Inductor

Coils are in electrical windings on the one hand and winding material, which are adapted to generate a magnetic field or detect. They are part of an electrical component or device such as a transformer, relay, electric motor or a speaker.

On the other hand, are separate coils inductive passive components whose essential feature is a defined inductance. They are used mainly in the area of ​​signal processing for frequency-determining circuits, such as LC resonant circuits, low-pass, high-pass, band-pass filters, the signal phase response correction for interference suppression, the current flow smoother or as energy storage in switching power supplies and many other electrical and electronic equipment. See also choke ( Electrical Engineering).

The frequency of use of the coil is considerably smaller than the resistors and capacitors, as they are often less expensive and easier to manufacture and cheaper be integrated in electronic semiconductor circuits. The use of coils is avoided when using capacitors, resistors and active devices (transistors ) can be simulated, for example by means of a gyrator circuit - in the electronic circuit design is therefore often - if at all possible.

Most coils consist of at least one winding of a conductor wire, enameled copper wire, silver plated copper wire or Litz, the most on a coil former ( bobbin carrier ) is wound and is mainly provided with a soft magnetic core. The winding arrangement and shape, the wire diameter, the winding and the core material set the value of the inductance and more (quality ) characteristics of the coil.

In addition, also helically applied conductors on printed circuit boards, which are optionally surrounded by surrounding ferrite, " coil " in the sense of an inductive passive component. The turns of a coil must always be opposite each other and be insulated from the often electrically conductive coil core to prevent a winding short, which would impair the function of the coil material. In coils and transformers with multiple layers of turns or windings of enameled copper wire, the individual layers of turns or windings are also at voltage differences from about 50 volts, for example, often additionally insulated by paint paper against voltage breakdown.

• 5.1 coils for oscillators

• 6.1 coils equation
• 6.2 parasitic

• 9.1 coils with a fixed inductance
• 9.2 Variable Inductors 9.2.1 variometer
• 9.2.2 Balance coil
• 9.2.3 roller inductor
• 9.2.4 transducers

Structure, component names

A classic is a coil around a fixed body ( bobbin ) wound wire. This body must not necessarily be present. If there is no bobbin or it is non-magnetic material, it is called in the mechanical or electrical sense of air coils. The bobbin serves mostly the mechanical stabilization of the wire and, in contrast to the coil core to magnetic influence.

Coil is also available in flat spiral shape and of rectangular or any other desired shaped coil cross-section. They may be implemented as a spiral conductor on a circuit board directly.

Coils have a certain inductance, this inductance can be the ultimate objectives (eg, inductors, filters, coils) or only secondary property (eg, transformers, pull type solenoids, relay coils ).

Next to the wound wires and the coil, the coil body in the interior often has a ( coil ) on the core (see below), to increase the inductance.

The word coil has the shape ( (see coil role ) ) out.

The inductance of a coil is measured in units of Henry ( see Henry (unit) ).

Operation

The main feature of coils whose inductance. The inductance resulting from the number of turns of the coil and from the area enclosed by the coil material and the dimensions. The magnetic linkage ( flux-linkage ) of the individual coils to one another, due to the physically close arrangement of the individual coils, the inductance of the wound coil increases with the square of the winding number of theoretical. Doubling the number of turns in the same geometrical dimensions thus causes a fourfold increase in inductance.

If the coil wire through which a time-varying current, produced by the electrical conductor a time- varying magnetic flux. Any change in the current produced a self-induction voltage at the ends of the electrical conductor. This voltage is directed so that they their cause ( the current) counteracts ( Lenz's Law ). An increase in the rate of change of the current leads to the increase of the voltage, which counteracts the power. The proportionality between time- varying current through the conductor and the resulting self-induction voltage is called inductance.

