Electrolytic capacitors are can-shaped components that store and filter electric charges in many kinds of electronic devices. If you open a computer’s case and see capacitors with their sides and top bulging, these components have failed. Excess voltage in the circuits can make chemical compounds in the capacitors deteriorate and expand, swelling and breaking them open.
A capacitor contains two plates made of metal foil, rolled up tightly, with a layer of an electrical insulator between them. The insulating material, called a dielectric, varies with different kinds of capacitors. An electrolytic capacitor uses a chemical paste that forms a thin coating of aluminum oxide on the plates. Under normal conditions, the aluminum oxide stores electric charges between the plates but prevents current from moving between them. If the chemicals break down or are poorly made, the insulator turns into a conductor, and the capacitor short-circuits, damaging itself and surrounding electronic components.
A thin-walled metal case contains a capacitor’s chemicals. It has a hollow cylindrical shape with an open bottom. At the bottom, a plastic plug seals the chemicals and the capacitor’s electrical leads. If the component develops a problem, hydrogen gas accumulates in the can, eventually causing the sides to bulge. Pressure may force some of the chemicals past the plastic seal.
Because of their chemical nature, electrolytic capacitors have lifetimes limited to about 10 to 20 years. After this, the chemical paste dries out, and the component loses its ability to hold a charge. If a device sits unused for many years, the capacitors can dry out. Later, when you turn the equipment on, the electric current produces heat and chemical reactions in the capacitor, making it bulge.
Modern electrolytic capacitors have X-shaped score lines cut into the top of the metal case. If pressure builds in the capacitor, the component will break safely and gradually at this weak spot, relieving the pressure, though not saving the component. Without the score lines, pressure continues to build in the capacitor and can cause it to explode or burn violently.
A capacitor’s dielectric has a maximum voltage called the breakdown voltage. Past this point, the electrons have so much energy they can jump across or burn through the insulating layer between plates. Every electrolytic capacitor has a breakdown voltage rating; electronic designers know this and use capacitors only in circuits where the voltage levels suit the parts. An electrolytic capacitor’s chemicals boil and expand when exposed to excessive voltage. This causes the whole capacitor to bulge.