Vacuum variable capacitor
Encyclopedia
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.
applications 5000 volts(5kV) and above. They are used in equipment such as high powered broadcast
transmitter
s, Amateur radio
RF Amplifiers
and large Antenna tuner
s. The main applications today are RF plasmas of 2 to 160MHz where the vacuum capacitor is used as the impedance variation part in an automatic matching network.
Other variations of vacuum capacitors include fixed value capacitors. Fixed value capacitors are designed very much like the variable versions with the exception of an adjustment mechanism.
s, the air moving around the plates may change the value slightly; often it is not much but in some applications its enough to cause undesirable effects. Another common problem with air variables is dust and insects. Dust collected on the plates or a curious insect could have disastrous effects. An insect in contact with the plates is likely to cause a short circuit with the open frame design used on most air variable capacitors.
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
and glass
, some may use other materials such as ceramic
s and metal
s such as gold
and silver
. Vacuum variables also vary in adjustment mechanisms which range from coarse to fine.
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
variable capacitor
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 , or as a variable reactance, e.g...
uses a high vacuum as the dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
instead of air or other insulating
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
material. This allows for a higher voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
rating and/or capacitance
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
value using a smaller total volume. In addition to the higher voltage rating a vacuum dielectric greatly reduces the chance of arcing
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
between the plates. There are several different designs in vacuum variables, the most common geometry
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....
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
Electron tube
Electron tube can be used to describe either of two things:* Vacuum tube* Gas-filled tube...
. A metal bellows
Bellows
A bellows is a device for delivering pressurized air in a controlled quantity to a controlled location.Basically, a bellows is a deformable container which has an outlet nozzle. When the volume of the bellows is decreased, the air escapes through the outlet...
is used to maintain a vacuum seal while allowing positional control for the moving parts of the capacitor.
Common Applications
Vacuum variable capacitors are commonly used in high voltageVoltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
applications 5000 volts(5kV) and above. They are used in equipment such as high powered broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
s, Amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
RF Amplifiers
Valve RF amplifier
A valve RF amplifier or tube amplifier , is a device for electrically amplifying the power of an electrical signal, typically radio frequency signals....
and large Antenna tuner
Antenna tuner
An antenna tuner, transmatch or antenna tuning unit is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna...
s. The main applications today are RF plasmas of 2 to 160MHz where the vacuum capacitor is used as the impedance variation part in an automatic matching network.
Other Variations
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 Variable capacitorVariable 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 , or as a variable reactance, e.g...
s, the air moving around the plates may change the value slightly; often it is not much but in some applications its enough to cause undesirable effects. Another common problem with air variables is dust and insects. Dust collected on the plates or a curious insect could have disastrous effects. An insect in contact with the plates is likely to cause a short circuit with the open frame design used on most air variable capacitors.
Vacuum variable capacitors are generally more expensive than air variable capacitors
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 , or as a variable reactance, e.g...
. This is primarily due to their design and the materials used. Although most use copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, some may use other materials such as ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s and metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s such as gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
and silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
. Vacuum variables also vary in adjustment mechanisms which range from coarse to fine.