An electrolytic capacitor ("electrolytic") is a capacitor in which one electrode is made of a metal on which a thin oxide layer forms. This layer acts as the capacitor's dielectric. An electrolyte covers the surface of the oxide layer and also serves as the second electrode.
Electrolytics have very high volumetric capacitance compared to other types. They find extensive use in modern electronic devices. They may serve as filter and reservoir elements in power supplies, to couple signals between amplifier stages, or to store energy as in a flashlamp.
- 1 Basic information
- 2 Types and features of electrolytic capacitors
- 3 History
- 4 Electrical characteristics
- 4.1 Series-equivalent circuit
- 4.2 Capacitance, standard values and tolerances
- 4.3 Rated and category voltage
- 4.4 Surge Voltage
- 4.5 Transient Voltage
- 4.6 Reverse voltage
- 4.7 Impedance
- 4.8 ESR and dissipation factor tan δ
- 4.9 Ripple current
- 4.10 Current surge, peak or pulse current
- 4.11 Leakage current
- 4.12 Dielectric absorption (soakage)
- 5 Reliability and life time
- 6 Failure modes, self-healing mechanism and application rules
- 7 Additional information
- 8 See also
- 9 References
Electrolytics store electric energy in an electric field in the dielectric oxide layer between two electrodes like other conventional capacitors. The electrolyte acts as the cathode. The cathode and the storage principle distinguish them from electrochemical supercapacitors, in which the electrolyte is the conductive connection between two electrodes and storage occurs via double-layer capacitance and pseudocapacitance.
Electrolytics are polarized and may only be operated with DC voltage. Reverse polarity, or excess ripple current can destroy the dielectric and thus the device. Any ripple voltage must not cause polarity reversal. The destruction of electrolytics can produce an explosion and/or fire.
Bipolar electrolytic capacitors, which may be operated with AC voltage, use two anodes connected in reverse polarity.
Their large capacitance makes electrolytics particularly suitable for passing or bypassing low-frequency signals and storing large amounts of energy. They are widely used in power supplies and interconnecting stages of amplifiers at audio frequencies.
Electrolytic capacitors are made using a chemical feature of some metals, earlier called “valve metals”, on which an oxide insulating layer can be formed by anodisation to serve as a dielectric. Three different metals support electrolytics. Aluminium, in the form of foil; tantalum in the form of a sintered pellet (or 'slug') made from high purity tantalum powder with tantalum pentoxide as a dielectric and Niobium used in a similar manner to tantalum.
All anode materials are either etched or sintered to roughen the surface, producing a larger surface area compared to a smooth surface. Applying a positive voltage to the anode in an electrolytic bath forms an oxide barrier layer whose thickness is proportional to the voltage. This oxide layer acts as the dielectric. Its properties are given in the following table:
|Aluminum||Aluminum oxide Al2O3||9.6||amorphous||710||1.4|
|Tantalum||Tantalum pentoxide Ta2O5||27||amorphous||625||1.6|
|Niobium pentoxide Nb2O5||41||amorphous||400||2.5|
The electrolyte/cathode has to match the rough insulating oxide surface. Many different electrolytes are in use. Generally they are either “non-solid” or “solid”. Non-solid electrolytes are liquids that conduct by moving ions. Conductive solid electrolytes are made to contact the dielectric by chemical processes such as pyrolysis for manganese dioxide or polymerization for conducting polymers.
Every electrolytic in principle is a "plate capacitor" whose capacitance is a function of the electrode area A, the permittivity ε the thickness (d) of the dielectric.
The dielectric thickness of electrolytic capacitors is in the range of nano-meters per volt. Otherwise the voltage strengths of the oxide layer is quite high. This dielectric oxide layer, combined with a sufficiently high dielectric strength, achieves high volumetric capacitance. This is one reason that electrolytics achieve higher capacitance values than conventional capacitors.
Etching/sintering increases anode surface area. That increases the capacitance value by a factor of up to 200. The larger surface area is the second reason for electrolytics relatively high capacitance values.
Because the forming voltage defines the oxide layer thickness, the voltage rating of the resultant capacitor is easily determined. The volume of a capacitor is defined by the product of capacitance and voltage, the so-called “CV-Volume”.
However, comparing the permittivities of the different oxide materials, reveals that tantalum pentoxide has approximately 3 times higher permittivity than aluminum oxide. Tantalum electrolytic capacitors of a given CV value therefore are smaller than aluminum electrolytic capacitors, while tantalum and niobium electrolytic capacitors may have similar CV-volumes.
Basic construction of non-solid aluminum electrolytic capacitors
Basic construction of solid tantalum electrolytic capacitors
Types and features of electrolytic capacitors
There are three basic types: aluminum, tantalum and niobium. Each type uses non-solid and solid manganese dioxide or solid polymer electrolytes enabling a variety of combinations of anode material and electrolyte.
The non-solid or so-called "wet" aluminum electrolytic capacitors have always been the least expensive for high capacitance or voltage values for decoupling and buffering purposes. They are also insensitive to low ohmic charging and discharging as well as to low-energy transients. The non-solid electrolytics can be found in nearly all types of electronic devices save military applications.
"Solid" tantalum electrolytic capacitors are mainly used in electronic devices as surface-mountable chip capacitors 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/space applications only tantalum electrolytic capacitors can be used.
Niobium is in direct competition with tantalum, because niobium is more readily available. Their properties are comparable.
