capacitor

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capacitor

or

condenser,

device for the storage of electric charge. Simple capacitors consist of two plates made of an electrically conducting material (e.g., a metal) and separated by a nonconducting material or dielectric (e.g., glass, paraffin, mica, oil, paper, tantalum, or air). The Leyden jarLeyden jar
, form of capacitor invented at the Univ. of Leiden in the 18th cent. It consists of a narrow-necked glass jar coated over part of its inner and outer surfaces with conductive metal foil; a conducting rod or wire passes through an insulating stopper in the neck of the
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 is a simple capacitor. If an electrical potential (voltage) is applied to the plates of a capacitor (e.g., by connecting one plate to the positive and the other to the negative terminal of a storage battery), the plates will become charged, one positively and one negatively. If the externally applied voltage is then removed, the plates of the capacitor remain charged, and the presence of the electric charge induces an electrical potential between the plates. This phenomenon is called electrostatic induction. The capacity of the device for storing electric charge (i.e., its capacitancecapacitance,
in electricity, capability of a body, system, circuit, or device for storing electric charge. Capacitance is expressed as the ratio of stored charge in coulombs to the impressed potential difference in volts.
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) can be increased by increasing the area of the plates, by decreasing their separation, or by changing the dielectric. The dielectric constant of a particular dielectric is the measure of the dielectric's unit capacitance. It describes the ratio of the capacitance of a dielectric-filled capacitor to a capacitor of the same size with a vacuum between the plates. Capacitors are used in many electrical and electronic devices. The main capacitor classifications are non-polarized (used for AC circuits) and polarized (used for DC circuits). Capacitors can also be classified as fixed or variable. One type of variable capacitor, formerly used in radio and television tuning circuits, consisted of two sets of semicircular plates, one fixed and the other mounted on a movable shaft. By rotating the shaft the area of overlap of the two plates increases or decreases, thus increasing or decreasing the capacitance. These devices have largely been replaced by frequency synthesizers and a special type of solid-state diodediode
, two-terminal electronic device that permits current flow predominantly in only one direction. Most diodes are semiconductor devices; diode electron tubes are now used only for a few specialized applications.
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, known as a varactor, whose capacitance changes with the reverse-biased voltage across it.

Capacitor

An electrical device capable of storing electrical energy. In general, a capacitor consists of two metal plates insulated from each other by a dielectric. The capacitance of a capacitor depends primarily upon its shape and size and upon the relative permittivity εr of the medium between the plates. In vacuum, in air, and in most gases, εr ranges from one to several hundred. See Capacitance, Permittivity

One classification of capacitors comes from the physical state of their dielectrics, which may be gas (or vacuum), liquid, solid, or a combination of these. Each of these classifications may be subdivided according to the specific dielectric used. Capacitors may be further classified by their ability to be used in alternating-current (ac) or direct-current (dc) circuits with various current levels.

Capacitors are also classified as fixed, adjustable, or variable. The capacitance of fixed capacitors remains unchanged, except for small variations caused by temperature fluctuations. The capacitance of adjustable capacitors may be set at any one of several discrete values. The capacitance of variable capacitors may be adjusted continuously and set at any value between minimum and maximum limits fixed by construction. Trimmer capacitors are relatively small variable capacitors used in parallel with larger variable or fixed capacitors to permit exact adjustment of the capacitance of the parallel combination.

Made in both fixed and variable types, air, gas, and vacuum capacitors are constructed with flat parallel metallic plates (or cylindrical concentric metallic plates) with air, gas, or vacuum as the dielectric between plates. Alternate plates are connected, with one or both sets supported by means of a solid insulating material such as glass, quartz, ceramic, or plastic. Gas capacitors are similarly built but are enclosed in a leakproof case. Vacuum capacitors are of concentric-cylindrical construction and are enclosed in highly evacuated glass envelopes.

