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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. 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 Telephone s. 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 Sign s 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. 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. ELECTRICAL BEHAVIOR OF ELECTROLYTICS A common modelling circuit for an electrolytic capacitor has the following Schematic : where Rleakage is the leakage resistance, RESR is the Equivalent Series Resistance , LESL the Equivalent Series Inductance (L being the conventional symbol for inductance). 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 all the effects associated with real electrolytic capacitors. Since the electrolytes evaporate, Design Life is most often rated in Hour s 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. 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:
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