Aluminum Electrolytic Capacitors

®1 Description of Electric Double LayerCapacitor1-1 Basic ConceptsGenerally capacitors are constructed with a dielectricplaced between opposed electrodes, functioning ascapacitors by accumulating charges in the dielectricmaterial. Aluminum electrolytic and tantalum electrolyticcapacitors, for example, use an aluminum oxidefilm and a tantalum oxide film as the dielectric,respectively.On the other hand, Electric Double Layer Capacitorshave no visible dielectric in a general sense but utilizethe state referred to as the electric double layer, whichis developed naturally on the interface betweensubstances, as the function of dielectric.1-2 Operating PrincipleThe Electric Double Layer represents the state inwhich positive and negative charges exist at a veryshort distance on the boundary where contact occursbetween two different substances (e.g. solid andliquid). By externally applying a voltage below acertain voltage to the boundary, higher charges canbe accumulated. Accordingly, charge and dischargeof electric double layer capacitors utilize adsorptionand desorption of ions to the ionic adsorption layer(Electric Double Layer) formed on the electrodesurface of the activated carbon used for electrodes.ElectrolyteELECTRIC DOUBLELAYER CAPACITORSTECHNICAL NOTEApplying DC voltage externally across the electrodesof the Electric Double Layer allows almost no passageof current up to a certain voltage, exhibiting acondition like insulation.However, the application of voltages exceeding thecertain voltage causes electrolysis to occur in theelectrolyte, resulting in abrupt passage of current.This voltage determines the resistance of voltage ofan Electric Double Layer Capacitor. We use anorganic electrolyte and its standard electrolysis occursat the voltage of about 2.53V.1-3 Advantages and Disadvantages of ElectricDouble Layer Capacitor[Advantages](1) Small size and capacitance in farads (F) availableby utilizing the activated carbon electrode with alarge surface area(2) No special charging circuit and constrains duringdischarge are required.(3) No effect on the life through overcharging andoverdischarging(4) Environmentally clean energy[Disadvantage](1) The life is limited due to the use of electrolyte.(2) Series connection is required when used with alow resistance of voltage at a high voltage.(3) Cannot be used in AC circuits due to high internalresistance unlike aluminum electrolytic capacitors.ActivatedcarbonActivatedcarbonChargeDischargeFig.1 Schematic of Principle of Electric Double Layer CapacitorElectric Double Layer CapacitorsTECHNICAL NOTECAT.No.2010/2011E