# Coil (electrical engineering)

Coils in different forms, with or without ferrite core ( high-frequency chokes )

In electrical engineering, coils are, on the one hand, windings and winding goods that are suitable for generating or detecting a magnetic field . They are electrical components or are part of a device such as a transformer , relay , electric motor or loudspeaker .

On the other hand, separate coils are inductive passive components whose essential property is a defined inductance . They are mainly used in the field of signal processing for frequency-determining circles, e.g. B. LC resonant circuits , low-pass filters , high-pass , band-pass filters , signal phase characteristic correcting, for interference suppression, to the current flow smoothing or as an energy store in switching power supplies, and many other electrical and electronic equipment used. See also throttle (electrical engineering) .

The frequency of use of the coils is, however, much lower than that of resistors and capacitors , since these are often cheaper and easier to manufacture and can also be integrated more cheaply in electronic semiconductor circuits. In electronic circuit design, the use of coils is therefore often avoided - if at all possible - if they can be simulated with capacitors, resistors and active components (transistors), for example by means of a gyrator circuit.

Most coils consist of at least one winding of a current conductor made of wire , enamelled copper wire , silver-plated copper wire or high-frequency stranded wire , which is usually wound on a bobbin ( bobbin ) and is predominantly provided with a soft magnetic core. The winding arrangement and shape, the wire diameter, the winding and the core material determine the value of the inductance and other (quality) properties of the coil.

In addition, spiral-shaped conductor tracks on printed circuit boards, which are optionally surrounded by surrounding ferrite cores , are "coils" in the sense of an inductive passive component. The turns of a coil must always be insulated from one another and from the coil core, which is often electrically conductive, in order to prevent a winding short , which would significantly impair the function of the coil. In the case of coils and transformers with several layers of turns or windings made of enamelled copper wire, the individual layers of windings or windings z. B. additionally isolated against voltage breakdown by lacquer paper.

Coil as a printed circuit with a ferrite core

## Structure, component names

Circuit symbol for coils left to IEC  617-4 (1983), the right to the IEC 617-4 (1996) and DIN  EN 60617-4 (1997)

A coil is a wound wire with the turns isolated from one another. One turn is one revolution including the supply lines. There are only integer numbers of turns. A winding body ( coil body ) does not necessarily have to be present. If the winding body is missing or if it is made of non-magnetic material, one speaks of air-core coils in the mechanical or electrical sense . The coil body mostly only serves to mechanically stabilize the wire and, unlike the coil core, has no magnetic influence.

Coils are also available in a flat spiral shape and with a rectangular or any other shaped coil cross-section. You can also be implemented as a spiral conductor track directly on a circuit board .

Coils have a certain inductance ; this inductance can be their actual purpose (e.g. choke coils , filter coils ) or only a secondary property (e.g. transformers , pull magnets , relay coils ).

In electric motors , the coils are used as a winding and z. B. in the Pupin coil referred to as a coiled line .

In addition to the wound wire and the bobbin, the coil often has a (coil) core (see below) inside to increase the inductance.

The word coil indicates the type of construction (see coil (roll) ).

The inductance of a coil is measured in the unit henry (see henry (unit) ).

## functionality

The main property of coils is their inductance . The inductance results from the number of turns of the coil, the material enclosed by the coil and the dimensions. Due to the magnetic linkage (flux linkage) of the individual windings with one another, due to the close proximity of the individual windings, the inductivity of wound coils theoretically increases as the square of the number of windings. A doubling of the number of turns with the same geometric dimensions thus results in a quadrupling of the inductance.

If an electrical voltage is applied to the terminals of the coil , the current (which flows through the voltage source in this experiment) does not change suddenly. With an ideal coil with an inductance of 1 H and a voltage of 1 V, the current has increased to 1 A after 1 s. A voltage can also arise at the ohmic resistance (internal resistance) of a non-ideal coil itself, or at a resistor inserted in the circuit of the coil. The change in the current due to the applied voltage or the voltage drop only comes to a standstill when the current at the internal resistance creates a corresponding counter-voltage. A short-circuited ideal coil (compare: superconductor ) theoretically never discharges. Simultaneously with the current through the coil wire, a magnetic field is created in the coil.

A changing magnetic flux of an externally applied magnetic field generates an induction current on a (short-circuited) conductor loop and a corresponding self- induction voltage at the ends of the open electrical conductor . This voltage is directed in such a way that it counteracts its cause (the current) ( Lenz's rule ). An increase in the rate of change in the magnetic flux leads to an increase in the voltage that opposes the current. The proportionality factor between the current that changes over time through the conductor and the resulting self-induction voltage is called inductance.

