Editing Inductor (section)

==Types==
===Air-core inductor===
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Radio transmitter tank coil.png
| caption1 = High Q tank coil in [[tuned circuit]] of radio transmitter
| width1   = 220
| image2   = Antenna tuning coil - station WOR.jpg
| caption2 = An [[antenna tuner|antenna tuning]] coil at an AM radio station. 
| width2   = 140
| footer   = These coils illustrate high power [[Q factor|high Q]] construction: single layer winding with turns spaced apart to reduce [[Proximity effect (electromagnetism)|proximity effect]] losses, made of silver-plated wire or tubing to reduce [[skin effect]] losses, supported by narrow insulating strips to reduce [[dielectric losses]]
}}

The term ''air core coil'' describes an inductor that does not use a [[magnetic core]] made of a ferromagnetic material. The term refers to coils wound on plastic, ceramic, or other nonmagnetic forms, as well as those that have only air inside the windings. Air core coils have lower inductance than ferromagnetic core coils, but are often used at high frequencies because they are free from energy losses called [[core loss]]es that occur in ferromagnetic cores, which increase with frequency. A side effect that can occur in air core coils in which the winding is not rigidly supported on a form is 'microphony': mechanical vibration of the windings can cause variations in the inductance.

====Radio-frequency inductor====
[[Image:Hf spoler og transformatorer.jpg|thumb|upright=1.5|Collection of RF inductors, showing techniques to reduce losses. The three top left and the [[loop antenna|ferrite loopstick]] or rod antenna,<ref>{{cite web
  | title = An Unassuming Antenna – The Ferrite Loopstick
  | publisher = Radio Time Traveller
  | date = January 23, 2011
  | url = http://radio-timetraveller.blogspot.com/2011/01/unassuming-antenna-ferrite-loopstick.html
  | access-date = March 5, 2014 
}}</ref><ref name="Frost">{{cite web
  | last = Frost
  | first = Phil
  | title = What's an appropriate core material for a loopstick antenna?
  | work = Amateur Radio beta
  | publisher = Stack Exchange, Inc.
  | date = December 23, 2013
  | url = http://ham.stackexchange.com/questions/1156/whats-an-appropriate-core-material-for-a-loopstick-antenna
  | access-date = March 5, 2014
}}</ref><ref name="Poisel">{{cite book
  | last = Poisel
  | first = Richard 
  | title = Antenna Systems and Electronic Warfare Applications
  | publisher = Artech House
  | date = 2011
  | pages = 280
  | url = https://books.google.com/books?id=1YA1NZuo6u0C&q=%22ferrite+rod+loop+antenna&pg=PA280
  | isbn = 978-1608074846
}}</ref><ref name="Yadava">{{cite book
  | last = Yadava
  | first = R. L. 
  | title = Antenna and Wave Propagation
  | publisher = PHI Learning Pvt. Ltd
  | date = 2011
  | pages = 261
  | url = https://books.google.com/books?id=MMtjYYrE2r8C&q=%22ferrite+loop+antenna&pg=PA261
  | isbn = 978-8120342910
}}</ref> bottom, have basket windings.]]