The electrical properties of aluminum, tantalum and niobium electrolytic capacitors have been greatly improved by polymer electrolytes.
An outline of the main characteristics of the different types is shown in the table below.
|Non-solid, organic electrolyte,
e.g. GBL, DMF, DMA,
|Non-solid, e.g. borax, glycol||0.1…2,700,000||630||85/105|
|Non-solid, water based||1…18,000||100||85/105|
|Hybrid, polymer and non-solid||6.8…1,000||125||105/125|
|Non-solid, sulfuric acid||0.1…18.000||630||125/200|
|Solid, manganese dioxide||0.1…3,300||125||125/150|
Capacitors with the same dimensions and of similar capacitance and voltage are compared in the following table. In such a comparison the values for ESR and permissible ripple current are the most important parameters. The lower the ESR, the higher the allowed ripple current per volume and better circuit functionality. However, better electrical parameters imply a higher price.
100 kHz, 20 °C
|Max. Ripple current
|Max. Leakage current
after 2 Min. 2)
|„wet“ Al-e-caps 1978 3)
Ethylene glycol/borax electrolyte
|Vishay, 036 RSP,
Ethylene glycol/borax electrolyte
|„wet“ Al-e-caps, SMD
Ethylene glycol/borax electrolyte
|„wet“ Al-e-caps, SMD
Multianode, MnO2 electrolyte
Polymer + non-solid electrolyte
1) 100 µF/10 V, unless otherwise specified,
2) calculated for a capacitor 100 µF/10 V,
3) out of a datasheet from 1978
Aluminum electrolytics form the majority of capacitors used in electronics because of the diversity of sizes and low costs. Tantalum capacitors, usually in surface mount configurations, have a higher specific capacitance than the aluminum capacitors and are used in devices with limited space or flat design such as laptops. They are also used in military technology, mostly in ahermetically sealed axial style. Niobium chip capacitors compete with tantalum chip capacitors.
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The phenomenon that an oxide layer in an electrochemical process blocks an electric current in one direction but allows it to flow in the other direction was discovered in 1875 by French researcher Eugène Ducretet. He coined the term "valve metal" for such metals.
Charles Pollak (born Karol Pollak), a producer of accumulators, discovered that the oxide layer on an aluminum anode remained stable in a neutral or alkaline electrolyte, even when any polarising voltage was removed. In 1896 he patented his idea of an Electric liquid capacitor with aluminium electrodes (de: Elektrischer Flüssigkeitskondensator mit Aluminiumelektroden).
The first practical electrolytic capacitors consisted of a metallic box cathode filled with a borax electrolyte dissolved in water, in which a folded aluminum anode plate was inserted. By applying a DC voltage, the oxide layer was formed on the anode surface. The advantage of these capacitors was that they were significantly smaller and cheaper than other capacitors relative to their capacitance. This construction remained largely unchanged into the 1930s and was called the “wet” electrolytic capacitor, in the sense of containing a high water content.
The first common application of wet aluminum 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 greater-capacitance (for the time) and high-voltage capacitors for use with valve circuitry, typically at least 4 microfarads and rated up to around 500 volts DC. Waxed paper and oiled silk film capacitors were available, but devices with that order of capacitance and voltage rating were bulky and prohibitively expensive.
The ancestor of the modern electrolytic capacitor was patented by Samuel Ruben in 1925 in partnership with Philip Mallory, the founder of the Mallory Battery Company (now known as Duracell International). Ruben's idea adopted the stacked construction of a silver mica capacitor. He introduced a separated second foil to contact the electrolyte adjacent the anode foil instead of using the electrolyte filled container as cathode. The stacked second foil got its own terminal in additional to that of the anode. The container now had no electrical function. This type of electrolytic capacitor combined with a liquid or gel-like electrolyte of a non-aqueous nature and therefore dry, in the sense of reduced water content, became known as the “dry” type.
With Ruben's invention, together with the invention of wound foils separated with a paper spacer in 1927 by A. Eckel of the Hydra-Werke (Germany) company, the actual development of modern electrolytic capacitors began.
William Dubilier whose first patent for electrolytic capacitors was filed in 1928 turned the new ideas for electrolytic capacitors into manufacturable items and started the first commercial production in 1931 as Cornell-Dubilier (CD) factory in Plainfield, NJ. At the same time in Germany, Berlin, the “Hydra-Werke”, an AEG company, had started volume production.
In his 1896 patent Pollak wrote that capacitance can be increased by roughening the surface of the anode foil. Today: the electro-chemical etching of low voltage foils can be produce up to a 200-fold increase in surface area. This etching process enabled an ongoing size reduction for a given capacitance.
The first tantalum capacitors were developed in 1930 by Fansteel Metallurgical Corporation for military purposes They adopted the basic construction of a wound cell and used a tantalum anode foil with a tantalum cathode foil separated with a paper spacer that is impregnated with a liquid electrolyte.
The principal development of solid electrolyte tantalum capacitors began some years after the transistor was invented in 1947. They were developed by Bell Laboratories in the early 1950s as a miniaturized and more reliable low-voltage capacitor to complement the transistor. R. L. Taylor and H. E. Haring from Bell labs found a solution in early 1950 based on their experience with ceramics. They ground tantalum to a powder, pressed it into a cylindrical form and then sintered the powder particles at high temperature between 1500 and 2000 °C under vacuum to a pellet (“slug”).
These first sintered tantalum capacitors used a non-solid electrolyte that did not fit the concept of solid electronics. In 1952' D. A. McLean and F. S. Power developed a solid electrolyte based on manganese dioxide.
The first commercially viable tantalum capacitors were made by researchers of the Sprague Electric Company. Preston Robinson, Sprague's Director of Research, is considered to be the official inventor of tantalum capacitors in 1954. His invention was supported by R. J. Millard, who introduced the “reform” step in 1955, a significant improvement in which the dielectric was reformed after each dip-and-convert cycle of manganese dioxide deposition. This dramatically reduced the leakage current of the finished capacitors.
This first solid manganese dioxide electrolyte had a conductivity 10 times better than all types of non-solid electrolyte. This influenced the development of aluminum electrolytic capacitors. In 1964 Philips developed the first aluminum electrolytic capacitors with solid electrolyte. 
Intel launched their first microcomputer, the MCS 4 in 1971. Hewlett Packard followed a year later with one of the first pocket calculators, the HP 35. The requirements for capacitors in these applications required lower losses. The ESR for bypass and decoupling capacitors of standard electrolytic capacitors had to decrease.
Although solid tantalum capacitors offered capacitors with lower ESR and leakage current values than the aluminum electrolytic capacitors, in 1980 a tantalum price increase raised the cost of tantalum capacitors, especially in the entertainment industry.  The industry switched back to aluminum.
From 1970 to 1990, aluminum electrolytics were developed with low leakage current, long life, or higher temperature rating (up to 125 °C) that were specifically suited to certain[vague] industrial applications.
The next step in ESR reduction took place in 1975 with the development of conducting polymers by Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa. The conductivity of conductive polymers such as polypyrrole (PPy) or PEDOT was 100 to 500 times better than that of TCNQ, and approaching the conductivity of some metals.
In 1983, when Sanyo introduced their "OS-CON" aluminum capacitors a new step of ESR reduction was achieved. These capacitors used a solid organic conductor, the charge transfer salt TTF-TCNQ (tetracyanoquinodimethane) that improved conductivity by a factor of 10 compared to manganese dioxide.
In 1991, Panasonic introduced their "SP-Cap" range called polymer aluminum electrolytic capacitors. These aluminum capacitors with polymer electrolytes reached ESR values comparable to ceramic multilayer capacitors (MLCCs). They were less expensive than tantalum capacitors and with their surface mount design competed directly with tantalum chip capcitors for use in laptops and cell phones.
Tantalum capacitors with PPy polymer electrolyte cathode followed three years later. In 1993, NEC introduced SMD polymer tantalum electrolytic capacitors called "NeoCap". Sanyo followed in 1997 with the "POSCAP" polymer tantalum chips.
A new conductive polymer for tantalum polymer capacitors was presented by Kemet at the "1999 Carts" conference. This device used the organic conductive polymer PEDT Poly(3,4-ethylenedioxythiophene), also known as PEDOT (trade name Baytron®)
Another price explosion for tantalum in 2000/2001 sped the development of niobium electrolytic capacitors with manganese dioxide electrolyte, which have been available since 2002. Niobium is much more abundant in nature than tantalum and is less expensive. The high availability of the base metal in the Soviet Union in the late '60s led to the development of niobium capacitors there instead of tantalum capacitors as in the West. The materials and processes used to produce niobium-dielectric capacitors are essentially the same as for tantalum-dielectric capacitors. The characteristics of the two are roughly comparable.
With the goal of reducing ESR for inexpensive non-solid capacitors, from the mid-1980s new water-based electrolytes were developed in Japan. Water is an effective solvent for electrolytes and improves electrolyte conductivity. In the 1990s Japanese manufacturer Rubycon developed new water-based electrolyte systems with enhanced conductivity.
In the first part of the 21st century, a stolen formula of such a water-based electrolyte, in which the important stabilizing substances were absent, led to the problem of "mass-bursting" of capacitors in computers and power supplies, which became known under the term "Capacitor Plague". In these capacitors, the water reacts aggressively and even violently with aluminum accompanied by strong heat and gas development and often has led to explosions.
The electrical characteristics of capacitors are harmonized by the international generic specification IEC 60384-1. In this standard, the electrical characteristics of capacitors are described by an idealized series-equivalent circuit with electrical components which model all ohmic losses, capacitive and inductive parameters of an electrolytic capacitor:
- C, the capacitance
- RESR, the equivalent series resistance which summarizes all ohmic losses, usually abbreviated as "ESR"
- LESL, the equivalent series inductance which is the effective self-inductance, usually abbreviated as "ESL".
- Rleak, the resistance representing the leakage current
Capacitance, standard values and tolerances
The electrical characteristics of electrolytic capacitors depend on structure of the anode and used electrolyte. This influences the capacitance value of electrolytic capacitors which depends on measuring frequency and temperature. Electrolytic capacitors with non-solid electrolytes show a broader aberration over frequency and temperature than capacitors with solid electrolytes.
The basic unit of an electrolytic capacitors' capacitance is the Farad. However: for the majority of purposes, this is an inconveniently large unit. Most capacitors encountered will be in the microfarad range (abbreviated μF, or less correctly uF or MFD). The capacitance value specified in data sheets is called the rated capacitance CR or nominal capacitance CN and is the value for which the capacitor has been designed.
