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An electrolytic capacitor is a polarized capacitor whose anode or positive plate is made of a metal that forms an insulating oxide layer through anodization. This oxide layer acts as the dielectric of the capacitor. A solid, liquid, or gel electrolyte covers the surface of this oxide layer, serving as the cathode or negative plate of the capacitor. Because of their very thin dielectric oxide layer and enlarged anode surface, electrolytic capacitors have a much higher capacitance-voltage (CV) product per unit volume than ceramic capacitors or , and so can have large capacitance values. There are three families of electrolytic capacitor: aluminium electrolytic capacitors, tantalum electrolytic capacitors, and niobium electrolytic capacitors.
The large capacitance of electrolytic capacitors makes them particularly suitable for passing or bypassing low-frequency signals, and for storing large amounts of energy. They are widely used for decoupling or noise filtering in Power supply and DC link circuits for variable-frequency drives, for coupling signals between amplifier stages, and storing energy as in a flashlamp.
Electrolytic capacitors are polarized components because of their asymmetrical construction and must be operated with a higher potential (i.e., more positive) on the anode than on the cathode at all times. For this reason the polarity is marked on the device housing. Applying a reverse polarity voltage, or a voltage exceeding the maximum rated working voltage of as little as 1 or 1.5 volts, can damage the dielectric causing catastrophic failure of the capacitor itself. Failure of electrolytic capacitors can result in an explosion or fire, potentially causing damage to other components as well as injuries. Bipolar electrolytic capacitors which may be operated with either polarity are also made, using special constructions with two anodes connected in series. A bipolar electrolytic capacitor can be made by connecting two normal electrolytic capacitors in series, anode to anode or cathode to cathode, along with .
To increase their capacitance per unit volume, all anode materials are either etched or sintered and have a rough surface structure with a much higher surface area compared to a smooth surface of the same area or the same volume. By applying a positive voltage to the above-mentioned anode material in an electrolytic bath an oxide barrier layer with a thickness corresponding to the applied voltage will be formed (formation). This oxide layer acts as the dielectric in an electrolytic capacitor. The properties of these oxide layers are given in the following table:
| + Characteristics of the different oxide layers in aluminium, tantalum and niobium electrolytic capacitorsJ.L. Stevens, A.C. Geiculescu, T.F. Strange, Dielectric Aluminum Oxides: Nano-Structural Features and Composites PDF T. Kárník, AVX, NIOBIUM OXIDE FOR CAPACITOR MANUFACTURING, METAL 2008, 13. –15. 5. 2008, PDF
!Anode- material ! Dielectric ! Oxide structure ! Relative permittivity ! Breakdown voltage (V/μm) ! Electric layer thickness (nm/V) |
| 1.4 |
| 1.25...1.0 |
| 1.6 |
| 2.5 |
After forming a dielectric oxide on the rough anode structure, a counter electrode has to match the rough insulating oxide surface. This is accomplished by the electrolyte, which acts as the cathode electrode of an electrolytic capacitor. There are many different electrolytes in use. Generally they are distinguished into two species, “non-solid” and “solid” electrolytes. As a liquid medium which has ion conductivity caused by moving ions, non-solid electrolytes can easily fit the rough structures. Solid electrolytes which have electron conductivity can fit the rough structures with the help of special chemical processes like pyrolysis for manganese dioxide or polymerization for conducting .
Comparing the permittivities of the different oxide materials it is seen that tantalum pentoxide has a permittivity approximately three times higher than aluminium oxide. Tantalum electrolytic capacitors of a given capacitance-voltage (CV) product value theoretically are therefore smaller than aluminium electrolytic capacitors. In practice different safety margins to reach reliable components makes a comparison difficult.
The anodically generated insulating oxide layer is destroyed if the polarity of the applied voltage changes.
The dielectric thickness of electrolytic capacitors is very small, in the range of Meter per volt. On the other hand, the voltage strengths of these oxide layers are quite high. With this very thin dielectric oxide layer combined with a sufficiently high dielectric strength the electrolytic capacitors can achieve a high volumetric capacitance. This is one reason for the high capacitance values of electrolytic capacitors compared to conventional capacitors.
All etched or sintered anodes have a much higher surface area compared to a smooth surface of the same area or the same volume. That increases the capacitance value, depending on the rated voltage, by a factor of up to 200 for non-solid aluminium electrolytic capacitors as well as for solid tantalum electrolytic capacitors.A. Albertsen, Jianghai Europe, Keep your distance – Voltage Proof of Electrolytic Capacitors, PDF I.Horacek, T.Zednicek, S.Zednicek, T.Karnik, J.Petrzilek, P.Jacisko, P.Gregorova, AVX, High CV Tantalum Capacitors - Challenges and Limitations [6] The large surface compared to a smooth one is the second reason for the relatively high capacitance values of electrolytic capacitors compared with other capacitor families.
