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Capacitors, Part 7 Electrolytic Capacitors [2]

■Characteristics of aluminum electrolytic capacitors

Wet type aluminum electrolytic capacitors are widely used because they offer high capacitance and are inexpensive. However, compared to other capacitor types, they have the following characteristics which need to be carefully considered when designing applications.

● Limited service life
Drying (evaporation) of the electrolyte causes a drop in capacitance, also known as capacitance loss. The service life is commonly considered to be about 10 years. Electrolyte leaks can also cause a drop in circuit insulation and other problems.
Arrhenius law (doubling for every 10ºC)
The degree of loss in electrolyte is related to the temperature, roughly in accordance with the so-called Arrhenius law or equation for the temperature dependent speed of chemical reactions. This states that for every rise in usage temperature by 10 degrees centigrade, the service life is shortened to one half, and conversely it is doubled for every 10 degree drop.
Expected service life of electrolytic capacitors according to Arrhenius equation
● Electrolytic capacitors have polarity
When a voltage with opposite polarity is applied, internal temperature will rise and gas will be produced which raises internal pressure and can lead to destruction of the capacitor.
● Ripple current causes inherent temperature rise
Aluminum electrolytic capacitors have large ESR (equivalent series resistance) which leads to high thermal losses when subject to ripple current. The resulting rise in inherent temperature can shorten the life of the capacitor.
● Self-healing function of oxide layer
The oxide film anode used as dielectric in the aluminum electrolytic capacitor can be damaged by the application of opposite polarity voltage or by voltage exceeding the rated value. The electrolyte has both acid and basic components. The oxidation effect of the acid component causes healing of the oxide layer, a phenomenon that is referred to as self-healing.
● Capacitance and ESR are highly temperature dependent
As the graphs below demonstrate, the capacitance variation is larger at low temperatures, and the ESR value also is high.
Temperature vs. capacitance trend / Frequency vs. capacitance trend

■TDK (EPCOS brand) aluminum electrolytic capacitor types and major applications

TDK (EPCOS brand) aluminum electrolytic capacitor types and major applications
Various types of TDK (EPCOS brand) aluminum electrolytic capacitors
Electronics ABC