INDUCTORS AND THEIR USES

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This article is not intended to be a definitive treatise on the subject, but is intended to give a general overview, which I hope will be particularly interesting and helpful to the newcomer to Amateur Radio.

There are many different types of inductors, so I will limit the scope of this article to a brief description of the various types and their uses.  Also, I will deal with transformers separately, in the next section, so as to avoid confusion.  The different types of inductor have different circuit symbols, as shown below.  The alternative symbol is now often used on many circuit diagrams and each of the variants shown can be applied to it.  Note that pre-set inductors are sometimes represented by the variable inductor symbol.  To be pedantic, preset inductors are those requiring a trimming tool to adjust their value, whereas a variable inductor is one having an operator control which varies the inductance.  Strictly speaking, the goniometer and the variometer are both transformers but they are described in the inductor section because the variometer is always configured for use as a variable inductor and the goniometer is similar in many respects to the variometer.

As with resistors and capacitors, inductors come in a large range of physical forms and may be either fixed or variable.  Many of the small, air-cored, types are available with axial or radial leads, or are primarily intended for printed circuit mounting or are fitted with threaded bushes or solder tags.  The much larger and heavier iron-cored types are fitted with mechanical mounting arrangements to enable them to be mounted into equipment.  There are encapsulated variants of all types for use in adverse environmental conditions.

But first, what is an inductor?  For the purposes of this article, we can regard an inductor as a length of wire wound onto a former, which may or may not have an iron or ferrite core.  This winding may be a single layer (solenoid), or may be several layers separated by layers of insulating material or the winding may be "wave" or "pile" wound.  The insulation material used between layers is normally acid-free paper or plastic film, such as Mylar.  The entire component, or at least the winding, is often encapsulated in wax or varnish to prevent the ingress of moisture or other possible contaminants, which could attack the insulation or even the wire itself.

There are several different types of wire used in inductors.  The commonest type of conductor used is copper wire, which may be a single solid wire or several strands but other materials, such as solid aluminium wire and resistance wire, are also to be found in specialised applications.  All wires are available in a large range of diameters and coverings.  Outside of the USA, the thickness of wire, or its gauge, is normally stated in millimetres.   However, Imperial Standard Wire Gauge (SWG), based on inch sizes, was commonly used in the UK until a few years ago.  The USA still uses the American Wire Gauge (AWG), which is also based on inch sizes.  There are other gauges in use, such as Birmingham Wire Gauge, Brown and Sharpe Wire Gauge and Piano Wire Gauge, but these are not used for copper or aluminium winding wires or resistance wires.  Winding wires are generally available with diameters between 0.025mm and 5.0mm, although smaller diameters down to 0.009mm and larger diameters up to 12.7mm are available.

Winding wires may be bare, enameled or enameled with a cotton, silk or rayon covering, depending on application.  Glass covering is also used in very special applications where the insulation must withstand very high temperatures or chemical attack.  There are several types of enamel used, the oldest being shellac based.  Modern synthetic enamels are often "self-fluxing", where the enamel melts at normal soldering temperatures, acting as a soldering flux in the process.  For higher temperature applications, non self-fluxing enamels such as Lewmex, Bicalex or one of their equivalents, are often used.  Important parameters of wire coverings are mechanical thickness, abrasion resistance, colour and flexibility, temperature rating and voltage breakdown.

The wire often found in components intended for use between about 50kHz and 1MHz is either bunched or litz wire.  In litz wire, several solid, insulated, wires are woven or twisted together and then several of these bundles are woven or twisted together.  Sometimes, several of these composite bundles are then woven or twisted together, the process being repeated until the desired overall thickness of wire has been achieved.  It is essential that the individual wires are insulated from each other, except at the end connections, where it is essential that they are all connected.  The 60kHz transmitter at the time-code broadcasting station WWVB uses 6075/36 (6075 strands of 36 AWG wire).  This type of wire results in a very much higher Q for a given coil than if an equivalent solid conductor had been used, by reducing the effective skin resistance of the conductor.  The weaving or twisting pattern of litz wire is designed so that individual wires will reside for short intervals on the outside of the cable and for short intervals on the inside of the cable.  This allows the interior of the litz wire to contribute to the cable's conductivity.  Litz wire is extremely expensive.

Bunched wire, or "poor man's litz wire", also uses a number of insulated solid conductors, but these are twisted together as one bundle.  This also results in higher Q coils but is not as effective as litz wire.  However, bunched wire is considerably cheaper than litz wire.

