Aluminum electrolytic capacitors are made of two aluminum foils and a paper soaked in electrolyte. The anode aluminum foil is anodized to form a very thin oxide layer on one side and the unanodized aluminum acts as cathode; the anode and cathode are separated by paper soaked in electrolyte, as shown in Fig. 8.10A and B.
The aluminum foil which forms the anode of the capacitor will have its surface chemically highly etched to increase its surface area, and therefore its capacitance.
Until the wound construction of aluminum foil capacitors, this type of capacitor was bulky and heavy. There are different sizes of capacitor ranging from 3 mm in diameter for 5 mm in height up to 90 mm for 210 mm .
In non-polar aluminum electrolytic capacitors and motor-start aluminum electrolyte capacitors a second anode foil substitutes for the cathode foil to achieve a non-polar capacitor in a single case. These figures show typical constructions of the non-surface-mount aluminum electrolytic capacitors.
Aluminum electrolytic capacitor construction delivers colossal capacitance because etching the foils can increase surface area more than 100 times and the aluminum-oxide dielectric is less than a micrometer thick. Thus the resulting capacitor has very large plate area and the plates are intensely close together.
Other types of aluminum electrolytic capacitors not cov-ered include the obsolete wet types without separator mem-branes, “hybrid” aluminum electrolytic capacitors containing both polymer and liquid electrolyte components and sol-id-polymer electrolytic capacitors.