In addition to the actual desired inductance, real coils also have other, generally undesirable electrical properties such as electrical resistance , parasitic capacitances and thus at least one electrical resonance point (natural resonance, parallel resonant circuit ) or, in the case of a coil core that increases the inductivity, a disruptive remanence and eddy current losses. All these parameters are temperature and working frequency dependent. It is therefore only sensible to use them up to a component-typical maximum cut-off frequency, where there is still a sufficient inductive reactance or phase angle in the corresponding insert circuit.

If, on the other hand, a high-quality resistor, consisting of a long wound (resistance) wire, has a particularly low inductance, the mechanical resistance wire carrier, e.g. B. a porcelain tube with contact clips , bifilar wound with a wire running in the opposite direction. The opposing magnetic fluxes almost cancel each other out. This method is used, for example, for wired load resistors for the high low frequency range up to about 100 kHz.

## Magnetic field and current flow

Magnetic field of a coil

The following mnemonics can be used to determine which end of a coil forms a magnetic north pole and which end forms a south pole when a direct current flows through it (the technical direction of the current , i.e. from the plus to the minus pole, is to be used as the direction of the current ):

• If you look at a coil end and this in  traversed clockwise from electricity, so there arises a magnetic south pole.
• If you look at a coil end and this is against  traversed clockwise from electricity, so there arises a magnetic north pole.
• If you grasp the turns of the coil with your right hand in such a way that the fingers (except for the thumb) are directed along the turns in the technical direction of the current, the thumb points in the direction of the magnetic north pole of the coil.

Inside a slim coil (length much larger than diameter) the length  with  turns in which an electric current  flows, the magnetic field is created with the field strength ${\ displaystyle l}$${\ displaystyle n}$${\ displaystyle I}$

${\ displaystyle H = I \ cdot {\ frac {n} {l}}}$

The flux density B  resulting from the coil core (s. U.) Dependent material constants μ r and the magnetic field constant μ 0 = 4π x 10 -7  H / m thus

${\ displaystyle B = \ mu _ {r} \ cdot \ mu _ {0} \ cdot H = \ mu _ {r} \ cdot \ mu _ {0} \ cdot I \ cdot {\ frac {n} {l }}}$

## Coil cores

Coil cores have the task of increasing or reducing the inductance of the coil. The increase in inductance achieved by a magnetic core leads to a reduction in the number of turns or conductor length required for a certain inductance value and thus to a reduction in the interfering electrical resistance of the coil.

Cores made of electrical conductors such as copper or aluminum , which reduce the inductance by displacement of the field, are used to tune ( resonant circuit ) coils in the high frequency range, e.g. B. used in FM tuners.

### Coil with iron core

Eddy currents in the iron block (above) and in laminated sheets (below)
Coil with pot core
Fixed inductors with colored rings.
Top: 6.8 µH
Middle: 22 µH
Bottom: 2.2 µH

If an iron core is inserted into a coil , its ferromagnetic properties increase the permeability and thus also the magnetic flux density in the coil. This means that you get by with significantly fewer turns and thus with much less component volume in order to achieve the required inductance . Above a certain material-dependent flux density, however, a disruptive saturation magnetization of the core occurs.

Because the iron of the core is an electrical conductor, an undesired eddy current is induced in it, like in a short-circuit coil through which an alternating current flows , which heats the iron core. This eddy current can be reduced if the core does not consist of a solid piece of iron, but of a stack of iron sheets. These must be isolated from each other by layers of lacquer or (previously) paper in order to interrupt the eddy current.

At very high frequencies, the coil is filled with electrically non-conductive powder press material or ferrimagnetic material such as ferrite in order to increase the inductance.

These magnetic core materials typically have a hysteresis effect (remanence), which leads to electrical losses because the core has to be remagnetized with each period of an alternating current. In addition, this results in a deformation of the current curve with additional peaks in each period, which is unwelcome in some applications because it increases the distortion factor . The losses that occur due to eddy currents and hysteresis are called iron losses .