Because the forming voltage defines the oxide layer thickness, the desired voltage rating can be produced very simply. Electrolytic capacitors have high volumetric efficiency, the so-called "CV product", defined as the product of capacitance and voltage divided by volume.
| + Overview of the key features of the different types of electrolytic capacitor |
| 105/125/150 |
| 85/105 |
| 85/105 |
| 105 |
| 105/125 |
| 125/200 |
| 125/150 |
| 105 |
| 105 |
| 105 |
The non-solid or so-called "wet" aluminium electrolytic capacitors were and are the cheapest among all other conventional capacitors. They not only provide the cheapest solutions for high capacitance or voltage values for decoupling and buffering purposes but are also insensitive to low ohmic charging and discharging as well as to low-energy transients. Non-solid electrolytic capacitors can be found in nearly all areas of electronic devices, with the exception of military applications.
Tantalum electrolytic capacitors with solid electrolyte as surface-mountable chip capacitors are mainly used in electronic devices in which little space is available or a low profile is required. They operate reliably over a wide temperature range without large parameter deviations. In military and space applications only tantalum electrolytic capacitors have the necessary approvals.
Niobium electrolytic capacitors are in direct competition with industrial tantalum electrolytic capacitors because niobium is more readily available. Their properties are comparable.
The electrical properties of aluminium, tantalum and niobium electrolytic capacitors have been greatly improved by the polymer electrolyte.
| + Comparison of the most important characteristics of different types of electrolytic capacitors
! Electrolytic capacitor family ! Type 1) ! Dimension DxL, WxHxL (mm) ! Max. ESR 100 kHz, 20 °C (mΩ) ! Max. ripple current 85/105 °C (mA) ! Max. leakage current after 2 min. 2) (μA) |
| 10 (0.01CV) |
| 10 (0.01CV) |
| 10 (0.01CV) |
| 10 (0.01CV) |
| 10 (0.01CV) |
| 10 (0.01CV) |
| 10 (0.01CV) |
| 10 (0.01CV) |
| 100 (0.1CV) |
| 100 (0.1CV) |
| 20 (0.02CV) |
| 20 (0.02CV) |
| 100 (0.1CV) |
| 40 (0.04CV) |
| 200 (0.2CV) |
| 10 (0.01CV) |
1) Manufacturer, series name, capacitance/voltage
2) calculated for a capacitor 100 μF/10 V,
3) from a 1976 data sheet
Charles Pollak (born Karol Pollak), a producer of accumulators, found out that the oxide layer on an aluminium anode remained stable in a neutral or alkaline electrolyte, even when the power was switched off. In 1896, he filed a patent for an "Electric liquid capacitor with aluminium electrodes" (de: Elektrischer Flüssigkeitskondensator mit Aluminiumelektroden) based on his idea of using the oxide layer in a polarized capacitor in combination with a neutral or slightly alkaline electrolyte.Pollack, Charles. "Elektrischer Flüssigkeitskondesator mit Aluminiumelektroden" Electrical with aluminium electrodes]. D.R.P. 92564, filed: 14. January 1896, granted: 19. May 1897.
The first more common application of wet aluminium electrolytic capacitors was in large telephone exchanges, to reduce relay hash (noise) on the 48 volt DC power supply. The development of AC-operated domestic radio receivers in the late 1920s created a demand for large-capacitance (for the time) and high-voltage capacitors for the valve amplifier technique, typically at least 4 microfarads and rated at around 500 volts DC. Waxed paper and oiled silk were available, but devices with that order of capacitance and voltage rating were bulky and prohibitively expensive.
With Ruben's invention, together with the invention of wound foils separated with a paper spacer 1927 by A. Eckel of Hydra-Werke (Germany), Elektrolytischer Kondensator mit aufgerollten Metallbändern als Belegungen, Alfred Eckel Hydra-Werke, Berlin-Charlottenburg, DRP 498 794, filed May 12, 1927, granted May 8, 1930 the actual development of electrolytic capacitors began.
