An aluminum electrolytic capacitor is made by sandwiching a paper separator soaked in electrolyte between two strips of etched aluminum foil, as shown in Fig. 1-15. The paper spacer prevents a short circuit between the cathode and anode foils. The layers of materials are wound in jelly-roll fashion and inserted in an aluminum case. External connections are made from the electrodes to the outside terminals of the case. Direct current is passed through the terminals of the capacitor, causing a thin dielectric layer of aluminum oxide to form on the anode. The electrolyte in contact with the metal foil is the cathode. A plus sign marks the positive terminal of an aluminum electrolytic capacitor.
These capacitors offer high CV ratios and are low in cost. but they exhibit high DC leakage and low insulation resistance. They also have limited shelf lives, and their capacitance values deteriorate with time. Standard units are available in radial- or axial-leaded cases in a wide range of sizes and values. The most commonly specified values are between 4.7 and 2200 μF with working voltages up to 50 VDC. These capacitors are polarized, and this property must be observed when connecting the capacitor in a circuit or it will be destroyed,
Figure 1-15 Aluminum electrolytic capacitor.
Nonpolarized aluminum electrolytic capacitors are available for use in AC circuits for such applications as speaker crossovers and audio filtering. Two polarized capacitors are placed in series with their cathode terminals connected. The anode terminals form the external circuit connections, and the cathode terminals are isolated from the external circuit by an insulator. These capacitors are rated from 1 to 10 μF with maximum working voltages of 50 VDC.
Electrolytic capacitors are specified where high values of capacitance are required in the least amount of space (high volumetric efficiency). This property is called high CV ratio. They are formed by electrochemical processes in which oxide dielectrics are grown in and on porous aluminum and tantalum foil and pellets. The metal foils are acid etched to make them porous, increasing their effective exposed areas from 6 to 20 times. High CV ratios are made possible by the thin oxide layers formed on the plates of the capacitors. The pellets are also made so that they are porous or spongelike and have large exposed surfaces. However, electrolytic capacitors have higher leakage current than electrostatic capacitors because of the impurities embedded in the foil and the electrolyte. This current increases with temperature while voltage breakdown decreases with temperature. Electrolytic capacitors also have higher power factors than electrostatic capacitors, causing losses called equivalent series resistance (ESR).