The switch-on behavior of coils with an iron core is also much more complex because, depending on the state of the core before switching on, there is almost no magnetization or a noticeable magnetization already acts as remanence, which either corresponds to the current polarity or can also be opposite and then through the inrush current must first be remagnetized. These effects mean that in extreme cases when a voltage is switched on, fuses respond beforehand due to a possible inrush current until the nominal, current-limiting inductance is reached, although there is actually no overload case. In the case of larger inductances, such as transformers or reactors with iron cores, special precautions must therefore often be taken in AC power applications, especially when switching on, see for example transformer switching relays . However, self-induction voltages that occur when switching off must also be observed in terms of the circuitry. In small-signal applications, the hysteresis effects only lead to a reduced quality of the component at the moment it is switched on. With coils and especially with transformers of higher power, starting from a few watts, an annoying acoustic noise generation of the core material often occurs in the low frequency range, which is referred to as mains hum . It is caused by small mechanical changes in size of the nucleus due to the changing magnetic field, see magnetostriction . This effect can be reduced by vacuum impregnation with a special lacquer, which at the same time increases the dielectric strength between different (transformer) coils.

The elementary magnets in the iron core are aligned with the poles of the coil. If the north pole is on the left, the north poles of the elementary magnets are also on the left. The field lines emerge at the north pole and reenter the inside of the coil at the south pole. Inside the coil, the field lines run from south to north. In the case of an elongated coil with many turns, the magnetic field inside is homogeneous; it is similar to the magnetic field between the legs of a horseshoe magnet. Outside, the coil field is similar to that of a bar magnet.

### Cores in high frequency coils

Usually a core made of pressed magnetic powder ( powder core ) or ferrite is used for this purpose . Toroidal coils or toroidal core chokes are used to filter high-frequency interference .

With tunable coils, ferrite cores with a thread are used; Details can be found in the adjustment coil section .

## High frequency coils

Cross-wound coil made of HF litz wire with trimmable iron powder core for the medium wave range

With increasing frequency, the currents are increasingly displaced to the surface of the wire ( skin effect ). The wire surface then increasingly determines the quality of the coil. From approx. 100 kHz, high-frequency stranded wire is therefore often used as the winding material to reduce losses ; it consists of several fine wires isolated from one another. From around 50 MHz, the coils are mostly self-supporting with thicker wire. A silver-plated surface can also reduce losses. Cores for high frequency coils consist of a ferromagnetic, electrically non-conductive material. This prevents eddy currents in the core. The design can also make a coil suitable for high frequencies by reducing parasitic capacitances in those with a high number of turns (for example for the medium wave range) using special winding shapes (honeycomb, basket bottom or cross wound coils ).

### Coils for oscillators

Coils in oscillators or band filters should basically maintain their inductance as precisely as possible. A low temperature coefficient that is still present, which is mainly caused by the core material used, can be almost completely compensated for by an opposing temperature coefficient of the resonant circuit capacitance used with the appropriate component selection and dimensioning of the partial capacitors.

Air-core coils can cause frequency modulation due to the slightest changes in inductance when vibrated . They are therefore wound onto a bobbin, fixed with lacquer or glue, or completely embedded in wax .

## AC behavior

Consumer arrow system: current and voltage arrows point in the same direction in the component

If an AC voltage is applied to a coil , the current and the magnetic field also change their direction periodically. There is a relationship between the time course of the coil current i (t) and the terminal voltage u (t)

${\ displaystyle u (t) = L \ cdot {\ frac {\ mathrm {d} i (t)} {\ mathrm {d} t}}}$,

where t is the time and L is the self-inductance of the coil. Here, current and voltage, as is usual with passive components, are indicated in the consumer meter arrow system.

Since the current can only rise or fall gradually because of the energy transport in the magnetic field, it always follows the course of the voltage with a time delay; he is out of phase . Under ideal conditions (with negligibly small ohmic resistance ) the alternating voltage leads the current by 90 °. There is an inertia of the coil against current changes. (Memorandum: "With inductivities the currents are delayed".)

When current flows through a coil, energy is stored in the magnetic field:

${\ displaystyle E _ {\ mathrm {mag}} = {\ tfrac {1} {2}} LI ^ {2}}$

Mathematically, the phase shift follows from the derivation rules for trigonometric functions: For example, if a sinusoidal current

${\ displaystyle i (t) = I_ {0} \ cdot \ sin (\ omega t)}$

impressed in the coil, the voltage on the coil results from mathematical derivation

${\ displaystyle u (t) = L \ cdot {\ frac {\ mathrm {d} i (t)} {\ mathrm {d} t}} = \ omega L \ cdot I_ {0} \ cdot \ cos (\ omega t)}$.

