Inductor 1

Inductor
Inductor

A selection of low-value inductors

Type Passive

Working principle Electromagnetic induction

First production Michael Faraday (1831)

Electronic symbol

An inductor (or reactor) is a passive electrical component that can store energy in a magnetic field created by the
electric current passing through it. An inductor's ability to store magnetic energy is measured by its inductance, in
units of henries. Typically an inductor is a conducting wire shaped as a coil; the loops help to create a strong
magnetic field inside the coil due to Ampere's Law. Due to the time-varying magnetic field inside the coil, a voltage
is induced, according to Faraday's law of electromagnetic induction, which by Lenz's Law opposes the change in
current that created it. Inductors are one of the basic components used in electronics where current and voltage
change with time, due to the ability of inductors to delay and reshape alternating currents. Inductors called chokes
are used as parts of filters in power supplies or to block AC signals from passing through a circuit.

Overview
Inductance (L) results from the magnetic field forming around a current-carrying conductor which tends to resist
changes in the current. Electric current through the conductor creates a magnetic flux proportional to the current, and
a change in this current creates a corresponding change in magnetic flux which, in turn, by Faraday's Law generates
an electromotive force (EMF) that opposes this change in current. Inductance is a measure of the amount of EMF
generated per unit change in current. For example, an inductor with an inductance of 1 henry produces an EMF of 1
volt when the current through the inductor changes at the rate of 1 ampere per second. The number of loops, the size
of each loop, and the material it is wrapped around all affect the inductance. For example, the magnetic flux linking
these turns can be increased by coiling the conductor around a material with a high permeability such as iron. This
can increase the inductance by 2000 times.
Inductor 2

Ideal and real inductors

An "ideal inductor" has inductance, but no resistance or capacitance, and does not dissipate or radiate energy. A real
inductor may be partially modeled by a combination of inductance, resistance (due to the resistance of the wire and
losses in core material), and capacitance. At some frequency, some real inductors behave as resonant circuits (due to
their self capacitance). At some frequency the capacitive component of impedance becomes dominant. Energy is
dissipated by the resistance of the wire, and by any losses in the magnetic core due to hysteresis. Practical iron-core
inductors at high currents show gradual departure from ideal behavior due to nonlinearity caused by magnetic
saturation. At higher frequencies, resistance and resistive losses in inductors grow due to skin effect in the inductor's
winding wires. Core losses also contribute to inductor losses at higher frequencies. Practical inductors work as
antennas, radiating a part of energy processed into surrounding space and circuits, and accepting electromagnetic
emissions from other circuits, taking part in electromagnetic interference. Circuits and materials close to the inductor
will have near-field coupling to the inductor's magnetic field, which may cause additional energy loss. Real-world
inductor applications may consider the parasitic parameters as important as the inductance.

Applications
Inductors are used extensively in analog circuits and signal processing.
Inductors in conjunction with capacitors and other components form
tuned circuits which can emphasize or filter out specific signal
frequencies. Applications range from the use of large inductors in
power supplies, which in conjunction with filter capacitors remove
residual hums known as the mains hum or other fluctuations from the
direct current output, to the small inductance of the ferrite bead or torus
installed around a cable to prevent radio frequency interference from
being transmitted down the wire. Smaller inductor/capacitor
combinations provide tuned circuits used in radio reception and
broadcasting, for instance.

Two (or more) inductors that have coupled magnetic flux form a
transformer, which is a fundamental component of every electric utility
power grid. The efficiency of a transformer may decrease as the
An inductor with two 47mH windings, as may be
frequency increases due to eddy currents in the core material and skin
found in a power supply.
effect on the windings. Size of the core can be decreased at higher
frequencies and, for this reason, aircraft use 400 hertz alternating
current rather than the usual 50 or 60 hertz, allowing a great saving in weight from the use of smaller transformers.[1]

An inductor is used as the energy storage device in some switched-mode power supplies. The inductor is energized
for a specific fraction of the regulator's switching frequency, and de-energized for the remainder of the cycle. This
energy transfer ratio determines the input-voltage to output-voltage ratio. This XL is used in complement with an
active semiconductor device to maintain very accurate voltage control.
Inductors are also employed in electrical transmission systems, where they are used to depress voltages from
lightning strikes and to limit switching currents and fault current. In this field, they are more commonly referred to as
reactors.
Larger value inductors may be simulated by use of gyrator circuits.
Inductor 3