Wave winding is normally used for multi-layer coils used at radio frequencies and produces a type of coil possessing low self-capacitance and therefore a high self- resonant frequency.  Wave-wound coils are not close-wound but have gaps between each turn.  The enameled wire, which is usually cotton, silk or rayon covered, traverses the width of the winding once every half turn, each layer sitting on top of the underlying one, rather than occupying the gaps between its turns.  This results in decreased inter-turn and inter-layer capacitance.

Pile winding is used for low frequency and DC applications, such as LF chokes and transformers, magnetic actuators and relays.  As the name implies, pile wound coils are usually wound on a bobbin, or former with end cheeks, with the turns being semi-randomly wound to fill the space between the end cheeks.  The wire is usually enamel covered and if the intended application will result in a high voltage across the ends of the winding, care must be taken to ensure that the start and finish turns are not in close contact so as to avoid possible voltage breakdown of the enamel.

The inductance (L) of an air cored solenoid, where the length is much greater than the diameter, is proportional to the square of the number of turns and the cross- sectional area of the coil but is inversely proportional to its length.

The inductance of cored inductors is also related to the permeability of the core material.  A vacuum, and for all practical purposes, any non-ferrous substance, have permeabilities of 1, all ferrous materials having values greater than unity.  Where all the magnetic flux passes through a ferrous core, the inductance is given by:

It should be noted that, in practice, it is very unlikely that all the magnetic flux would pass through the core and hence the actual inductance of a ferrous cored coil will be less than the calculated figure.

The Q value ("quality") of an inductor is another important parameter.

It should be noted that a single straight wire has inductance, a property that is often important at VHF and UHF frequencies.  The formula below gives the inductance of a single straight wire in free space.  For all practical purposes, this formula can be used for real-life applications but placing the wire in close proximity to a metal plate will substantially alter the effective inductance and will introduce capacity effects.  These points should be considered when resonant frequency calculations are undertaken.

I have not included any formulae relating to inductors with multiple layers or inductors used at high frequencies as these are very complex subjects, needing an appreciation of many factors, including self capacity, self-resonance, flux density, etc.  However, a formula for calculating the self-resonant frequency of an inductor is given later in this article.  Inductors are often referred to as "coils" or "chokes", depending on their application.

Inductance values (L) are stated in Henrys (H), using the normal decimal sub- multipliers of micro (), nano (n) and milli (m).  As the Henry is a fairly large unit, it is not normally necessary to use multipliers.  It should be noted that the smaller types of chokes often look exactly like resistors and are often colour coded or printed with alpha-numerics in a very similar manner.  One has to know whether the component is a resistor or an inductor.

Thus, for example, 1.0 micro-henry is normally written as 10, 1.5 microhenrys as 15, 1000 microhenrys as 1n, 1.0 millihenry as 1mH, 1.0 henry as 1H and 100 henrys as 100H.  Alpha-numeric marking is used for larger value types, whereas standard colour coding or alpha-numeric markings are used to indicate component values on low value types.  The latter method is self evident and in the former method, a series of coloured bands is used to indicate value and tolerance, with the first band located near one end of the component and often wider than the other bands.

The Standard EIA Colour Code Table per EIA-RS-279 is as follows:-
     Colour   1st band   2nd band      3rd band       4th band      Temperature
(Multiplier) (tolerance) Coefficient
Black 0 0 1 1%
Brown 1 1 10 2% 100 ppm Red 2 2 100 50 ppm
Orange 3 3 1000 15 ppm Yellow 4 4 100000 25 ppm
Green 5 5 1000000 0.5% Blue 6 6 10000000 0.25%
Violet 7 7 100000000 0.1% Grey 8 8 1000000000 0.05%
White 9 9 10000000000
Gold 0.1 5% Silver 0.01 10%
None 20%
Although it is theoretically possible to manufacture inductors having any nominal value of inductance, smaller value chokes are normally produced with "standard" microhenry or millihenry values.  These values are normally in the E12, also known as the 10% series (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82).  Larger value types are normally manufactured with "decade" henry values, e.g. 1.0, 10, 100 or 1000.  Tuned circuit coils have values depending on application requirements and are not normally marked with their inductances.

Before continuing, let us consider skin-effect, series resistance, self-capacity, current carrying capability and temperature rating.  Most people understand the inductance value of an inductor but many do not appreciate that inductors also have series resistance, self-capacity and are subject to maximum current ratings.