The purpose of a high vacuum, or a gas under pressure, is to increase the voltage breakdown value for a given plate spacing. For high-voltage applications, when increasing the spacing between plates is undesirable, the breakdown voltage of air capacitors may be increased by rounding the edges of the plates. Air, gas, and vacuum capacitors are used in high-frequency circuits. Fixed and variable air capacitors incorporating special design are used as standards in electrical measurements. See Capacitance measurement, Electrical units and standards

Solid-dielectric capacitors use one of several dieletrics such as a ceramic, mica, glass, or plastic film. Alternate plates of metal, or metallic foil, are stacked with the dielectric, or the dielectric may be metal-plated on both sides.

A large capacitance-to-volume ratio and a low cost per microfarad of capacitance are chief advantages of electrolytic capacitors. These use aluminum or tantalum plates. A paste electrolyte is placed between the plates, and a dc forming voltage is applied. A current flows and by a process of electrolysis builds up a molecule-thin layer of oxide bubbles on the positive plate. This serves as the dieletric. The rest of the electrolyte and the other plate make up the negative electrode. Such a device is said to be polarized and must be connected in a circuit with the proper polarity. Polarized capacitors can be used only in circuits in which the dc component of voltage across the capacitors exceeds the crest value of the ac ripple.

Another type of electrolytic capacitor utilizes compressed tantalum powder and the baking of manganese oxide (MnO2) as an electrolyte. Nonpolarized electrolytic capacitors can be constructed for use in ac circuits. In effect, they are two polarized capacitors placed in series with their polarities reversed.

Thick-film capacitors are made by means of successive screen-printing and firing processes in the fabrication of certain types of microcircuits used in electronic computers and other electronic systems. They are formed, together with their connecting conductors and associated thick-film resistors, upon a ceramic substrate. Their characteristics and the materials are similar to those of ceramic capacitors.

Thin-film dielectrics are deposited on ceramic and integrated-circuit substrates and then metallized with aluminum to form capacitive components. These are usually single-layer capacitors. The most common dielectrics are silicon nitride and silicon dioxide.

Capacitor

 

a system of two or more electrodes (plates) sepa-rated by a dielectric whose thickness is small in comparison to the dimensions of the plates. Such a system has a mutual capacitance. Capacitors in the form of ready-made components are used in electric circuits wherever a lumped capacitance is required. Gases, liquids, or insulating solids, as well as semiconductors, may be used as the dielectrics.

In capacitors with a gaseous or liquid dielectric, the electrodes are a system of metal plates separated by a constant gap. In capacitors with a solid dielectric, the electrodes may consist of a thin metal foil or a layer of metal deposited directly on the dielectric. In some types a thin layer of dielectric is deposited on a metal foil (the first plate), and the second plate is formed by a film of a metal or semiconductor deposited on the other side of the dielectric or by an electrolyte in which the oxidized foil is immersed. Two basically new types of capacitor—diffusion capacitors and metal-oxide-semiconductor (MOS) capacitors— are being used in integrated circuits. Diffusion capacitors use the capacitance produced by the diffusion method in the p-n junction, which depends on the applied voltage. In MOS capacitors a layer of silicon dioxide grown on the surface of a silicon plate is used as the dielectric. A substrate of low specific resistivity (silicon) and a thin aluminum film are used as the plates.

If a capacitor is connected to a DC source, an electric charge Q = C · U accumulates on the capacitor plates. If Q is expressed in coulombs and U (the voltage across the capacitor plates) is expressed in volts, the result obtained for C (capacitance) will be in farads. For a capacitor with two plane-parallel, flat electrode plates, the capacitance in picofarads (pF) is

where ∈0 is the dielectric constant of a vacuum (∈0 = 8.85 X 10-3 pF/mm), ∈ is the relative dielectric constant of the dielectric (∈ ≥ 1), S is the area of a flat electrode plate in sq mm, and b is the distance between the electrode plates in mm.

