electrolytic capacitor is a type of that uses an ionic conducting liquid as one of its plates with a larger capacitance per unit volume than other types. They are valuable in relatively high-current and low-frequency electrical . This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations in output. They are also widely used as coupling capacitors in circuits where should be conducted but should not.

Electrolytic capacitors can have a very high capacitance, allowing filters made with them to have very low .

Aluminum electrolytic capacitors are constructed from two conducting foils, one of which is coated with an insulating layer, and a paper spacer soaked in . The foil insulated by the oxide layer is the while the electrolyte and the second foil act as . This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminium casing. The two most popular geometries are axial leads coming from the center of each circular face of the cylinder, or two radial leads or lugs on one of the circular faces. Both of these are shown in the picture.

In aluminum electrolytic capacitors, the layer of insulating 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 . The aluminum oxide layer can withstand an electric field strength of the order of 10⁹ volts per meter. The combination of high capacitance and high voltage result in high energy density.

Most electrolytic capacitors are polarized and may catastrophically fail if voltage is incorrectly applied. 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 reactions). Following the loss of the dielectric material, the capacitor will , and with sufficient short circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst.This because if the aluminium foil with a layer of aluminium oxide on it is made +ve the oxide ion will get oxidised and will convert into oxygen gas generating a high pressure and hence may burst up the capacitor. This is same as the electrochemical principle in an electrolytes with 2 electrodes.

To minimize the likelihood of a polarized electrolytic being incorrectly inserted into a circuit, polarity is indicated on the capacitor's exterior by a stripe with and possibly arrowheads adjacent to the negative lead or terminal. Also, the negative terminal lead of a radial electrolytic is shorter than the positive lead. On a , 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 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 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. These should not be used in sensitive circuits without a constant bias, since the diode action can cause distortion. They are primarily used in circuits where the bias changes occasionally.

Modern capacitors have a , 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. 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. A constant forward bias is preferable, and will increase the life of the capacitor.

The 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 ; however, this unit has been too large for general use until the invention of the , so , nanofarad and are more commonly used. These are usually abbreviated to μF (or uF), nF, and pF.

Many conditions determine a capacitor's value, such as the thickness of the and the . In the manufacturing process, electrolytic capacitors are made to conform to a set of . By multiplying these base numbers by a , 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.

Aluminum electrolytics have their problems however; noise, high leakage, high temperature drift, high dielectric absorption and high inductance. 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 -25C than at 25C. Most 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 is implicated in various reliability issues as well.

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