Standardized measuring condition for capacitors is an AC measuring method with 0.5 V at a frequency of 100/120 Hz and 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 of ≤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.[why?] Therefore the capacitance values of electrolytic capacitors are not directly comparable and differ from those of film capacitors or ceramic capacitors , whose capacitance is measured at 1 kHz or higher.
Measured with an AC measuring method with 100/120 Hz the measured 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 DC capacitance. The DC capacitance is about 10% higher than the 100/120 Hz AC capacitance.[why?] 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 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.
- rated capacitance, series E3, tolerance ±20%, letter code "M“
- rated capacitance, series E6, tolerance ±20%, letter code "M“
- rated capacitance, series E12, tolerance ±10%, letter code "K“
The permitted tolerance is determined by the particular application. Electrolytic capacitors, which are often used for filtering and bypassing capacitors don’t have the need for narrow tolerances because they are mostly not used for accurate frequency applications like oscillators.
Rated and category voltage
Referring to IEC/EN 60384-1 standard the allowed operating voltage for electrolytic capacitors is called "rated voltage UR " or "nominal voltage UN". The rated voltage UR is the maximum DC voltage or peak pulse voltage that may be applied continuously at any temperature within the rated temperature range TR.
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 specify 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 right.
Applying a higher voltage than specified may destroy electrolytic capacitors.
Lower voltage applied may have positive influences to electrolytic capacitors. For aluminum electrolytic capacitors in some cases a lower voltage applied can extend the lifetime. For tantalum electrolytic capacitors lowering the voltage applied increases the reliability and reduce the expected failure rate. I
The surge voltage indicates the maximum peak voltage value that may be applied to electrolytic capacitors during their application for a limited number of cycles. The surge voltage is standardized in IEC/EN 60384-1. For aluminum capacitors with a rated voltage of up to 315 V, the surge voltage is 1.15 times the rated voltage, and for capacitors with a rated voltage exceeding 315 V, the surge voltage is 1.10 times the rated voltage.
For tantalum capacitors the surge voltage is 1.3 times the rated voltage, rounded off to the nearest volt. The surge voltage applied to tantalum capacitors may influence the capacitors failure rate.
Aluminum electrolytic capacitors with non-solid electrolyte are relatively insensitive to high and short-term transient voltages higher than surge voltage, if the frequency and the energy content of the transients is low. This ability depends on rated voltage and component size. Low energy transient voltages lead to a voltage limitation similar like a zener diode. An unambiguously and general specification of tolerable transients or peak voltages is not possible. In every case, transients arises, the application has to be approved very carefully.
Electrolytic capacitors with solid manganese oxide or polymer electrolyte, aluminum as well as tantalum electrolytic capacitors can not withstand transients or peak voltages higher than surge voltage. Transients for this type of e-caps may destroy the components.
Standard electrolytic capacitors, are polarized capacitors and generally require anode electrode voltage to be positive relative to the cathode voltage.
Nevertheless, electrolytic capacitors can withstand reverse voltage of short duration for a limited number of cycles. Aluminum capacitors with non-solid electrolyte can withstand a reverse voltage of about 1 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.
Solid tantalum capacitors can also withstand reverse voltages for short periods. The most common guidelines for tantalum reverse voltage are:
- 10 % of rated voltage to a maximum of 1 V at 25 °C,
- 3 % of rated voltage to a maximum of 0.5 V at 85 °C,
- 1 % of rated voltage to a maximum of 0.1 V at 125 °C.
At no time should a reverse voltage be applied regularly such as with an AC voltage.
To minimize the likelihood of a polarized electrolytic being incorrectly inserted into a circuit, the polarity has to be very clearly indicated on the case, see #Polarity marking.
Special bipolar aluminum electrolytic capacitors designed for bipolar operation are available, 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 adaptable for mainstream AC applications. For power applications, capacitors with metallized polymer film or paper dielectric are used instead.
In general, a capacitor is seen as a storage component for electric energy. But this is only one capacitor function. A capacitor can also act as an AC resistor. Especially aluminum electrolytic capacitors in many applications are used as a decoupling capacitors to filter or bypass undesired biased AC frequencies to the ground or for capacitive coupling of audio AC signals. Than the dielectric is used only for blocking DC. For such applications the AC resistance, the impedance is as important as the capacitance value.
The impedance Z is the vector sum of reactance and resistance, 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 to pass alternating currents and can be used like Ohms law.
With other words, the impedance is a frequency dependent AC resistance and possesses both magnitude and phase at a particular frequency.
In data sheets of electrolytic capacitors only the impedance magnitude |Z| is specified, and simply written as "Z". Regarding to 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.
Besides measuring the impedance can be calculated using the idealized components out 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 therefore 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
In the special case of resonance, in which the both reactive resistances XC and XL have the same value (XC=XL), then the impedance will only be determined by ESR. With frequencies above the resonance the impedance increases again due to the ESL. The capacitor becomes to an inductance.
ESR and dissipation factor tan δ
The equivalent series resistance (ESR) summarizes all resistive losses. These are the terminal resistances, the contact resistance of the electrode contact, the line resistance of the electrodes, the electrolyte resistance, and the dielectric losses in the dielectric oxide layer.
For electrolytic capacitors generally the ESR decreases with increasing frequency and temperature.
ESR influences the remaining superimposed AC ripple behind smoothing and may influence the circuit functionality. Related to the capacitor ESR is accountable for internal heat generation if a #ripple current flow over the capacitor. This internal heat reduces the #lifetime of non-solid aluminum electrolytic capacitors or influences the reliability of solid tantalum electrolytic capacitors.