The capacitance (in pF) of a cylindrical capacitor (two coaxial hollow cylinders separated by a dielectric) is

where l is the length of the cylinder in mm, D2 is the inside diameter of the outer cylinder in mm, and D1 is the outside diameter of the inner cylinder in mm. No allowance is made for distortions of the homogeneity of the electric field at the edges of the electrode plates (the edge effect); therefore, these calculations yield values for C that are somewhat low. The accuracy of calculation increases with a decrease in the ratio b/S (for a flat capacitor) or (D2D1)/l (for a cylindrical capacitor).

Capacitors are often connected in groups; for capacitors connected in parallel the total capacitance of a group is Cg = C1 + C2 + … + Cn, and for a series connection, Cg = 1/[(1/C1) + (1/C2) + … +(1/Cn)] where C1, C2, … , Cn are the capacitances of the single capacitors of which the group is composed. If an alternating voltage with a frequency of f hertz (Hz) is connected to the circuit, a reactive (capacitive) current I = U/x will flow through the capacitor (U is the voltage across the electrode plates of the capacitor and xr is the reactance of the capacitor; xr = 1/2πfC ohms, if f is given in hertz and C in farads).

The dependence of capacitor reactance on frequency is used in electric filters. The vector of the current flowing through a capacitor leads the vector of the voltage across the capacitor plates by an angle ϕ ≈ 90°, which makes possible the use of capacitors as a means for increasing the power factor of industrial installations with inductive load, for inline compensation of transmission lines, and in asynchronous capacitor motors. The reactive power of a capacitor in volt-amperes reactive (VAR) is Pr = 2πfU2C, where U is in volts, f is in hertz, and C is in farads. Among the main parameters of a capacitor (see Table 1) are the rated capacitance Cr; the rated capacitance tolerance

where Cm is the measured capacitance value; the operating (nominal) voltage Ur, at which the capacitor operates reliably over a prolonged time interval (usually more than 1,000 hr); the test voltage Ut, which the capacitor must withstand over a certain time period (2–5 sec; sometimes up to 1 min) without break-down of the dielectric; the breakdown voltage Ub (direct current), which causes a dielectric breakdown within a period of several seconds; the loss angle δ (the greater δ, the greater the fraction of energy that goes to heat the capacitor); the active power loss Pa = 2 πfU2·Cr. tan δ (W), where δ is the loss angle, U is in volts, Cr is in farads, and/is in hertz; the temperature coefficient of capacitance (TCC), which characterizes the temperature dependence of capacitance changes; and the insulation resistance Ri between the capacitor terminals when they are connected to a constant voltage.

Capacitors have inductance L. As a result, the impedance of a capacitor is often not predominantly capacitive in any frequency range. The use of capacitors is expedient only at frequencies f < f0 (f0 is the natural resonance frequency of the capacitor), since for f > f0 the impedance is predominantly inductive. The reliability of a capacitor is determined by the probability of failure-free operation for the guaranteed service period; sometimes reliability is expressed in terms of the failure rate. The specific capacity Csp = Cr/Va (cu cm), where Va is the active volume, and the specific cost, which is the cost of the capacitor related to the energy stored in the capacitor or to its charge, are also used as relative quality criteria. The specific cost of a capacitor always decreases with an increase in dimensions.

Capacitors may be classified as follows, according to the type of use: (1) low-voltage, low-frequency (high specific capacity Csp); (2) low-voltage, high-frequency (low TCC and tan δ; high Csp); (3) high-voltage DC (high Ri); and (4) high-voltage, high-

