The Two Types of Capacitors

Capacitors come in a variety of shapes, but there are basically two types: electrolytic and non-electrolytic.

Capacitors are electronic components that store a charge. They may be canisters, discs or block-shaped, but they always have two wires. Capacitors consist of two large-surface conductors, called plates, separated by an insulator, called a dielectric. Capacitors can function in a variety of applications. There are two types of capacitors: electrolytic capacitors, through which current must flow in one direction only; and non-electrolytic capacitors, through which current can flow either way.

Non-Electrolytic Capacitors

Capacitors are actually breaks in a circuit, but because of the large area of the plates and the thin separation of insulator between them they have some interesting and useful characteristics. A voltage applied to one plate will stack up electrons on that plate, which then repulses electrons from the other plate. This means that the capacitor stores a charge. When the current is removed, the stored charge gradually leaks away. Capacitors also have the ability to pass high-frequency signals while restricting the flow of direct current or lower-frequency signals.

Electrolytic Capacitors

In electrolytic capacitors, one plate is an electrolyte -- a liquid that conducts electricity. To increase capacitance, manufacturers experimented with making the dielectric as thin as possible. One technique was to grow the dielectric as an oxide on the surface of a liquid or foam. When current first flows through the capacitor, it causes a thin oxide film to form on the surface of the electrolyte. The oxide is an insulator, so the resulting device is a capacitor formed of a plate (the electrolyte), a dielectric (the oxide), and another plate. Electrolytic capacitors have the largest capacity of any type of capacitor, but they do have problems. One is that the current through the capacitor must flow in one direction only. Reverse flow breaks down the oxide film and ruins the capacitor.

Tantalum Capacitors

The newest type of electrolytic capacitor is the tantalum capacitor. The tantalum is made into a foam--with lots of nooks and crannies. Tantalum forms an oxide on its surface. With the enormous surface area of the foam plus the thinness of the oxide layer, tantalum capacitors have the highest capacitance of any capacitor.

Applications

Capacitors are used for a variety of applications. In conjunction with a coil, a capacitor helps change the 12 volts of an automobile battery to the 10,000-volt spike needed for the spark plug. Capacitors are an integral part of the contact buttons used to call elevators -- when you touch the button, it changes the capacitance. Electrolytic capacitors are used where high capacitance is needed but the current is strictly DC, such as in power supplies. Non-electrolytic capacitors must be used whenever the current frequently reverses direction, such as in radio frequency circuits.

References

• TheElectronicsClub: Capacitors
• WilliamsonLabs: Capacitors
• PicList: Capacitors

Resources

• Rice: Basic Electronics
• IonCapa: Capacitors
• RadioElectronics: Capacitor Types