The ratio of maximum coil voltage and maximum coil current is with sinusoidal excitation

${\ displaystyle {\ frac {U_ {0}} {I_ {0}}} = \ omega L}$.

A complex alternating current resistance (impedance) can thus be assigned to the coil , which, however, in contrast to an ohmic resistance, does not convert any power into heat ( power loss ). This is due to the fact that energy is absorbed by the coil during a quarter period and released again in the next quarter period. As a result, the energy only shuttles back and forth without doing any work. This special form of resistance is called reactance and the current is called reactive current . ${\ displaystyle {\ underline {Z}} = \ mathrm {j} \ omega L}$

For a coil of inductance L and an alternating current of frequency f , the reactance is calculated

${\ displaystyle X = \ Im {({\ underline {Z}})}}$

to

${\ displaystyle X = 2 \ pi f \ cdot L = \ omega \ cdot L}$

with the dimension [V / A].

${\ displaystyle \ omega = 2 \ pi f}$

is called the angular frequency or circular frequency .

The reactance increases with increasing frequency, with the ohmic wire resistance remaining the same. Therefore, a coil designed for AC voltage has a much lower resistance at an equally large DC voltage (f = 0 Hz), since only the wire resistance hinders the current.

### Coil equation

Relationship between self-induced voltage and terminal voltage
Area of ​​a coil with three turns

The coil equation

${\ displaystyle u (t) = L \ cdot {\ frac {\ mathrm {d} i (t)} {\ mathrm {d} t}}}$.

results in the specified form only with a linear material behavior of the core with and with a negligibly small electrical field strength in the winding wire. This is to be shown in the following with the help of the law of induction and flow. ${\ displaystyle B = \ mu _ {0} \ mu _ {r} H}$

The induction law says in general terms: . In this case it should be used for a stationary contour line and can therefore also be used in the special form ${\ displaystyle \ oint \ limits _ {\ partial A} {\ vec {E}} {\ text {d}} {\ vec {s}} = - \ iint \ limits _ {A} {\ frac {\ partial {\ vec {B}}} {\ partial t}} \ mathrm {d} {\ vec {A}}}$${\ displaystyle \ partial A}$

${\ displaystyle \ oint \ limits _ {\ partial A} {\ vec {E}} {\ text {d}} {\ vec {s}} = - {\ frac {\ text {d}} {{\ text {d}} t}} \ iint \ limits _ {A} {\ vec {B}} \ mathrm {d} {\ vec {A}}}$

be noted.

As the integration path, we choose the path shown in the adjacent figure with dashed lines ( instead of there ). The associated coil area is illustrated in the associated video. ${\ displaystyle \ partial r}$${\ displaystyle \ partial A}$

If you take into account that the electrical field strength in the conductor is approximately zero, the ring integral is fed exclusively from the negative terminal voltage via the electrical field strength . The negative sign comes from the fact that the integration path is traversed against the direction of the arrow of the terminal voltage. Thus: ${\ displaystyle {\ vec {E}}}$${\ displaystyle -u (t)}$

${\ displaystyle u (t) = {\ frac {\ text {d}} {{\ text {d}} t}} \ iint \ limits _ {A} {\ vec {B}} \ mathrm {d} { \ vec {A}}}$

With a linear core behavior, the magnetic flux through the entire coil and the current are strictly proportional to one another, so that a proportionality factor (the so-called inductance) can be introduced. The following then applies: ${\ displaystyle \ Psi = \ iint \ limits _ {A} {\ vec {B}} \ mathrm {d} {\ vec {A}}}$${\ displaystyle i (t)}$${\ displaystyle L}$

${\ displaystyle u (t) = {\ frac {\ text {d}} {{\ text {d}} t}} \ left (L \ cdot i (t) \ right)}$

If the core material does not change its behavior over time and its position relative to the loops remains constant, L is independent of time, and one can also write:

${\ displaystyle u (t) = L \ cdot {\ frac {\ text {d}} {{\ text {d}} t}} \ left (i (t) \ right)}$

### Parasitic elements

Vector diagram of the impedance Z of a coil

Real coils show a phenomenon in an AC circuit that can be explained with the help of the topological vector diagram. The equivalent ohmic series resistance (ESR), which can be determined as copper resistance with direct current, appears to be higher in alternating current operation. Reasons for this are the design and material-related additional losses (eddy current and magnetic reversal losses in the core, skin effect and proximity effect ). They lead to a smaller change in the phase position of the current or a higher active component of the electrical power loss than would be expected due to the copper resistance.

