What is a Capacitor?

(Last edited 5/14/2026)

An electrolytic capacitor uses a thin sheet (20–100μm thick) of high-purity aluminum foil (at least 99.99% pure) as its anode.  Smaller, more costly variants may use tantalum for an anode instead. The foil’s surface is chemically etched and then covered with a layer of aluminum oxide, which serves as the dielectric. The etching boosts the effective surface area compared to a smooth foil, thereby raising the capacitance. 

A second piece of aluminum foil functions as the negative electrical plate. Sandwiched between this cathode foil and the oxide-coated anode is a porous paper soaked in a liquid electrolyte; this wetted paper provides the physical path for negative charge. The anode foil, cathode foil, and electrolyte-soaked separator are rolled together and housed inside an aluminum can, which is finally slipped into an insulating sleeve.

To form the initial dielectric oxide layer, a positive voltage is applied to the etched anode foil in an electrolytic bath. This process creates a dielectric oxide layer on the anode foil, with its thickness corresponding to the applied voltage.

Diagram showing the inside of an aluminum electrolytic capacitor

When DC voltage is applied to the capacitor, the dielectric generates an electrical field that produces a polarized charge between the two electrodes. However, the dielectric oxide layer can have imperfections, allowing the possibility of current leakage, known as leakage current, after applying the correct polarity DC voltage. Because the oxide layer is formed by applying voltage to the anode, if capacitors remain unused for extended periods without voltage, the dielectric oxide layer can degrade. The acidic electrolyte reacts with the oxide layer, breaking it down. As this deterioration progresses, leakage current increases when the rated voltage is applied. Excessive leakage current through the capacitor with a degraded dielectric can ultimately lead to capacitor failure.

What Makes a Capacitor, “Super”?

I’ve often seen the bulk capacitors in a PSU called “supercapacitors.” In reality, the bulk caps are simply high-voltage aluminum electrolytics: physically big and offering hundreds of microfarads, but they’re still built on the same basic aluminum-foil-and-oxide principle. Supercapacitors, by contrast, employ electrodes with enormously increased surface area, typically activated carbon, and separate them with an electrolyte-soaked porous layer rather than a true dielectric film.

Supercapacitors also differ in their electrical ratings: they generally provide capacitance in the millifarad to farad range but are limited to low voltages (typically ~2.5-3.0 V per cell, requiring series stacking for higher voltages). That rules them out for switch-mode power supplies, which need capacitors that handle high-voltage and deliver values in the low-to-mid microfarad range. On top of that, most supercapacitors tolerate only 60–70°C (some ruggedized types up to 85°C), whereas power-supply designs in PCs demand capacitors rated at 85–105°C for reliable, long-term operation.