An electrolytic capacitor is a capacitor that uses an electrolyte (an ionic conducting liquid) as one of its plates to achieve a larger capacitance per unit volume than other types. The large capacitance of electrolytic capacitors makes them 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. An electrolytic capacitor will generally have higher leakage current than a comparable (dry) capacitor, and may have significant limitations in its operating temperature range, parasitic resistance and inductance, and the stability and accuracy of its capacitance value.
Supercapacitors provide the highest capacitance of any practically available capacitor, up to thousands of farads, with working voltages of a few volts. Electrolytic capacitors range downwards from tens (exceptionally hundreds) of thousands of microfarads to about 100 nanofarads—smaller sizes are possible but have no advantage over other types. Other types of capacitor are available in sizes typically up to about ten microfarads, but the larger sizes are much larger and more expensive than electrolytics (film capacitors of up to thousands of microfarads are available, but at very high prices). Electrolytic capacitors are available with working voltages up to about 500V, although the highest capacitance values are not available at high voltage. Working temperature is commonly 85°C for standard use and 105° for high-temperature use; higher temperature units are available, but uncommon.
Most electrolytic capacitors require that the voltage applied to one terminal (the anode) never become negative relative to the other (they are said to be "polarized"), so cannot be used with AC signals without a DC polarizing bias. Non-polarized electrolytic capacitors are available for special purposes.
Capacitance tolerance and stability, equivalent series resistance (ESR) and dissipation factor are significantly inferior to other types of capacitors, leakage current is higher and working life is shorter. Capacitors can lose capacitance as they age and lose electrolyte, particularly at high temperatures. A common failure mode which causes difficult-to-find circuit malfunction is progressively increasing ESR without change of capacitance, again particularly at high temperature. Large ripple currents flowing through the ESR generate harmful heat.
Two types of electrolytic capacitor are in common use: aluminum and tantalum. Tantalum capacitors have generally better performance, higher price, and are available only in a more restricted range of parameters. Solid polymer dielectric aluminum electrolytic capacitors have better characteristics than wet-electrolyte types—in particular lower and more stable ESR and longer life—at higher prices and more restricted values.
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The principle of the electrolytic capacitor was discovered in 1886 by Karol 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 aluminum and the electrolyte solution. A major problem was that most electrolytes tended to dissolve the oxide layer again when the power is removed, but he eventually found that sodium borate (borax) would allow the layer to be formed and not attack it afterwards. He was granted a patent for the borax-solution aluminum electrolytic capacitor in 1897.
The first application of the technology was in making starting capacitors for single-phase alternating current (AC) motors. Although most electrolytic capacitors are polarized, that is, they can only be operated with direct current (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 in AC systems.
Nineteenth and early twentieth century electrolytic capacitors bore little resemblance to modern types, their construction being 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 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) high-voltage capacitors, typically at least 4 microfarads 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 expensive.
The ancestor of the modern electrolytic capacitor was patented by Julius Lilienfeld in 1926. Lilienfeld's design resembled that of a silver mica capacitor, but with electrolyte-soaked paper sheets in place of the mica dielectric. However, it proved impractical to adequately seal the devices, and in the hot conditions inside typical mains-operated radio receivers the capacitors quickly dried out and failed.
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 (condenser was the earlier term for capacitor) was constructed like 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 units are referred to as "wet electrolytics," and those with liquid still inside are prized by vintage radio collectors.
"Mershons" were an immediate success and the name "Mershon Condenser" was, for a short time, synonymous with quality radio receivers in the late 1920s. However, 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 World War II, when sufficient resources were finally applied to finding the causes of electrolytic capacitor unreliability, that they started to become as reliable as they are today. A major advance was the process of etching and pre-anodizing the foil prior to assembly, which allowed the use of much less corrosive electrolyte solutions, which in turn meant the devices could be left unenergized for long periods without deterioration. Modern electrolytic capacitors can remain usable after lying idle for decades, whereas the original Mershons could not tolerate more than a few months without a polarizing voltage. Elaborate "re-forming" procedures were necessary to avoid damage to receivers that had not been used for some time.
Aluminum electrolytic capacitors are constructed from two conducting aluminium 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 acts as the cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminum 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.
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. This oxide has a dielectric constant of 10, which is several times higher than most common polymer insulators. It can withstand an electric field strength of the order of 25 megavolts per meter which is an acceptable fraction of that of common polymers. This combination of high capacitance and reasonably high voltage result in high energy density.
Most electrolytic capacitors are polarized and require one of the electrodes to be positive relative to the other; they may catastrophically fail if voltage is reversed. This is because a reverse-bias voltage above 1 to 1.5 V will destroy the center layer of dielectric material via electrochemical reduction (see redox reactions). Following the loss of the dielectric material, the capacitor will short circuit, and with sufficient short circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst, often in a spectacularly dramatic fashion.
