![Condensateur-variable-a-air 375x240](https://upload.wikimedia.org/wikipedia/commons/a/a7/Condensateur-variable-a-air_375x240.gif)
A variable capacitor is a
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
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 it is sometimes called a tuning capacitor or tuning condenser), or as a variable
reactance, e.g. for
impedance matching
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 si ...
in
antenna tuner
An antenna tuner (and any of the names in the list below) is a device that is inserted between a transmitter, radio transmitter and its antenna (radio), antenna; when placed close by the antenna and properly adjusted (tuned) it optimizes power ...
s.
Mechanically controlled capacitance
![Forgokondenzator rajz en](https://upload.wikimedia.org/wikipedia/commons/3/38/Forgokondenzator_rajz_en.svg)
In mechanically controlled variable capacitors, the distance between the plates, or the amount of plate surface area which overlaps, can be changed.
The most common form arranges a group of semicircular metal plates on a rotary axis ("
rotor
Rotor may refer to:
Science and technology
Engineering
* Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator
*Helicopter rotor, the rotary wing(s) of a rotorcraft ...
") that are positioned in the gaps between a set of stationary plates ("
stator") so that the area of overlap can be changed by rotating the axis. Air or plastic foils can be used as
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 ...
material.
By choosing the shape of the rotary plates, various functions of capacitance vs. angle can be created, e.g. to obtain a linear frequency scale. Various forms of reduction
gear
A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic ...
mechanisms are often used to achieve finer tuning control, i.e. to spread the variation of capacity over a larger angle, often several turns. Maximum capacitance is achieved when the plates are "meshed" together, that is, they are inter-laced. Minimum capacitance is achieved when the plates are "unmeshed", that is, they are not inter-laced.
File:Forgokondenzator1.jpg, ''C''min = 29 pF
File:Forgokondenzator2.jpg, ''C'' = 269 pF
File:Forgokondenzator3.jpg, ''C''max = 520 pF
A vacuum variable capacitor uses a set of plates made from concentric cylinders that can be slid in or out of an opposing set of cylinders (sleeve and plunger). These plates are then sealed inside of a non-conductive envelope such as glass or ceramic and placed under 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 di ...
. The movable part (plunger) is mounted on a flexible metal membrane that seals and maintains the vacuum. A screw shaft is attached to the plunger; when the shaft is turned the plunger moves in or out of the sleeve and the value of the capacitor changes. The
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 di ...
not only increases the working
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 ...
and
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
handling capacity of the
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
, it also greatly reduces the chance of
arcing
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. An ...
across the plates. The most common usage for vacuum variables are in high-powered
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s such as those used for
broadcasting
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 beg ...
,
military
A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. It is typically authorized and maintained by a sovereign state, with its members identifiable by their distinct ...
and
amateur radio, as well as high-powered
RF tuning networks. Vacuum variables can also be more convenient; since the elements are under a vacuum, the working voltage can be higher than an air variable the same size, allowing the size of the vacuum capacitor to be reduced.
Very cheap variable capacitors are constructed from layered aluminium and plastic foils that are variably pressed together using a screw. These so-called ''squeezers'' cannot provide a stable and reproducible capacitance, however. A variant of this structure that allows for linear movement of one set of plates to change the plate overlap area is also used and might be called a ''slider''. This has practical advantages for makeshift or home construction, and may be found in resonant-loop antennas or crystal radios.
Small variable capacitors operated by screwdriver (for instance, to precisely set a resonant frequency at the factory and then never be adjusted again) are called
trimmer capacitors. In addition to air and plastic, trimmers can also be made using a solid dielectric, such as
mica.
Special forms of mechanically variable capacitors
Multiple sections
Very often, multiple stator/rotor sections are arranged behind one another on the same axis, allowing for several tuned circuits to be adjusted using the same control, e.g. a preselector, an input filter and the corresponding oscillator in a receiver circuit. The sections can have identical or different nominal capacitances, e.g. 2 × 330 pF for AM filter and oscillator, plus 3 × 45 pF for two filters and an oscillator in the FM section of the same receiver. Capacitors with multiple sections often include trimmer capacitors in parallel to the variable sections, used to adjust all tuned circuits to the same frequency.
Butterfly
A butterfly capacitor is a form of rotary variable capacitor with two independent sets of stator plates opposing each other, and a
butterfly
Butterflies are insects in the macrolepidopteran clade Rhopalocera from the order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The group comprise ...
-shaped rotor arranged so that turning the rotor will vary the capacitances between the rotor and either stator equally.
Butterfly capacitors are used in symmetrical tuned circuits, e.g.
RF power
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...
stages in
push-pull configuration or symmetrical
antenna tuner
An antenna tuner (and any of the names in the list below) is a device that is inserted between a transmitter, radio transmitter and its antenna (radio), antenna; when placed close by the antenna and properly adjusted (tuned) it optimizes power ...
s where the rotor needs to be "cold", i.e. connected to RF (but not necessarily
DC)
ground potential. Since the peak RF current normally flows from one stator to the other without going through wiper contacts, butterfly capacitors can handle large resonance RF currents, e.g. in
magnetic loop antennas.
In a butterfly capacitor, the stators and each half of the rotor can only cover a maximum angle of 90° since there must be a position without rotor/stator overlap corresponding to minimum capacity, therefore a turn of only 90° covers the entire capacitance range.
