Electrolytic capacitor
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2008-07-11T05:00:48Z
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Spell recieve => receive
[[Image:Capacitors electrolytic.jpg|right|thumb|Axial (top) and radial (bottom) electrolytic capacitors]]
An '''electrolytic capacitor''' is a type of [[capacitor]] typically with a larger capacitance per unit volume than other types, making them valuable in relatively high-current and low-frequency electrical [[electrical network|circuits]]. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations, in [[rectifier]] output, and especially in the absence of rechargeable [[Battery (electricity)|batteries]] that can provide similar low-frequency current capacity. They are also widely used as coupling capacitors in circuits where [[alternating current|AC]] should be conducted but [[direct current|DC]] should not; the large value of the capacitance allows them to pass very low frequencies.
== History ==
There is no clear inventor of the electrolytic capacitor. It is one of the many technologies that spent many years as a laboratory curiosity, the classic "solution looking for a problem".
The principle of the electrolytic capacitor was discovered in 1886 by [[Charles Pollak]], as part of his research into [[anodizing]] of aluminum and other metals. Pollack discovered that due to the thinness of the aluminum oxide layer produced, there was a very high capacitance between the aluminium and the electrolyte solution. A major problem was that most electrolytes tend to dissolve the oxide layer again when the power is removed, but he eventually found that sodium perborate ([[borax]]) would allow the layer to be formed and not attack it afterwards. He was granted a patent for the borax-solution aluminium electrolytic capacitor in 1897.
The first application of the technology was in making [[motor start]] capacitors for single-phase [[alternating current]] motors. Although most electrolytic capacitors are polarized, that is, they can only be operated with DC, by separately anodizing aluminum plates and then interleaving them in a borax bath, it is possible to make a capacitor that can be used as a capacitive ballast for AC systems.
19th and early 20th century electrolytic capacitors bore little resemblance to modern types, being constructed more along the lines of a car battery. The [[borax]] electrolyte solution had to be periodically topped up with [[distilled water]], again reminiscent of a [[lead acid battery]]
The first major application of DC versions of this type of capacitor was in large telephone exchanges, to "quieten" relay hash on the 48 volt DC power supplies.
The development of AC-operated domestic radio receivers in the late 1920s required the production of fairly large capacitance (for the time) high voltage capacitors, typically at least 4 micofarads and rated at around 500 volts DC. Waxed paper and oiled silk capacitors were available but devices with that order of capacitance and voltage rating were bulky and prohibitively expensive. The first attempt at a modern electrolytic capacitor was patented by Hungarian-Canadian Engineer [[Julius Lilienfeld]] in 1926. Lilienfeld's design was constructed rather along the lines of a [[silver mica capacitor]], but with electrolyte-soaked paper sheets in place of the mica. It proved impractical to adequately seal the devices, and in the hot conditions inside typical AC operated radio receivers they quickly dried out and failed.
Retired US Navy engineer Ralph D Mershon is credited with developing the first commercially available "radio" electrolytic capacitor that was used in any quantity, (although other researchers produced broadly similar devices). The "Mershon Condenser" as it was known, was constructed similarly to a conventional [[paper capacitor]], with two long strips of aluminum foil interwound with strips of insulating paper, but with the paper saturated with electrolyte solution instead of wax. Rather than trying to hermetically seal the devices, Mershon's solution was to simply fit the capacitor into an oversize aluminum or copper can, half-filled with extra electrolyte. (These are referred to by [[vintage radio]] enthusiasts as "wet electrolytics", and ones with liquid still inside are prized collectors items).
Although "Mershons" were an immediate success, (and the name "Mershon Condenser" was for a short time synonomous with quality radio receivers in the late 1920s), due to a number of manufacturing difficulties their service life turned out to be quite short and Mershon's company went bankrupt in the early 1930s.
It was not until [[WWII]] when sufficient resources were finally applied to finding the causes of electrolytic capacitor unreliability, that they became the reliable components they are today.
== Construction ==
Aluminum electrolytic capacitors are constructed from two conducting [[aluminum]] foils, one of which is coated with an insulating [[oxide]] layer, and a paper spacer soaked in [[electrolyte]]. The foil insulated by the oxide layer is the [[anode]] while the [[liquid]] electrolyte and the second foil act as [[cathode]]. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminium casing. The two most popular geometries are axial leads coming from the center of each circular face of the cylinder, or two radial leads or lugs on one of the circular faces. Both of these are shown in the picture.
== Polarity ==
In aluminum electrolytic capacitors, the layer of insulating [[aluminum oxide]] on the surface of the aluminum plate acts as the dielectric, and it is the thinness of this layer that allows for a relatively high capacitance in a small [[volume]]. The aluminum oxide layer can withstand an electric field strength of the order of 10<sup>9</sup> volts per meter. The combination of high capacitance and high voltage result in high energy density.
