Capacitors come in all shapes and sizes, but nearly all of them function in the same way.
These components, separated by a dielectric material, act as a pair of surfaces that attract and repel electrical charges. At their simplest, they are a pair of flat conductive surfaces separated by air or plastic. We express the governing equation behind a capacitor’s effectiveness this way:
C = εA/d
In this equation, capacitance (C) = permittivity (ε) times surface area (A), divided by distance between the parallel plates (d).
What is an Electrolytic Capacitor?
The larger the surface area and the shorter the distance between anode and cathode, the larger the capacitance of a component will be. While traditional capacitor construction has advanced over the last century, electrolytic capacitors take the idea of a thin dielectric and a large surface volume to an extreme. Instead of attempting to form a thin dielectric barrier by mechanical means, this type of component forms a dielectric directly on the anode through oxidation, AKA rust.
Carefully controlling the oxidation process leads to an extremely thin dielectric, which allows for a low value for the “d” in a capacitor’s governing equation. Oxidation allows the surface area, “A”, to be very high compared to a component’s volume. To contact this non-uniform surface, construct the cathode using either an electrolytic solution or a process whereby a normally solid material can fill in these tiny surfaces. Electrolytic capacitors are polarized, which means that connecting the leads in a voltage orientation opposite the way it was intended can quickly destroy their capacitive properties.
Aluminum Electrolytic Capacitors
Electrolytic capacitors are normally made from one of three different materials: aluminum, tantalum, and niobium. Aluminum is one of three metals manufacturers use for electrolytic capacitors for several reasons:
-Aluminum acts as a so-called “valve” metal, where a positive voltage in an electrolytic bath allows it to form a thin oxide layer that acts as a dielectric.
-The aluminum anode is made from pure aluminum foil, which can form many capacitive layers. Along with this layering, you can etch the aluminum, forming a rough surface for the oxide to form, increasing an effective surface area up to 200 times more than a flat surface.
-Aluminum capacitors can also act as a solid component, using manganese dioxide or a polymer to form a solid cathode instead of a liquid electrolytic solution.
Tantalum Electrolytic Capacitors
Surface-mount devices (SMDs) for use in computing applications account for most tantalum capacitors manufactured today. Tantalum capacitor advantages for manufacturers include:
-Tantalum exhibits many of the same properties as aluminum, most importantly that you can oxidize it to form a thin dielectric layer.
-Unlike aluminum, you don’t form a tantalum capacitor by etching layers of anodes and rolling them up with cathode layers. Instead, you press tantalum powder together on a conductive wire and sinter it. Oxide will form on the surface and within cavities inside this combined material. Like the aluminum etching process, this allows for a very high surface area, and thus a high capacitance relative to its volume.
-Tantalum has a significantly higher permittivity than aluminum and a slightly lower breakdown voltage.
-Tantalum capacitors are used in applications that require extremely high quality, such as military, medical, and space components.
Unfortunately, tantalum is a relatively rare material, meaning that these components tend to be more expensive than those made out aluminum.
Niobium Electrolytic Capacitors
Niobium shares several properties with tantalum and occurs more frequently in nature. While this may seem to make an excellent substitute for expensive tantalum capacitors, only the Soviet Union did much research into niobium-based capacitor manufacturing techniques until the 1990s. We’ve seen an increase in niobium capacitors because:
-A spike in the price of tantalum in the early 2000s helped encourage niobium usage in the West.
-Like tantalum capacitors, niobium anodes include a mass of material formed around a wire conductor. This porous material undergoes oxidization to form a dielectric. Add an electrolytic solution or solid material to act as the cathode to result in a completed capacitor.
-Niobium-based dielectrics exhibit a higher relative permittivity than tantalum components but require increased dielectric thickness for a given voltage rating.
Electrolytic capacitors are only one type of capacitor in use today. For an introduction to the subject of capacitors in general, be sure to check out this article on capacitor basics. If you’d like a hands-on demonstration of how capacitance works, check out our instructions for how to make your own variable capacitor.