electrolytic capacitor
A capacitor that uses an electrolyte as one of its capacitative plates.
electrolytic capacitor: high capacity, polarized, and prone to failure
An electrolytic capacitor achieves high capacitance in a small volume by using a thin oxide layer as its dielectric, with an electrolyte (a chemical paste or liquid) serving as one of the two conducting plates. The other plate is typically aluminum or tantalum foil. This asymmetry means electrolytic capacitors are polarized: they must be installed with correct polarity, or the oxide layer breaks down and the capacitor fails, often explosively.
The oxide dielectric is only nanometers thick, which is why these capacitors pack 1 to 10,000 microfarads into packages smaller than a AA battery. A comparable film capacitor would be the size of a brick. This makes electrolytics essential in power supplies, audio amplifiers, and any circuit that must filter or store moderate amounts of charge. Aluminum electrolytics are cheap and common; tantalum electrolytics offer better stability and reliability but cost more and are less forgiving of overvoltage.
Electrolytic capacitors degrade steadily during use. The electrolyte dries out or chemically degrades, raising the equivalent series resistance (ESR) and reducing capacitance. Heat accelerates this: a rule of thumb is that capacitor life halves for every 10 degrees Celsius rise in operating temperature. A capacitor rated for 105°C at 2000 hours might last only 500 hours at 125°C. Ripple current also heats the capacitor internally, and exceeding the rated ripple current is a common cause of premature failure in power supplies.
Reverse voltage, even briefly, oxidizes the electrolyte and creates a short circuit. Overvoltage spikes rupture the oxide layer. Once degraded, the capacitor exhibits leakage current, bulges at the case (a visible warning sign), and eventually vents or explodes. For this reason, electrolytics are replaced routinely in aging equipment, especially in high-ripple-current applications.
Despite their limitations, electrolytics remain the workhorse of analog circuits because no other technology offers the same combination of capacitance, cost, and size. Modern designs specify them conservatively: using 80 percent of rated voltage, keeping temperatures below 85°C, and selecting units rated for sufficient ripple current. In high-reliability applications, they are paired with film capacitors to handle voltage spikes and extend overall circuit life.