Real coils have, in addition to the actual desired inductance also other, usually undesired electrical properties, such as an electrical resistance, parasitic capacitance, and thus at least one electric resonance point ( parallel resonance circuit ) or an inductance -increasing bobbin disruptive remanence and eddy current losses. All these parameters are temperature - and frequency-dependent labor. Their use is therefore only up to a maximum cutoff frequency devices typical sense, where a sufficient inductive reactance and phase angle acts in the corresponding application circuit.

If a high resistance, consisting of a long coiled (resistance) wire, on the other hand have a very low inductance, must the mechanical resistance wire carrier, such as a porcelain tube with contact clamps are bifilar wound with a counter-rotating wire. Thus, the oppositely directed magnetic fluxes almost cancel. This method is used for example for wire load resistors for the high low-frequency range up to about 100 kHz.

Magnetic field and current flow

The following mnemonics can be used to determine which end of a coil a magnetic north and which end a south pole is at a current flowing through them direct current ( as current direction is the direction of current, that is, from the positive to the negative pole to use ):

• If you look at a coil end and flows through this clockwise from the electricity, so there arises a magnetic south pole.
• If you look at a coil end and this is traversed counterclockwise from electricity, so there arises a magnetic north pole.
• Comprises one 's right hand, the turns of the coil so that the fingers are each directed in the direction of current (not the thumb ) along the turns, as shown by the thumb in the direction of the magnetic north pole of the coil.

Inside a slim coil ( length much greater than diameter) of the length of coils in which an electric current flows, the magnetic field with field strength formed

The flux density B is obtained from the core ( see below) -dependent material constants ĩr and the magnetic field constant μ0 = 4π · 10-7 H / m thus

Cores

Cores have the task to increase the inductance of the coil or decrease. The achieved through a magnetic core of the inductance increase results in a reduction of the required number of turns for a given inductance value, or length of conductor and hence to reduce the interfering electrical resistance of the coil.

Cores of electrical conductors such as copper or aluminum, which reduce field displacement by the inductance, are used for tuning ( tuned circuit ) coils in the high frequency range, eg for FM tuners.

Coil with an iron core

An iron core inserted into a coil, the permeability and thus the magnetic flux density in the coil is increased due to its ferromagnetic properties. Thus, it is made with substantially fewer turns and thus with much less volume of components in order to achieve a required inductance. From a certain material-dependent flux density but occurs a disturbing saturation magnetization of the core.

Because the iron of the core is an electrical conductor, an undesirable eddy current is induced in it as in a short circuit through which an alternating current coil, which heats the iron core. This eddy current can reduce it if the core is not made of a solid piece of iron, but of a stack of iron plates. This must be insulated from each through paint layers or ( earlier) paper to interrupt the eddy current.

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

These magnetic core materials typically have a hysteresis effect ( remanence ) that leads to electrical losses, because at each period of an alternating current, the core has to be demagnetised. Moreover, this is a distortion of the current waveform with additional peaks materialize in each period, which is unwelcome in some applications, because they increase the harmonic distortion. The losses that occur due to eddy currents and hysteresis, called iron losses.

Also, the switch-on of coils with iron core is much more complex, because, depending on the state of the core is before turning almost no magnetization or as remanence is already casting a noticeable magnetization that either the current polarity can also be opposite corresponds to or, then by the inrush current must be re-magnetized only. These effects lead to the fact that in the extreme case when switching a voltage fuses already appeal on the basis of a possible inrush current until reaching the nominal time, only later current limiting inductance previously, although actually there is no overload. For larger inductors, such as transformers or choke coils with iron core, in AC power applications must therefore be taken frequently especially for Einschaltfall special precautions, see for example in transformer switching relay. But in both cases occurring self-induction voltage circuitry are to be observed. In small-signal applications, the hysteresis effects only lead to a decreased quality of the component when turned on. In coils and transformers especially for higher power, starting at a few watts starting, often disturbing acoustic noise generation of the core material occurs in the low frequency range, which is referred to as mains hum. It is due to low mechanical changes in size of the core due to the changing magnetic field, see magnetostriction. Can reduce this effect by vacuum impregnation with special paint, which increases even more the dielectric strength between different ( transformer ) coils simultaneously.

