A vacuum variable capacitor is a

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 (therefo ...

which uses a high

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

as the

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

instead of

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

or other insulating material. This allows for a higher

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

rating than an air dielectric using a smaller total volume. However, many dielectrics have higher breakdown field strengths than vacuum: 60-170 MV/m for

teflon Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemou ...

, 470-670 MV/m for

fused silica Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...

and 2000 MV/m for

diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...

, compared with 20-40 MV/m for vacuum. There are several different designs in vacuum variables. The most common form is inter-meshed concentric cylinders, which are contained within a glass or ceramic vacuum envelope, similar to an electron tube. A metal

bellows A bellows or pair of bellows is a device constructed to furnish a strong blast of air. The simplest type consists of a flexible bag comprising a pair of rigid boards with handles joined by flexible leather sides enclosing an approximately airtigh ...

is used to maintain a vacuum seal while allowing positional control for the moving parts of the capacitor.

Invention

Nikola Tesla Nikola Tesla ( ; ,"Tesla"
''

Applications

Vacuum variable capacitors are commonly used in high-

applications: 5000

volts The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Definit ...

(5 kV) and above. They are used in equipment such as high-powered

broadcast Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum ( radio waves), in a one-to-many model. Broadcasting began ...

transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...

amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...

RF amplifiers and large

antenna tuner An antenna tuner (and any of the names in the list below) is a device that is inserted between a radio transmitter and its antenna; when placed close by the antenna and properly adjusted (tuned) it optimizes power transfer by matching the im ...

s. Industrially they are used in plasma generating equipment, for

dielectric heating Dielectric heating, also known as electronic heating, radio frequency heating, and high-frequency heating, is the process in which a radio frequency (RF) alternating electric field, or radio wave or microwave electromagnetic radiation heats a diel ...

, and in semiconductor manufacturing. The main applications today are RF plasmas of 2 to 160 MHz where the vacuum capacitor is used as the impedance variation part in an automatic

matching network In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...

in the fabrication of chips and flat panel displays. Fixed value vacuum capacitor

Other variations of vacuum capacitors include fixed-value capacitors, which are designed very much like the variable versions with the exception that the adjustment

mechanism Mechanism may refer to: *Mechanism (engineering), rigid bodies connected by joints in order to accomplish a desired force and/or motion transmission *Mechanism (biology), explaining how a feature is created *Mechanism (philosophy), a theory that a ...

is omitted.

Comparison

When compared to other variable capacitors, vacuum variables tend to be more precise and more stable. This is due to the vacuum itself. Because of the sealed chamber, the dielectric constant remains the same over a wider range of operating conditions. With air

s, the air moving around the plates may change the value slightly; often it is not much but in some applications it is enough to cause undesirable effects. Vacuum variable capacitors are generally more expensive than air variable capacitors. This is primarily due to their design and the materials used. Although most use

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

and

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

, some may use other materials such as

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

s and

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

s such as

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...

and

silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

. Vacuum variables also vary in adjustment mechanisms.

References

External links

* *
Webarchive: Meidensha products

Introduction to capacitors
{{DEFAULTSORT:Vacuum Variable Capacitor Capacitors Inventions by Nikola Tesla de:Vakuumkondensator#Variable Vakuumkondensatoren