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How does an electrolytic capacitor work?
Ultracap-Knowledge | Rainer Hake | reading time: 4 minutes
Structure & function of an eCap
Electrolytic capacitors belong to the group of electro-chemical capacitors. As is the case for all capacitors, the capacitance increases with the value of the electrode surface A and the dielectric constant ε and decreases with a higher distance of d.
ULTRACAPs are related to electrolytic capacitors much like cousins in a family. They are similar in principle, but in many aspects they are very different. Due to their higher insulation resistance, polarised electrolytic capacitors can reach voltage values of several hundred volts, although the capacitance of ULTRACAPs is millions of times bigger. Unlike ULTRACAPs, electrolytic capacitors are destroyed (burst) immediately if their polarity is reversed.
A large surface equals large specific capacitance
Anode metal is the basic material used for electrolytic capacitors, which consists of aluminium foil or a polymer in aluminium electrolytic capacitors. Both form a roughened anode, with a significantly larger surface area than a smooth surface would have. This increased surface area is an important factor in the relatively high specific capacitance of electrolytic capacitors compared to other capacitor families. Due to the high dielectric strength of the oxidation layer, the dielectric can be extremely thin. This very thin layer is the second important factor for the relatively high specific capacitance of the electrolytic capacitors.
Multiple quality features of electrolytic capacitors
The basic technology of electrolytic capacitors has been known for decades. A detailed description is given in Wikipedia. It should be borne in mind, however, that an incredible amount of expertise has gone into the development of these complex components. Developers are often surprised to find that a circuit has suddenly stopped working after a few months and the electrolytic capacitor no longer does what it’s supposed to. One of the reasons for this could be that the capacitor has dried out, which dramatically reduces the capacitance. Incorrect formation during production can also cause problems.
In fact, there are a large number of factors affecting quality and value for money that can accidentally be overlooked. One thing is clear: complex electrical assemblies can only be developed and marketed successfully if customers can be sure that dependable technology is used and manufacturing is performed to strict quality standards. And this is the case with our technology partners.
In general, electrolytic capacitors are always polarized, i.e. they can only be used for DC voltage. When an AC voltage is applied or the voltage source is incorrectly polarized, the insulating oxide layer is destroyed, the electrolyte evaporates and the capacitor bursts open. In many applications, however, there is a need for a higher capacitance, which cannot be sensibly achieved with the classic bipolar capacitors (polyester, polypropylene capacitors) (installation space volume). For this reason, bipolar capacitors have been developed, which in principle consist of two capacitors connected in series.
Bipolar electrolytic capacitors are often used in crossovers and as coupling capacitors in amplifier circuits where the polarity is not clear. The current load is rather secondary in these applications, but the filter function or signal quality is more the criterion. What is important here is a high capacity and small design, rather than the load capacity. One would also like to have the lowest possible AC resistance (reactance).
The capacity of the first capacitor is formed by the inner aluminium foil (cathode) and the electrolyte. The oxide layer on the aluminium foil is the dielectric. The second capacitor is formed by the electrolyte and the outer aluminium foil (anode). The oxide layer on the aluminium foil is also the dielectric. The outer aluminium foil is therefore not only used for contacting, but is a capacitor coating in its own right.
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