection of Stimulus (physiology), stimuli. Although, in some cultures, five human senses were traditio ...
those factors.


Changing the dielectric

The effects of varying the characteristics of the dielectric can also be used for sensing and measurement. Capacitors with an exposed and porous dielectric can be used to measure humidity in air. Capacitors are used to accurately measure the fuel level in
airplane An airplane (American English), or aeroplane (Commonwealth English), informally plane, is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, Propeller (aircraft), propeller, or rocket engine. Airplanes come in a vari ...
s; as the fuel covers more of a pair of plates, the circuit capacitance increases.


Changing the distance between the plates

A capacitor with a flexible plate can be used to measure strain or pressure or
weight In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition. Some sta ...
. Industrial pressure transmitters used for
process control Industrial process control (IPC) or simply process control is a system used in modern manufacturing which uses the principles of control theory and physical industrial control systems to monitor, control and optimize continuous Industrial processe ...
use pressure-sensing diaphragms, which form a capacitor plate of an oscillator circuit. Capacitors are used as the
sensor A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
in
condenser microphone A microphone, colloquially called a mic (), or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publi ...
s, where one plate is moved by air pressure, relative to the fixed position of the other plate. Some
accelerometer An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...
s use
microelectromechanical systems MEMS (micro-electromechanical systems) is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size (i.e., 0.001 to 0.1 mm), and MEMS devices ...
(MEMS) capacitors etched on a chip to measure the magnitude and direction of the acceleration vector. They are used to detect changes in acceleration, e.g. as tilt sensors or to detect free fall, as sensors triggering
airbag An airbag is a vehicle occupant-restraint system using a bag designed to inflate in milliseconds during a collision and then deflate afterwards. It consists of an airbag cushion, a flexible fabric bag, an inflation module, and an impact sensor. ...
deployment, and in many other applications. Some fingerprint sensors use capacitors.


Changing the effective area of the plates

Capacitive touch switches are now used on many consumer electronic products


Oscillators

{{Further, Hartley oscillator A capacitor can possess spring-like qualities in an oscillator 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 the capacitance value of the capacitor together control the oscillatory frequency.


Hazards and safety

Capacitors may retain a charge long after power is removed from a circuit; this charge can cause dangerous or even potentially fatal shocks or damage connected equipment. For example, even a seemingly innocuous device such as a disposable camera flash unit powered by a 1.5 volt AA battery contains a capacitor which may be charged to over 300 volts. This is easily capable of delivering a shock. Service procedures for electronic devices usually include instructions to discharge large or high-voltage capacitors. Capacitors may also have built-in discharge resistors to dissipate stored energy to a safe level within a few seconds after power is removed. High-voltage capacitors are stored with the terminals shorted, as protection from potentially dangerous voltages due to dielectric absorption. Some old, large oil-filled capacitors contain
polychlorinated biphenyl Polychlorinated biphenyls (PCBs) are organochlorine compounds with the formula Carbon, C12Hydrogen, H10−''x''Chloride, Cl''x''; they were once widely used in the manufacture of carbonless copy paper, as heat transfer fluids, and as dielectri ...
s (PCBs). It is known that waste PCBs can leak into groundwater under landfills. Capacitors containing PCB were labelled as containing "Askarel" and several other trade names. PCB-filled capacitors are found in very old (pre 1975) fluorescent lamp ballasts, and other applications. High-voltage capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or as they reach their normal end of life. Dielectric or metal interconnection failures may create arcing that vaporizes dielectric fluid, resulting in case bulging, rupture, or even an explosion. Capacitors used in RF or sustained high-current applications can overheat, especially in the center of the capacitor rolls. Capacitors used within high-energy capacitor banks can violently explode when a short in one capacitor causes sudden dumping of energy stored in the rest of the bank into the failing unit. High voltage vacuum capacitors can generate soft X-rays even during normal operation. Proper containment, fusing, and preventive maintenance can help to minimize these hazards. High-voltage capacitors can benefit from a pre-charge to limit in-rush currents at power-up of high voltage direct current (HVDC) circuits. This will extend the life of the component and may mitigate high-voltage hazards.


See also

*
Glitch removal Glitch removal is the elimination of Glitch#Electronics glitch, glitchesunnecessary signal transitions without functionalityfrom electronic circuits. Power dissipation of a gate occurs in two ways: static power dissipation and dynamic power dissip ...


References

Capacitors