Table 1. Main parameters of constant-capacitance capacitors made In the USSR
*Values shown as ranges are inclusive
†TCC not indicated for types of capacitors in which the changes in capacitance with temperature are relatively large and nonlinear
Capacitor typeRated capacitance limits (ρF)*Voltage limits (V)*Average specific capacitance (ρF/cm3)TCC x 106 [(*C-1]*†tan δ x 104 at frequency f
tan δ x 104*f(Hz)*
Air.................5 x 101-4 x 103102-1030.1+(20-100)0.1-5.0108
Vacuum.................10-103103-4.5 x 1040.1+(20-30)0.1-3.0108
Vitreous-enamel.................10-103102-103103+65 to -13015108
 (normalized) 
Glass-ceramic.................10-5 x 103102-5 x 102104±(30-300)20-30108
High-frequency ceramic.................1-105102-103103+120 to -1,30012-15108
 (normalized) 
Low-frequency ceramic.................102-108102-3 x 102105350103
Mica.................10-4 x 105102-104103±50 to ±20010-20108
Paper.................102-107102 -1.5 x 103104100103
Metallized-paper.................2.5 x 104-108102 -1.5 x 103105150103
Polystyrene film.................102-1046 x 10-1.5 x 104103-20010103-106
PETF film.................102-108102-1.6 x 104104-20020103
Varnish-film.................105-10810-102108150103
Electrolytic aluminum.................105-10104-5 x 1021082 x 10350
Tantalum.................105-1093-6 x 1022 x 10810350
Oxide-semiconductor.................104-1091.5-30.01085 x 10250

and low-frequency (high specific reactive power). Capacitors are manufactured with constant capacitance, variable capacitance and semivariable capacitance (trimmers). The parameters, design, and area of use of a capacitor are determined by the dielectric separating its plates; therefore, capacitors are classified primarily by the type of their dielectric. Capacitors with a gaseous dielectric (air-filled, gas-filled, or vacuum) have very low values of tan δ and a high capacitance stability (see Table 1). Constant-capacitance air capacitors are used in measurement technology primarily as capacitance standards. They are recommended for use at voltages not exceeding 1,000 V. In high-voltage electric circuits (more than 1,000 V), capacitors filled with gas (nitrogen, Freon, and others) and vacuum capacitors are used. Vacuum capacitors have lower losses and a low TCC and are more vibration-resistant than gas-filled capacitors. The operating voltage for constant-capacitance vacuum capacitors is 5–45 kV. The most appropriate frequency range for operation of vacuum capacitors is 1–10 MHz. In vacuum capacitors the breakdown voltage does not depend on atmospheric pressure; therefore, they are widely used in aircraft equipment. The main disadvantage of capacitors with a gaseous dielectric is their very low specific capacity.

Capacitors with a liquid dielectric have higher capacitance than capacitors of the same dimensions with a gaseous dielectric, since the dielectric constant of liquids is greater than that of gases. However, such capacitors have a high TCC and high dielectric losses; therefore they are not promising.

Capacitors with a solid inorganic dielectric may have glass, vitreous enamel, glass ceramics, ceramics (for low and high frequencies), or mica. Glass, vitreous-enamel, and glass-ceramic capacitors are multilayer packets consisting of alternating layers of dielectric and plate material (silver or other metal). Capacitor-grade glass, low-frequency or high-frequency vitreous enamel, or glass ceramics may be the dielectric. Such capacitors have relatively low losses and low TCC, are moisture-resistant and temperature-resistant, and have a high insulation resistance. The life of such capacitors is not less than 5,000 hr at rated voltage and maximum operating temperature. A ceramic capacitor consists of a polycrystalline ceramic dielectric on which the plate material (silver, platinum, or palladium) is deposited by firing. The terminals are soldered to the plates, and the entire assembly is covered by a moisture-proof layer. Ceramic capacitors are classified as low-voltage, high-frequency (low losses, high resonance frequency, small size, and low weight); low-voltage, low-frequency (high specific capacity and relatively large losses); and high-voltage (4—30 kV), using special ceramics with a high breakdown voltage.

