Capacitors are passive two-terminal electrical devices that store energy electrostatically in electric fields. They have numerous forms but all their kinds contain at least two electrical conductor materials that are separated by dielectric insulator elements. The most common type of capacitor is made of two metal foils separated by a thin layer of insulating film. Capacitors are used in virtually all electrical circuits that are found in the electrical devices we use in our day-to-day activities. The principle behind their operation is the creation of a potential difference across them that develop electric fields across the dielectric insulator material, leading to positive charges collecting on one plate and negative ones on the other plate. They store energy in the resultant electrostatic field, and good capacitors have single constant capacitance values. Capacitance is the ratio of the electric charge on each conductor to the potential difference between them, and its SI unit is the Farad or coulomb per volt.
Super capacitors like the electric double layers capacitors that are also known as electrochemical double layer capacitors (EDLCs) or ultra-capacitors are currently in production. EDLCs are electrochemical capacitors that have an unusually high energy densities compared to common types of capacitors, in most cases several orders of magnitude greater than high capacity electrolytic capacitors. They include dielectric spacers which boost their capacitance, the dielectric spacers are the most common insulators. However, low capacitance devices are available with vacuums between their plates that allow extremely high voltage operations and minimal losses.
Variable capacitors that had plates open to the atmosphere were long used in radio tuners circuits, but nowadays designs have been developed that use polymer foil dielectric materials between the moving and stationary plates but have no significant air space between them. These capacitor designs have maximized the charges they hold by using dielectric materials that have high permittivity and high breakdown voltages.
In earlier capacitor designs paper was used as the solid dielectric materials because it offered relatively high voltage performance, but they were susceptible to water absorption and have since been replaced with plastic films. Nowadays, solid dielectric materials range from plastic, mica, glass to ceramic materials. Plastics offer better stability, live longer and do not lose their efficiency with time, thus becoming the most preferred in timer circuits though they have the disadvantage of not being able to operate in high temperatures and frequencies. Ceramic are relatively inexpensive, small in sizes and work well in high frequency applications, though they lose their efficiency with time and their capacitance varies strongly with voltage.
Aluminum and Tantalum plates with oxide dielectric layers are now being used in electrolytic capacitors. Their other electrodes are liquid electrolytes that are connected to the circuits via some foil plates. Electrolytic capacitors offer very high capacitances, but their downside is that they suffer from reduced efficiency when subjected to heat, have poor tolerance, high instability and high current leakages. Their conductivity drops in low temperatures increasing their series resistance. They’re mainly used for power-supply conditioning.