For electrolytic capacitors, out of historical reasons sometimes the dissipation factor tan δ will be specified in the relevant data sheets, instead of the ESR. The dissipation factor is determined by the tangent of the phase angle between the capacitive reactance XC minus the inductive reactance XL and the ESR. If the inductance ESL is small, the dissipation factor can be approximated as:
The dissipation factor is used for capacitors with very low losses in frequency determining circuits where the reciprocal value of the dissipation factor is called the quality factor (Q) which represents a resonator's bandwidth.
A "ripple current" is the RMS value of a superimposed AC current of any frequency and any waveform of the current curve for continuous operation within the specified temperature range. It arises mainly in power supplies (including switched-mode power supplies) after rectifying an AC voltage and flows as charge and discharge current through the decoupling or smoothing capacitor.
Ripple currents generates heat inside the capacitor body. These dissipation power loss PL is caused by ESR and is the squared value of the effective (RMS) ripple current IR.
This internal generated heat, additional to the ambient temperature and possibly other external heat sources leads to a capacitor body temperature having a temperature difference of Δ T against the ambient. This heat has to be distributed as thermal losses Pth over the capacitors surface A and the thermal resistance β to the ambient.
The internal generated heat has to be distributed to the ambient by thermal radiation, convection, and thermal conduction. The temperature, which is established on the balance between heat produced and distributed, shall not exceed the capacitors maximum specified temperature.
The ripple current is specified as an effective (RMS) value at 100 or 120 Hz or at 10 kHz at upper category temperature. Non-sinusoidal ripple currents have to be analyzed and separated into their single sinusoidal frequencies by means of Fourier analysis and summarized by squared addition the single currents.
In non-solid electrolytic capacitors the heat generated by the ripple current force the evaporation of electrolytes, shortening the life times. Exceeding the limit tends to result in explosive failure.
In solid tantalum electrolytic capacitors with manganese dioxide electrolyte the heat generated by the ripple current influences reliability. Exceeding the limit tends to result in catastrophic failures with shorts and burning components.
The heat generated by the ripple current also influences the life time of aluminum and tantalum electrolytic capacitors with solid polymer electrolytes. Exceeding the limit tends to result in catastrophic failures with short components.
Current surge, peak or pulse current
Aluminum electrolytic capacitors with non-solid electrolytes normally can be charged up to the rated voltage without any current surge, peak or pulse limitation. This property is a result of the limited ion movability in the liquid electrolyte, which slow down the voltage ramp across the dielectric, and the capacitors ESR. Only the frequency of peaks integrated over the time must not exceed the maximal specified ripple current.
Solid tantalum electrolytic capacitors with manganese dioxide electrolyte as well as with polymer electrolyte are damageable against peak or pulse currents. Solid Tantalum capacitors which are exposed to surge, peak or pulse currents, f. e. in in highly inductive circuits, should be used with a voltage derating. If possible the voltage profile should be a ramp turn-on, as this reduces the peak current seen by the capacitor.
For electrolytic capacitors the DC leakage current (DCL) is a special characteristic the other conventional capacitors don’t have. This current is represented by the resistor Rleak in parallel with the capacitor in the series-equivalent circuit of electrolytic capacitors.
The reasons for leakage current are different between electrolytic capacitors with non-solid and with solid electrolyte or more common for “wet” aluminum and for “solid” tantalum electrolytic capacitors with manganese dioxide electrolyte as well as for electrolytic capacitors with polymer electrolytes. For non-solid aluminum electrolytic capacitors the leakage current includes all weakened imperfections of the dielectric caused by unwanted chemical processes happened during the time without voltage applied (storage time) between operating cycles. This unwanted chemical processes depend on the kind of electrolyte. Electrolytes with water contend or water based electrolytes are more aggressive against the aluminum oxide layer than electrolytes based on organic liquids. This is the reason different electrolytic capacitor series specify different storage time without reforming instructions.
Applying a positive voltage to a "wet" capacitor a reforming process (self-healing) repairs all weakened dielectric layers, and the leakage current remain on a low level.
Although the leakage current of non-solid e-caps is higher than current flow over insulation resistance in ceramic or film capacitors, the self-discharge of modern non-solid electrolytic capacitors with organic electrolytes takes several weeks.
The main causes of DCL for solid tantalum capacitors are f. e. electrical breakdown of the dielectric, conductive paths due to impurities or due to poor anodization, bypassing of dielectric due to excess manganese dioxide, due to moisture paths or due to cathode conductors (carbon, silver). This “normal” leakage current in solid electrolyte capacitors couldn’t be reduced by “healing”, because under normal conditions solid electrolytes don’t can deliver oxygen for forming processes. This statement should not be confused with the self-healing process during field crystallization, see #Reliability, Failure rate.
The specification of the leakage current in datasheets often will be given by multiplication of the rated capacitance value CR with the value of the rated voltage UR together with an addendum figure, measured after a measuring time of 2 or 5 minutes, for example:
The leakage current value depends on the voltage applied, on temperature, and on measuring time. Leakage current in solid MnO2 tantalum electrolytic capacitors generally drops very much faster than for non-solid electrolytic capacitors but remain on the reached level.