In the 1960's the development of semiconductor technology, which mainly uses operating voltages of 30 V or less, led to widespread use of ceramic capacitors based on thin ceramic films (about 0.2 mm). The use of ferroceramics as dielectrics made possible the achievement of a specific capacity on the order of 0.1 μF/cm3. This type of capacitor is recommended for use in low-voltage, low-frequency circuits. In mica capacitors, mica split into plates as thin as 0.01 mm serves as the dielectric. Mica capacitors have low losses, high breakdown voltage, and high resistance of insulation. The electrodes of mica capacitors are made of foil or are deposited on the mica by evaporation of the metal in a vacuum or by firing. Mica capacitors are widely used in radio engineering (electric filters and blocking circuits). The disadvantage of mica capacitors is their low time and temperature stability of capacitance, particularly for capacitors with electrodes made of foil.

Capacitors with a solid organic dielectric are manufactured by winding long, thin ribbons of dielectric and foil (capacitor plate); plates in the form of a layer of metal (zinc or aluminum) 0.03–0.05 micron thick deposited on the dielectric are sometimes used. In paper capacitors special capacitor-grade paper is the dielectric; paper capacitors have relatively high losses and a higher specific cost. Effective use of paper capacitors is possible at frequencies up to 1 MHz. Paper capacitors are widely used in low-frequency, high-voltage circuits with high current intensity—for example, to increase the power factor (cos ϕ).

In metallized-paper capacitors a high specific capacity (as compared with paper capacitors) is achieved by the use of metallized electrode plates, but the insulation resistance is decreased. Metallized-paper capacitors have the property of “self-healing” after localized breakdowns. The use of paper or metallized-paper capacitors in circuits with very low voltage (as compared to the rated voltage) is not recommended.

In film capacitors a synthetic film (polystyrene or fluoroplastic) is the dielectric. Film capacitors have high insulation resistance, high TCC, low losses, and relatively low specific cost. In combined (paper-film) capacitors the combined use of paper and film increases the insulation resistance and breakdown voltage, thus improving the reliability. Lacquer-film capacitors with thin metallized films have the highest specific capacity. The specific capacity of such capacitors approaches that of electrolytic capacitors, but film capacitors have better electrical characteristics and permit operation with sign-variable voltage.

In electrolytic (oxide) capacitors the dielectric is an oxide film deposited electrolytically on the surface of an aluminum, tantalum, niobium, or titanium plate that serves as one of the capacitor plates. A liquid, semiliquid, or paste electrolyte or semiconductor serves as the second plate. Electrolytic capacitors have high specific capacity, losses, and leakage current and a low capacitance stability. Oxide-semiconductor electrolytic capacitors have the best electrical characteristics; however, their specific cost is still high. The operation of electrolytic capacitors is possible only for a certain polarity of the voltage across the electrode plates, which in fact limits the permissible magnitude of the alternating component of the operating voltage. Therefore, electrolytic capacitors are usually used only in DC circuits or in circuits with currents pulsating at a low frequency (up to 20 kHz) as blocking capacitors, in decoupling circuits, and in electrical filters.

Capacitors with variable or semivariable capacitance are manufactured with both mechanical and electrical adjustment of capacitance. In capacitors with mechanical adjustment, a change in capacitance is most frequently achieved by changing the area of the capacitor plates or (less frequently) by changing the gap between the plates. The most common types of variable capacitors have an air dielectric; they consist of two groups of parallel plates, one of which (the rotor) may be moved in such a way that its plates move into the gaps between the plates of the other group (the stator). The capacitance is varied by changing the relative angular position of the rotor and the stator. Variable capacitors with a solid dielectric (ceramics, mica, glass, or film) are used primarily for minor changes in capacitance (as semivariable or trimming capacitors).

Two types of solid dielectric are used for capacitors with electrical capacitance control: ferroelectrics (variconds) and semiconductors with a barrier layer (varicaps, semicaps, and so on). Variconds increase their capacitance upon an increase in the voltage across the capacitor plates. In varicaps the dependence of the width of the p-n junction on the applied voltage is used to change the capacitance: it decreases as the voltage increases, because of an increase in the width of the p-n junction. Varicaps have a greater capacitance stability and lower high-frequency losses than variconds.

