The dielectric in aluminum electrolytic capacitors is an electrochemically “formed” so called γ′-Aluminum oxide as show in Fig. 5. One important property of an Aluminum Electrolytic Capacitor is the leakage current flowing thru the dielectric when an DC voltage is applied.
An aluminum electrolytic capacitor consists of cathode aluminum foil, capacitor paper (separator), electrolyte, and an aluminum oxide film, which acts as the dielectric, formed on the anode foil surface. A very thin oxide film formed by electrolytic oxidation (formation) offers superior dielectric constant and has rectifying properties.
Equation (1) shows that the capacitance (C) increases as the dielectric constant (ε) and/or its surface area (S) increases and/or the dielectric thickness (d) decreases. An aluminum electrolytic capacitor comprises a dielectric layer of aluminum oxide (Al 2 O 3 ), the dielectric constant (ε) of which is 8 to 10.
The electrolytic capacitor differs from other types of capacitors in that only one of its surfaces is a metallic plate whereas the other is a chemical solution, the so-called electrolyte. The dielectric medium is a thin layer of aluminum oxide prepared on aluminum foils.
When DC voltage is applied, leakage current flows through the capacitors. The leakage current of aluminum electrolytic capacitors is larger than other types of capacitors; Leakage current changes according to the temperature, applied voltage and application time.
Aluminum electrolytic capacitors with non-solid electrolytes normally can be charged up to the rated voltage without any current limitation. This property is a result of the limited ion movability in the liquid electrolyte, which slows down the voltage ramp across the dielectric, and the capacitor's ESR.
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As to the basic construction principles of electrolytic capacitors, there are three different types: aluminium, tantalum, and niobium capacitors. Each of these three capacitor families uses non-solid and solid manganese dioxide or solid polymer electrolytes, so a great spread of different combinations of anode material and solid or non-solid electrolytes is available.