Inductor construction
An inductor is usually constructed as a coil of conducting material, typically
copper wire, wrapped around a core either of air or of ferromagnetic or
ferrimagnetic material. Core materials with a higher permeability than air
increase the magnetic field and confine it closely to the inductor, thereby
increasing the inductance. Low frequency inductors are constructed like
transformers, with cores of electrical steel laminated to prevent eddy currents.
'Soft' ferrites are widely used for cores above audio frequencies, since they do
not cause the large energy losses at high frequencies that ordinary iron alloys
do. Inductors come in many shapes. Most are constructed as enamel coated
wire (magnet wire) wrapped around a ferrite bobbin with wire exposed on the
outside, while some enclose the wire completely in ferrite and are referred to
as "shielded". Some inductors have an adjustable core, which enables
changing of the inductance. Inductors used to block very high frequencies are Inductors. Major scale in centimetres.

sometimes made by stringing a ferrite cylinder or bead on a wire.

Small inductors can be etched directly onto a printed circuit board by laying out the trace in a spiral pattern. Some
such planar inductors use a planar core.
Small value inductors can also be built on integrated circuits using the same processes that are used to make
transistors. Aluminium interconnect is typically used, laid out in a spiral coil pattern. However, the small dimensions
limit the inductance, and it is far more common to use a circuit called a "gyrator" that uses a capacitor and active
components to behave similarly to an inductor.

Types of inductors

Air core coil

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 actually have 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 losses 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 inductors

At high frequencies, particularly radio frequencies (RF), inductors have higher resistance and other losses. In
addition to causing power loss, in resonant circuits this can reduce the Q factor of the circuit, broadening the
bandwidth. In RF inductors, which are mostly air core types, 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. Radio frequency alternating current does not penetrate far into the body of a conductor but
travels along its surface. Therefore, in a solid wire, most of the cross sectional area of the wire is not used to
conduct the current, which is in a narrow annulus on the surface. This effect increases the resistance of the wire in
the coil, which may already have a relatively high resistance due to its length and small diameter.
• Proximity effect: Another similar effect that also increases the resistance of the wire at high frequencies is
proximity effect, which occurs in parallel wires that lie close to each other. The individual magnetic field of
adjacent turns induces eddy currents in the wire of the coil, which causes the current in the conductor to be
Inductor 4

concentrated in a thin strip on the side near the adjacent wire. Like skin effect, this reduces the effective
cross-sectional area of the wire conducting current, increasing its resistance.
• 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.
To reduce parasitic capacitance and proximity effect, 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.
• Honeycomb 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-weave coils.
• 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 several 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 braided together. The braid pattern ensures that each wire strand spends
the same amount of its length on the outside of the braid, so skin effect distributes the current equally between the
strands, resulting in a larger cross-sectional conduction area than an equivalent single wire.

Ferromagnetic core coil

Ferromagnetic-core or iron-core inductors use a magnetic core made of a ferromagnetic or ferrimagnetic material
such as iron or 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:
• Core losses: 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:
• Eddy currents: 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 resistance
of the core material. The amount of energy lost increases with the area inside the loop of current.
• Hysteresis: Changing or reversing the magnetic field in the core also causes losses due to the motion of the tiny
magnetic domains 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.
For both of these processes, the energy loss per cycle of alternating current is constant, so core losses increase
linearly with frequency.
• Nonlinearity: If the current through a ferromagnetic core coil is high enough that the magnetic core saturates, the
inductance will not remain constant but will change with the current through the device. This is called
nonlinearity and results in distortion of the signal. For example, audio signals can suffer intermodulation
Inductor 5

distortion in saturated inductors. To prevent this, in linear circuits the current through iron core inductors must be
limited below the saturation level. Using a powdered iron core with a distributed air gap allows higher levels of
magnetic flux which in turn allows a higher level of direct current through the inductor before it saturates.[2]

Laminated core inductor

Low-frequency inductors are often made with laminated cores to prevent eddy currents, using construction similar to
transformers. The core is made of stacks of thin steel sheets or laminations 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
the energy loss greatly. The laminations are made of low-coercivity silicon steel, to reduce hysteresis losses.