At [[high frequency|high frequencies]], particularly [[radio frequency|radio frequencies]] (RF), inductors have higher resistance and other losses. In addition to causing power loss, in [[resonant circuit]]s this can reduce the [[Q factor]] of the circuit, broadening the [[Bandwidth (signal processing)|bandwidth]]. In RF inductors specialized construction techniques are used to minimize these losses. The losses are due to these effects:
*'''Skin effect''': The resistance of a wire to [[high frequency]] current is higher than its resistance to [[direct current]] because of [[skin effect]].<ref name="Zurek1">{{cite web
  | last = Zurek
  | first = Stan
  | title = Skin effect
  | website = Encyclopedia Magnetica website
  | publisher = 
  | date = 2023
  | url = http://www.e-magnetica.pl/doku.php/proximity_effect
  | format = 
  | doi = 
  | accessdate = 21 May 2024}}</ref><ref name="Kazimierczuk">{{cite book
  | last = Kazimierczuk
  | first = Marian K.
  | title = High-Frequency Magnetic Components
  | publisher = John Wiley and Sons
  | date = 2011
  | location = 
  | pages = 
  | language = 
  | url = https://books.google.com/books?id=t2TgU-uuNQ0C&pg=PA141
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 978-1-119-96491-9
  | mr = 
  | zbl = 
  | jfm =}}</ref>{{rp|p.141}}  Due to induced [[eddy current]]s, radio frequency alternating current does not penetrate far into the body of a conductor but travels along its surface.  For example, at 6&nbsp;MHz the skin depth of copper wire is about 0.001 inches (25&nbsp;μm); most of the current is within this depth of the surface.  Therefore, in a solid wire, the interior portion of the wire may carry little current, effectively increasing its resistance.
*'''Proximity effect''': Another similar effect that also increases the resistance of the wire at high frequencies is [[proximity effect (electromagnetism)|proximity effect]], which occurs in parallel wires that lie close to each other.<ref name="Zurek2">{{cite web
  | last = Zurek
  | first = Stan
  | title = Proximity effect
  | website = Encyclopedia Magnetica website
  | publisher = 
  | date = 2023
  | url = http://www.e-magnetica.pl/doku.php/proximity_effect
  | format = 
  | doi = 
  | accessdate = 21 May 2024}}</ref><ref name="Kazimierczuk" />{{rp|p.98}} The individual magnetic field of adjacent turns induces [[eddy current]]s in the wire of the coil, which causes the current density in the conductor to be displaced away from the adjacent surfaces.  Like skin effect, this reduces the effective cross-sectional area of the wire conducting current, increasing its resistance.
*'''Dielectric losses''': The high frequency electric field near the conductors in a [[LC circuit|tank coil]] can cause the motion of polar molecules in nearby insulating materials, dissipating energy as heat.  For this reason, coils used for tuned circuits may be suspended in air, supported by narrow plastic or ceramic strips rather than being wound on coil forms.
*'''Parasitic capacitance''': The capacitance between individual wire turns of the coil, called [[parasitic capacitance]], does not cause energy losses but can change the behavior of the coil. Each turn of the coil is at a slightly different potential, so the [[electric field]] between neighboring turns stores charge on the wire, so the coil acts as if it has a capacitor in parallel with it. At a high enough frequency this capacitance can resonate with the inductance of the coil forming a [[tuned circuit]], causing the coil to become [[self-resonant frequency|self-resonant]].

{{multiple image
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| total_width = 300
| image1   = Spider coil.jpg
| image2   = Kreuzwickelspule.png
| footer   = ''(left)'' Spiderweb coil ''(right)'' Adjustable ferrite slug-tuned RF coil with basketweave winding and litz wire
}}
To reduce parasitic capacitance and proximity effect, [[Q factor|high Q]] RF coils are constructed to avoid having many turns lying close together, parallel to one another. The windings of RF coils are often limited to a single layer, and the turns are spaced apart. To reduce resistance due to skin effect, in high-power inductors such as those used in transmitters the windings are sometimes made of a metal strip or tubing which has a larger surface area, and the surface is silver-plated.
; Basket-weave coils: To reduce proximity effect and parasitic capacitance, multilayer RF coils are wound in patterns in which successive turns are not parallel but crisscrossed at an angle; these are often called ''honeycomb'' or ''[[Basket winding|basket-weave]]'' coils.  These are occasionally wound on a vertical insulating supports with dowels or slots, with the wire weaving in and out through the slots.
; Spiderweb coils: Another construction technique with similar advantages is flat spiral coils. These are often wound on a flat insulating support with radial spokes or slots, with the wire weaving in and out through the slots; these are called ''spiderweb'' coils. The form has an odd number of slots, so successive turns of the spiral lie on opposite sides of the form, increasing separation. 
; Litz wire: To reduce skin effect losses, some coils are wound with a special type of radio frequency wire called [[litz wire]]. Instead of a single solid conductor, litz wire consists of a number of smaller wire strands that carry the current. Unlike ordinary [[stranded wire]], the strands are insulated from each other, to prevent skin effect from forcing the current to the surface, and are twisted or braided together. The twist pattern ensures that each wire strand spends the same amount of its length on the outside of the wire bundle, so skin effect distributes the current equally between the strands, resulting in a larger cross-sectional conduction area than an equivalent single wire.

; Axial Inductor
Small inductors for low current and low power are made in molded cases resembling resistors.  These may be either plain (phenolic) core or ferrite core.  An ohmmeter readily distinguishes them from similar-sized resistors by showing the low resistance of the inductor.

===Ferromagnetic-core inductor===
{{See also|Magnetic core}}
[[Image:Aplikimi i feriteve.png|thumb|A variety of types of ferrite core inductors and transformers]]

