An electrolytic capacitor is a type of capacitor that uses an ionic conducting liquid as one of its plates. Typically with a larger capacitance per unit volume than other types, they are valuable in relatively high-current and low-frequency electrical circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations, in rectifier output. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not.
Electrolytic capacitors can have a very high capacitance, allowing filters made with them to have very low corner frequencies.
The principle of the electrolytic capacitor was discovered in 1886 by Charles Pollak, as part of his research into anodizing of aluminum and other metals. Pollack discovered that due to the thinness of the aluminum oxide layer produced, there was a very high capacitance between the aluminium and the electrolyte solution. A major problem was that most electrolytes tend to dissolve the oxide layer again when the power is removed, but he eventually found that sodium perborate (borax) would allow the layer to be formed and not attack it afterwards. He was granted a patent for the borax-solution aluminium electrolytic capacitor in 1897.
The first application of the technology was in making motor start capacitors for single-phase alternating current motors. Although most electrolytic capacitors are polarized, that is, they can only be operated with DC, by separately anodizing aluminum plates and then interleaving them in a borax bath, it is possible to make a capacitor that can be used for AC systems. 19th and early 20th century electrolytic capacitors bore little resemblance to modern types, being constructed more along the lines of a car battery. The borax electrolyte solution had to be periodically topped up with distilled water, again reminiscent of a lead acid battery
The first major application of DC versions of this type of capacitor was in large telephone exchanges, to "quieten" relay hash on the 48 volt DC power supplies.
The development of AC-operated domestic radio receivers in the late 1920s required the production of fairly large capacitance (for the time) high voltage capacitors, typically at least 4 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 prohibitively expensive. The first attempt at a modern electrolytic capacitor was patented by Julius Lilienfeld in 1926. Lilienfeld's design was constructed rather along the lines of a silver mica capacitor, but with electrolyte-soaked paper sheets in place of the mica. It proved impractical to adequately seal the devices, and in the hot conditions inside typical AC operated radio receivers they quickly dried out and failed.
Retired US Navy engineer Ralph D Mershon is credited with developing the first commercially available "radio" electrolytic capacitor that was used in any quantity, (although other researchers produced broadly similar devices). The "Mershon Condenser" as it was known, was constructed similarly to a conventional paper capacitor, with two long strips of aluminum foil interwound with strips of insulating paper, but with the paper saturated with electrolyte solution instead of wax. Rather than trying to hermetically seal the devices, Mershon's solution was to simply fit the capacitor into an oversize aluminum or copper can, half-filled with extra electrolyte. (These are referred to by vintage radio enthusiasts as "wet electrolytics", and ones with liquid still inside are prized collectors items).
Although "Mershons" were an immediate success, (and the name "Mershon Condenser" was for a short time synonymous with quality radio receivers in the late 1920s), due to a number of manufacturing difficulties their service life turned out to be quite short and Mershon's company went bankrupt in the early 1930s.
It was not until World War II when sufficient resources were finally applied to finding the causes of electrolytic capacitor unreliability, that they became the reliable components they are today.
Unlike most capacitors, electrolytic capacitors have a voltage polarity requirement. The correct polarity is indicated on the packaging by a stripe with minus signs and possibly arrowheads, denoting the adjacent terminal that should have lower electrical potential (i.e. negative terminal). Also the negative terminal lead of radial electrolytic capacitors are shorter. This is necessary 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). Without the dielectric material the capacitor will short circuit, and if the short circuit current is excessive, then the electrolyte will heat up and either leak or cause the capacitor to explode.
Special capacitors designed for AC operation are available, usually referred to as "non-polar" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminium foil strips prior to assembly. On the alternate halves of the AC cycles, one or the other of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one. Essentially, a 10 microfarad AC capacitor behaves like two 20 microfarad DC capacitors in inverse series.
Modern capacitors have a safety valve, typically either a scored section of the can, or a specially designed end seal to vent the hot gas/liquid, but ruptures can still be dramatic. Electrolytics can withstand a reverse bias for a short period of time, but they will conduct significant current and not act as a very good capacitor. Most will survive with no reverse DC bias or with only AC voltage, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time. A constant forward bias is preferable, and will increase the life of the capacitor.
These are the different schematic symbols for electrolytic capacitors. Some schematic diagrams do not print the "+" adjacent to the symbol. Electrolytic capacitors are marked to show the polarity of the leads.
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.
RESR must be as small as possible since it determines the loss power when the capacitor is used to smooth voltage. Loss power scales quadratically with the ripple current flowing through and linearly with RESR. Low ESR capacitors are imperative for high efficiencies in power supplies.
It should be pointed out that this is only a simple model and does not include dielectric absorption (soakage) and other non-ideal effects associated with real electrolytic capacitors.
Since the electrolytes evaporate, design life is most often rated in hours at a set temperature. For example, typically as 2000 hours at 105 degrees Celsius (which is the highest working temperature). Design life doubles for each 10 degrees lower , reaching 15 years at 45 degrees. However a great number of capacitors much older than this are still in service. Most Electrolytic capacitors are rated for 85 degrees Celsius maximum.
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.
A standardized set of capacitor base numbers was devised so that the value of any modern electrolytic capacitor could be derived from multiplying one of the modern conventional base numbers 1.0, 1.5, 2.2, 3.3, 4.7 or 6.8 by a power of ten. Therefore, it is common to find capacitors with values of 10, 15, 22, 33, 47, 68, 100, 220, and so on. Using this method, values ranging from 0.1 to 4700 are common in most applications. Values are generally in microfarads (µF).
Many electrolytic capacitors have a tolerance range of 20 %, meaning that the manufacturer is stating that the actual value of the capacitor lies within 20 % of its labeled value. Selection of the preferred series ensures that any capacitor can be sold as a standard value, within the tolerance. Also many electrolytic caps have asymmetric tolerances, typically -20% but with much larger positive tolerance. This eliminates any need to test and grade individual caps.
Unlike capacitors that use a bulk dielectric made from an intrinsically insulating material, the dielectric in electrolytic capacitors depends on the formation and maintenance of a microscopic metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume, but maintaining the integrity of the dielectric usually requires the steady application of the correct polarity of direct current else the oxide layer will break down and rupture, causing the capacitor to fail. In addition, electrolytic capacitors generally use an internal wet chemistry and they will eventually fail if the water within the capacitor evaporates.
Electrolytic capacitance values are not as tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytics, it is quite common to see an electrolytic capacitor specified as having a "guaranteed minimum value" and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification is acceptable.
As with bulk dielectric capacitors, electrolytic capacitors come in several varieties: