Capacitors are one of the most common electronic components and are available in a wide variety of types of capacitors. Each type of capacitor has a set of characteristics and properties that make them suitable for certain applications, environments, and products. Capacitors are typically categorized by their form factor and the dielectric material used in the capacitor. Each type of capacitor has significant differences in typical and available values for capacitance tolerance, voltage rating, temperature stability, equivalent series resistance (ESR), size, and reliability that effect how they behave in the real world. These differences have an impact on capacitor selection, making some capacitors great in some applications and a source of trouble in others.
Film capacitors are one of the more common types of capacitors. Film capacitors include a large family of capacitors with the main difference being the dielectric materials used. Common materials used include polyester (mylar), polystyrene, polypropylene polycarbonate, metalized paper and teflon. Film capacitors are available in values from pF (picoFarads) up to 100's of uF (microFarads). High voltage film capacitors are also available, with voltage ratings exceeding 500 volts. The advantage of film capacitors, especially the film capacitors that use plastic films, are long life and very stable capacitance values.
Film capacitors are available in several package sizes and form factors. The most common form factors for film capacitors are cylindrical, oval, round, and rectangular and most form factors are available with axial and radial style leads.
Electrolytic capacitors have some of the highest capacitance values of any type of capacitors. Electrolytic capacitors are constructed with thin metallic films and an electrolytic semi-liquid solution. The flexibility of these materials allows them to be rolled up and provide a large surface area and therefore help to create a large capacitance. Since the electrolytic solution is conductive and used as the second electrode in an electrolytic capacitor, a thin dielectric oxide layer is grown on the metallic film, to prevent the metallic film from shorting to the electrolytic solution. The dielectric film is very thin which greatly increases the capacitance of an electrolytic capacitor.
Electrolytic capacitors do come with a couple of key limitations, polarization and voltage ratings. The downside of electrolytic capacitors are that most of them are polarized and care must be taken to ensure they are used correctly. Placing an electrolytic capacitor in backwards will result in a very rapid destruction of the capacitor, often violently with the potential to cause damage to anything nearby. All polarized electrolytic capacitors have their polarity marked on them with a negative sign which indicates which pin must be kept at the lowest electrical potential. The voltage rating of most electrolytic capacitors is low, but they can be found with voltage ratings up to several hundred volts.
The two most common types of electrolytic capacitors are aluminum electrolytic capacitors and tantalum capacitors. Tantalum capacitors differ from most electrolytic capacitors in that they look more like ceramic capacitors. Unlike ceramic capacitors, tantalum capacitors are polarized. However, tantalum capacitors are much more resilient for reversed polarities than aluminum electrolytic capacitors and are sometimes placed in series with both negative terminals connected to form a "non-polarized" tantalum capacitor. Tantalum capacitors are much smaller than aluminum electrolytic capacitors and have lower leakage currents which make them a better choice for many signal blocking, by-passing, decoupling, filtering, and timing applications.
Ceramic capacitors are some of the most common capacitors used, especially in surface mount applications. They are made by coating a ceramic disc or plate with a conductor and connecting several together. The ceramic used has a very high dielectric constant, which lets ceramic capacitors to have a relatively high capacitance value in a small size. Unlike electrolytic capacitors, ceramic capacitors are not polarized but their capacitance goes through a non-linear shift as their temperature changes. For these reasons, ceramic capacitors are often used as decoupling or by-pass capacitors. Ceramic capacitors are available in values ranging from a few pF to several uF and have voltage ratings from a few volts to tens of thousands of volts.
Other Types of Capacitors
Several specialty types of capacitors are available for more specialized applications. Trimmer or variable capacitors are capacitors with adjustable capacitance and are useful for fine tuning or compensation in the circuit. Ultra capacitors are capacitors with very high capacitance values, typically with capacitance greater than one farad. They are often low voltage but store enough energy to replace batteries in certain applications.