William Dubilier, whose first patent for electrolytic capacitors was filed in 1928,William Dubilier, Electric Condenser, US Patent 468787 industrialized the new ideas for electrolytic capacitors and started the first large commercial production in 1931 in the Cornell-Dubilier (CD) factory in Plainfield, New Jersey. At the same time in Berlin, Germany, the "Hydra-Werke", an AEG company, started the production of electrolytic capacitors in large quantities. Another manufacturer, Ralph D. Mershon, had success in servicing the radio-market demand for electrolytic capacitors.Henry B.O. Davis (1983) Electrical and Electronic Technologies: A Chronology of Events and Inventors from 1900 to 1940, p 111: "The Mershon Company put electrolytic capacitors on the market. The capacitors packed a high capacitance in a very small space compared to existing paper capacitors.
In his 1896 patent Pollak already recognized that the capacitance of the capacitor increases when roughening the surface of the anode foil. Today (2014), electrochemically etched low voltage foils can achieve an up to 200-fold increase in surface area compared to a smooth surface. Advances in the etching process are the reason for the dimension reductions in aluminium electrolytic capacitors over recent decades.
For aluminium electrolytic capacitors the decades from 1970 to 1990 were marked by the development of various new professional series specifically suited to certain industrial applications, for example with very low leakage currents or with long life characteristics, or for higher temperatures up to 125 °C.Philips Data Handbook PA01, 1986, the first 125 °C series "118 AHT"J. Both, The modern era of aluminum electrolytic capacitors, Electrical Insulation Magazine, IEEE, Volume:31, Issue: 4, July–August 2015, ieeexplore.ieee.org
The relevant development of solid electrolyte tantalum capacitors began some years after William Shockley, John Bardeen and Walter Houser Brattain invented the transistor in 1947. It was invented by Bell Laboratories in the early 1950s as a miniaturized, more reliable low-voltage support capacitor to complement their newly invented transistor. The solution found by R. L. Taylor and H. E. Haring at Bell Labs in early 1950 was based on experience with ceramics. They ground tantalum to a powder, which they pressed into a cylindrical form and then sintering at a high temperature between 1500 and 2000 °C under vacuum conditions, to produce a pellet ("slug").R. L. Taylor and H. E. Haring, "A metal semi-conductor capacitor", J. Electrochem. Soc., vol. 103, p. 611, November, 1956.E. K. Reed, Jet Propulsion Laboratory, Characterization of Tantalum Polymer Capacitors, NEPP Task 1.21.5, Phase 1, FY05
These first sintered tantalum capacitors used a non-solid electrolyte, which does not fit the concept of solid electronics. In 1952 a targeted search at Bell Labs by D. A. McLean and F. S. Power for a solid electrolyte led to the invention of manganese dioxide as a solid electrolyte for a sintered tantalum capacitor.D. A. McLean, F. S. Power, Proc. Inst. Radio Engrs. 44 (1956) 872
Although fundamental inventions came from Bell Labs, the inventions for manufacturing commercially viable tantalum electrolytic capacitors came from researchers at the Sprague Electric Company. Preston Robinson, Sprague's Director of Research, is considered to be the actual inventor of tantalum capacitors in 1954.Preston Robinson, Sprague, US Patent 3066247, 25. Aug. 1954 - 27. Nov. 1962Sprague, Dr. Preston Robinson Granted 103rd Patent Since Joining Company In 1929 [11] His invention was supported by R. J. Millard, who introduced the "reform" step in 1955,A. Fraioli, Recent Advances in the Solid-State Electrolytic Capacitor, IRE Transactions on Component Parts, June 1958R. J. Millard, Sprague, US Patent 2936514, October 24, 1955 - May 17, 1960 a significant improvement in which the dielectric of the capacitor was repaired after each dip-and-convert cycle of MnO2 deposition, which dramatically reduced the leakage current of the finished capacitors.
Although solid tantalum capacitors offered capacitors with lower ESR and leakage current values than the aluminium electrolytic capacitors, a 1980 price shock for tantalum dramatically reduced the applications of tantalum electrolytic capacitors, especially in the entertainment industry.W. Serjak, H. Seyeda, Ch. Cymorek, Tantalum Availability: 2000 and Beyond, PCI, March/April 2002, [12] The industry switched back to using aluminium electrolytic capacitors.
With the beginning of digitalization, Intel launched its first microcomputer, the MCS 4, in 1971. In 1972 Hewlett Packard launched one of the first pocket calculators, the HP 35.K. Lischka, Spiegel 27.09.2007, 40 Jahre Elektro-Addierer: Der erste Taschenrechner wog 1,5 Kilo, [13] The requirements for capacitors increased in terms of lowering the equivalent series resistance (ESR) for bypass and decoupling capacitors.Larry E. Mosley, Intel Corporation, Capacitor Impedance Needs For Future Microprocessors, CARTS USA 2006, ecadigitallibrary.com
It was not until 1983 when a new step toward ESR reduction was taken by Sanyo with its "OS-CON" aluminium electrolytic capacitors. These capacitors used a solid organic conductor, the charge transfer salt TTF-TCNQ (tetracyanoquinodimethane), which provided an improvement in conductivity by a factor of 10 compared with the manganese dioxide electrolyte.
