About Capacitors

Resonant capacitor

A resonant capacitor is an acoustics and electrical engineering term for a frequency resonant inductor. Although they are actually inductors, they are called capacitors because they are capable of storing significant amounts of charge (electrons) in an electrical network (circuit) whereas an inductor stores energy in a magnetic field. Resonant capacitors are typically used to introduce resonant frequencies of a given base frequency to spread signal energy over the frequency spectrum, effectively reducing the energy at the base frequency and increasing tolerance to EMI, Above about the Resonant capacitor information content.

Capacitor voltage transformer

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transformers would be uneconomical. In practice the first capacitor, C₁, is often replaced by a stack of capacitors connected in series. This results in a large voltage drop across the stack of capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor, C₂, and hence the secondary terminals, Above about the Capacitor voltage transformer information content.

Vacuum variable capacitor

A vacuum variable capacitor uses a high vacuum as the dielectric instead of air or other insulating material. This allows for a higher voltage rating and/or capacitance value using a smaller total volume. In addition to the higher voltage rating a vacuum dielectric greatly reduces the chance of arcing between the plates. There are several different designs in vacuum variables, the most common geometry of the capacitor plates is usually inter-meshed concentric cylinders. The meshed cylinders are contained within a glass or ceramic vacuum envelope, similar to an electron tube. A metal bellows is used to maintain a vacuum seal while allowing positional control for the moving parts of the capacitor, Above about the Vacuum variable capacitor information content.

Ferroelectric capacitor

Ferroelectric capacitor is a capacitor based on a ferroelectric material. In contrast, traditional capactors are based on dielectric materials. Ferroelectric devices are used in digital electronics as part of ferroelectric RAM, or in analog electronics as tunable capacitors, Above about the Ferroelectric capacitor information content.

Reservoir capacitor

The pulsating DC from an AC rectifier (either half-wave or full-wave) usually needs to be smoothed before use. This can be done using a Reservoir capacitor.

Photoflash capacitor

A photoflash capacitor is a capacitor used in flash cameras, professional flashes, and solid-state laser power supplies. Their typical purpose is to power a high voltage flash tube, which then illuminates a photographic target or optically pumps a laser rod. As flash tubes require very high current to operate, photoflash capacitors are designed to safely supply high discharge current pulses without excessive internal heating. Compared with electrolytic capacitors usually used for power supply filtering at power frequency, a photoflash capacitor is designed to have lower series inductance, similar or higher capacitance value, (often) a higher rated voltage, and a low series resistance, Above about the photoflash capacitor information content.

Switched capacitor

Switched Capacitor is a circuit design technique for discrete time signal processing. It works by moving charges between different capacitors when switches are opened (off) and closed (on). Usually, non-overlapping signals are used to control the switches, so that not all switches are on simultaneously, Above about the Switched capacitor information content.

Variable capacitor

A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefore they are sometimes called tuning capacitors), or as a variable reactance, e.g. for impedance matching in antenna tuners, Above about the Variable capacitor information content.

Electrochemical capacitors

Historical introduction

Electrochemical capacitors provide a mode of electrical charge- and energy-storage and delivery, complementary to that by batteries. The first electrochemical capacitor device was disclosed in a General Electric Co. patent in 1957 to Becker but was of a crude nature, employing porous carbon. Later work by Sohio (1969) described a so-called "electrokinetic capacitor" utilizing porous carbon in a non-aqueous electrolyte which enabled it to be charged up to about 3 V, though the operation of the device was not "electrokinetic" in nature, a misnomer. In 1971, Trasatti and Buzzanca recognized that the electrochemical charging behavior of ruthenium dioxide films was like that of capacitors. Between 1975 and 1980, the present author and his co-workers, under contract with the then Continental Group Inc., carried out extensive fundamental and development work on the ruthenium oxide type of electrochemical capacitor (Conway, 1997) which behaves as a surface- redox pseudocapacitance (see below). The whole field has burgeoned since about 1990 and is very active in fundamental, and R&D directions, Above about the Electrochemical capacitors information content.

Decoupling capacitor

A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another. Noise caused by other circuit elements is shunted through the capacitor reducing the effect they have on the rest of the circuit, Above about the Decoupling capacitor information content.