The system established in the USSR for abbreviating model designations of capacitors with constant capacitance consists of four indexes. The first index (a letter) is K (for capacitor; from Russian kondensator), the second (a numeral) describes the capacitor group in terms of its dielectric, the third (a letter) denotes the purpose of the capacitor (P—for operation in both AC and DC circuits, Ch—for operation in AC circuits, U—for operation in DC, AC, and pulsed circuits, and I—for pulsed operation; capacitors without this index letter are intended for operation in DC circuits and for pulsed operation), and the fourth is a design serial number. An example of type designation is K15I = 1, which denotes a constant-capacitance ceramic capacitor for pulsed operation.

For mechanically adjustable, variable-capacitance capacitors the following designations have been established: the first two indexes (letters) are KT for trimming (semivariable) capacitors and KP for variable capacitors; the third index (a numeral) designates the type of dielectric used. The designation KN (nonlinear capacitor) has been established for capacitors with electrically adjustable capacitance; the third index denotes a primary capacitor parameter (amplification factor), and the fourth denotes the purpose of the capacitor.

REFERENCE

Renne, V. T. Elektricheskie kondensatory, 3rd ed. Leningrad, 1969.

A. V. KOCHEROV

capacitor

[kə′pas·əd·ər]
(electricity)
A device which consists essentially of two conductors (such as parallel metal plates) insulated from each other by a dielectric and which introduces capacitance into a circuit, stores electrical energy, blocks the flow of direct current, and permits the flow of alternating current to a degree dependent on the capacitor's capacitance and the current frequency. Symbolized C. Also known as condenser; electric condenser.

Capacitor

An electrical device capable of storing electrical energy. In general, a capacitor consists of two metal plates insulated from each other by a dielectric. The capacitance of a capacitor depends primarily upon its shape and size and upon the relative permittivity εr of the medium between the plates. In vacuum, in air, and in most gases, εr ranges from one to several hundred.

One classification of capacitors comes from the physical state of their dielectrics, which may be gas (or vacuum), liquid, solid, or a combination of these. Each of these classifications may be subdivided according to the specific dielectric used. Capacitors may be further classified by their ability to be used in alternating-current (ac) or direct-current (dc) circuits with various current levels.

Capacitors are also classified as fixed, adjustable, or variable. The capacitance of fixed capacitors remains unchanged, except for small variations caused by temperature fluctuations. The capacitance of adjustable capacitors may be set at any one of several discrete values. The capacitance of variable capacitors may be adjusted continuously and set at any value between minimum and maximum limits fixed by construction. Trimmer capacitors are relatively small variable capacitors used in parallel with larger variable or fixed capacitors to permit exact adjustment of the capacitance of the parallel combination.

Made in both fixed and variable types, air, gas, and vacuum capacitors are constructed with flat parallel metallic plates (or cylindrical concentric metallic plates) with air, gas, or vacuum as the dielectric between plates. Alternate plates are connected, with one or both sets supported by means of a solid insulating material such as glass, quartz, ceramic, or plastic. Gas capacitors are similarly built but are enclosed in a leakproof case. Vacuum capacitors are of concentric-cylindrical construction and are enclosed in highly evacuated glass envelopes.

The purpose of a high vacuum, or a gas under pressure, is to increase the voltage breakdown value for a given plate spacing. For high-voltage applications, when increasing the spacing between plates is undesirable, the breakdown voltage of air capacitors may be increased by rounding the edges of the plates. Air, gas, and vacuum capacitors are used in high-frequency circuits. Fixed and variable air capacitors incorporating special design are used as standards in electrical measurements.

Solid-dielectric capacitors use one of several dieletrics such as a ceramic, mica, glass, or plastic film. Alternate plates of metal, or metallic foil, are stacked with the dielectric, or the dielectric may be metal-plated on both sides.