The series resistance of an inductor is a combination of the intrinsic resistance of the wire and any effective increase due to skin-effect.  Formulae relating to skin-effect are extremely complicated and are beyond the scope of this article.  At low frequencies, the series resistance is equal to the DC resistance of the wire, but as the frequency increases, the effective resistance also increases.  This is because, at high frequencies, the signal current only flows in the outer layer, or "skin", of the conductor.  The skin-effect only becomes important above about 50kHz, becoming progressively more important as the frequency increases.  At frequencies above 100MHz, the signal current is only flowing in the outermost few microns of the conductor and, for this reason, conductors intended for use at these frequencies are usually silver plated copper, although the conductivity of the base material is unimportant, unless there is also a high DC current flowing, as the signal current only flows in the plating layer.  The Q of an inductor is inversely proportional to the effective resistance and any current flowing in this resistance will generate heat, which could result in excessive temperature rise.

The self-capacity of an inductor is a combination of the capacity existing between individual turns of the winding and any capacity existing between the start and finish lead-out wires and associated terminals.  Self-capacity is normally only important at high frequencies but it should be noted that all inductors have a self-resonant frequency.  An empirically derived formula, which should give a value within ±10% of the true value, for coils having lengths much greater than their diameters, is given below.

Any DC current flowing in a winding having a ferrous material core will magnetise the core material.  If this current is high enough, the core material will "saturate", causing the inductance to decrease considerably.  The same will occur if the maximum permitted flux density in the core is exceeded due to large signal current peaks.  This effect is used in "saturable reactor" applications.  If saturation occurs in an inductor used at audio frequencies, considerable distortion to the audio waveforms will occur.

This effect is used when a waveform such as the sawtooth current driving the line scanning coils in a TV receiver or CRT monitor needs to be made non-linear to compensate for aberrations in other parts of a circuit.  Often a small bar magnet is used to magnetically bias the coil core.  The physical position of this magnet, relative to the core, is usually adjustable, allowing the waveform shape to be selectively varied.

Ferrite materials exhibit the "Curie Effect" where their permeability decreases dramatically when the Curie point temperature is reached.  This effect is used in some temperature controlled soldering irons.

The ability of all inductors to maintain their values or pass current can be adversely affected by excessively low or high temperatures.  Hence inductors often have stated temperature and current ratings.

Iron dust cores are, as their name implies, made of powdered iron, compressed together with a binding medium, to form a "slug" of material.  Terminal wires are often bonded into the ends of the slug if they are to be used in fixed inductors or a thread may be moulded into the outside surface to enable them to be used in variable inductors, where they are threaded into non-magnetic formers upon which the actual coil is wound.  A variation of this is where a threaded brass rod is bonded into one end of the slug.  The purpose of using iron dust is to reduce the eddy-current losses.  A specialised type of iron dust core is the pot-core, which is described in the appropriate sections below.

Ferrite cores can be made of a variety of ferrite materials depending upon the frequency of intended use and the required relative permeability.  Otherwise, the comments about iron-dust cores apply.  Specialised types of ferrite core exist, namely, pot-cores, ferrite rods, as used in ferrite rod aerials, single hole ferrite beads, multiple hole ferrite beads and ferrite rings or toroids.  The uses of these specialised types are described in the appropriate sections below.

Iron cores usually take the form of a stack of thin, silicon-iron, laminations, arranged to form the core of low frequency chokes.  Individual laminations are insulated from one another, usually by a very thin oxide layer, to reduce eddy current losses.  Laminations are made in "C" and "T", "E" and "I" or modified "E" and "T" combinations, with other styles being available for specific purposes.

In an inductor intended for low frequency use, where the winding carries no DC current, each successive set of laminations is rotated 180 degrees as the stack is built up.  This has the effect of virtually elininating any air gap in the magnetic path as the butt-joint in one set of laminations is covered by the solid part of the sets on either side.  If DC current flows through the winding as, for example, in a smoothing choke, successive sets of laminations are not rotated through 180 degrees.  The core therefore consists of a stack of "E" laminations and a stack of "I" laminations butted together with a small air gap.  It is normal to insert a layer of insulating material in this gap, prior to clamping up the complete core assembly, to ensure an air gap of consistent width.  The inductance of a gapped choke will be less than an otherwise identical component having no gap.

