Electrolytic capacitors - Basic electronics circuits

The electrolytic capacitor is made of several metals, notably aluminium and tantalum, can have very thin films of their respective oxides formed on the surface when a voltage is applied in the correct polarity (metal positive) between the metal and a slightly acidic liquid.

These very thin films then insulate the metal from the conducting Hquid, the electrolyte, forming a capacitor; an electrolytic capacitor. The name comes from the resemblance to an electrolytic (metal plating) cell.

The most common type of electrolytic capacitor makes use of aluminium foil, which can be etched, dimpled or corrugated to increase the effective area, enclosed in an aluminium can which is filled with a slightly acid solution of ammonium perborate in jelly form.

The capacitor is formed by applying a slowly rising voltage to the capacitor, with the foil positive and the case negative until the voltage reaches its rated level and the DC current falls to a minimum, indicating that the insulation is as good as it is ever likely to be. From then on, when the capacitor is used, it must have a DC (polarizing) voltage applied in the same polarity so as to maintain the insulating film. If the capacitor is used with the voltage reversed, the film will be dissolved, removing any insulation and allowing large currents to pass through the liquid, which will vaporize, destroying the can.

Because of the very fragile nature of the insulating film, which can be only a few atoms thick, electrolytic capacitors are always liable to have a large amount of leakage, so that leakage current at rated voltage is quoted rather than power factor or dissipation factors. Leakage is often related to the capacitance value and working voltage, and the formula:
I leakage = 4 + (0.006 x C x V) where I in µA, C in F and V in volts.

Many manufacturers also quote a life expectancy in excess of 100 000 hours, at 40°G and rated voltage, for electrolytics, since there is still some prejudice against their use for anything other than consumer electronics.

Electrolytic capacitors are used predominantly as reservoir and smoothing capacitors for mains-frequency power supplies, so that their most important parameters, other than capacitance and voltage rating, concern the amount of ripple current that they can pass.