History
Capacitors began as devices for storing static electric charge and evolved into one of the basic building blocks of electronic circuits. Older literature often uses the word condenser, a name that remained common in radio and automotive work for many decades.
- 1740s — Leyden jar. The Leyden jar was the first practical capacitor. Glass acted as the dielectric and conductive surfaces on the inside and outside stored opposite charges. Its ability to store and then suddenly release charge transformed electrical experiments.
- Benjamin Franklin's experiments. Franklin and other investigators studied Leyden jars to understand where the charge was stored and how multiple jars could be connected. Groups of jars were among the early electrical devices described as a “battery.”
- 19th century — Condensers and electrical measurement. As telegraphy, induction coils, motors, and laboratory instruments developed, capacitors were built from glass, paper, foil, and other insulating materials. Better measurement of charge and capacitance followed.
- Late 19th and early 20th centuries — Paper and mica. Foil-and-paper capacitors became practical for power and radio equipment. Mica's excellent insulation and stability made it valuable in early tuned radio-frequency circuits.
- 20th century — Electrolytic capacitors. Electrochemical formation of an extremely thin oxide dielectric allowed much larger capacitance in a compact package. Electrolytics became central to rectifier filtering and power supplies.
- Mid-20th century — Tantalum and improved ceramics. Tantalum oxide enabled compact solid capacitors, while ceramic dielectric development produced inexpensive components spanning tiny RF values through much larger bypass values.
- Film-capacitor era. Plastic films such as polyester and polypropylene replaced many paper dielectrics, improving moisture resistance, stability, pulse behavior, and service life.
- Surface-mount electronics. Multilayer ceramic capacitors and molded tantalum or polymer capacitors became common in automated assembly. Components grew physically smaller while circuit switching speeds increased, making ESR and ESL increasingly important design parameters.
- Modern power electronics. Capacitors now serve everything from milliwatt sensor circuits to megawatt-class converters. High-current DC-link and pulse capacitors are designed around low ESR, low inductance, thermal management, and long life.
The farad
The SI unit of capacitance is the farad, named for Michael Faraday. One farad is a very large capacitance for traditional electronic circuits, which is why microfarads, nanofarads, and picofarads appear so often.
From jars to multilayer structures
The basic principle has changed surprisingly little: two conductors separated by a dielectric store electrical energy in an electric field. What changed dramatically was the choice of dielectric, electrode geometry, manufacturing precision, packaging, sealing, and the ability to make enormous effective electrode area inside a small package.