Electrolytic capacitors are small and readily available, but using them wrong could be an explosive failure. Learn how they work and how to use them.
This article is part of The engineer’s complete guide to capacitors. If you’re unsure of what type of capacitor is best for your circuit, read How to choose the right capacitor for any application.
What is an aluminum electrolytic capacitor?
Aluminum electrolytic capacitors, often called electrolytic capacitors, are usually selected because they offer a relatively large capacitance for a relatively small physical size. Aluminum electrolytic capacitors tend to be readily available, and with high voltage values (on the order of 700 V). These polarized capacitors usually have a wide tolerance (± 20%), tend to exhibit large leakage currents (5 to 20 µA per µF), and have low self-resonant frequency (fRES < 100 kHz). Aluminum electrolytic capacitors tend to have larger values of equivalent series resistance (ESR).
How aluminum electrolytic capacitors work
A capacitor is physically created when two conductors are separated by an insulator known as a dielectric. While it may at first appear that an electrolytic capacitor comprises two conductive aluminum foils separated by an insulating fluid, this is not the case, because the electrolyte is ionically conductive and is not an insulating fluid.
The electrochemical action produces a thin oxide film on the aluminum anode. The anode and apparent cathode are etched to increase the plate area. The thin oxide layer serves as the dielectric. The paper separator (spacer) holds the liquid electrolyte. The electrolyte is the actual cathode. The electrode is just the apparent cathode.
If an aluminum electrolytic capacitor has a reverse voltage applied, its capacitance will decrease, its leakage current will increase and the capacitor may explode.
Besides the application of a reverse voltage, overvoltage, AC voltage or capacitor failure, internal gas pressure can be generated at a rate the package cannot contain mechanically. To reduce the risk of violent rupturing of the case itself, aluminum electrolytic capacitors are usually equipped with pressure-relief vent structures that limit the maximum internal pressure. These safety vents are intended to rupture and release the gas pressure. After rupture the capacitor has limited life because its electrolyte evaporates. Vents with blow plugs are usually included at the top of the capacitor to release the pressure during a failure.
Applications of aluminum electrolytic capacitors
Aluminum electrolytic capacitors are used in filter applications like line-operated DC power supplies, DC/DC converters and in DC links. The degradation of the capacitors depends on their operating conditions including temperature, ripple current and the DC bias voltage. These conditions can have a strong influence on capacitor failures. In power converters, the semiconductor and magnetics are robust, and the weakest link is often posed by the aluminum electrolytic capacitors. These capacitors also find applications in energy storage, low-frequency bypassing and coupling in amplifiers with a signal chain operating under 100 kHz.
Alternatives to aluminum electrolytic capacitors
Tantalum electrolytic capacitors offer electrical characteristics that often exceed the performance of aluminum electrolytic capacitors. However, they tend to be more expensive and have longer lead times. When tantalum capacitors are not available, niobium electrolytic capacitors are often the next best option with a maximum voltage of 10 V. Hybrid polymer electrolytic capacitors can also be used in lieu of aluminum electrolytic capacitors, but they tend to be more expensive.