Ferromagnetic-core or iron-core inductors use a magnetic core made of a [[ferromagnetic]] or [[ferrimagnetic]] material such as iron or [[Ferrite (magnet)|ferrite]] to increase the inductance. A magnetic core can increase the inductance of a coil by a factor of several thousand, by increasing the magnetic field due to its higher [[magnetic permeability]]. However the magnetic properties of the core material cause several side effects which alter the behavior of the inductor and require special construction:
  {{glossary}}
  {{term|[[Core loss]]es}}{{defn|A time-varying current in a ferromagnetic inductor, which causes a time-varying magnetic field in its core, causes energy losses in the core material that are dissipated as heat, due to two processes:{{glossary}}
  {{term|[[Eddy current]]s}}{{defn|From [[Faraday's law of induction]], the changing magnetic field can induce circulating loops of electric current in the conductive metal core. The energy in these currents is dissipated as heat in the [[electrical resistance|resistance]] of the core material. The amount of energy lost increases with the area inside the loop of current.}}
{{term|[[Hysteresis loop|Hysteresis]]}}{{defn|Changing or reversing the magnetic field in the core also causes losses due to the motion of the tiny [[magnetic domain]]s it is composed of. The energy loss is proportional to the area of the hysteresis loop in the BH graph of the core material. Materials with low [[coercivity]] have narrow hysteresis loops and so low hysteresis losses.}}
  {{glossary end}}
Core loss is non-linear with respect to both frequency of magnetic fluctuation and magnetic flux density. Frequency of magnetic fluctuation is the frequency of AC current in the electric circuit; magnetic flux density corresponds to current in the electric circuit. Magnetic fluctuation gives rise to hysteresis, and magnetic flux density causes eddy currents in the core. These nonlinearities are distinguished from the threshold nonlinearity of saturation. Core loss can be approximately modeled with [[Steinmetz's equation]]. At low frequencies and over limited frequency spans (maybe a factor of 10), core loss may be treated as a linear function of frequency with minimal error. However, even in the audio range, nonlinear effects of magnetic core inductors are noticeable and of concern. }}
{{term|Saturation}}{{defn|If the current through a magnetic core coil is high enough that the core [[Saturation (magnetic)|saturates]], the inductance will fall and current will rise dramatically. This is a nonlinear threshold phenomenon and results in distortion of the signal. For example, [[audio signal]]s can suffer [[intermodulation distortion]] in saturated inductors. To prevent this, in [[linear circuit]]s the current through iron core inductors must be limited below the saturation level. Some laminated cores have a narrow air gap in them for this purpose, and powdered iron cores have a distributed air gap. This allows higher levels of magnetic flux and thus higher currents through the inductor before it saturates.<ref>{{cite web |url=http://www.newark.com/pdfs/techarticles/vishay/Inductors101.pdf |title=Inductors 101 |publisher=vishay |access-date=2010-09-24}}</ref>}}
{{term|Curie point demagnetization}}{{defn|If the temperature of a ferromagnetic or ferrimagnetic core rises to a specified level, the magnetic domains dissociate, and the material becomes paramagnetic, no longer able to support magnetic flux.  The inductance falls and current rises dramatically, similarly to what happens during saturation.  The effect is reversible: When the temperature falls below the Curie point, magnetic flux resulting from current in the electric circuit will realign the magnetic domains of the core and its magnetic flux will be restored.  The Curie point of ferromagnetic materials (iron alloys) is quite high; iron is highest at 770{{nbsp}}°C. However, for some ferrimagnetic materials (ceramic iron compounds – [[ferrite (magnet)|ferrite]]s) the Curie point can be close to ambient temperatures (below 100{{nbsp}}°C).{{citation needed|date=February 2018}} }}
{{glossary end}}

====Laminated-core inductor====
[[Image:Vorschaltdrossel Kvg.jpg|thumb|upright=0.8|Laminated iron core [[ballast (electrical)|ballast]] inductor for a [[metal halide lamp]] ]]
Low-frequency inductors are often made with [[laminated core]]s to prevent eddy currents, using construction similar to [[transformer]]s. The core is made of stacks of thin steel sheets or [[lamination]]s oriented parallel to the field, with an insulating coating on the surface. The insulation prevents eddy currents between the sheets, so any remaining currents must be within the cross sectional area of the individual laminations, reducing the area of the loop and thus reducing the energy losses greatly. The laminations are made of low-conductivity [[silicon steel]] to further reduce eddy current losses.

====Ferrite-core inductor====
{{main|Ferrite core}}
For higher frequencies, inductors are made with cores of ferrite. Ferrite is a ceramic ferrimagnetic material that is nonconductive, so eddy currents cannot flow within it. The formulation of ferrite is xxFe<sub>2</sub>O<sub>4</sub> where xx represents various metals. For inductor cores [[soft ferrite]]s are used, which have low coercivity and thus low hysteresis losses.

