Electrolytic capacitor

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An electrolytic capacitor is a type of capacitor with a larger capacitance per unit volume than other types, making them 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, and especially in the absence of rechargeable batteries that can provide similar low-frequency current capacity. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not; the large value of the capacitance allows them to pass very low frequencies without carrying DC.

1 Construction
2 Polarity
3 Safety
4 Electrical behavior of electrolytics
5 Variants
6 See also
7 External links
8 References

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Construction

Aluminium 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 act as cathode. 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.

Tantalum capacitors are more expensive than aluminum-based capacitors, and generally only usable at low voltage, but they have much higher capacitance per unit volume and thus are popular in miniature applications such as cellular telephones.

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Polarity

Electrolytic capacitors have a polarity, unlike most capacitors. This is due to the fact that the aluminum oxide layer is held in place by the electric field, and when reverse-biased, it dissolves into the electrolyte. This allows a short circuit between the electrolyte and the aluminum. The liquid heats up and the capacitor may explode. The aluminium oxide layer is the dielectric, and the thinness of this layer, along with its ability to withstand an electric field strength of the order of 10⁹ volts per metre, is what produces the high capacitance. Modern capacitors have a safety valve on one circular face to vent the hot gas/liquid, but the rupture is still loud. The correct polarity is indicated on the packaging by a stripe with minus signs and possibly arrowheads, indicating the lead that should be more negative than the other.

This is the only reason for the polarity requirement. Electrolytics will behave like any other capacitor if reverse biased, up to the point that they are destroyed. Most survive with no DC bias or with only AC, and can even withstand a reverse bias for a period of time, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time. A constant forward bias also increases the life of the capacitors.

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Safety

The electrolyte is usually boric acid or sodium borate in water with some sugars or ethylene glycol added to retard evaporation. While you should not eat this, nor get it in your eyes, it is not very corrosive or dangerous. Simply wash it off your skin after coming into contact with it. It is important, however, to always be careful and the wearing of safety glasses is always advised. Wet-slug tantalum electrolytics contain sulfuric acid.

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Electrical behavior of electrolytics

A common modelling circuit for an electrolytic capacitor has the following schematic:

Image:Electrolytic capacitor model.png

where R_leakage is the leakage resistance, R_ESR is the equivalent series resistance, L_ESL the equivalent series inductance (L being the conventional symbol for inductance).

R_ESR 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 R_ESR. 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 all the 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.

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Variants

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 as 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 1,000,000 µF 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. Over a long time, the liquid can dry out, causing the capacitor to fail. Bipolar electrolytics contain two capacitors connected in series opposition and are used for coupling AC signals.

Tantalum: compact, low-voltage devices up to about 100 µF, these have a lower energy density and are more accurate than aluminum electrolytics. Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance and little DC leakage, and very low impedance at low frequencies. However, unlike aluminum electrolytics, they are intolerant of voltage spikes and are destroyed (often exploding violently) if connected backwards or exposed to spikes above their voltage rating. Tantalum capacitors are also polarized because of their dissimilar electrodes. The cathode 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 anode electrode is formed 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[1].

Electrolytic double-layer capacitors (EDLCs), also known as Supercapacitors or Ultracapacitors, have very high capacitance values but low voltage ratings. They use a molecule-thin layer of electrolyte, rather than a manufactured sheet of material, as the dielectric. As the energy stored is inversely proportional to the thickness of the dielectric, these capacitors have an extremely high energy density. The electrodes are made of activated carbon, which has a high surface area per unit volume, further increasing the capacitor's energy density.
Individual EDLCs can have capacitances of hundreds or even thousands of farads. For example, the Korean company NessCap offers units up to 5000 farads (5 kF) at 2.7 V, useful for electric vehicles and solar energy applications. Smaller units (in the 0.1 F – 10 F range) are frequently used instead of (or in addition to) batteries to supply standby power to memory circuits and clocks.
The electrodes for EDLCS could also be made by transition metal oxides, eg. RuO₂, IrO₂, NiO, etc. Electrodes made by metal oxides store the charges by two mechanism: double layer effect, the same with active carbon, and pseudocapacitance, which can store more energy than double layer effects.
Aerogel capacitors, using carbon aerogel to attain immense electrode surface area, can attain huge values, up to thousands of farads. EDLCs can be used as replacements for batteries in applications where a high discharge current is required, e.g. in electrically powered vehicles. They can also be recharged hundreds of thousands of times, unlike conventional batteries which last for only a few hundred or thousand recharge cycles. However, capacitor voltage drops faster than battery voltage during discharge, so a DC-to-DC converter may be used to maintain voltage and to make more of the energy stored in the capacitor usable.