Electrolytic capacitors

All of the above-mentioned types of capacitors provide relatively small values of capacitance. They are also nonpolarized, meaning that they can be hooked up in a circuit in either direction. An electrolytic capacitor provides considerably greater capacitance than any of the above types, but it must be connected in the proper direction in a circuit to work right. Therefore, an electrolytic capacitor is a polarized component.
Electrolytic capacitors are made by rolling up aluminum foil strips, separated by paper saturated with an electrolyte liquid. The electrolyte is a conducting solution. When dc flows through the component, the aluminum oxidizes because of the electrolyte. The oxide layer is nonconducting, and forms the dielectric for the capacitor. The layer is extremely thin, and this results in a high capacitance per unit volume.
Electrolytic capacitors can have values up to thousands of microfarads, and some units can handle thousands of volts. These capacitors are most often seen in audiofrequency circuits and in dc power supplies.