====Powdered-iron-core inductor <span class="anchor" id="powdered_iron_anchor"></span>====
{{see also|Carbonyl iron}}
Another material is powdered iron cemented with a binder. [[Medium frequency]] equipment almost exclusively uses powdered iron cores, and inductors and transformers built for the lower [[shortwave]]s are made using either cemented powdered iron or [[Ferrite core|ferrite]]s.{{citation needed|date=September 2022}}

====Toroidal-core inductor====
{{main|Toroidal inductors and transformers}}
[[File:3Com OfficeConnect ADSL Wireless 11g Firewall Router 2012-10-28-0869.jpg|thumb|Toroidal inductor in the power supply of a wireless router]]

In an inductor wound on a straight rod-shaped core, the [[magnetic field lines]] emerging from one end of the core must pass through the air to re-enter the core at the other end. This reduces the field, because much of the magnetic field path is in air rather than the higher permeability core material and is a source of [[electromagnetic interference]]. A higher magnetic field and inductance can be achieved by forming the core in a closed [[magnetic circuit]]. The magnetic field lines form closed loops within the core without leaving the core material. The shape often used is a [[toroid]]al or doughnut-shaped ferrite core. Because of their symmetry, toroidal cores allow a minimum of the magnetic flux to escape outside the core (called ''[[leakage flux]]''), so they radiate less electromagnetic interference than other shapes. Toroidal core coils are manufactured of various materials, primarily ferrite, powdered iron and laminated cores.<ref>{{cite web|url=http://www.vishay.com/docs/34053/definit.pdf |title=Inductor and Magnetic Product Terminology |publisher=Vishay Dale |access-date=2012-09-24}}</ref>

===Variable inductor===
{{multiple image
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| total_width = 230
| image1   = Ferrite slug tuned inductor with pot core.JPG
| image2   = Variometer.jpg
| footer   = ''(left)'' Inductor with a threaded ferrite slug ''(visible at top)'' that can be turned to move it into or out of the coil, 4.2&nbsp;cm high. ''(right)'' A variometer used in radio receivers in the 1920s
}}
[[Image:Rollspule.jpg|thumb|A "roller coil", an adjustable air-core RF inductor used in the [[tuned circuit]]s of radio transmitters. One of the contacts to the coil is made by the small grooved wheel, which rides on the wire. Turning the shaft rotates the coil, moving the contact wheel up or down the coil, allowing more or fewer turns of the coil into the circuit, to change the inductance.]]

Probably the most common type of variable inductor today is one with a moveable ferrite magnetic core, which can be slid or screwed in or out of the coil. Moving the core farther into the coil increases the [[Permeability (electromagnetism)|permeability]], increasing the magnetic field and the inductance. Many inductors used in radio applications (usually less than 100&nbsp;MHz) use adjustable cores in order to tune such inductors to their desired value, since manufacturing processes have certain tolerances (inaccuracy). Sometimes such cores for frequencies above 100&nbsp;MHz are made from highly conductive non-magnetic material such as aluminum.<ref>{{cite web |url=http://www.coilcraft.com/pdfs/uni5.pdf |series=Coilcraft catalog |title=page with aluminum cores |access-date=10 July 2015 }}{{Dead link|date=September 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> They decrease the inductance because the magnetic field must bypass them.

Air core inductors can use sliding contacts or multiple taps to increase or decrease the number of turns included in the circuit, to change the inductance. A type much used in the past but mostly obsolete today has a spring contact that can slide along the bare surface of the windings. The disadvantage of this type is that the contact usually [[short circuit|short-circuits]] one or more turns. These turns act like a single-turn short-circuited transformer [[secondary winding]]; the large currents induced in them cause power losses.

A type of continuously variable air core inductor is the ''variometer''. This consists of two coils with the same number of turns connected in series, one inside the other. The inner coil is mounted on a shaft so its axis can be turned with respect to the outer coil. When the two coils' axes are collinear, with the magnetic fields pointing in the same direction, the fields add and the inductance is maximum. When the inner coil is turned so its axis is at an angle with the outer, the mutual inductance between them is smaller so the total inductance is less. When the inner coil is turned 180° so the coils are collinear with their magnetic fields opposing, the two fields cancel each other and the inductance is very small. This type has the advantage that it is continuously variable over a wide range. It is used in [[antenna tuner]]s and matching circuits to match low frequency transmitters to their antennas.

Another method to control the inductance without any moving parts requires an additional DC current bias winding which controls the permeability of an easily saturable core material. ''See'' [[Magnetic amplifier]].

===Choke===
[[File:Two inductors (437342545).jpg|thumbnail|An MF or HF radio choke for tenths of an ampere, and a ferrite bead VHF choke for several amperes.]]
A [[choke (electronics)|choke]] is an inductor designed specifically for blocking high-frequency alternating current (AC) in an electrical circuit, while allowing DC or low-frequency signals to pass. Because the inductor restricts or "chokes" the changes in current, this type of inductor is called a choke. It usually consists of a coil of insulated wire wound on a magnetic core, although some consist of a donut-shaped "bead" of ferrite material strung on a wire. Like other inductors, chokes resist changes in current passing through them increasingly with frequency. The difference between chokes and other inductors is that chokes do not require the high [[Q factor]] construction techniques that are used to reduce the resistance in inductors used in tuned circuits.