The elementary magnets in the iron core are determined by the poles of the coil. Is the north pole of the left, the north poles of the elementary magnets are also left. The field lines appear therefore from the North Pole and the South Pole penetrate back into the coil interior. In the interior of the coil, the field lines run from south to north. In an elongated coil having many turns, the magnetic field in the interior is homogeneous, it is similar to the magnetic field between the legs of a horseshoe magnet. Outside, the coil field resembles that of a bar magnet.

Nuclei in high-frequency coil

A core usually made ​​of pressed magnetic powder ( powder core ) or ferrite is used for this purpose. In order to filter high-frequency interference, among other toroidal coils and toroidal chokes are used.

In tunable coil ferrite cores are used with a thread: by screwing or unscrewing the inductance of such a coil are increased or decreased. When an RF coil having a core made ​​of aluminum ( or other electrically conductive material ) for matching, the screwing of the core reduces the inductance. The reason is that the core acts as a shorted secondary of a transformer. A deeper screwing causes a displacement of the magnetic field of the coil.

RF coil

With increasing frequency, the currents are more and more to the surface of the wire displaces (skin effect). The wire surface is increasingly then determines the quality of the coil. From about 100 kHz is therefore often used to reduce the losses Litz as wrapping material; It consists of several, mutually insulated thin wires. From about 50 MHz, the coils are usually designed cantilever with thicker wire. A silver-plated surface can reduce the losses further. Cores for high -frequency coils are made of a ferromagnetic, electrically nonconductive material. Thus, eddy currents are prevented in the core. Even with the design you can make a high-frequency coil fit by by special winding forms decreased in those with high numbers of turns parasitic (eg, for the medium-wave band ) capacity ( honeycomb, basket floor or cross-wound coils).

Coils for oscillators

Coils in oscillators or band filters should generally comply with their inductance as accurately as possible. A small still existing temperature coefficient, which is mainly caused by the core material used, can be almost completely compensated by an oppositely directed temperature coefficient of the resonant circuit capacitance used in the corresponding components selection and sizing of the capacitor elements.

Air coils can cause through the smallest inductance modulation at a frequency vibration. They are therefore wound onto a spool, fixed with glue or varnish, or completely embedded in wax.

AC behavior

If a coil of AC voltage is applied, the current and the magnetic field is also periodic change their direction. Between the change of the coil current I ( t) and the terminal voltage U (t) is the connection

Where t is time, and L is the self inductance of the coil. Here are the current and voltage, as with passive components common to specify the consumption arrow system.

Since the current can only rise or fall gradually due to the transport of energy in the magnetic field, it follows the course of the voltage is always delayed; It is phase-shifted. Under ideal conditions (for a small negligible ohmic resistance), the AC voltage lags the current by 90 °. There is an inertia of the coil to power changes. (Mnemonic: " When inductors the currents be late .")

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

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

Impressed into the coil, so there is the voltage across the coil through mathematical derivation to

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

The coil can be as a complex alternating current resistance (impedance ) are associated, which, however, in contrast to an ohmic resistor does not implement power into heat ( dissipation). This is because energy is absorbed during a quarter period of the coil and released back into the next quarter period. This commutes energy only back and forth, without being consumed. We call this special form of resistance and reactance of the current reactive current.

For a coil of the inductor L and an alternating current of frequency f to the reactance ( reactance ) is calculated

To

With the dimension [V / A].

Is called the angular frequency or angular frequency.

The reactance increases with increasing frequency, the ohmic resistance wire remains the same. Therefore designed for AC voltage coil on an equal DC voltage ( f = 0 Hz) has a much lower resistance, since only the wire resistance impedes the flow.