The next step in ESR reduction was the development of conducting polymers by Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa in 1975.About the Nobel Prize in Chemistry 2000, Advanced Information, October 10, 2000,[15] The conductivity of conductive polymers such as polypyrrole (PPy) Y. K. ZHANG, J. LIN，Y. CHEN, Polymer Aluminum Electrolytic Capacitors with Chemically-Polymerized Polypyrrole (PPy) as Cathode Materials Part I. Effect of Monomer Concentration and Oxidant on Electrical Properties of the Capacitors, PDF or PEDOTU. Merker, K. Wussow, W. Lövenich, H. C. Starck GmbH, New Conducting Polymer Dispersions for Solid Electrolyte Capacitors, ecadigitallibrary.com is better than that of TCNQ by a factor of 100 to 500, and close to the conductivity of metals.
In 1991 Panasonic released its "SP-Cap", series of polymer aluminium electrolytic capacitors. These aluminium electrolytic capacitors with polymer electrolytes reached very low ESR values directly comparable to ceramic multilayer capacitors (MLCCs). They were still less expensive than tantalum capacitors and with their flat design for and competed with tantalum chip capacitors as well.
Tantalum electrolytic capacitors with PPy polymer electrolyte cathode followed three years later. In 1993 NEC introduced its SMD polymer tantalum electrolytic capacitors, called "NeoCap". In 1997 Sanyo followed with the "POSCAP" polymer tantalum chips.
A new conductive polymer for tantalum polymer capacitors was presented by Kemet at the "1999 Carts" conference. John Prymak, Kemet, Replacing MnO2 with Polymers, 1999 CARTS This capacitor used the newly developed organic conductive polymer PEDT Poly(3,4-ethylenedioxythiophene), also known as PEDOT (trade name Baytron®) F. Jonas, H.C.Starck, Baytron, Basic chemical and physical properties, Präsentation 2003, www.hcstarck.de
target="_blank" rel="nofollow">[20]
From 1999 through at least 2010, a stolen recipe for such a water-based electrolyte, in which important stabilizersJ. L. Stevens, T. R. Marshall, A. C. Geiculescu m, C. R. Feger, T. F. Strange, Carts USA 2006, The Effects of Electrolyte Composition on the Deformation Characteristics of Wet Aluminum ICD Capacitors, [21] Alfonso Berduque, Zongli Dou, Rong Xu, KEMET, Electrochemical Studies for Aluminium Electrolytic Capacitor Applications: Corrosion Analysis of Aluminium in Ethylene Glycol-Based Electrolytes PDF were absent, led to the widespread problem of "bad caps" (failing electrolytic capacitors), leaking or occasionally bursting in computers, power supplies, and other electronic equipment, which became known as the "capacitor plague". In these electrolytic capacitors the water reacts quite aggressively with aluminium, accompanied by strong heat and gas development in the capacitor, resulting in premature equipment failure, and development of a cottage industry repair industry.
The basic unit of an electrolytic capacitor's capacitance is the Farad (μF). The capacitance value specified in the data sheets of the manufacturers is called the rated capacitance CR or nominal capacitance CN and is the value for which the capacitor has been designed.
The standardized measuring condition for electrolytic capacitors is an AC measuring method with 0.5 V at a frequency of 100/120 Hz at a temperature of 20 °C. For tantalum capacitors a DC bias voltage of 1.1 to 1.5 V for types with a rated voltage ≤2.5 V, or 2.1 to 2.5 V for types with a rated voltage of >2.5 V, may be applied during the measurement to avoid reverse voltage.
The capacitance value measured at the frequency of 1 kHz is about 10% less than the 100/120 Hz value. Therefore, the capacitance values of electrolytic capacitors are not directly comparable and differ from those of or ceramic capacitors, whose capacitance is measured at 1 kHz or higher.
Measured with an AC measuring method at 100/120 Hz the capacitance value is the closest value to the electrical charge stored in the e-caps. The stored charge is measured with a special discharge method and is called the Direct current capacitance. The DC capacitance is about 10% higher than the 100/120 Hz AC capacitance. The DC capacitance is of interest for discharge applications like photoflash.