Unlike most capacitors, electrolytic capacitors have a voltage polarity requirement. The correct polarity is indicated on the packaging by a stripe with [[minus sign]]s and possibly arrowheads, denoting the adjacent terminal that should have lower electrical potential (i.e. negative terminal). This is necessary because a reverse-bias voltage above 1 to 1.5 V<ref>http://electrochem.cwru.edu/ed/encycl/misc/c04-appguide.pdf</ref><ref>http://yarchive.net/electr/electrolytic_caps.html</ref><ref>http://www.rubycon.co.jp/en/products/alumi/faq.html</ref> will destroy the center layer of dielectric material via electrochemical reduction (see [[redox]] reactions). Without the dielectric material the capacitor will [[short circuit]], and if the short circuit current is excessive, then the electrolyte will heat up and either leak or cause the capacitor to explode.
Special capacitors designed for AC operation are available, usually referred to as "non-polar" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminium foil strips prior to assembly. On the alternate halves of the AC cycles, one or the other of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one. Essentialy, a 10 microfarad AC capacitor behaves like two 20 microfarad DC capacitors in inverse series.
Modern capacitors have a [[safety valve]], typically either a scored section of the can, or a specially designed end seal to vent the hot gas/liquid, but ruptures can still be dramatic. Electrolytics can withstand a reverse bias for a short period of time, but they will conduct significant current and not act as a very good capacitor. Most will survive with no reverse DC bias or with only AC voltage, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time. A constant forward bias is preferable, and will increase the life of the capacitor.
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These are the different schematic symbols for electrolytic capacitors. The minus or N marked side of the physical capacitor is equivalent to the node opposite to the plus sign on its symbolic equivalent. '''Tip:''' Take notice of the shape of the symbols and the placement of the positive and negative nodes, because most schematics do not print the "+", but rely on the symbol itself instead.
[[Image:cap-elko-smd-polarity.jpg|thumb|note: caps in metal can have the color mark at the minus side !]]
Polarity of caps with wires:
* axial: the minus wire is connected to the case, the plus wire is isolated.
* radial = single ended: a vertical color stripe indicates the minus side.
For the polarity of [[Surface-mount technology|SMD]] caps see pic:
==Electrolyte==
The electrolyte is usually [[boric acid]] or [[sodium borate]] in aqueous solution together with various sugars or [[ethylene glycol]] which are added to retard evaporation. Getting a suitable balance between chemical stability and low internal electrical resistance is very tricky and in fact, the exact composition of high-performance electrolyte is a closely guarded trade secret. It took many years of painstaking research before reliable devices were developed.
Care should be taken to avoid ingestion of or eye contact with the electrolyte, and any areas of the body where skin contact has occurred should be washed in good time. It is important to follow safe working practice and to use appropriate protective equipment, notably gloves and safety glasses, when working with the electrolyte. Some very old tantalum electrolytics, often called "Wet-slug", contain the more hazardous sulfuric acid, however most of these are no longer in service due to [[corrosion]].
==Electrical behavior of electrolytics==
A common modeling circuit for an electrolytic capacitor has the following [[schematic]]:
[[Image:Electrolytic capacitor model.svg]]
where R<sub>leakage</sub> is the leakage resistance, R<sub>ESR</sub> is the [[equivalent series resistance]], L<sub>ESL</sub> the [[equivalent series inductance]] (L being the conventional symbol for inductance).
R<sub>ESR</sub> must be as small as possible since it determines the [[loss power]] when the capacitor is used to smooth voltage. Loss power scales [[quadratic growth|quadratically]] with the ripple [[current (electricity)|current]] flowing through and linearly with R<sub>ESR</sub>.
Low ESR capacitors are imperative for high efficiencies in power supplies.
It should be pointed out that this is only a simple model and does not include [[Capacitor_%28component%29#Non-ideal_properties_of_practical_capacitors | dielectric absorption (soakage) and other non-ideal effects]] associated with real electrolytic capacitors.
Since the electrolytes evaporate, [[design life]] is most often rated in [[hour]]s at a set [[temperature]]. For example, typically as 2000 hours at 105 degrees Celsius (which is the highest working temperature). Design life doubles for each 10 degrees lower[http://www.niccomp.com/Catalog/AlumApplInfoCautions1105.pdf], reaching 15 years at 45 degrees. Most Electrolytic capacitors are rated for 85 degrees Celsius maximum.
==Capacitance==
The [[capacitance]] value of any capacitor is a measure of the amount of electric charge stored per unit of potential difference between the plates. The basic unit of capacitance is a [[farad]], however this unit has been too large for general use until the invention of the [[Double-layer capacitor]], so [[microfarad]], nanofarad and [[picofarad]] are more commonly used. These are usually abbreviated to μF or uF, nF and pF.
Many conditions determine a capacitor's value, such as the thickness of the [[dielectric]] and the [[plate area]]. In the manufacturing process, electrolytic capacitors are made to conform to a set of [[preferred number]]s. By multiplying these base numbers by a [[Order of magnitude|power of ten]], any practical capacitor value can be achieved, which is suitable for most applications.
