Electrolytic Capacitors
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An aluminum electrolytic capacitor obtains a large capacitance from a very thin oxide dielectric formed on aluminum foil. The electrolyte provides contact to the oxide system, and the foils are usually wound into a compact cylindrical package. This construction gives a great deal of capacitance for its size and cost.
Polarity
Most aluminum electrolytic capacitors are polarized. The negative terminal is commonly identified by a stripe on the sleeve, while the positive lead may be longer on a new radial part. The circuit-board marking and the manufacturer's datasheet should always be checked rather than relying on one visual clue.
Where electrolytics are useful
- Bulk filtering after a rectifier.
- Energy storage on DC power rails.
- Low-frequency coupling and bypassing.
- Motor drives and power converters.
- Audio amplifiers and power supplies.
ESR and ripple current
The internal foil, electrolyte, contacts, and dielectric losses appear partly as equivalent series resistance (ESR). AC ripple current flowing through ESR produces heat. For power-supply applications the ripple-current rating can be just as important as capacitance and voltage.
Life and temperature
Electrolytic capacitors age as electrolyte and sealing systems change over time. High temperature and ripple heating generally shorten useful life. Datasheets often specify endurance at a particular temperature and rated conditions, with application guidance for estimating life at lower temperatures.
Leakage and reforming
A real electrolytic capacitor passes a small DC leakage current. Long storage, temperature, capacitance, voltage rating, and construction affect leakage. Very old equipment may contain electrolytics whose oxide dielectric has deteriorated; service procedures for vintage equipment sometimes use controlled current-limited reforming, but questionable capacitors are often safer and more reliable to replace.
Bipolar electrolytics
Special non-polar or bipolar electrolytic capacitors are made for AC situations such as some loudspeaker crossovers and motor applications. They are not the same as ordinary polarized electrolytics simply connected backward.
Selection checklist
- Capacitance and tolerance.
- Working voltage with adequate margin.
- Ripple-current rating at the actual frequency and temperature.
- ESR or impedance requirements.
- Temperature rating and expected service life.
- Physical size, lead spacing, mounting, and vent clearance.