Dielectric absorption (soakage)
Dielectric absorption occurs when a capacitor that has remained charged for a long time discharges only incompletely when briefly discharged. Although an ideal capacitor would reach zero volts after discharge, real capacitors develop a small voltage from time-delayed dipole discharging, a phenomenon that is also called dielectric relaxation, "soakage" or "battery action".
|Type of capacitor||Dielectric Absorption|
|Tantalum electrolytic capacitors with solid electrolyte||2 to 3%, 10%|
|Aluminium electrolytic capacitor with non solid electrolyte||10 to 15%|
Dielectric absorption may a problem in circuits, were very small currents are used for in the function of an electronic circuit such as long-time-constant integrators or sample-and-hold circuits. In most applications of electrolytic capacitors supporting power supply lines dielectric absorption is not a problem.
But especially for electrolytic capacitors with high rated voltage the voltage at the terminals generated by the dielectric absorption can be a safety risk to personnel or circuits. In order to prevent shocks most very large capacitors are shipped with shorting wires that need to be removed before they are used.
Reliability and life time
Reliability (failure rate]
The reliability of a component is a property that indicates how reliable this component performs its function in a time interval. It is subject to a stochastic process and can be described qualitatively and quantitatively; it is not directly measurable. The reliability of electrolytic capacitors are empirically determined by identifying the failure rate in production accompanying endurance tests, see Reliability engineering#Reliability testing
The reliability normally is shown in a bathtub curve and is divided into three areas: Early failures or infant mortality failures, constant random failures and wear out failures. Failures totalized in a failure rate are short circuit, open circuit and degradation failures (exceeding electrical parameters).
The reliability prediction is generally expressed in a Failure rate λ, abbreviation FIT (Failures In Time]. This is the number of failures that can be expected in one billion (109) component-hours of operation (e.g. 1000 components for 1 million hours, or 1 million components for 1000 hours which is 1 ppm/1000 hours) at fixed working conditions during the period of constant random failures. These failure rate model implicitly assume the idea of "random failure". Individual components fail at random times but at a predictable rate.
Billions of tested capacitors unit-hours would be needed to establish failure rates in the very low level range which are required today to ensure the production of large quantities of components without failures. This requires about a million units over a long time period which needs great personal staff and a lot of money. The tested failure rates often are complemented with figures resulting from big users feedback of failed component returns from the field (field failure rate) which mostly results in a lower failure rate than tested.
The reciprocal value of FIT is MTBF (Mean Time Between Failures).
The standard operation conditions for the failure rate FIT are 40 °C and 0.5 UR. For other conditions of applied voltage, current load, temperature, capacitance value, circuit resistance (for tantalum capacitors), mechanical influences and humidity the FIT figure can recalculated with acceleration factors standardized for industrial or military contexts. As higher f. e. temperature and applied voltage as higher is the failure rate.
The most often cited source for recalculation the failure rate is the MIL-HDBK-217F, the “bible” of failure rate calculations for electronic components. SQC Online, the online statistical calculators for acceptance sampling and quality control gives an online tool for short examination to calculate given failure rate values to application conditions.
It is good to know that for tantalum capacitors often the failure rate is specified in essence at 85 °C and rated voltage UR as reference conditions and expressed as per cent failed components per thousand hours (n %/1000 h). That is “n” number of failed components per 105 hours or in FIT the ten-thousand-fold value per 109 hours.
It should be noted that industrial produced tantalum capacitors nowadays are very reliable components. Continuous improvement in tantalum powder and capacitor technologies have resulted in a significant reduction in the amount of impurities present which formerly have caused most of the field crystallization failures. Commercial available industrial produced tantalum capacitors now have reached as standard products the high MIL standard “C” level which is 0.01%/1000h at 85 °C and UR or 1 failure per 107 hours at 85 °C and UR. Recalculated in FIT with the acceleration factors coming from MIL HDKB 217F at 40 °C and 0.5 UR is this failure rate for a 100 µF/25 V tantalum chip capacitor used with a series resistance of 0.1 Ω the failure rate is 0.02 FIT.
Aluminum electrolytic capacitors are not familiar with the specification in "% per 1000 h at 85 °C and UR". They use the FIT specification with 40 °C and 0.5 UR as reference conditions. Also for aluminum electrolytic capacitors should be noted that they are very reliable components. Published figures show for low voltages types (6.3…160 V) FIT rates in the range of 1 to 20 FIT and for high voltage types (>160 …550 V) FIT rates in the range of 20 to 200 FIT. Field failure rates for aluminum e-caps are in the range of 0.5 to 20 FIT.
The published figures show that both capacitor types, tantalum and aluminum, are reliable components, comparable with other electronic components achieving safe operations of decades under normal conditions. But it exist a great difference in case of wear-out failures. Tantalum capacitors with solid electrolyte have no wear-out mechanism so that the constant failure rate least up to the point all capacitors have failed. Electrolytic capacitors with non-solid electrolyte however have a limited time of constant random failures up to that point the wear-out failures starts. This time of the constant random failure rate correspondents with the life time or service life of “wet” aluminum electrolytic capacitors.
The life time, service life, load life or useful life of electrolytic capacitors is a special characteristic of non-solid aluminum electrolytic capacitors, which liquid electrolyte can evaporate over the time. Lowering the electrolyte influences the electrical parameters. The capacitance decreases and the impedance and ESR increases with decreasing electrolyte. This very slowly electrolyte drying-out depends on the temperature, the applied ripple current load, and the applied voltage. The lower these parameters compared with their maximum values the longer the capacitors “life”. The “end of life” point is defined by the appearance of wear-out failures or degradation failures when either capacitance, impedance, ESR or leakage current exceed their specified change limits.