A variation of the laminated inductor core is the C-core, where a continuous strip of core material is wound into a stack which is then divided into two parts.  The cut faces are then polished such that the two halves form a continuous magnetic path, with virtually no air gap, when tightly clamped together.  The photograph below shows the clamping straps on a typical C-core inductor.

Solid metal cores or coil formers are never used as they would be coupled to the coil and would cause large eddy current and "shorted-turn" losses.  Brass cores are a special case, used only in variable value inductors for use at VHF.  The photo below shows a four coil turret, which was part of a professional signal generator, where all four coils are tuned by brass slugs.

 

FIXED INDUCTORS

These types are categorised according to the core materials used and most of these can be sub-divided into many different types.

Air Cored

These types of inductor are usually referred to as coils and are often used as the inductive element in tuned circuits.  It is now more common to use iron-dust or ferrite cored coils in low power applications as they are normally smaller than their air-cored equivalents and tuning adjustments may be carried out using the core, rather than by using a large and relatively expensive variable trimmer capacitor.  However, in the early years of radio (wireless), coils were always air cored and therefore tended to be much larger than their modern equivalents.  Three examples of medium and long wave domestic receiver coils are shown below.   These photographs were kindly supplied by Eddie Wilson G0ECW.  The actual components are owned by the Newhaven Fort Wireless Museum.  The "pancake" or "basket" coil would normally have had a cover, so that the coil would be completely enclosed but this has been removed in this photograph so that the coil proper can be seen.

Another use of air cored inductors is when the inductor consists of a few turns of wire used as a choke in UHF circuits or it consists of a number of self-supporting sections for use as a choke at lower frequencies.  Chokes with inductance values of a few microhenrys to several millihenrys are manufactured.

Air cored coils used in tuned circuits may be a number of turns of self-supporting wire or they may be turns of wire wound on non-magnetic formers made of plastic or ceramic materials.  Self-supporting coils normally use solid wire or strip, whereas coils wound on formers use solid, stranded, bunched or litz wire.  Coils used in the high power circuits of transmitters almost invariably use air cored coils, although exceptions are sometimes encountered.

A type of directional aerial, often used in pre-war radio receivers, was the frame aerial.  This was essentially a large, multi-turn, air cored coil wound on a wooden frame, which was often up to 30cms square.  This coil was tuned and formed the combined input tuned circuit and aerial of the receiver.  Frame aerials are still used in some amateur 160m and 80m direction finding receivers.  Frame aerials were often wound around receiver or loudspeaker cabinets, as shown in the photograph below, which was kindly supplied by Eddie Wilson G0ECW.  This example shows a long and medium wave aerial, the wide, open-wound coil being the medium wave section, while the narrow, close-wound coil is the long-wave section.  The actual equipment is owned by the Newhaven Fort Wireless Museum.  A schematic diagram of a single-band frame aerial is also shown.

              

Eddie has also pointed out a modern use for a very low frequency frame aerial.  One could debate whether this application is an aerial or the primary winding of two coils coupled together to form a tuned transformer with variable coupling but lets not be pedantic.  The photograph shows one of the anti-theft devices installed in many super-markets and shops to detect items being illegally removed from the shop.  The item to be protected is fitted with a security tag incorporating a small coil and a capacitor and a device to disable the resulting parallel tuned circuit when the tag is de-activated at the check-out.  The system works by feeding the large coils in the detectors installed at the shop exit with a VLF signal at the resonant frequency of the tuned circuit in the security tag, usually around 50kHz.  If an "active" tag passes through the field from the detector coils, considerable absorption will occur, the resulting signal strength reduction being detected and used to trigger an alarm.  A similar system is used in some types of metal detector, where if a metallic object magnetically couples to the search coil, the Q of the coil will be reduced, causing the amplitude of the energising signal applied to the coil to be reduced.

Two orthogonally configured multi-turn loop aerials are still used in professional low and medium frequency direction finding equipment, where they are always enclosed in Faraday screens.  The two loops are usually connected to a goniometer, which is calibrated in degrees of bearing.  Further details of a goniometer are given in the variable air cored inductance section below.  A schematic diagram of a single loop aerial and a photograph of two crossed loops are shown below.  Note that for clarity in the schematic, only one turn is shown, although, in practice, several turns would be enclosed within the Faraday screen.  The crossed loop aerial shown in the photograph is one fitted onto a preserved lifeboat, which can be seen on display in the No.3 Covered Slip at the Historic Dockyard, Chatham, Kent.