Apparently, the ESR (the real part of Z ) changes compared to the value determined with direct current. These parasitic components can be detected, for example, with a measuring bridge that is able to measure real and imaginary parts separately.

Equivalent circuit diagram of a coil with a magnetizable core

In the equivalent circuit diagram of the coil with the inductance L, the ESR can be represented as a series connection of the copper resistance R Cu and a frequency-dependent core resistance R Fe . The core resistance is composed of the eddy loss, the hysteresis and the aftereffect component.

Another parasitic effect is the capacitance between the turns and between the turns and connections. These parasitic capacitances of the coil are summarized as winding capacitance C P in the equivalent circuit diagram and are parallel to the inductance. The parasitic capacitances significantly influence the impedance of a coil. When the frequency is increased from zero, the impedance initially increases as would be expected based on the inductance. At the natural resonance frequency, it then reaches its maximum value, only to then decrease again - now the coil shows capacitive behavior.

This phenomenon is disadvantageous in filter and interference suppression applications, where it is necessary that even very high frequencies are still sufficiently attenuated by the coil. The effect is reduced by making the coil single-layer and elongated or cross-layered. The distributed winding of several chambers one after the other is also common. In filter applications (e.g. line filters ), different coil designs often have to be combined in order to achieve high inductance on the one hand and low parasitic capacitance on the other.

## Connection and disconnection processes with direct voltage

Connection and disconnection process on a real coil ( R wire  = 10 Ω) with "ideal" free-wheeling diode ; above: self-induction voltage, middle: current, below: supply voltage; the time axis is scaled in units standardized to the time constant

If you connect a real (that is: lossy) coil to a DC voltage, the current and voltage take the following time course:

• when switching on:
${\ displaystyle i _ {\ mathrm {L}} (t) = I_ {0} \ cdot {\ biggl (} 1- \ mathrm {e} ^ {- {\ frac {t} {\ tau}}} {\ biggr)} = {\ frac {U_ {0}} {R _ {\ mathrm {L}}}} \ cdot {\ biggl (} 1- \ mathrm {e} ^ {- {\ frac {t \ cdot R_ { \ mathrm {L}}} {L}}} {\ biggr)}}$
${\ displaystyle u _ {\ mathrm {L}} (t) = U_ {0} \ cdot e ^ {- {\ frac {t} {\ tau}}} = U_ {0} \ cdot e ^ {- {\ frac {t \ cdot R _ {\ mathrm {L}}} {L}}}}$
• when switching off:
${\ displaystyle i _ {\ mathrm {L}} (t) = I_ {0} \ cdot e ^ {- {\ frac {t} {\ tau}}} = {\ frac {U_ {0}} {R_ { \ mathrm {L}}}} \ cdot e ^ {- {\ frac {t \ cdot R _ {\ mathrm {L}}} {L}}}}$
${\ displaystyle u _ {\ mathrm {L}} (t) = - U_ {0} \ cdot e ^ {- {\ frac {t} {\ tau}}} = - U_ {0} \ cdot e ^ {- {\ frac {t \ cdot R _ {\ mathrm {L}}} {L}}}}$

With:

• ${\ displaystyle \ tau = {\ tfrac {L} {R _ {\ mathrm {L}}}}}$( Time constant )
• ${\ displaystyle L}$ - inductance of the coil
• ${\ displaystyle t}$ - Time
• ${\ displaystyle R _ {\ mathrm {L}}}$ - ohmic (wire) resistance of the coil
• ${\ displaystyle I_ {0} = {\ tfrac {U_ {0}} {R _ {\ mathrm {L}}}}}$
• ${\ displaystyle U_ {0}}$ - DC voltage

This relationship shows that the current flowing in a coil cannot change abruptly. When switching on a DC circuit with a coil, the induction voltage counteracting the operating voltage prevents a rapid increase in current. This follows the laws of an exponential function . If it assumes a high value, it becomes smaller , so the current increase to the final value is sooner completed. ${\ displaystyle R _ {\ mathrm {L}}}$${\ displaystyle \ tau}$ ${\ displaystyle I_ {0}}$

A sudden switch-off of the coil current ( ) is not possible. In reality, when trying to interrupt the current, a voltage peak of opposite polarity occurs, the magnitude of which depends only on the parasitic capacitance of the coil and other voltage-limiting effects ( electrical breakdown , flashovers, switching arcs ). They can cause damage from overvoltage. ${\ displaystyle - {\ tfrac {\ mathrm {d} i} {\ mathrm {d} t}} \ to \ infty}$

Coils operated with direct current are therefore often protected by a protective diode connected in parallel , which enables the (coil) current to continue flowing when the (supply) current is switched off and the magnetic energy stored in the coil

${\ displaystyle W = {\ tfrac {1} {2}} L \; I ^ {2}}$

converted into thermal energy mostly in the coil wire and to a small extent in the diode . This prevents the high voltage peak at the coil connections, but it takes longer for the current to drop to a low level.