To minimize the likelihood of a polarized electrolytic being incorrectly inserted into a circuit, polarity has to be very clearly indicated on the case.
In radial electrolytic capacitors a bar across the side of the capacitor is commonly used to indicate the negative terminal side and the negative terminal lead is shorter than the positive terminal lead. On a printed circuit board it is customary to indicate the correct orientation by using a filled half circle to indicate the minus lead side and the + marker to indicate the plus lead side.
In axial electrolytic capacitors a bar across the side with arrow pattern pointing to the negative lead end is commonly used to indicate the negative terminal lead end and the positive lead end of the capacitor usually has a narrow part close to the edge of the positive end. Similarly the negative terminal lead is shorter than the positive terminal lead. On a printed circuit board it is customary to indicate the correct orientation by using a square through-hole pad for the positive lead and a round pad for the negative.
Special bipolar capacitors designed for AC operation are available, usually referred to as "non-polarized" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminum foil strips prior to assembly. On the alternate halves of the AC cycles, one of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one.
Modern capacitors have a safety valve which is 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.
The above are the most common schematic symbols for electrolytic capacitors. Some schematic diagrams do not print the "+" adjacent to the symbol. Older circuit diagrams show electrolytic capacitors as a small positive plate surrounded below and on the sides by a larger dish-shaped negative electrode, usually without "+" marking.
Designing for reverse bias
Polarized electrolytic capacitors are sometimes adapted for bipolar AC use.
When the positive terminals (or the negative terminals) of two aluminum electrolytic capacitors are connected, the result is effectively a non-polar capacitor. Each of the two capacitors rectify the applied voltage and act as if it had been bypassed by diodes -- the correct polarity capacitor gets the full voltage. :2
Two identical polarized electrolytic capacitors are connected back to back to form a bipolar capacitor with half the nominal capacitance of either. However, the anode film can only withstand a small reverse voltage. This arrangement can lead to premature failure as the anode film is broken down during the reverse-conduction phase and partially rebuilt during the forward phase. A factory-made non-polarized electrolytic capacitor has both plates anodized so that it can withstand rated voltage in both directions; such capacitors also have about half the capacitance per unit volume of polarized capacitors.
An electrolytic can withstand a reverse bias for a short period, but 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.
The electrolyte is usually boric acid or sodium borate in aqueous solution, together with various sugars or ethylene glycol which are added to slow down evaporation. Getting a suitable balance between chemical stability and low internal electrical resistance is not a simple matter; the exact compositions of high-performance electrolytes are closely guarded trade secrets. It took years of research before reliable devices were developed. The electrolytic solvent has to have high dielectric constant, high dielectric strength, and low resistivity; a solute of ionic conductivity facilitators is mixed within.
Electrolytes may be toxic or corrosive. Working with the electrolyte requires safe working practice and appropriate protective equipment such as gloves and safety glasses. Some very old tantalum electrolytics, often called "Wet-slug", contain corrosive sulfuric acid; however, most of these are no longer in service due to corrosion.
There are three major types of water-based electrolytes for aluminium electrolytic capacitors: standard water-based (with 40-70% water), and those containing ethylene glycol or dipropyl ketone (both with less than 25% water). The water content helps lowering the equivalent series resistance, but can make the capacitor prone to generating gas, especially if the electrolyte formulation is faulty; this is a leading cause of capacitor plague, to which the high water content electrolytes are more susceptible. The lower voltage ratings (thinner oxide layer) and lower operating voltage (slower regeneration of oxide layer) are further aggravating factors.
There are a number of non-aqueous electrolytes, which use only a small amount of water. The electrolytes are generally composed of a weak acid, a salt of weak acid, and a solvent, and optional thickening agent and other additives. The electrolyte is usually soaked into an electrode separator. The weak acids are usually organic acid (glacial acetic acid, lactic acid, propionic acid, butyric acid, crotonic acid, acrylic acid, phenol, cresol, etc.) or boric acid. The salts employed are often ammonium or metal salts of organic acids (ammonium acetate, ammonium citrate, aluminium acetate, calcium lactate, ammonium oxalate, etc.) or weak inorganic acids (sodium perborate, trisodium phosphate, etc.). Solvent-based electrolytes may be based on alkanolamines (monoethanolamine, diethanolamine, triethanolamine,...) or polyols (diethylene glycol, glycerol, etc.).
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 (μF, or less correctly uF), nanofarad (nF) and picofarad (pF) are more commonly used.
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 numbers. By multiplying these base numbers by a power of ten, any practical capacitor value can be achieved, which is suitable for most applications.
Passive electronic components, including capacitors, are usually produced in preferred values (e.g., IEC 60063 E6, E12, etc. series).
The capacitance of aluminum electrolytic capacitors tends to change over time, and they usually have a tolerance range of 20%. Some have asymmetric tolerances, typically −20% but with much larger positive tolerance as many circuits merely require a capacitance to be not less than a given value; this can be seen on datasheets for many consumer-grade capacitors. Tantalum electrolytics can be produced to tighter tolerances and are more stable.