The percentage of allowed deviation of the measured capacitance from the rated value is called the capacitance tolerance. Electrolytic capacitors are available in different tolerance series, whose values are specified in the E series specified in IEC 60063. For abbreviated marking in tight spaces, a letter code for each tolerance is specified in IEC 60062.
The required capacitance tolerance is determined by the particular application. Electrolytic capacitors, which are often used for filtering and bypassing, do not have the need for narrow tolerances because they are mostly not used for accurate frequency applications like in .
The voltage proof of electrolytic capacitors decreases with increasing temperature. For some applications it is important to use a higher temperature range. Lowering the voltage applied at a higher temperature maintains safety margins. For some capacitor types therefore the IEC standard specifies a "temperature derated voltage" for a higher temperature, the "category voltage UC". The category voltage is the maximum DC voltage or peak pulse voltage that may be applied continuously to a capacitor at any temperature within the category temperature range TC. The relation between both voltages and temperatures is given in the picture at right.
Applying a higher voltage than specified may destroy electrolytic capacitors.
Applying a lower voltage may have a positive influence on electrolytic capacitors. For aluminium electrolytic capacitors a lower applied voltage can in some cases extend the lifetime. For tantalum electrolytic capacitors lowering the voltage applied increases the reliability and reduces the expected failure rate.Ch. Reynolds, AVX, Technical Information, Reliability Management of Tantalum Capacitors, PDF I
For tantalum electrolytic capacitors the surge voltage can be 1.3 times the rated voltage, rounded off to the nearest volt. The surge voltage applied to tantalum capacitors may influence the capacitor's failure rate.A. Teverovsky, Perot Systems Code 562, NASA GSFCE, Effect of Surge Current Testing on Reliability of Solid Tantalum Capacitors ecadigitallibrary.com
Electrolytic capacitors with solid manganese oxide or polymer electrolyte, and aluminium as well as tantalum electrolytic capacitors cannot withstand transients or peak voltages higher than the surge voltage. Transients may destroy this type of electrolytic capacitor.
Nevertheless, electrolytic capacitors can withstand for short instants a reverse voltage for a limited number of cycles. Specifically, aluminium electrolytic capacitors with non-solid electrolyte can withstand a reverse voltage of about 1 V to 1.5 V. This reverse voltage should never be used to determine the maximum reverse voltage under which a capacitor can be used permanently.Nichicon. "General Description of Aluminum Electrolytic Capacitors" PDF section "2-3-2 Reverse Voltage".Rubycon. "Aluminum Electrolytic Capacitors FAQ"CDM Cornell Dubilier. "Aluminum Electrolytic Capacitor Application Guide" p. 4 and p. 6 and p. 9
Solid tantalum capacitors can also withstand reverse voltages for short periods. The most common guidelines for tantalum reverse voltage are:
But in no case, for aluminium as well as for tantalum and niobium electrolytic capacitors, may a reverse voltage be used for a permanent AC application.
To minimize the likelihood of a polarized electrolytic being incorrectly inserted into a circuit, polarity has to be very clearly indicated on the case, see the section on polarity marking below.
Special bipolar aluminium electrolytic capacitors designed for bipolar operation are available, and usually referred to as "non-polarized" or "bipolar" types. In these, the capacitors have two anode foils with full-thickness oxide layers connected in reverse polarity. On the alternate halves of the AC cycles, one of the oxides on the foil acts as a blocking dielectric, preventing reverse current from damaging the electrolyte of the other one. But these bipolar electrolytic capacitors are not suitable for main AC applications instead of power capacitors with metallized polymer film or paper dielectric.
The impedance Z is the vector sum of reactance and resistance; it describes the phase difference and the ratio of amplitudes between sinusoidally varying voltage and sinusoidally varying current at a given frequency. In this sense impedance is a measure of the ability of the capacitor to pass alternating currents and can be used like Ohm's law.
In other words, impedance is a frequency-dependent AC resistance and possesses both magnitude and Phasor at a particular frequency.
In data sheets of electrolytic capacitors only the impedance magnitude |Z| is specified, and simply written as "Z". Regarding the IEC/EN 60384-1 standard, the impedance values of electrolytic capacitors are measured and specified at 10 kHz or 100 kHz depending on the capacitance and voltage of the capacitor.
Besides measuring, the impedance can be calculated using the idealized components of a capacitor's series-equivalent circuit, including an ideal capacitor C, a resistor ESR, and an inductance ESL. In this case the impedance at the angular frequency ω is given by the geometric (complex) addition of ESR, by a capacitive reactance XC
and by an inductive reactance XL (Inductance)
.
Then Z is given by
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