page_title: Electrolytic capacitor
section_title: Impedance
hierarchical_section_title: Electrolytic capacitor / Electrical characteristics / Impedance
caption_attribution_description: Impedanzkurven verschiedener Kapazitätswerte aus unterschiedlichen Kondensatorfamilien
context_page_description: An electrolytic capacitor is a polarized capacitor whose anode or positive plate is made of a metal that forms an insulating oxide layer through anodization. This oxide layer acts as the dielectric of the capacitor. A solid, liquid, or gel electrolyte covers the surface of this oxide layer, serving as the cathode or negative plate of the capacitor. Due to their very thin dielectric oxide layer and enlarged anode surface, electrolytic capacitors have a much higher capacitance-voltage product per unit volume than ceramic capacitors or film capacitors, and so can have large capacitance values. There are three families of electrolytic capacitor: aluminum electrolytic capacitors, tantalum electrolytic capacitors, and niobium electrolytic capacitors.
  The large capacitance of electrolytic capacitors makes them particularly suitable for passing or bypassing low-frequency signals, and for storing large amounts of energy. They are widely used for decoupling or noise filtering in power supplies and DC link circuits for variable-frequency drives, for coupling signals between amplifier stages, and storing energy as in a flashlamp.
context_section_description: In general, a capacitor is seen as a storage component for electric energy. But this is only one capacitor function. A capacitor can also act as an AC resistor. Especially aluminum electrolytic capacitors in many applications are used as decoupling capacitors to filter or bypass undesired biased AC frequencies to the ground or for capacitive coupling of audio AC signals. Then the dielectric is used only for blocking DC. For such applications the AC resistance, the impedance, is as important as the capacitance value.
  The impedance Z is the vector sum of reactance and resistance; it describes the phase difference and the ratio of amplitudes between sinusoidally varying voltage and sinusoidally varying current at a given frequency. In this sense impedance is a measure of the ability of the capacitor to pass alternating currents and can be used like Ohm's law.
  In other words, the impedance is a frequency-dependent AC resistance and possesses both magnitude and phase at a particular frequency.
  In data sheets of electrolytic capacitors only the impedance magnitude |Z| is specified, and simply written as "Z". Regarding the IEC/EN 60384-1 standard, the impedance values of electrolytic capacitors are measured and specified at 10 kHz or 100 kHz depending on the capacitance and voltage of the capacitor.
  Besides measuring, the impedance can be calculated using the idealized components of a capacitor's series-equivalent circuit, including an ideal capacitor C, a resistor ESR, and an inductance ESL. In this case the impedance at the angular frequency ω is given by the geometric (complex) addition of ESR, by a capacitive reactance XC
  and by an inductive reactance XL (Inductance)
  .
  Then Z is given by
  .
  In the special case of resonance, in which the both reactive resistances XC and XL have the same value (XC=XL), then the impedance will only be determined by ESR. With frequencies above the resonance the impedance increases again due to the ESL of the capacitor. The capacitor becomes an inductance.
caption: Typical impedance curves for different capacitance values over frequency. The higher the capacitance the lower the resonance frequency.
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