Split stator
The closely related split stator variable capacitor does not have the limitation of 90° angle since it uses two separate packs of rotor electrodes arranged axially behind one another. Unlike in a capacitor with several sections, the rotor plates in a split stator capacitor are mounted on opposite sides of the rotor axis. While the split stator capacitor benefits from larger electrodes compared to the butterfly capacitor, as well as a rotation angle of up to 180°, the separation of rotor plates incurs some losses since RF current has to pass the rotor axis instead of flowing straight through each rotor vane.
Differential
Differential variable capacitors also have two independent stators, but unlike in the butterfly capacitor where capacities on both sides increase equally as the rotor is turned, in a differential variable capacitor one section's capacity will increase while the other section's decreases, keeping the sum of the two stator capacitances constant. Differential variable capacitors can therefore be used in capacitive
potentiometric
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
The measuring instrum ...
circuits.
History
The variable capacitor with air dielectric was invented by the
Hungarian engineer
Dezső Korda
Dezső Korda (french: Désiré or Desider Korda) (8 January 1864 – 1 April 1919) was a mechanical engineer born in Kisbér, Hungary.
Korda finished his education in 1885 at the Budapest, in ''Royal Joseph Technical University'' (present-da ...
. He received a
German
German(s) may refer to:
* Germany (of or related to)
** Germania (historical use)
* Germans, citizens of Germany, people of German ancestry, or native speakers of the German language
** For citizens of Germany, see also German nationality law
**Ge ...
patent for the invention on 13 December 1893.
Electronically controlled capacitance
Voltage tuned capacitance
The thickness of the depletion layer of a reverse-biased semiconductor
diode varies with the DC voltage applied across the diode. Any diode exhibits this effect (including p/n junctions in transistors), but devices specifically sold as variable capacitance diodes (also called varactors or
varicap
In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction.
Applications
Va ...
s) are designed with a large junction area and a doping profile specifically designed to maximize capacitance.
Their use is limited to low signal amplitudes to avoid obvious distortions as the capacitance would be affected by the change of signal voltage, precluding their use in the input stages of high-quality RF communications receivers, where they would add unacceptable levels of
intermodulation
Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency comp ...
. At
VHF/
UHF frequencies, e.g. in FM Radio or TV tuners, dynamic range is limited by noise rather than large signal handling requirements, and varicaps are commonly used in the signal path.
Varicaps are used for
frequency modulation
Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing.
In analog fre ...
of oscillators, and to make high-frequency
voltage controlled oscillator
A microwave (12–18GHz) voltage-controlled oscillator
A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillat ...
s (VCOs), the core component in
phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
(PLL)
frequency synthesizer
A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in many modern devices such as radio receivers, televisions, mobile telephones, radiotelephon ...
s that are ubiquitous in modern communications equipment.
BST device are based on Barium Strontium Titanate and vary the capacitance by applying high voltage to the device. They have a dedicated analogue control input and therefore introduce less non-linearities than varactor diodes, especially for higher signal voltages. The limitations for BST are stability over temperature and linearity in demanding applications.
Digitally tuned capacitance
A digitally tuned capacitor is an
IC variable capacitor based on several technologies.
MEMS
Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
, BST and
SOI
''Soi'' ( th, ซอย ) is the term used in Thailand for a side-street branching off a major street (''thanon'', th, ถนน). An alley is called a ''trok'' ( th, ตรอก).
Overview
Sois are usually numbered, and are referred to by th ...
/
SOS devices are available from a number of suppliers and vary in capacitance range, quality factor and resolution for different RF tuning applications.
MEMS devices have the highest quality factor and are highly linear, and therefore are suitable for antenna aperture tuning, dynamic impedance matching, power amplifier load matching and adjustable filters. RF tuning MEMS are still a relatively new technology and has not yet been accepted broadly.
SOI/SOS tuning devices are constructed as solid state FET switches built on insulated CMOS wafers and use MIM caps arranged in binary-weighted values to achieve different capacitance values. SOI/SOS switches have high linearity and are well suited to low power applications where high voltages are not present. High voltage endurance requires multiple FET devices in series which adds series resistance and lowers the quality factor.
The capacitance values are designed for
antenna impedance matching
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 si ...
in multi-band
LTE GSM
The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...
/
WCDMA
The Universal Mobile Telecommunications System (UMTS) is a 3G, third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the I ...
cellular handsets and mobile TV receivers that operate over wide frequency ranges, such as the European
DVB-H and Japanese
ISDB-T mobile TV systems.
Transducers
Variable capacitance is sometimes used to convert physical phenomena into electrical signals.
* In a capacitor
microphone
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publ ...
(commonly known as a
condenser microphone
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
), the diaphragm acts as one plate of a capacitor, and vibrations produce changes in the distance between the diaphragm and a fixed plate, changing the voltage maintained across the capacitor plates.
* Some types of industrial sensors use a capacitor element to convert physical quantities such as
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
,
displacement
Displacement may refer to:
Physical sciences
Mathematics and Physics
* Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...
or
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
to an electrical signal for measurement purposes.
* Capacitive sensors can also be used in the place of
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
es, e.g. in computer keyboards or "touch buttons" for elevators that have no user-movable parts.
Notes
# A makeshift version of this design, using two tin cans with slightly different diameter isolated by cardboard, has been used to make variable capacitors in emergencies, e.g. by concentration camp inmates.
# While mechanical variable capacitors have been used and are still used extensively in electronics, their use in radios is diminishing. Radios now often use digital frequency synthesizers for tuning rather than analog (tank) oscillators.
References
External links
Build your own air variable capacitorIntroduction to capacitors
{{DEFAULTSORT:Variable Capacitor
Capacitors
Wireless tuning and filtering
Hungarian inventions
19th-century inventions
pt:Capacitor#Capacitores variáveis