1. Basic Overview of Aluminum Electrolytic Capacitors
1-1. Basic Principle of Capacitors The basic principle of a capacitor can be described by Figure 1-1. When a voltage is applied to two opposing metal electrodes, charge is stored according to the magnitude of the voltage: Q = CV Q: Charge (C) V: Voltage (V) C: Capacitance (F) C: Capacitance of the capacitor, which can be expressed by the electrode area S [m²], dielectric thickness t [m], and relative permittivity ε C[F] = ε₀ × ε × S/t ε₀: Dielectric constant of the dielectric in vacuum (=8.85 x 10⁻¹² F/m) The relative permittivity of aluminum oxide film is 7~8. To obtain a larger capacitance, the surface area S can be increased or its thickness t reduced. Table 1-1 lists the relative permittivity of several typical dielectrics commonly used in capacitors. In many cases, the naming of capacitors is usually determined by the material used in the dielectric, such as aluminum electrolytic capacitors, tantalum capacitors, etc. Dielectric Relative Permittivity Dielectric Relative Permittivity Aluminum Oxide Film 7 ~ 8 Ceramic 10~120 Thin film resin 3.2, polystyrene 2.5, mica 6~8, tantalum oxide film 10~20. Although aluminum electrolytic capacitors are very small, they have relatively large capacitance because the surface area of the electrode foil is expanded after electrochemical corrosion, and its dielectric oxide film is very thin. Figure 1-2 illustrates the basic components of an aluminum electrolytic capacitor. 1-2 Equivalent Circuit of a Capacitor The equivalent circuit diagram of a capacitor can be represented by Figure 2 below. R1: Resistance of electrodes and leads R2: Resistance of anodic oxide film and electrolyte R3: Insulation resistance of damaged anodic oxide film D1: Anodic oxide film with unidirectional conductivity C1: Capacitance of anode foil C2: Capacitance of cathode foil L: Equivalent inductance caused by electrodes and leads 1-3 Basic Electrical Properties 1-3-1 Capacitance The capacitance of a capacitor is determined by the impedance presented when measuring AC capacitance. AC capacitance varies with frequency, voltage, and measurement method. The capacitance of an aluminum electrolytic capacitor decreases with increasing frequency. Similar to frequency, the temperature during measurement has a certain impact on the capacitance of a capacitor. As the measurement temperature decreases, the capacitance decreases. On the other hand, DC capacitance, which can be obtained by measuring its charge by applying a DC voltage, has a slightly larger capacitance than AC capacitance at room temperature and exhibits superior stability characteristics. 1-3-2 Tan δ (Loss Tangent) In the equivalent circuit, the ratio of the series equivalent resistance ESR to the capacitive reactance 1/wC is called Tan δ, and its measurement conditions are the same as for capacitance. tan δ =RESR/ (1/wC) = wC RESR Where: RESR = ESR (120 Hz) w = 2πf f = 120Hz tan δ increases with increasing measurement frequency and increases with decreasing measurement temperature. Impedance (Z): At a specific frequency, the resistance that impedes the passage of alternating current is called impedance (Z). It is closely related to capacitance and inductance, and also to the equivalent series resistance ESR. The specific expression is as follows: Where: Xc = 1/ wC = 1/ 2πfC XL=wL=2πfL Leakage current: The dielectric of a capacitor has a significant impedance to direct current. However, because the aluminum oxide film is impregnated with electrolyte, a small current called leakage current is generated when voltage is applied, during the reformation and repair of the oxide film. The leakage current is relatively large when voltage is first applied, but it gradually decreases and eventually stabilizes over time.Read next
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2026-02-22