Ferrite-core inductor
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 xxFe2O4 where xx represents
various metals. For inductor cores soft ferrites are used, which have low coercivity and thus low hysteresis losses.
Another similar material is powdered iron cemented with a binder.

Toroidal core coils

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 reenter 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. 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 toroidal 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, Kool Mu MPP, powdered iron and laminated cores.[3]

Variable inductor
A variable inductor can be constructed by making one of the terminals of the device a sliding spring contact that can
move along the surface of the coil, increasing or decreasing the number of turns of the coil included in the circuit. An
alternate construction method is to use a moveable magnetic core, which can be slid in or out of the coil. Moving the
core farther into the coil increases the permeability, increasing the inductance. Many inductors used in radio
applications (usually less than 100 MHz) use adjustable cores in order to tune such inductors to their desired value,
since manufacturing processes have certain tolerances (inaccuracy).

Core loss
Core loss calculators[4] can be used to determine the type of inductor required. Using inputs such as input voltage,
output voltage, output current, frequency, ambient temperature, and inductance these calculators can predict the
losses of the inductors core and AC/DC based on the operating condition of the circuit being used.[5]

In electric circuits
The effect of an inductor in a circuit is to oppose changes in current through it by developing a voltage across it
proportional to the rate of change of the current. An ideal inductor would offer no resistance to a constant direct
current; however, only superconducting inductors have truly zero electrical resistance.
The relationship between the time-varying voltage v(t) across an inductor with inductance L and the time-varying
current i(t) passing through it is described by the differential equation:
Inductor 6

When there is a sinusoidal alternating current (AC) through an inductor, a sinusoidal voltage is induced. The
amplitude of the voltage is proportional to the product of the amplitude (IP) of the current and the frequency (f) of the
current.

In this situation, the phase of the current lags that of the voltage by π/2.
If an inductor is connected to a direct current source with value I via a resistance R, and then the current source is
short-circuited, the differential relationship above shows that the current through the inductor will discharge with an
exponential decay:

Laplace circuit analysis (s-domain)

When using the Laplace transform in circuit analysis, the impedance of an ideal inductor with no initial current is
represented in the s domain by:

where
L is the inductance, and
s is the complex frequency.
If the inductor does have initial current, it can be represented by:
• adding a voltage source in series with the inductor, having the value:

(Note that the source should have a polarity that is aligned with the initial current)
• or by adding a current source in parallel with the inductor, having the value:

where
L is the inductance, and
is the initial current in the inductor.
Inductor 7

Inductor networks
Inductors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent
inductance (Leq):

The current through inductors in series stays the same, but the voltage across each inductor can be different. The sum
of the potential differences (voltage) is equal to the total voltage. To find their total inductance:

These simple relationships hold true only when there is no mutual coupling of magnetic fields between individual
inductors.
Inductor 8

Stored energy
The energy (measured in joules, in SI) stored by an inductor is equal to the amount of work required to establish the
current through the inductor, and therefore the magnetic field. This is given by:

where L is inductance and I is the current through the inductor.

This relationship is only valid for linear (non-saturated) regions of the magnetic flux linkage and current relationship.

Q factor
An ideal inductor will be lossless irrespective of the amount of current through the winding. However, typically
inductors have winding resistance from the metal wire forming the coils. Since the winding resistance appears as a
resistance in series with the inductor, it is often called the series resistance. The inductor's series resistance converts
electric current through the coils into heat, thus causing a loss of inductive quality. The quality factor (or Q) of an
inductor is the ratio of its inductive reactance to its resistance at a given frequency, and is a measure of its efficiency.
The higher the Q factor of the inductor, the closer it approaches the behavior of an ideal, lossless, inductor.
The Q factor of an inductor can be found through the following formula, where R is its internal electrical resistance
and is capacitive or inductive reactance at resonance:

By using a ferromagnetic core, the inductance is greatly increased for the same amount of copper, multiplying up the
Q. Cores however also introduce losses that increase with frequency. A grade of core material is chosen for best
results for the frequency band. At VHF or higher frequencies an air core is likely to be used.
Inductors wound around a ferromagnetic core may saturate at high currents, causing a dramatic decrease in
inductance (and Q). This phenomenon can be avoided by using a (physically larger) air core inductor. A well
designed air core inductor may have a Q of several hundred.
An almost ideal inductor (Q approaching infinity) can be created by immersing a coil made from a superconducting
alloy in liquid helium or liquid nitrogen. This supercools the wire, causing its winding resistance to disappear.
Because a superconducting inductor is virtually lossless, it can store a large amount of electrical energy within the
surrounding magnetic field (see superconducting magnetic energy storage). Bear in mind that for inductors with
cores, core losses still exist.

Inductance formulae
The table below lists some common simplified formulas for calculating the approximate inductance of several
inductor constructions.
Inductor 9

Construction Formula Dimensions Notes

[6]
Cylindrical air-core coil • L = inductance in henries (H)
• μ0 = permeability of free space = 4
× 10−7 H/m
[6]
• K = Nagaoka coefficient
• N = number of turns
• A = area of cross-section of the coil in
square metres (m2)
• l = length of coil in metres (m)

[7] • L = inductance exact if ω = 0 or

Straight wire conductor
• l = cylinder length ω=∞
• c = cylinder radius
• μ0 = vacuum permeability =
nH/cm
• μ = conductor permeability
• p = resistivity
• ω = phase rate

• L = inductance (µH) • Cu or Al
-0+3% • l = length of conductor (mm) • l > 100 d
• d = diameter of conductor (mm) • d2 f > 1 mm2
• f = frequency MHz

• L = inductance (µH) • Cu or Al
+0-3% • l = length of conductor (mm) • l > 100 d
• d = diameter of conductor (mm) • d2 f < 1 mm2
• f = frequency MHz

• L = inductance (µH)
Short air-core cylindrical
[8] • r = outer radius of coil (in)
coil
• l = length of coil (in)
• N = number of turns

Multilayer air-core coil • L = inductance (µH)

• r = mean radius of coil (in)
• l = physical length of coil winding (in)
• N = number of turns
• d = depth of coil (outer radius minus
inner radius) (in)

Flat spiral air-core coil • L = inductance (µH)

• r = mean radius of coil (cm)
• N = number of turns
• d = depth of coil (outer radius minus
inner radius) (cm)

• L = inductance (µH)
• r = mean radius of coil (in)
• N = number of turns
• d = depth of coil (outer radius minus
inner radius) (in)
Inductor 10

Toroidal core (circular • L = inductance (H)

cross-section) • μ0 = permeability of free space = 4
× 10−7 H/m
• μr = relative permeability of core
material
• r = radius of coil winding (m)
• N = number of turns
• D = overall diameter of toroid (m)

Synonyms
• choke
• coil
• reactor

Notes
[1] "Aircraft electrical systems" (http:/ / www. wonderquest. com/ expounding-aircraft-electrical-systems. htm). Wonderquest.com. . Retrieved
2010-09-24.
[2] "Inductors 101" (http:/ / www. newark. com/ pdfs/ techarticles/ vishay/ Inductors101. pdf). vishay. . Retrieved 2010-09-24.
[3] "Inductor and Magnetic Product Terminology" (http:/ / www. datasheetarchive. com/ datasheet-pdf/ 072/ DSA00379445. html).
Datasheetarchive.com. . Retrieved 2010-09-24.
[4] Vishay. "Products - Inductors - IHLP inductor loss calculator tool landing page" (http:/ / www. vishay. com/ inductors/ calculator-home-list/
). Vishay. . Retrieved 2010-09-24.
[5] View: Everyone Only Notes. "IHLP inductor loss calculator tool" (http:/ / www. element-14. com/ community/ docs/ DOC-17923).
element14. . Retrieved 2010-09-24.
[6] Nagaoka, Hantaro (1909-05-06). The Inductance Coefficients of Solenoids (http:/ / www. g3ynh. info/ zdocs/ refs/ Nagaoka1909/ index.
html#31). 27. Journal of the College of Science, Imperial University, Tokyo, Japan. p. 18.
[7] The Self and Mutual Inductances of Linear Conductors, By Edward B. Rosa, Bulletin of the Bureau of Standards, Vol.4, No.2, 1908,
p301-344 (http:/ / www. g3ynh. info/ zdocs/ refs/ Rosa1908/ index. html)
[8] ARRL Handbook, 66th Ed. American Radio Relay League (1989).