Coils equation

The coils equation

Results in the specified form solely for linear material behavior of the core and at a negligible electric field strength in the coil wire. This is in the following using induction and Ampere's law are shown.

The law of induction is in general form. It is to be applied in this case for a stationary contour line, and can therefore be used in the particular form

Be noted.

When we choose the path of integration in the picture drawn with dashed lines way ( there instead ). The associated coil area is illustrated in the accompanying video.

Taking into account that the electric field strength in the conductor is approximately equal to zero, the integral ring fed through the electric field strength only of the negative clamping voltage. The negative sign comes from the fact that the path of integration is run against the arrow direction of the terminal voltage. Thus, the following applies:

For linear core behavior of the magnetic flux through the coil and the total current is strictly proportional to each other, so that one can introduce a proportionality factor ( the so-called inductance ). We then have:

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

Parasitic

Reliable coils show in an AC circuit, a phenomenon 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 AC operation. Reasons for this are design - and material-induced additional losses ( eddy current and hysteresis losses in the core, skin effect and proximity effect ). They lead to a smaller change in the phase angle of the current or a higher active component of the electrical power loss occurs, as would be expected because of the copper resistance.

Apparently the ESR varies accordingly ( the real part of Z) compared to the value given with direct current. These parasitic components can be detected, for example with a measuring bridge which is able to measure the real and imaginary parts separated.

In the equivalent circuit of the coil with the inductance L of the ESR as a series circuit from the copper resistance RCu and a frequency-dependent resistance RFe core can be displayed. The core resistance is composed of the vertebral loss, the hysteresis and the aftereffect share.

Another parasitic effect are the capacitances between the windings and between the windings and connections. These parasitic capacitances of the coil are summarized as winding capacitance CP in the equivalent circuit diagram and lie parallel to the inductance. The parasitic capacitances affect the impedance of a coil significantly. By increasing the frequency of zero to the apparent resistance initially increases as would be expected due to the inductance. In the self-resonant frequency, he then attains its maximum value in order to then fall again - now the coil shows capacitive behavior.

This phenomenon is disadvantageous in filter and Entstöranwendungen, where it is required that even very high frequencies are not sufficiently attenuated by the coil. In reducing the effect by carrying out the single-layer coil and is elongated or kreuzlagig. Also, the distributed - winding succession of several chambers is common. Often you have ( eg line filter ) combine different coil designs to achieve both high inductance and on the other hand a low parasitic capacitance in filter applications.

See also: Reactive power compensation and complex AC circuit analysis

For switching on and off in DC

If you switch a real ( ie: lossy ) coil to a DC voltage, current and voltage take the following time characteristics:

• During the switch:

• The switch-off:

With:

• (Time constant)
• - Inductance of the coil
• - time
• - Resistive ( wire ) resistance of the coil
• - DC

This relationship shows that the current flowing in a coil current can not change abruptly. When switching a DC circuit with a coil of the operating voltage counteracting induction voltage prevents current peaks. This follows the laws of an exponential function. If a high value is assumed is small, hence the power increase is more closed to the final value.

A sudden shutdown of the coil current () is not possible. In reality arises when attempting to interrupt the current, a voltage peak reverse polarity, the amount of which depends only on the parasitic capacitance of the coil and other voltage-limiting effects ( electrical breakdown, arcing, switching arc ). They can cause damage by overvoltage.

DC power supplied coil therefore often protected by a protection diode connected in parallel, which allows the continued flow of the ( coil ) current during turn-off of the ( feed ) current and the magnetic energy stored in the coil

Largely converted in the coil wire, and to a small extent in the diode into heat energy. The high voltage surge at the terminals of the coil is thus prevented, but it takes longer until the current has fallen to low values.

