The core of the matter – Clive VK6CSW

Look at the innards of almost any piece of today’s electronic gear and you see inductors of all shapes and sizes. Some are air-wound, some have cores, some are totally enclosed in ferrite pots or metal laminations. This article is an attempt to look briefly at some of the whys and wherefores of coils and their various cores. It is intended simply as a overview and not as a definitive text. I am no expert in this subject, but it is amazing what you can turn up once you start to dig around a bit. But let’s start at the beginning……..

One April evening in 1820, Hans Christian Oersted was giving a lecture on electricity at Copenhagen University. As he passed a current through a wire the needle on a nearby compass swung away from North. The induction of a magnetic field by electricity had been discovered. The idea was seized upon by the leading physicists of the day, and Michael Faraday intuitively realised the connection between magnetism and electricity. By 1832 Faraday had worked out the principles of electromagnetic induction and was demonstrating his famous two-coil experiment whereby a rising or collapsing magnetic field in one coil would “induce” a brief pulse of electricity in the second coil even though no electrical connection existed between the two coils.

The further realisation of the association between magnetism, electricity, and mechanical energy subsequently led to the invention of the electric motor, dynamo, alternator, the discovery of Hertzian waves, and ultimately to the electronic age. The significance of Oersted’s chance observation together with Faraday’s intuition was immense. Many others were involved in the development of these sciences of course, some of whose names are now immortalised in our hobby; Volta, Ohm, Gauss, Maxwell, Ampere, Coulomb, Henry, Hertz, Tesla, to name but a very few.

Faraday quickly realised that the induction effect was much greater when the coils were wound on a magnetic material such as soft iron rather than just being air-wound, (actually he used wooden formers which are magnetically transparent) the reason being that the iron permitted a greater flux density to exist. In effect, all cores are a refinement of this technique in that they permit control of the inductance of the coil to achieve a specific aim.

It is not my intention to go into the physics of inductance to any extent, but we do need to look at a few basics. Consider an air-wound coil. A particular current flow will induce a magnetic field in and around the coil, much as shown in Fig 1. The field strength stabilises when the energising (magnetomotive) force equals the magnetic path resistance, or reluctance. The field strength can be considered as so many magnetic lines of force per unit area, or flux density, and any material which can reduce the reluctance and thus increase the flux density will also increase the inductance of the coil.

Electrical energy is required to create a magnetic field, and this field represents stored energy; a magnetic field moving across a conductor will create electrical energy within that conductor by giving up some or all of this energy. A single coil has self-inductance; two or more coils which share a common magnetic field have mutual inductance.

Ferro-magnets are substances whose natural magnetism increases in the presence of a magnetising field. Iron is the best-known ferro-magnet, but nickel and cobalt exhibit a similar quality and are also classified as ferro-magnets, and nowadays there are many man-made compounds which are highly ferro-magnetic, AlNiCo and samarium, for example. The effect is caused by alignment of electron spin in regions known as domains, and the residual magnetic effect of the applied field may be either temporary or permanent. Energy is required to alter this alignment.

When ferro-magnetic material is introduced into the magnetic field of an air-wound coil by placing it in the core, the reluctance is reduced and the flux density increased. Thus the inductance of the coil has been increased for the same number of turns. The ratio of the flux density in the air coil core to the increased flux density in the ferro-magnetic core due to the same magnetising field is called the permeability of the material. If we say that the permeability of air is 1, then, by definition, any material with a permeability greater than 1 is ferro-magnetic. (Materials which reduce the flux density, such as brass, are called dia-magnetic). Soft iron has a permeability of around 800. Thus a coil wound on this material would have an inductance 800 times that of the same coil with an air core. Just as copper has a high electrical conductivity, so iron has a high magnetic conductivity.

Unlike air, which has no magnetic saturation limit, all ferromagnetic materials do have a flux density limit. This means that beyond a certain point, no amount of increase in the magnetising current will further increase the flux density. The material is magnetically saturated at this point and this leads to non-linear operation of the inductor. An example of this effect is an antenna balun with a ferrite core, which if driven with too high an RF current will cause the transformer action to be no longer linear. The sinusoidal RF waves become distorted, causing generation of harmonics and possible TVI. Some devices use deliberately saturated cores, the transformer in a battery powered fluorescent light being one example.

Because magnetic materials can be saturated, and because there is a limit to the rate at which the domains can follow a changing magnetic field, permeability varies with both magnetising force and frequency. Cores which work at low frequency may be “transparent” at RF.