The life time is a specification of a collective of tested capacitors and delivers an expectation of the behavior of similar types. This life time definition corresponds with the time of the constant random failure rate in the bathtub curve.
But even after exceeding the specified limits and the capacitors have reached their “end of life” the electronic circuit is not in an immediate danger, only the functionality is reduced. With today's high levels of purity in the manufacture of electrolytic capacitors it is not to be expected that after end-of-life-point with progressive evaporation combined with parameter degradation short circuits occur.
The life time of non-solid aluminum electrolytic capacitors is specified in terms of “hours per temperature" like “2,000h/105 °C”. With this specification the life time at operational conditions can be estimated by special formulas or graphs specified in the data sheets of serious manufacturers. They use different ways for specification, some give special formulas, others specify their e-caps life time calculation with graphs, which consider the influence of applied voltage. Basic principle for calculating the time under operational conditions is the so-called “10-degree-rule”.
This rule also is well known as Arrhenius rule. It characterizes the change of thermic reactions speed. For every 10 °C lower temperature evaporation halves. That means for every 10 °C lower temperature the life time of capacitors doubles. That means if a life time specification of an electrolytic capacitor is f. e. 2000 h/105 °C the capacitors life time at 45 °C can be ”calculated” with 128,000 hours – that is roughly 15 years - by using the 10-degrees-rule.
However, also solid polymer electrolytic capacitors, aluminum as well as tantalum and niobium electrolytic capacitors do have a life time specification. The polymer electrolyte have a small deterioration of conductivity by a thermal degradation mechanism of the conductive polymer. The electrical conductivity decreased, as a function of time, in agreement with a granular metal type structure, in which aging is due to the shrinking of the conductive polymer grains. The life time of polymer electrolytic capacitors is specified in similar terms like non-solid e-caps but it’s life time calculation follows other rules leading to much longer operational life times.
Tantalum electrolytic capacitors with solid manganese dioxide electrolyte don't have wear-out failures so they don't have a life time specification in the sense of non-solid aluminum electrolytic capacitors. Also tantalum capacitors with non-solid electrolyte, the “wet tantalums” don’t have a life time specification because they are hermetically sealed and evaporation of electrolyte is minimized.
Electrolytic capacitors with solid electrolyte don't have wear-out failures so they don't have a life time specification in the sense of non-solid aluminum electrolytic capacitors. Also tantalum capacitors with non-solid electrolyte, the “wet tantalums” don’t have a life time specification because they are hermetically sealed and evaporation of electrolyte is minimized.
Failure modes, self-healing mechanism and application rules
The many different types of electrolytic capacitors show a different behavior in point of electrical long-term behavior, their inherent failure modes and their self-healing mechanism. Application rules for types with an inherent failure mode are specified to ensure capacitors high reliability and long life.
|Drying out over time,
|New generated oxide (forming)
by applying a voltage
solid polymer electrolyte
|Deterioration of conductivity,
|Insulating of faults
in the dielectric
by oxidation or evaporation
of the polymer electrolyte
solid MnO2 electrolyte
|Thermally induced insulating
of faults in the dielectric
by oxidization of the electrolyte MnO2
into insulating MnO2O3
if current availability is limited
|Voltage derating 50%
Series resistance 3 Ω/V
solid polymer electrolyte
|Deterioration of conductivity,
|Insulating of faults
in the dielectric by oxidation or evaporation
of the polymer electrolyte
|Voltage derating 20 %
solid MnO2 electrolyte
|Thermally induced insulation of faults
in the dielectric
by oxidation of Nb2O5
into insulating NbO2
voltage derating 50%
niobium oxide anode:
voltage derating 20 %
solid polymer electrolyte
|Deterioration of conductivity,
|Insulating of faults
in the dielectric
by oxidation or evaporation
of the polymer electrolyte
voltage derating 50%
niobium oxide anode:
voltage derating 20 %
|Hybrid aluminum e-caps,
solid polymer + non-solid electrolyte
|Deterioration of conductivity,
drying out over time,
|New generated oxide (forming)
by applying a voltage
Smaller or low voltage electrolytic capacitors may be connected in parallel without any safety correction action. Large sizes capacitors, especially large sizes and high voltage types should be individual guarded against sudden energy charge of the whole capacitor bank due to a failed specimen.
Some applications like AC/AC converters with DC-link for frequency controls in three-phase grids needs higher voltages aluminum electrolytic capacitors usually offer. For such applications electrolytic capacitors can be connected in series for increased voltage withstanding capability. During charging, the voltage across each of the capacitors connected in series is proportional to the inverse of the individual capacitor’s leakage current. Since every capacitor differs a little bit in individual leakage current the capacitors with a higher leakage current will get less voltage. The voltage balance over the series connected capacitors is not symmetrically. Passive or active voltage balance has to be provided in order to stabilize the voltage over each individual capacitor.
