In this type of capacitors the dielectric consists of an extremely thin film of aluminium oxide formed on one of its aluminium foil plates. Intimate contact with the other plates is achieved by impregnating the paper between the foils between electrolytes in the form of viscous substance, such as ammonium borate. The sandwich is then rolled into cylindrical element and housed in either plastic or ceramic, metallic cardboard, protective tube. It is divided into:
(a) Plain foil type electrolytic capacitors
(b) Etched foil type capacitor
(c) Tantalum electrolytic capacitor
Plain foil type electrolytic capacitor: A thin film of aluminium oxide is deposited on the surface of aluminium plate. Aluminium electrode is placed in a solution of ammonium borate and a constant voltage is applied. Initially current is large. It drops gradually as the film grows. The thickness of the film depends upon the forming voltage. Working voltage is about 90% of forming voltage. A thickness film is formed at about 600V. Thus maximum working is 550V.
Construction: Film of aluminium oxide 0.01 cm thick is coated on both sides of aluminium foil. Two strips of aluminium foil are used separated by two layers of porous paper soaked with electrolyte. It is rolled; ends closed with wax and sealed in a container.
Etched foil type capacitors: The construction is similar to plain foil type capacitor but anode is mildly etched before forming. This increases the anode surface area.
Range of capacitance up to 6μf/cm2 for aluminium foil capacitors 50μf/cm2
For capacitors for etched foil capacitors
Power factor 2 to 4%
Working voltage max 550V
Variation in capacitance 20 to + 50% of nominal value
Advantages: High capacitance volume ratio with reasonably low cost.
Disadvantages: High power factor, large variation in capacitance, small safety factor, and high leakage current.
Application: Filtering, decoupling and bypass.
Tantalum electrolytic capacitor: Pellet type tantalum powder is pressed in vacuum and then mounted inside silver cup containing sulphuric acid as electrolyte.
Foil type: Construction is similar to that of aluminium foil type capacitors. These capacitors do not need reforming. They are made in polarized and non-polarized types.
Solid electrolyte type: this uses solid semiconductor in place of a liquid electrolyte. The pellets are pressed and sintered. They are then impregnated with aqueous solution of Magnus salt. A carbon compound is applied to allow metal cathode to be formed.
Range of capacitance 200μf
Working voltage 6V to 150V
Power factor at 50 Hz 3 to 12%
Maximum operating temperature 700C
Leakage current < 10μf
Applications: These are used as decoupling capacitors with transistors, and integrated circuits.