Since iron is a conductor, when an alternating current flows through it an EMF is induced in it, just as it would be in a wire. This means that a current, known as an eddy current, flows in the core. The core also has resistance and so this current flow generates heat, which is lost energy. These losses can be reduced by laminating the core and insulating the laminations from one another by means of varnish. It is worth bearing in mind that a similar effect occurs at RF, and core heating may be a cause of drift in oscillator circuits.

Not only is energy required to alter the magnetic domains, but, as mentioned earlier, there is a limit to the speed at which they can be made to follow changes in the magnetic field. The alignment of the domains lags slightly behind the magnetising field, and some residual magnetism occurs. This effect is called magnetic hysteresis and represents an energy loss. Early wireless sets used transformer-coupled audio stages which lacked upper register response because the iron laminations were less effective at the higher frequencies. Much R and D went into finding suitable alloys to alleviate the problem. Hypersil and Permalloy are examples of high quality core material, but even these are useless above a few tens of kilohertz and quite unsuitable for RF.

Around 1930 it was found that finely powdered iron particles held in an insulating binder made cores which would work into the low megahertz region, significantly increasing the coil’s Q. Q is reactance divided by resistance, and reactance is proportional to inductance times frequency. Thus by introducing a ferromagnetic core we achieve a required inductance with fewer turns and less resistance, thereby raising the Q factor. The earliest commercial use of powdered iron cores appears to have been in IF transformers in the superhets of the mid-1930s where the higher Q gave better selectivity. Early IFs had fixed cores and trimmer capacitors, but these soon gave way to the fixed capacitor with trimmable core that is still used today.

Most coils with or without cores have a significant field external to the coil which will cause unwanted coupling unless the coil is screened, usually in an aluminium or mumetal can. (Mumetal is the trade name of a high-permeability, low-saturation magnetic alloy having a high nickel content which is often used as both a low-frequency screening material and for relay and AC instrument cores.) This screening-can interferes with the external field and reduces the Q of the coil, but the idea soon arose to “pot” the entire coil assembly with a powdered iron moulding. This both improves the external magnetic path and raises the Q, and reduces the stray external field. With the development of higher communications frequencies, better powdered iron cores and pots were developed. Today, powdered iron cores useable to 200 Mhz are easily obtainable with permeabilities up to about 35, depending on frequency. The permeability is significantly lower than that of the iron itself because much of the magnetic path is, of necessity, through the non-magnetic binding material.

Ferrites are entirely different from powdered iron and are a ceramic iron oxide with the general formula where X is one or several of di-valent transition metals such as manganese, nickel, zinc, cobalt, etc. The required composition is moulded and fired at temperatures of around 1000 degrees C. Ferrites and powdered iron cores often possess similar characteristics at RF, but there is one important difference. If ferrite is over-driven it suffers a permanent change to its permeability and becomes useless for its designed purpose, whereas over-driven powdered iron will eventually recover to its original permeability. On the other hand, the permeability of a ferrite core can be much higher than that of a similar sized powdered iron core, allowing a smaller physical size for a given inductance, an important consideration in miniature equipment. Against that, small changes in temperature may result in significant inductance value changes.

Powdered iron tends to be less lossy than ferrite resulting in coils of higher Q, making this material the better choice for narrow band or tuned circuits application. The toroid or “doughnut-shaped” core has a particular advantage in that the magnetic field is contained wholly within the core and therefore such a coil is self-shielding. See Fig 2. This confers advantages in space-saving because shielding is unnecessary, but it is difficult to alter the inductance other than by adding or subtracting turns. Very small inductance changes can be achieved by compressing or spreading the turns, but this may not be practicable in all cases. Because the toroid core is self-shielding it is hard to couple to a GDO for  measurement. A one or two turn link around the toroid and the
GDO coil solves the problem.

Ferrite beads are small dowel-shaped devices widely used for RF suppression, shielding, de-coupling, and parasitic suppression. These beads offer little or no impedance to low frequencies or DC, but as the frequency rises their impedance rises rapidly. They operate as bulk devices and need no grounding or other connection, but just slip over the lead. The exact characteristics depend on the ferrite used, but they work by introducing a large hysteresis loss which dissipates the RF. Because they are dissipative rather than reactive, there is little risk of resonance at any frequency. They are often found on power leads as RF stoppers and on transistor legs as parasitic oscillation stoppers, for example. Larger devices which clamp around computer ribbon cable to suppress noise are also now available.

The foregoing is by no means exhaustive but may give some idea of the whys and wherefores of cores. Those seeking more detailed information should consult publications such as the ARRL Handbook, Amidon catalogues, and the Neosid Magnetic Components catalogue, to name a few, but at least this might have wetted the appetite!

Clive – VK6CSW.

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