A large capacitance-to-volume ratio and a low cost per microfarad of capacitance are chief advantages of electrolytic capacitors. These use aluminum or tantalum plates. A paste electrolyte is placed between the plates, and a dc forming voltage is applied. A current flows and by a process of electrolysis builds up a molecule-thin layer of oxide bubbles on the positive plate. This serves as the dieletric. The rest of the electrolyte and the other plate make up the negative electrode. Such a device is said to be polarized and must be connected in a circuit with the proper polarity. Polarized capacitors can be used only in circuits in which the dc component of voltage across the capacitors exceeds the crest value of the ac ripple.

Another type of electrolytic capacitor utilizes compressed tantalum powder and the baking of manganese oxide (MnO2) as an electrolyte. Nonpolarized electrolytic capacitors can be constructed for use in ac circuits. In effect, they are two polarized capacitors placed in series with their polarities reversed.

Thick-film capacitors are made by means of successive screen-printing and firing processes in the fabrication of certain types of microcircuits used in electronic computers and other electronic systems. They are formed, together with their connecting conductors and associated thick-film resistors, upon a ceramic substrate. Their characteristics and the materials are similar to those of ceramic capacitors. See Printed circuit

Thin-film dielectrics are deposited on ceramic and integrated-circuit substrates and then metallized with aluminum to form capacitive components. These are usually single-layer capacitors. The most common dielectrics are silicon nitride and silicon dioxide. See Integrated circuits

capacitor

An electric component which consists of conducting plates insulated from each other by a layer of dielectric material; introduces capacitance into a circuit.

capacitor

a device for accumulating electric charge, usually consisting of two conducting surfaces separated by a dielectric

capacitor

(electronics)
An electronic device that can store electrical charge. The charge stored Q in Coulombs is related to the capacitance C in Farads and the voltage V across the capacitor in Volts by Q = CV.

The basis of a dynamic RAM cell is a capacitor. They are also used for power-supply smoothing (or "decoupling"). This is especially important in digital circuits where a digital device switching between states causes a sudden demand for current. Without sufficient local power supply decoupling, this current "spike" cannot be supplied directly from the power supply due to the inductance of the connectors and so will cause a sharp drop in the power supply voltage near the switching device. This can cause other devices to malfunction resulting in hard to trace glitches.

capacitor

An electronic component that stores an electric charge and releases it when required. It comes in a huge variety of sizes and types for use in regulating power as well as for conditioning, smoothing and isolating signals. Capacitors are made from many different materials, and virtually every electrical and electronic system uses them.

Somewhat Like a Battery
Capacitors act like tiny storage batteries that charge and discharge rapidly. Made of two plates separated by a thin insulator or sometimes air, when one plate is charged negative and the other positive, a charge builds up and remains after the current is removed. When power is required, the circuit is switched to conduct current between the plates, and the charge is released. See ultracapacitor.

Many Applications
Big capacitors are used in computer power supplies. Tiny discrete ceramic and tantalum capacitors are built on the outside of the chip package or surround the chip on the motherboard. In signal processing, a capacitor and resistor smooth the spikes and sharp edges from a signal. In DRAM chips, capacitors are microscopic cells that hold the 0s and 1s (bits). Logic circuits, which are mostly transistors and resistors, may also contain capacitors. See tantalum capacitor and ferroelectric capacitor.


Capacitors
Looking like "silver cans," and acting like miniature storage batteries, capacitors are found on countless circuit boards such as this high-end display adapter. Wired between the power and ground planes, they quickly charge up when the computer is turned on. When more transistors switch simultaneously because the application demands extra processing, they are made to release their charge. (Image courtesy of NVIDIA Corporation.)






Silver Batteries
Looking like "silver cans," and acting like miniature storage batteries, capacitors are found on countless circuit boards such as this high-end display adapter. Wired between the power and ground planes, they quickly charge up when the computer is turned on. When more transistors switch simultaneously because the application demands extra processing, they are made to release their charge. (Image courtesy of NVIDIA Corporation.)
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