page_title: Aluminum electrolytic capacitor
section_title: Reverse voltage
hierarchical_section_title: Aluminum electrolytic capacitor / Electrical parameters / Reverse voltage
caption_attribution_description: English: A failed electrolytic from a broken LCD monitor. The power supply failed and overheated the cap causing explosion.
context_page_description: Aluminium electrolytic capacitors are polarized electrolytic capacitors whose anode electrode is made of a pure aluminum foil with an etched surface. The aluminum forms a very thin insulating layer of aluminium oxide by anodization that acts as the dielectric of the capacitor. A non-solid electrolyte covers the rough surface of the oxide layer, serving in principle as the second electrode of the capacitor. A second aluminum foil called “cathode foil” contacts the electrolyte and serves as the electrical connection to the negative terminal of the capacitor.
  Aluminum electrolytic capacitors are divided into three subfamilies by the type of electrolyte:
  non-solid aluminum electrolytic capacitors,
  solid manganese dioxide aluminum electrolytic capacitors, and
  solid polymer aluminium electrolytic capacitors.
  Aluminum electrolytic capacitors with non-solid electrolyte are the most inexpensive type and also those with widest range of sizes, capacitance and voltage values. They are made with capacitance values from 0.1 µF up to 2,700,000 µF, and rated voltages values from 4 V up to 630 V. The liquid electrolyte provides oxygen for re-forming or self-healing of the dielectric oxide layer.
context_section_description: Electrolytic capacitors are polarized capacitors and generally require an anode electrode voltage to be positive relative to the cathode voltage. However, the cathode foil of aluminum electrolytic capacitors is provided with a very thin, natural air-originated oxide layer. This oxide layer has a voltage proof of approximately 1 to 1.5 V. Therefore, aluminum electrolytic capacitors with non-solid electrolyte can continuously withstand a very small reverse voltage and, for example, can be measured with an AC voltage of about 0.5 V, as specified in relevant standards.
  At a reverse voltage lower than −1.5 V at room temperature, the cathode aluminum foil begins to build up an oxide layer corresponding to the applied voltage. This is aligned with generating hydrogen gas with increasing pressure. At the same time the oxide layer on the anode foil begins dissolution of the oxide, which weakens the voltage proof. It is now a question of the outside circuit whether the increasing gas pressure from oxidization leads to bursting of the case, or the weakened anode oxide leads to a breakdown with a short circuit. If the outside circuit is high-ohmic the capacitor fails and the vent opens due to high gas pressure. If the outside circuit is low-ohmic, an internal short-circuit is more likely. In every case a reverse voltage lower than −1.5 V at room temperature may cause the component to catastrophically fail due to a dielectric breakdown or overpressure, which causes the capacitor to burst, often in a spectacularly dramatic fashion. Modern electrolytic capacitors have a safety vent that is typically either a scored section of the case or a specially designed end seal to vent the hot gas/liquid, but ruptures can still be dramatic.
  To minimize the likelihood of a polarized electrolytic being incorrectly inserted into a circuit, polarity has to be very clearly indicated on the case, see the section headed "Polarity marking".
  Special bipolar capacitors designed for AC operation, usually referred to as "bipolar", "non-polarized" or "NP" types, are available. In these, the capacitors have two anode foils of opposite polarity connected in series. On each of the alternate halves of the AC cycle, one anode acts as a blocking dielectric, preventing reverse voltage from damaging the opposite anode. The voltage rating doesn't need to be symmetrical; "semi-polar" capacitors can be made with different thicknesses of oxide coatings, so they can withstand different voltages in each direction.  But these bipolar electrolytic capacitors are not adaptable for main AC applications instead of power capacitors with metallized polymer film or paper dielectric.
caption: An exploded electrolytic capacitor on a PCB
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