The following applies to the shutdown process with an "ideal" freewheeling diode:

${\ displaystyle i (t) = I_ {0} \ cdot \ mathrm {e} ^ {- {\ frac {t} {\ tau}}}}$.

The time constant is the quotient of inductance and wire resistance ; it can be a few seconds for large, high-quality inductances. The time constant is the same as that at the start of the switch-on curve and can be determined by a tangent applied to the start of the current / time curve at which this intersects the end value . At this point the value of the current rise curve is: ${\ displaystyle \ tau}$${\ displaystyle {\ tfrac {L} {R _ {\ mathrm {L}}}}}$${\ displaystyle I_ {0}}$${\ displaystyle t = \ tau}$

${\ displaystyle i (\ tau) = 0 {,} 6321 \ cdot I_ {0}}$.

The steepness of the tangent at the zero point is calculated from:

${\ displaystyle \ tan \ alpha = - {\ frac {I_ {0}} {\ tau}}}$.

This rate of increase in current (often specified in ) is an important value for a large number of applications, such as thyristor switches , switched-mode power supplies , voltage converters , suppressors. Here, coils are used everywhere to store energy or to limit the rate of increase in current. In practice, the coil current increases almost linearly with time at the beginning due to the usually relatively small real part of the coil impedance. Theoretically, the current through a coil at constant voltage would continue to increase, the stored energy would increase faster and faster (proportional to the square of the time). In practice, the energy that can be stored in a coil is limited for the following reasons: ${\ displaystyle {\ tfrac {\ mathrm {d} i} {\ mathrm {d} t}}}$${\ displaystyle \ mathrm {\ tfrac {A} {\ mu s}}}$

• The core material that may be present saturates above a certain flux density, as a result of which the inductance drops sharply (this leads to a rapid and strong increase in current).
• As the current through the coil increases, the total voltage across the electrical resistance of the coil wire finally drops , and the current cannot increase any further.${\ displaystyle R _ {\ mathrm {L}}}$

More and more electrical power is converted into heat output ( ) and there is a risk of overheating. ${\ displaystyle I ^ {2} \ cdot R _ {\ mathrm {L}}}$

Due to the properties described above, periodically switched coils can be used to generate high voltages from low voltages (for example: ignition coil , voltage converter, spark inductor , step-up converter and switching regulator ).

Conversely, they can be used to limit the current in AC voltage circuits ( series choke, commutator choke), and for low-loss reduction of voltages ( step-down converter ) and smoothing currents (filter choke).

## Printing / color codes

To indicate the inductance of a coil, color codes are sometimes used according to the following schemes:

Color code for coils according to IEC 62–1974
colour Inductance in µH tolerance
1st ring 2nd ring 3rd ring
(multiplier)
4th ring
"no" × - - - ± 20%
silver - - 1 · 10 −2 = 0.01 ± 10%
gold - - 1 · 10 −1 = 0.1 ± 5%
black 0 0 1 · 10 0 = 1 -
brown 1 1 1 · 10 1 = 10 -
red 2 2 1 · 10 2 = 100 -
orange 3 3 1 · 10 3 = 1,000 -
yellow 4th 4th 1 x 10 4 = 10,000 -
green 5 5 1 x 10 5 = 100,000 -
blue 6th 6th 1 · 10 6 = 1,000,000 -
violet 7th 7th 1 x 10 7 = 10,000,000 -
Gray 8th 8th 1 x 10 8 = 100,000,000 -
White 9 9 1 · 10 9  = 1,000,000,000 -
Color code for coils according to MIL-C-15305
colour Inductance in µH tolerance
1st ring
(wide)
2nd to 4th ring
number *
5th ring
(multiplier)
6th ring
"no" - - - ± 20%
silver Beginning - - ± 10%
gold - comma - ± 5%
black - 0 10 0 -
brown - 1 10 1 ± 1%
red - 2 10 2 ± 2%
orange - 3 10 3 -
yellow - 4th 10 4 -
green - 5 10 5 ± 0.5%
blue - 6th 10 6 -
violet - 7th 10 7 -
Gray - 8th 10 8 -
White - 9 10 9 -
* The 3rd digit is optional.