                     

Iron Dust Cored

Small coils are sometimes wound on fixed iron dust cores where they are invariably used as chokes.  These components are used at frequencies between about 100kHz and 100MHz and normally have inductances of less than 1 millihenry.  Coils for specific purposes are sometimes wound on pot-cores.

Ferrite Cored

Ferrite cores are used in a manner similar to iron dust cores, except that these components are used at frequencies between about 10kHz and 50MHz.  Coils for specific purposes are sometimes wound on pot-cores.

Special types of ferrite cored inductors are designed such that the core saturates at certain pre-determined signal levels.  These components are used in special applications, such as scan linearity correction circuits in CRT displays and were briefly described earlier in this article.

Another type of ferrite core is the ring or toroid.  These are used in the manufacture of RF chokes and transformers where there is little or no DC current flowing in the windings.  Mains filters often employ this type of inductor, where they are connected to provide good common mode signal rejection.

Single hole ferrite beads are used to suppress unwanted HF/VHF signals on wires.  The wire to be filtered is threaded through one or more beads and this has the effect of fitting a series inductor in the wire, without increasing the DC resistance of the wire.  Sometimes, the wire is passed through the hole in the bead several times to increased the effective inductance.

Split "beads", or rather tubes, are manufactured for retro-fitting to existing cables.  These are fitted over the cable and snapped shut.

Iron Cored

Iron cored inductors have many uses, ranging from relay coils and solenoid actuators to low frequency smoothing chokes.

In a relay, an iron-cored coil is arranged to attract a moving iron armature when current is passed through the coil, which, in turn, causes contacts to open or close.  Relays are available in a multitude of shapes and sizes and may be of open costruction, fitted with dust covers or fitted into fully hermetically sealed cans.  The inductance of the coil is normally not important but the "ampere-turns" (number of turns multiplied by the applied current) is.  It should be noted that when a relay is operated from a DC current, a large back-emf will be generated when the current is interrupted, which could be destructive to any semiconductor device driving the relay coil.  To prevent this, a correctly polarised diode is connected in parallel with the coil to limit the back-emf to about 0.6V.  Back-emf is dependent on coil inductance and the rate at which the current decays.  This is represented mathematically by the formula shown below and is true for all inductors, not only relay coils.

Relays designed to operate from AC coil currents are fitted with a copper "slug" coupled to the magnetic circuit, which prevents relay "chatter".  Clamping diodes are not necessary with AC relays.  Reed relays do not employ an iron core, other than the reed itself.  When a current is passed through the coil, the magnetic field produced causes the reed to operate.  The reed switch itself is enclosed in a sealed glass tube and is capable of extremely fast, but silent, operation.

Co-axial and high voltage relays are other variations on a theme, but all rely on an iron armature being moved by the magnetic field produced by the actuating coil.

Low frequency smoothing chokes are becoming less common, as most power supplies are now switched mode devices operating at frequencies up to 500kHz.  Smoothing chokes are still necessary but as the frequency is high, small ferrite cored components are normally used.  Classic circuits where a power transformer provides the desired output voltage, which is then rectified and smoothed are still encountered.  In these circuits, the smoothing choke is invariably a large, iron cored device, having an inductance of several henrys.  Usually, the choke is required to carry a fairly large DC current, which would saturate the core if counter-measures were not taken.  These components are normally provided with an air-gap of a few hundredths of a millimetre in the magnetic path of the laminated core to prevent saturation.  In some applications where the current varies appreciably during normal operation, the width of the air-gap is chosen such that the core does not saturate at low currents, resulting in relatively high inductance, but does saturate at higher currents, resulting in a lower inductance.  This type of inductance is called a "swinging choke".  Low frequency chokes are nearly always impregnated to prevent the ingress of moisture and may be of "open" construction, or may be enclosed in a sealed metal container.

 

VARIABLE INDUCTORS

Variable inductors are categorised according to the core materials used and most of these can be sub-divided into many different types.

Air cored

By definition, the inductance of air cored coils cannot easily be varied.  Variable inductance air cored coils are limited to small, self-supporting, types used at VHF, where the inductance is varied by adjusting the turn spacing.  Brass cores are a special case, also only used in variable value inductors for use at VHF.  The brass core is used in the same way as an iron dust or ferrite core but its introduction into a coil actually reduces the inductance by causing a limited "shorted-turn" effect.  Because the magnetic coupling factor between the coil and the core is low, excessive losses are avoided, although the use of brass cores does reduce the Q of the coil.