For the switch-off process with an "ideal" free-wheeling diode is true:

The time constant is the ratio of inductance and resistance wire, it can be a few seconds at large inductances of high quality. The time constant is equal to that at the beginning of Actuation curve and can be determined by a voltage applied to the beginning of the current / time curve tangent at which this intersects the final value. At this time, the value of the current rise curve:

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

This rate of current rise (often given in ) is an important value for a variety of applications, such as thyristor switches, switching power supply, voltage transformer, suppressors. Here coils are used for energy storage or for limiting the rate of current rise anywhere. The coil current rises in practice due to the generally relatively small real part of the coil impedance at the beginning almost linearly with time. Theoretically, the current would go through a coil at a constant voltage is always on, the stored energy would be faster ( in proportion to the square of the time ) is greater. In practice, the energy that can be stored in a coil, is limited for the following reasons:

• The optional device from a certain core material saturation flux density, so that the inductance decreases extremely ( the results in a fast and strong increase in current ).
• With increasing current through the coil finally falls on the electrical resistance of the coil wire from the total voltage, the current can not increase further.

It is becoming more electrical power converted into heat () and it threatens overheating.

Owing to their properties described above periodically connected coils can be used for generating high voltages from low voltages (for example, coil, voltage converters, induction coil, up-converter and the switching control).

Conversely, they can contribute to the current limitation in AC circuits ( series reactor, Kommutatordrossel ), and for low-loss reduction of voltages ( down converter ) and smoothing of flows ( choke ) can be used.

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

Alternatively, the inductance is (especially at higher values ​​) is indicated by a three -digit number. In this case, mean

• The first two digits of the value in uH
• The third digit, the number of appended zeros

For example, the imprint " 472 " means 4,7 mH.

Applications

Coils with a fixed inductance

Coils are, inter alia, in transformers, electromagnets, metering pumps, relays, contactors, electrodynamic and electromagnetic speakers, dynamic microphones ( moving coil ) pickups for electric guitars and basses, current transformers, as a deflection of television picture tubes, in galvanometers, moving coil instruments, moving iron works, electric motors, ignition coils and analog indicating quartz watches used. In electronic circuits, inter alia, they come as a frequency- determining element or as a throttle (Electrical ) to Siebungszwecken used.

Spiral electrical conductors in wire resistors, spiral antennas, spiral antennas, traveling wave tubes and filaments are not referred to as coils.

In the circle extending air coils are designated according to the mathematical body as a toroid.

Variable inductors

Variometer

A variable inductor used in the measurement and the historical wireless technology is referred to as vertical speed, and in one embodiment of two nested and cascaded coreless coils. The inner coil is rotated ( or along the longitudinal axis parallel sliding) bearings. The inductance maximum is reached when the winding planes parallel flow and the same direction of the current.

Another design variometers based on the motion of nuclei inside of cylindrical coils. These cores can be either made ​​of highly permeable material ( inductance increases when inserting moving ) or of highly conductive metal (inductance is reduced while placing moving through field displacement ). The first variant is used in the long-, medium - and short- wave range, the second in the VHF range.

The variometer tuning frequency is less sensitive and therefore more stable than voting by variable capacitor due to their higher mechanical stability against vibrations. Therefore, it was up in the 1970s used despite the higher complexity of the early 1950s, mainly in car radio, where it enabled a mechanical transmitter storage across multiple keys. In 1952, presented by Blaupunkt first FM car radio in the world, the car Super A 52 KU had a " self-service push button selector " for four channels with variometer tuning and cost 498 DM, which corresponds to purchasing power parity in today's money 1,200 euros.

Matching coil

Alignment coils are adjustable inductances for one-time adjustment ( calibration) of the frequency-determining elements are provided eg resonant circuits or band-pass filters and in this capacity comparable to trim capacitors which are also adjusted for one-time adjustment.

By turning in or out of the coil ferrite core with a non-magnetic balance cutlery the required inductance is adjusted and set as the desired resonant frequency of the resonant circuit and the passage width (bandwidth) of the bandpass filter. Chance of adjustment is carried out also by mechanical or pushing apart the turns of a coil without ferrite core (air coil).