Alternatively, the inductance (especially with higher values) is indicated by a three-digit number. Mean

• the first two digits the value in µH
• the third digit is the number of trailed zeros

Example: The imprint "472" means 4.7 mH.

## production

In coil winding technology, numerous methods and processes have been established for producing the coils. The most important are the linear, flyer and needle winding technology. The systems for coil winding cost between 150,000 euros for simple machines and up to 4 million euros for systems for large-scale production.

## Applications

### Fixed inductance coils

Coils are u. a. in transformers , electromagnets , metering pumps , relays , contactors , electro-dynamic and electromagnetic speakers , dynamic microphones ( moving coil ), pickups for electric guitars and basses, current transformers , as deflection in television picture tubes , in galvanometers , moving coil instruments , moving iron works , electric motors , coils and analog indicating Quartz used. In electronic circuits they come u. a. as a frequency-determining element or as a throttle (electrical engineering) for sieving purposes.

Coiled electrical conductors in wirewound resistors , helical antennas , spiral antennas , traveling wave tubes and filaments are not referred to as coils.

Circular air coils are also referred to as toroid after the geometric body .

### Variable inductances

#### Variometer

A variable inductance used in measurement technology and historical radio technology is called a variometer and, in one embodiment, consists of two coreless coils pushed into one another and connected in series. The inner coil is rotatably mounted (or displaceable in parallel along the longitudinal axis). The maximum inductance is reached when the winding levels are traversed in parallel and in the same direction by the current.

Another design of variometers is based on the movement of cores inside cylinder coils. These cores can either be made of highly permeable material (inductance increases when moving in) or of highly conductive metal (inductance decreases when moving in due to field displacement). The first variant is used in the long, medium and short wave range, the second in the VHF range.

Due to its higher mechanical stability, the variometer tuning is less sensitive to vibrations and therefore more stable in frequency than tuning by means of a variable capacitor . Therefore, despite the greater expense, it was mainly used in car radios from the beginning of the 1950s to the 1970s , where it also enabled mechanical station storage via several selection buttons. The first FM car radio in the world presented by Blaupunkt in 1952 , the Auto super A 52 KU, had a “self-service pushbutton selector” for four transmitters with variometer tuning and cost 498 DM, which is equivalent to 1,270 euros in today's currency after adjusting for purchasing power.

#### Alignment coil

Two alignment coils in a television set from 1980. The coils are about 8 mm high

Adjustment coils are adjustable inductances that are used for one-time adjustment (adjustment) of frequency-determining elements, e.g. B. resonant circuits or bandpass filters are provided and in this function comparable to trimming capacitors , which are also only adjusted for a one-off adjustment.

By screwing the coil ferrite core in or out with an amagnetic adjustment tool, the required inductance is set and the desired resonance frequency of the oscillating circuit or the bandwidth of the bandpass is determined. If an RF coil has an aluminum (or other electrically conductive material) core for alignment, screwing the core in will reduce the inductance. This is because the core acts like a short-circuited secondary winding of a transformer . Turning it deeper causes the magnetic field of the coil to be displaced. Occasionally, the adjustment is also carried out by mechanically pressing the turns of a coil together or apart without a ferrite core (air-core coil).

In the past, alignment coils were used in all areas of professional communications technology, in many electrical measuring devices and in entertainment electronics . Especially in the radio and television set production with its large quantities, the device comparison required a high level of personnel and instrumentation in the final production. With technical progress, the adjustable inductances have increasingly been replaced by special circuits such as the electronic phase-locked loop (PLL with quartz oscillator ) or the voltage-controlled oscillator (VCO), which offer high electrical long-term constancy and are also cheaper to manufacture. The adjustment of these circuits is greatly simplified and is usually implemented using digital (software) solutions.

#### Reel spool

A roller coil as used in adapters for transmitting antennas

A rolling coil is an electrical coil with adjustable inductance which is used in particular in the frequency range from a few kHz to a few MHz and at higher powers for coupling a transmitter in the long , medium and short wave range to a transmitting antenna . In this application, the roller coil is used in purely passively implemented matching networks in order to match the impedance of the transmitter amplifier to the antenna. Depending on the design, roller coils can be used in adapters in the power range from a few 10  W to a few 100 kW.