External links
General
• How stuff works (http://electronics.howstuffworks.com/inductor1.htm) The initial concept, made very simple
• Capacitance and Inductance (http://www.lightandmatter.com/html_books/4em/ch07/ch07.html) - A chapter
from an online textbook
• Spiral inductor models (http://www.mpdigest.com/issue/Articles/2005/aug2005/agilent/Default.asp).
Article on inductor characteristics and modeling.
• Online coil inductance calculator (http://www.66pacific.com/calculators/coil_calc.aspx). Online calculator
calculates the inductance of conventional and toroidal coils using formulas 3, 4, 5, and 6, above.
• AC circuits (http://www.phys.unsw.edu.au/~jw/AC.html)
• Understanding coils and transforms (http://www.mikroe.com/en/books/keu/03.htm)
Article Sources and Contributors 11

Article Sources and Contributors

Inductor Source: http://en.wikipedia.org/w/index.php?oldid=419788398 Contributors: A. Carty, AbJ32, Acronymsical, Aditya Cholan, Adpete, Aempirei, Aidanlister, Aitias, Alertjean, Alfred
Centauri, Amaraiel, Andonic, Antikon, Arch dude, Armstrong1113149, Army1987, ArnoldReinhold, Arthur Rubin, Ascidian, Atlant, Austin RS, BD2412, BabyBatter, Baseball Bugs,
Bdieseldorff, Bemoeial, Ben-Zin, Benjah-bmm27, Berean Hunter, Bernard François, Berrinam, Bert Hickman, BillC, Bobblewik, Bogdangiusca, Breakeydown, CAkira, Cbdorsett, Cfallin,
Chairboy, Chan siuman, CharlesC, Chetvorno, Chowbok, Christian75, Christopher Mahan, Congruence, Conversion script, Copeland.James.H, CosineKitty, Cpl Syx, CyrilB, Cyrius, DV8 2XL,
Dalstadt, DeadEyeArrow, Defrector, Dendodge, Dgrant, Dicklyon, Dlrohrer2003, Download, Dratman, Drhlajos, Ebraminio, Eclecticology, Elspec, Even stevenson, Fresheneesz, Fumitol, G-W,
Gail, Gaius Cornelius, Galoubet, Gene Nygaard, GeoGreg, Gerben49, Giftlite, Glenn, Gobonobo, Grandfatherclok, GreenSpigot, Gzuckier, Haham hanuka, Harland1, Harriv, HazardX21, Hefo,
Hephaestos, Heron, Highsand, Hmo, Hooperbloob, Immibis, InvertRect, Iwsh, J. W. Love, Jauhienij, Jiang, Jlg4104, Joel D. Reid, John of Reading, Junglecat, KDesk, KNfLrPnKNsT, Kar.ma,
Karthik262399, Katherine, Kbwikipedia, Keenan Pepper, Khalid Mahmood, Kieff, Kingpin13, Krishnavedala, Lexicon, Light current, Lightmouse, Lindosland, Lindseyrose, Little Mountain 5,
LizGere, Loggie, Lommer, Looxix, Lornova, Lovecz, LukeB 11, M jurrens, Manuel Trujillo Berges, Maralia, MarsRover, Maxzimet, Meggar, Meisongbei, Mhims, Michael Hardy, Mike Dill,
Mike1024, Mild Bill Hiccup, Mintguy, MisterSheik, Mkill, Mondebleu, Mormegil, MrStalker, Munozdj, Myanw, NawlinWiki, Nedim Ardoğa, Nemu, Neonil, Nickptar, Nikai, Nk, No such user,
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RandomXYZb, RaseaC, Reddi, Rich Farmbrough, RickK, Rickcwalker, Rjwilmsi, Rogper, Romanm, Ronhjones, Rtdrury, Salgueiro, Sankalpdravid, ScAvenger lv, Searchme, SebastianHelm,
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