Previously alignment coils were used in all areas of professional communications engineering, used in many electrical measuring instruments and consumer electronics. Especially in the radio and television production, with its large numbers of devices required balance cost and human instrumental effort in post-production. With the progress of the adjustable inductances were increasingly replaced by special circuits such as electronic phase-locked loop ( PLL quartz oscillator ) or the voltage-controlled oscillator (VCO ), which offer high electrical long -term stability and are next to manufacture cheaper too. The balance of these circuits is greatly simplified and is usually implemented by digital (software ) solutions.

Roller inductor

A roller inductor is an electrical coil with adjustable inductance. The component is used for example in the high-frequency technology for matching networks.

Transducers

Transducers allow the change in the inductance by a current flowing through a second winding direct current. They are also referred to as a magnetic amplifier, and based on the saturation of the core due to the bias of the controlling direct current. By this, the permeability of the core and thereby the inductance of the coil decreases.

Designations

As with many passive components and coils carry quite a few different names that have grown historically and to the design, the inventor, the application or, which is a special feature of coils can be traced back as a semi-finished product to the order made ​​component.

Specific design

• Bifilar coil (English: bifilar coil) is a coil with two parallel oppositely wound coils, which was applied in AB push-pull output stage NF transformers eg
• Chip inductor, coil the SMD surface mount
• Suitable Mikroinduktivität, coil in particularly small dimensions, mostly for automatic assembly
• Solenoid coil is a cylindrical coil for generating a spatially constant magnetic field as possible
• Voice coil (English: voice coil ), the drive unit of an electrodynamic sound transducer, such as a loudspeaker.
• Moving coil in a stationary magnetic field is resiliently suspended magnetic coils, which are deflected by the Lorentz force when current flows through.
• Spiral flat coil, Spiral wound coil of a conductor, a role model for coils on a printed circuit

Names of inventors

• Barker coil is a Helmholtz coil, and is solid in the nuclear magnetic resonance spectroscopy ( NMR spectroscopy also ENGL. Nuclear magnetic resonance ) was used.
• Braunbekspule used in geomagnetic research for magnetic field measurement on spacecraft.
• Garrettspule is used in metal detectors.
• Helmholtz coil is a separate coil system for generating an almost uniform magnetic field
• Pupin coil ( engl. loading coil) was a coil-loaded line in the telephone network, in order to reduce the attenuation of high - frequency components of the NF calls coils were used.
• Maxwell coil is a coil having a constant field gradient in the interior of the coil, see also Helmholtz coil
• Oudinspule (English Oudin coil) is a disruptive discharge coil for generating sparks at high frequencies
• Rogowski coil is a toroidal air-core coil and is used as part of electrical measuring instruments for the measurement of alternating current
• Tesla coil is excited at its resonant frequency secondary coil of a Tesla transformer for generating a rule high-frequency alternating currents of very high tension.

Application

• Choke is an inductor for throttling, attenuation and interference suppression of unwanted frequencies as well as to limit the current or energy storage is used.
• Degaussing coil is used for demagnetization magnetizable parts, such as hole or slot mask of a television picture tube.
• Single coil, a single-coil pickup (English for A -coil pickups ) is a single -coil assembly for each string for detecting the string vibration of an electric guitar
• Ignition coil or the induction coil is a component of the ignition system of a gasoline engine or a gas combustion system for generating a high voltage pulse
• Plug coil is a coil on a socket that is used by simply changing the frequency band switching in radio receivers and frequency meters

Purpose

Deflection coil, voice coil motor coil, the relay coil, the transformer coil, repeater coil and many others are semi-finished products ( windings usually on a winding support ) that are suitable to generate a magnetic field or detect, and part of a technical inductance of an inductive passive component such as z. instance a transformer or transformer is part of an electro-mechanical component such as a relay, motor, speaker or microphone or the pickup part of a picture tube ( deflection ) is.