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 voltage or the oxide layer will break down and rupture, causing the capacitor to lose its ability to withstand applied voltage (although it can often be "reformed"). 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 lose its ability to withstand voltage if voltage is not applied. A capacitor to which this has happened can often be "reformed" by connecting it to a voltage source through a resistor and allowing the resulting current to slowly restore the oxide layer.
- Bipolar electrolytics (also called Non-Polarised or NP capacitors) contain two anodized films, behaving like two capacitors connected in series opposition. These are used when one electrode can be either positive or negative relative to the other in different configurations, such as AC-coupling a plug that connects to unknown equipment with unknown output bias voltages. These are not needed on alternating current AC-coupling circuits when both sides are at the same bias, since the voltage across the capacitor is always near zero. Their performance extends to several MHz, but beyond this, bad frequency and temperature characteristics make them unsuited for high-frequency applications. Typical ESL values are a few nanohenries.
- Tantalum: compact, low-voltage devices up to several hundred µF, these have a lower energy density and are produced to tighter tolerances than aluminum electrolytics. Tantalum capacitors are also polarized because of their dissimilar electrodes. The anode 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 conductive plastic polymer (polypyrrole) that reduces internal resistance and eliminates a self-ignition failure.
Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance, little DC leakage, and very low impedance at high frequencies. However, unlike aluminum electrolytics, they are intolerant of positive or negative 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 aluminium-based (with liquid electrolyte) capacitors and generally only available in low-voltage versions, but because of their smaller size for a given capacitance and lower impedance at high frequencies they are popular in miniature applications such as cellular telephones.
- Solid Aluminum electrolytic capacitor with organic semiconductor electrolyte or OS-CON (which stands for OrganicSemi-CONductive) : a new generation capacitor in which the aluminum foil layers are not immersed in a liquid electrolyte solution but in a solid semiconductive material derived from isoquinoline. The single crystal N-n-butyl-isoquinoline is thermoformed to final shape enhancing its conductivity substantially, thus protecting the capacitor from excess heat surges, and finally the OS-CON cans are sealed with epoxy. These capacitors are stable in use between -55°C to 125°C. The main advantages of using this particular semiconductor are fairly low ESR, wider frequency range and greater stability in use compared to liquid electrolyte aluminum and tantalum solid polymer capacitors. OS-CON capacitors are often found as SMD.
Reliability and length of life
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Aluminum, and to a lesser extent tantalum, electrolytics have worse noise, leakage, drift with temperature and aging, dielectric absorption, and inductance than other types of capacitor. Additionally, low temperature is a problem for most aluminum capacitors: for most types, capacitance falls off rapidly below room temperature while dissipation factor can be ten times higher at −25 °C than at 25 °C. Most of the limitations can be traced to the electrolyte. At high temperature, the water can be lost to evaporation, and the capacitor (especially the small sizes) may leak outright. At low temperatures, the conductance of the salts declines, raising the ESR, and the increase in the electrolyte's surface tension can cause reduced contact with the dielectric. The conductance of electrolytes generally has a very high temperature coefficient, +2%/°C is typical, depending on size. The electrolyte, particularly if degraded, is implicated in various reliability issues as well.
High-quality aluminum electrolytics (computer-grade) have better performance and life than consumer-grade parts. High temperatures and ripple currents shorten life. Typical basic electrolytics are rated to work at temperatures up to 85 °C, and are rated for a worst-case life of about 2000 hours (a year is about 9000 hours); commonly available higher-temperature units are available for temperatures of 105 °C. One of the effects of aging is an increase in ESR; some circuits can malfunction due to a capacitor with correct capacitance but elevated ESR, although a capacitance meter will not find any fault (an ESR meter will). Runaway failure is possible if increased ESR increases heat dissipation and temperature.
Since the electrolytes evaporate, design life is most often rated in hours at a set temperature, for example, 2000 hours at 105 °C, which is the highest commonly used working temperature. Standard inexpensive consumer-grade electrolytic capacitors are rated for 85 °C maximum working temperature. Life in the operational environment is dictated by the Law of Arrhenius, which dictates that the capacitor life is a function of temperature and DC voltage. As a rule of thumb, the life doubles for each 10 °C lower operating temperature. In our example, it reaches 15 years at 45 °C (for caps rated at 105 °C). The operating temperature however is not just the ambient temperature. Ripple currents can increase it significantly. The actual operating temperature is a complex function of ambient temperature, air speed, ripple current frequency and amplitude, and also affected by material thermal resistance and the surface area of the can case. In general, high amplitude ripple currents shorten the life expectancy, whereas low frequency ripple is more detrimental than high frequency. The EIA IS-749 is a standard for testing electrolytic capacitor life.
|Internal structure of an electrolytic capacitor with non-solid electrolyte|
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