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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 measurement of the capacitance of a rotary capacitor c min 29 pf c 269 pf 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 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 not only increases the working voltage and current handling capacity of the capacitor it also greatly reduces the chance of arcing across the plates the most common usage for vacuum variables are in high powered transmitters such as those used for broadcasting military 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 edit various forms of variable capacitors multiple sections edit 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 edit a butterfly capacitor is a form of rotary variable capacitor with two independent sets of stator plates opposing each other and a butterfly 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 stages in push pull configuration or symmetrical antenna tuners 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 edit 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 edit 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 circuits history edit the variable capacitor with air dielectric was invented by the hungarian engineer dezső korda he received a german patent for the invention on 13 december 1893 1 electronically controlled capacitance edit voltage tuned capacitance edit 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 varicaps 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 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 of oscillators and to make high frequency voltage controlled oscillators vcos the core component in phase locked loop pll frequency synthesizers 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 edit a digitally tuned capacitor is an ic variable capacitor based on several technologies mems bst and soi 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 multi band lte gsm wcdma 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 2 transducers edit variable capacitance is sometimes used to convert physical phenomena into electrical signals in a capacitor microphone commonly known as a condenser microphone 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 displacement or relative humidity to an electrical signal for measurement purposes capacitive sensors can also be used in the place of switches e g in computer keyboards or touch buttons for elevators that have no user movable parts notes edit 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 edit george washington pierce principles of wireless telegraphy mcgraw hill book company new york 1910 p 114 photo of rotary capacitor of korda frenzel louis 29 january 2009 automatic digital antenna tuning fits multiple wireless applications electronic design archived from the original on 3 august 2014 retrieved 23 january 2017 external links edit wikimedia commons has media related to variable capacitors build your own air variable capacitor archived 14 march 2016 at the wayback machine high res images of historical variable capacitors introduction to capacitors v t e electronic components semiconductor devices mos transistors transistor nmos pmos bicmos biofet chemical field effect transistor chemfet complementary mos cmos depletion load nmos fin field effect transistor finfet floating gate mosfet fgmos insulated gate bipolar transistor igbt isfet ldmos mos field effect transistor mosfet multi gate field effect transistor mugfet power mosfet thin film transistor tft vmos umos qfet tunnel field effect transistor tfet high electron mobility transistor hemt rf cmos native transistor other transistors bipolar junction transistor bjt darlington transistor diffused junction transistor field effect transistor fet junction gate fet jfet organic fet ofet light emitting transistor let pentode transistor point contact transistor programmable unijunction transistor put static induction transistor sit tetrode transistor nanoscale vacuum channel transistor nvct single electron transistor set heterojunction bipolar transistor hbt mesfet heterostructure emitter bipolar transistor hebt itfet avalanche transistor junctionless nanowire transistor jlnt schottky transistor spin transistor ballistic collection transistor ballistic deflection transistor bdt drift field transistor organic electrochemical transistor oect spacistor surface barrier transistor synaptic transistor oxide thin film transistor nomfet grown junction transistor eosfet alloy junction transistor fe fet dna field effect transistor dnafet diodes avalanche diode shockley diode selenium rectifier fast diode single photon avalanche diode spad constant current diode cld crd gunn diode varicap impatt diode metal insulator metal diode mim diode transient voltage suppression diode tvs diode laser diode ld light emitting diode led organic light emitting diode oled photodiode pin diode p n diode tunnel diode avalanche photodiode apd solar cell multi junction solar cell mj hybrid solar cell schottky junction solar cell plasmonic solar cell heterojunction solar cell hjt schottky diode step recovery diode srd zener diode resonant tunneling diode rtd photoreflector backward diode geometric diode lr diode chua s diode baritt diode stabistor phosphorescent organic light emitting diode pholed thin film diode superluminescent diode sld josephson diode hole accumulation diode had silicon photomultiplier sipm metal rectifier bifacial solar cells bsc integrated circuits hybrid integrated circuit hic mixed signal integrated circuit mos integrated circuit mos ic three dimensional integrated circuit 3d ic photonic integrated circuit pic application specific integrated circuit asic field programmable gate array fpga system on a chip soc other devices diac heterostructure barrier varactor light emitting capacitor lec memistor memristor memtransistor memory cell metal oxide varistor mov organic semiconductor photodetector silicon controlled rectifier scr silicon controlled switch scs solaristor static induction thyristor sith thyristor trancitor triac varicap vertical cavity surface emitting laser vcsel gate turn off thyristor gto integrated gate commutated thyristor igct unijunction transistor ujt quantum cascade laser qcl photoresistor quantum dot display quantum dot solar cell photoelectrochemical cell dye sensitized solar cell plasmonic solar cell quantum dot laser quadrac interband cascade laser icl oscillistor mos composite static induction thyristor csmt mos controlled thyristor mct resistive opto isolator ro emitter turn off thyristor eto trisil quantum well infrared photodetector qwip quantum cascade detector qcd resonant cavity enhanced photo detector rce golay cell hybrid pixel detector semiconductor detector voltage regulators linear regulator low dropout regulator switching regulator buck boost buck boost split pi ćuk sepic charge pump switched capacitor vacuum tubes standard acorn tube audion beam tetrode barretter compactron diode fleming valve neutron tube nonode nuvistor pentagrid hexode heptode octode pentode photomultiplier pmt phototube tetrode triode x ray tube double diode triode additron tube computron tube micropup rf backward wave oscillator bwo cavity magnetron crossed field amplifier cfa gyrotron inductive output tube iot klystron maser sutton tube traveling wave tube twt vircator cathode ray tubes beam deflection tube charactron iconoscope magic eye tube monoscope selectron tube storage tube trochotron video camera tube williams tube gas filled tubes crossatron dekatron ignitron krytron mercury arc valve nixie tube thyratron trigatron voltage regulator tube geiger müller tube wire chamber neon lamp adjustable potentiometer digital variable capacitor varicap passive connector audio and video electrical power rf ferrite antifuse fuse resettable efuse resistor switch thermistor varistor reactive capacitor types ceramic resonator crystal oscillator inductor relay reed relay mercury relay other devices hall effect sensor squid spin valve magnetic tunnel junction mtj planar hall sensor ggnmos retrieved from https en wikipedia org w index php title variable_capacitor oldid 1352331862 categories capacitors wireless tuning and filtering hungarian inventions 19th century inventions hidden categories articles with short description short description is different 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