Performance after storage
All electrolytic capacitors with non-solid as well as with solid electrolyte are "aged" during manufacturing by applying rated voltage at high temperature for a sufficient time to repair all cracks and weaknesses that may have occurred during production. However, a particular problem with non-solid aluminum electrolytic capacitors may occur after storage or unused times without voltage applied. During storage or unused times potentially chemical processes (corrosion) can weaken the oxide layer, which may lead to a higher leakage current. However, today’s most electrolytic systems are chemically inert and don’t generate any corrosion problems, even after storage times of two years or longer. Especially non-solid electrolytic capacitors using organic solvents like GBL as electrolyte do not have problems with high leakage current after longer storage times. They can be specified with storage times up to 10 years without leakage current problems
The ability to reach longer storage times can be tested using an accelerated shelf-life testing, which requires the storage of capacitors without applied voltage at its upper category temperature for a certain period, usually 1000 hours. This shelf life test is a good indicator for the chemical stability of the electrolytic system and the protecting aluminum oxide layer because all chemical reactions are accelerated by high temperatures. Since decades nearly all today’s series of non-solid e-caps fulfill the 1000 hours shelf life test which comply with minimum fife years storage at room temperature. However, many e-cap series are specified only for a two years storage time. This is a standard storage time for electronic components for storing at room temperature caused by the oxidation of the terminals to ensure the solderability of the terminals.
Only for antique radio equipment or for very old e-caps built in the 1970s or earlier, "pre-conditioning" may be recommended. For this purpose, the rated voltage is applied to the capacitor via a series resistance of approximately 1 kΩ for a period of one hour. Applying a voltage via a safety resistor repairs the oxide layer by self-healing. If the capacitors don’t meet the leakage current requirements after preconditioning, it may be an indication of a mechanical damage.
Electrolytic capacitors with solid electrolytes don’t have any precondition instructions.
Electrolytic capacitors, like most other electronic components and if enough space is available, have imprinted markings to indicate manufacturer, type, electrical and thermal characteristics, and date of manufacture. If they are large enough the capacitor is marked with:
- manufacturer's name or trademark;
- manufacturer's type designation;
- polarity of the terminations (for polarized capacitors)
- rated capacitance;
- tolerance on rated capacitance
- rated voltage and nature of supply (AC or DC)
- climatic category or rated temperature;
- year and month (or week) of manufacture;
- certification marks of safety standards (for safety EMI/RFI suppression capacitors)
Polarized capacitors have polarity markings, usually "−" (minus) sign on the side of the negative electrode for electrolytic capacitors or a stripe or "+" (plus) sign, see #Polarity marking. Also, the negative lead for leaded "wet" e-caps is usually shorter.
Smaller capacitors use a shorthand notation. The most commonly used format is: XYZ J/K/M “V”, where XYZ represents the capacitance (calculated as XY × 10Z pF), the letters K or M indicate the tolerance (±10% and ±20% respectively) and “V” represents the working voltage.
- 105K 330V implies a capacitance of 10 × 105 pF = 1 µF (K = ±10%) with a working voltage of 330 V.
- 476M 100V implies a capacitance of 47 × 103 pF = 47 µF (M = ±20%) with a working voltage of 100 V.
Capacitance, tolerance and date of manufacture can be indicated with a short code specified in IEC/EN 60062. Examples of short-marking of the rated capacitance (microfarads): µ47 = 0,47 µF, 4µ7 = 4,7 µF, 47µ = 47 µF
The date of manufacture is often printed in accordance with international standards.
- Version 1: coding with year/week numeral code, "1208" is "2012, week number 8".
- Version 2: coding with year code/month code. The year codes are: "R" = 2003, "S"= 2004, "T" = 2005, "U" = 2006, "V" = 2007, "W" = 2008, "X" = 2009, "A" = 2010, "B" = 2011, "C" = 2012, "D" = 2013, “E” = 2014 etc. Month codes are: "1" to "9" = Jan. to Sept., "O" = October, "N" = November, "D" = December. "X5" is then "2009, May"
For very small capacitors no marking is possible. Here only the traceability of the manufacturers can ensure the identification of a type.
The standardization for all electrical, electronic components and related technologies follows the rules given by the International Electrotechnical Commission (IEC), a non-profit, non-governmental international standards organization.
The definition of the characteristics and the procedure of the test methods for capacitors for use in electronic equipment are set out in the Generic specification:
- IEC/EN 60384-1 - Fixed capacitors for use in electronic equipment
The tests and requirements to be met by aluminum and tantalum electrolytic capacitors for use in electronic equipment for approval as standardized types are set out in the following sectional specifications:
- IEC/EN 60384-3—Surface mount fixed tantalum electrolytic capacitors with manganese dioxide solid electrolyte
- IEC/EN 60384-4—Aluminium electrolytic capacitors with solid (MnO2) and non-solid electrolyte
- IEC/EN 60384-15—Fixed tantalum capacitors with non-solid and solid electrolyte
- IEC/EN 60384-18—Fixed aluminium electrolytic surface mount capacitors with solid (MnO2) and non-solid electrolyte
- IEC/EN 60384-24—Surface mount fixed tantalum electrolytic capacitors with conductive polymer solid electrolyte
- IEC/EN 60384-25—Surface mount fixed aluminium electrolytic capacitors with conductive polymer solid electrolyte
- IEC/EN 60384-26—Fixed aluminium electrolytic capacitors with conductive polymer solid electrolyte
The market of electrolytic capacitors in 2008 reach roughly 30% of the total market in value
- Aluminum electrolytic capacitors—US$3.9 billion (22%);
- Tantalum electrolytic capacitors—US$2.2 billion (12%);
In number of pieces this capacitors cover about 10% of the total capacitor market, which are about 100 to 120 billion pieces.
- Aluminum electrolytic capacitor
- Niobium capacitor
- Polymer capacitor
- Solid aluminum capacitor (SAL)
- Tantalum capacitor
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