There are different embodiments: Smaller reel coils, which are also used in amateur radio, for example in the shortwave area, are usually adjusted by hand. Larger reels or reels which have to be changed frequently during operation have a motor drive.

#### Transducers

Transductors allow the inductance to be changed by means of a direct current flowing through a second winding. They are also known as magnetic amplifiers and are based on the saturation of the core through the premagnetization due to the controlling direct current. This reduces the permeability of the core and thus the inductance of the coil.

## Designations

Coils of different designs

As with many passive components, coils also have many different names that have grown historically and can be traced back to the design, the inventor, the application or, a specialty of coils, as a semi-finished product to the component manufactured with them.

### Design specific

• Bifilar coil (engl .: bifilar coil ) is a coil with two parallel windings wound in opposite directions, the z. B. was used in AB push-pull NF power stage transformers
• Chip inductor , coil in SMD shape results for Surface Mount
• Micro-inductance , coil in particularly small dimensions, mostly suitable for automatic assembly
• Solenoid coil is a cylinder coil for generating a spatially constant magnetic field as possible
• Voice coil (engl .: voice coil ), the drive unit of an electrodynamic sound transducer , such. B. that of a loudspeaker.
• Plunger coils are resiliently suspended magnet coils in a stationary magnetic field, whichare deflectedby the Lorentz force when current flows throughthem.
• Spiral flat coil , spiral-shaped winding of a conductor, model for coils on printed circuits

### Inventor names

• Barker coil is a massive Helmholtz coil and is used in nuclear magnetic resonance spectroscopy (also known as NMR spectroscopy).
• Braunbek coil is used in geomagnetic research to measure magnetic fields on spacecraft.
• Garrett coil is used in metal detectors .
• Helmholtz coil is a special coil arrangement for generating an almost uniform magnetic field
• Pupin's coil (engl. Loading coil ) was a coiled line in the telephone network, in which coils were used to reduce the attenuation of the high LF frequency components of the telephone calls.
• Maxwell coil is a coil with a constant field gradient inside the coil, see also Helmholtz coil
• Oudin coil (engl. Oudin coil ) is a disruptive discharge coil for the generation of sparks with high frequencies
• Rogowski coil is a toroidal air-core coil and is used as a component of electrotechnical measuring devices for measuring alternating current
• Tesla coil is the secondary coil of a Tesla transformer, excited with its resonance frequency, for generating i. d. Usually high-frequency alternating currents with very high voltage.

### application

• Choke is an inductive component which is used for throttling, damping and radio interference suppression of undesired frequencies as well as for current limitation or for energy storage.
• Degaussing coil is used to demagnetize magnetizable parts, e.g. B. perforated or slotted mask of a television picture tube.
• Single Coil a single coil pickup for electric guitars.
• Ignition coil or induction coil is a component of the ignition system of a gasoline engine or a gas firing system for generating high pulse voltage
• Plug-in coil is a coil on a plug-in base which, by simply exchanging it, is used to switch the frequency band in radio receivers and frequency meters

### Intended use

Deflection coil, loudspeaker coil, motor coil, relay coil, transformer coil, transmitter coil and many others are semi-finished products (windings mostly on a winding carrier) that are suitable for generating or detecting a magnetic field and are part of a technical inductance , an inductive passive component such as e.g. . B. a transmitter or transformer , part of an electromechanical component such as a relay , motor , loudspeaker , microphone or pickup or part of a picture tube ( deflection coil ).

## literature

• Tadeusz Adamowicz: Handbook of Electronics, a comprehensive presentation for engineers in research, development and practice (original title: Poradnik inżyniera , translated by A. Dworak). Franzis, Munich 1979, ISBN 3-7723-6251-6 .
• The Brockhaus, Science + Technology. 2003, ISBN 3-7653-1060-3 .
• Dieter Sautter, Hans Weinerth: Lexicon Electronics and Microelectronics. VDI, Düsseldorf 1990, ISBN 3-18-400896-7 .
• Dieter Nührmann: work book electronics. Franzis, Munich 1981, ISBN 3-7723-6543-4 .
• Otto Zinke , Hans Seither: Resistors, capacitors, coils and their materials. Springer, Berlin 1982, ISBN 3-540-11334-7 .
• Martin Gerhard Wegener: Modern radio reception technology. Franzis, Munich 1985, ISBN 3-7723-7911-7 .