Electrolytic:

In electrolytic type of capacitor the dielectric is a very thin layer of aluminium oxide formed on the plates by a conducting chemical compound when a DC potential is applied. The surface of the plates, which are made of aluminium foil, may be etched to increase the surface area and hence the capacitance value. The large surface area and the very thin dielectric means that a very large capacitance value can be obtained. Another similar type uses tantalum oxide as the dielectric. Tantalums, range from 0.01 to 3000 µF with voltages up to 100 volts, while aluminium types range from 0.1 µF to nearly 1 F with a voltage range of from 3 to 700 volts. The higher the value of capacitance, the lower is the voltage range. In both types a leakage current is essential to maintain the dielectric, and they are generally used on a DC voltage for smoothing and de-coupling. The polarising voltage must also be high enough to keep the leakage current flowing, otherwise the capacitance value will be reduced.