Air cored variable inductors are normally only used in low power circuits, although they are sometimes used in HF transmitters, where they take the form of the so called "roller coaster" and in medium frequency transmitters, where they take the form of variometers.  Variometers are really transformers used as variable inductance coils and consist of two tightly coupled air spaced coils, one being capable of being turned through 180 degrees.  If these two coils are connected in series, a large variation of inductance can be obtained, depending on whether the mutual inductance is adding or subtracting from the total.

These devices depend on very close coupling between the coils and are usually wound with bunched or litz wire.  They are capable of very high Q values.

The radio goniometer is a device in which a rotatable search coil is coupled to two individual fixed coils arranged at 90 degrees to each other.  They are therefore really air cored RF transformers, but are describe in this section, as goniometers resemble variometers except they are not used as variable inductors but are used in conjunction with crossed loop DF aerials (the Bellini-Tosi system).  In this application, the two loops are connected to the two fixed coils and the rotating search coil is connected to the receiver.  Rotation of the search coil enables the receiver to be "connected" to the two loops in a continuously variable manner, effectively simulating a rotating loop aerial.  A photograph of an early goniometer and the schematic diagram are shown below.  A higher definition image and a considerable amount of interesting technical information is available on the Royal Navy Museum of Radar and Communications web-site.

                 

"Roller coasters" use a solid conductor coil, which may be rotated by means of a control shaft.  A small roller is arranged such that it runs on the coil and runs up and down the coil as it is rotated.  This roller forms an infinitely adjustable tapping point on the coil.  These components are nominally fairly high-Q devices but suffer from oxidation of the contacting surfaces, which creates electrical noise and lowers the Q.  This effect is considerably reduced if the roller is replaced by a small carbon brush, similar to those used in electric motors fitted with commutators or slip rings.  However, the carbon brush has resistance, which will lower the overall Q of the device.

Another variation on this theme is where a silver plated beryllium copper strip is wound on and off a former from a metal cylinder.  This type of inductor does not use an adjustable tapping point, the active inductor being those turns wound on the former.  The unused metal strip does not form shorted turns, hence the higher Q.  These inductors are less noisy than the roller coaster type and do not suffer the same degree of Q loss but they are considerably more mechanically complicated and therefore expensive.  The photo below is reproduced with the kind permission of Dave Knight G3YNH.  Further information and a host of other images may be found on his web site.

Iron Dust cored

One of the most common methods of providing a variable inductance coil is by the use of an adjustable iron dust core.  This type of inductor is normally only used in low power circuits.

An interesting "special case" was the very large iron dust core (78mm long x 43mm diameter) used for adjusting the inductance of the MF tuning coil fitted in the World War II transmitter type T1154.

Iron dust cores are used at frequencies between about 100kHz and 100MHz and normally coils using them have inductances of less than 1 millihenry.  These components are often housed in screening cans to eliminate interaction with other components.  Screening cans are usually manufactured from aluminium and therefore provide predominantly electrostatic screening.  If magnetic screening is needed, mu-metal cans are normally used but they are not often required.  Coils for specific purposes are sometimes wound on pot-cores.

Ferrite cored

Another very common type of variable inductor uses ferrite cores in a similar manner to iron dust cores.  This type of inductor is also normally used in low power circuits and they are used at frequencies between about 10kHz and 50MHz.  These components are often housed in screening cans to eliminate interaction with other components.  If magnetic screening is needed, mu-metal cans are normally used but they are not often required.  Coils for specific purposes are sometimes wound on pot-cores.

A specialised use of ferrite cores is in ferrite rod aerials or "ferri-loop-sticks" as they were originally called in the USA.  In this application, the core is extended in length and increased in diameter to form a rod.  The tuning coil is wound on a former, which can slide over the rod.  Sliding this coil up and down the rod varies the inductance, being at a maximum value when the coil is in the centre of the rod.  These devices are used as the internal aerials in modern portable broadcast radio receivers and are often used in 160m and 80m direction finding receivers.

Iron cored

Iron cores are seldom used in variable inductors.  An exception is in arc welding technology, where a saturable variable inductor is used to control the welding current.  These components use special laminated cores in which part of the core is mechanically movable.


This article is intended to give an overview of the subject and none of the items has been dealt with in depth.  Further information is available from inductor manufacturer's literature or from the Internet.

 

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