A standardized set of capacitor ''base numbers'' was devised so that the value of any modern electrolytic capacitor could be derived from multiplying one of the modern conventional base numbers '''1.0''', '''1.5''', '''2.2''', '''3.3''', '''4.7''' or '''6.8''' by a power of ten. Therefore, it is common to find capacitors with values of 10, 15, 22, 33, 47, 68, 100, 220, and so on. Using this method, values ranging from 0.1 to 4700 are common in most applications. Values are generally in microfarads (µF).
Most electrolytic capacitors have a ''tolerance'' range of 20 %, meaning that the manufacturer is stating that the actual value of the capacitor lies within 20 % of its labeled value. Selection of the preferred series ensures that any capacitor can be sold as a standard value, within the tolerance.
== Variants ==
[[Image:Electrolytic capacitors.jpg|thumb|Electrolytic capacitors of several sizes]]
Unlike capacitors that use a bulk dielectric made from an intrinsically insulating material, the dielectric in electrolytic capacitors depends on the formation and maintenance of a microscopic metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume, but maintaining the integrity of the dielectric usually requires the steady application of the correct polarity of [[direct current]] else the oxide layer will break down and rupture, causing the capacitor to fail. In addition, electrolytic capacitors generally use an internal wet chemistry and they will eventually fail if the water within the capacitor evaporates.
Electrolytic capacitance values are not as tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytics, it is quite common to see an electrolytic capacitor specified as having a "guaranteed minimum value" and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification is acceptable.
As with bulk dielectric capacitors, electrolytic capacitors come in several varieties:
*'''[[Aluminum]] electrolytic capacitor''': compact but lossy, these are available in the range of <1 µF to 1 F with working voltages up to several hundred volts DC. The dielectric is a thin layer of aluminum oxide. They contain corrosive liquid and can burst if the device is connected backwards. The oxide insulating layer will tend to deteriorate in the absence of a sufficient rejuvenating voltage, and eventually the capacitor will fail if voltage is not applied. Bipolar electrolytics (also called Non-Polarised or NP capacitors) contain two capacitors connected in series opposition and are used when the DC bias voltage must occasionally reverse. Bad frequency and temperature characteristics make them unsuited for high-frequency applications. Typical [[equivalent series inductance|ESL]] values are a few [[henry (unit)|nH]].<ref>[http://www.murata.com/emc/knowhow/pdfs/te04ea-1/12to16e.pdf The effect of non-ideal capacitors]. Murata technical document.</ref>
*'''[[Tantalum]]''': compact, low-voltage devices up to several hundred µF, these have a lower energy density and are more accurate than aluminum electrolytics. Tantalum capacitors are also polarized because of their dissimilar electrodes. The cathode electrode is formed of [[sintered]] tantalum grains, with the dielectric electrochemically formed as a thin layer of [[oxide]]. The thin layer of oxide and high surface area of the porous sintered material gives this type a very high capacitance per unit volume. The cathode electrode is formed either of a liquid electrolyte connecting the outer can or of a chemically deposited semi-conductive layer of [[manganese dioxide]], which is then connected to an external wire lead. A development of this type replaces the manganese dioxide with a [[conducting polymer|conductive plastic polymer]] ([[polypyrrole]]) that reduces internal resistance and eliminates a self-ignition failure.<ref>[http://www.niccomp.com/faq.html-ssi NIC components Corp. FAQ]</ref>
:Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance, little DC leakage, and very low [[Electrical impedance|impedance]] at high frequencies. However, unlike aluminum electrolytics, they are intolerant of voltage spikes and are destroyed (often exploding violently) if connected in the circuit backwards or exposed to spikes above their voltage rating.
:Tantalum capacitors are more expensive than aluminum-based capacitors and generally only usable at low voltage, but because of their higher capacitance per unit volume and lower [[Electrical impedance|impedance]] at high frequencies, they are popular in miniature applications such as [[cellular telephone]]s.
== See also ==
* [[Capacitor plague]]
* [[Supercapacitor]]
== External links ==
* [http://www.powerdesigners.com/InfoWeb/design_center/Design_Tips/Electrolytics/Caps.shtm Electrolytic Capacitors]
* [http://www.elna-america.com/tech_al_principles.php How Electrolytic Capacitors Work]
== References ==
{{reflist}}
* {{cite journal | author=Glenn Zorpette | title=Super Charged: A Tiny South Korean Company is Out to Make Capacitors Powerful enough to Propel the Next Generation of Hybrid-Electric Cars | journal=[[IEEE Spectrum]] | year=January 2005 | volume=42 No. 1 | url=http://spectrum.ieee.org/jan05/inthisissue }}
* [http://electrochem.cwru.edu/ed/encycl/art-c04-electr-cap.htm Electrochemistry Encyclopedia: Electrochemical Capacitors; Their Nature, Function, and Applications]
[[Category:Capacitors]]
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