Technical Handbook

Inductors

General

General Information

A Brief History of Inductance
Using Inductors in Loudspeakers
Inductor Cores
P-Cores
Background Design Information
Magnetic Properties of P-Cores
Sample Inductor Designs using P-Cores
 

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A Brief History of Inductance

The unit of inductance is the Henry (symbol H) named after American Joseph Henry (1797-1878) who, unlike the self-taught Faraday, was a trained physicist and mathematician. He actually discovered electro-magnetic induction before Faraday, but the latter was the first to publish and to receive the credit. Henry announced his discovery of self-inductance* in 1832, having already developed the first practical electromagnetic telegraph (1831) and one of the first electric motors. He was appointed secretary and director of the Smithsonian Institute (1846) where, amongst other things, he organised research into meteorology - work which later led to the foundation of the US Weather Bureau.

* Self-inductance (flux linkages per ampčre) is the parameter which defines the chokes or inductors used in loudspeaker design. It is represented by the symbol L, as distinct from Mutual-inductance (symbol M) which is relevant to the design of transformers and other magnetically linked coils. Both parameters are measured in Henrys.

1 Henry = 1,000 mH = 1,000,000 µH

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Using Inductors in Loudspeakers

The inductor is the most problematic of the passive electronic components used in the manufacture of loudspeaker crossover assemblies with the conflicting requirements of performance, size and cost always difficult to reconcile. However, the very flexibility of inductor specification can offer considerable benefits in achieving precisely the desired performance in the complete loudspeaker system.

No standard range of inductors is available for loudspeaker applications. For those manufacturers who lack the facilities to fabricate their own inductors, ECL can offer finished, wound inductors of an extremely high standard and consistency to suit any requirement. Components can be manufactured to your specification or we can offer a design service. Inductors may be specified as air-cored or wound on ferrite cores. We can also supply units wound on the high performance P-Cores detailed below.

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Inductor Cores

So-called air-cored inductors (which may in fact be wound on non-magnetic cores) can offer the ultimate in performance, with extremely low distortion levels. The penalty for this performance comes in low inductance for a given number of turns which, in turn results in a comparitively high DC resistance. Unfortunately, minimising an inductor's DC resistance is usually of prime importance in optimising a loudspeaker's overall efficiency, so the maximum practical inductance for an air-cored inductor is soon reached, but actual limits will depend entirely on design concept and cost.

An economic alternative to air-cored inductors is to wind on a magnetic material, of which the most common is ferrite: magnetic iron oxides bonded in a ceramic matrix. Ferrite cores or bobbins can thus increase inductance values for a given number of turns of wire (or DC resistance). This time, the penalty becomes the limit on magnetic field strength which can be induced in the ferrite material - its saturation level - which gives rise to non-linear performance, and significant audio distortion with rising current in the winding.

An alternative route is now available, for improved performance and/or cost-effectiveness using our range of P-Cores.

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P-Cores

P-Cores, comparable in shape and size to ferrite cores, are manufactured from high purity compressed iron powder bonded with small amounts of isolating material. With the use of these alternative magnetic materials, inductors wound on P-Cores achieve inductance values similar to ferrite designs but with much higher saturation levels and linear magnetic characteristics extending to much larger operating currents. This gives rise to a dramatic reduction in audio distortion (typically 20-30dB lower than with equivalent ferrite cores) and a significant improvement in audio performance at comparable or minimal additional cost.

P-Cores are supplied in cylindrical rod form with optional flanges (for most sizes) which may be glued to the rod to form a bobbin. Mounting accessories are also available for some sizes.

Careful optimisation of inductors wound on P-Cores can achieve perceived performance characteristics approaching that of air-cored inductors or, alternatively, the economies hitherto associated with using ferrite cores.

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Background Design Information

For any inductor, the inductance increase caused by using a magnetic core (rather than air, or non-magnetic material) is expressed by the apparent permeability factor µapp.

Although the value of  µapp is similar when using equivalent sized ferrite cores and P-Cores (or bobbins), the maximum current  Imax,  below which linear characteristics are maintained, is much higher for P-Cores. Audible distortion may be considered insignificant if Imax is not exceeded. Even if inductors wound on P-Cores are subjected to high current pulses in excess of Imax, they will still introduce only low distortion levels.

For P-Cores without flanges, the following empirical formula may be used to calculate the maximum recommended current Imax:

L = Length of core (mm)                       N = Number of turns in winding

When using rod cores, the core should protrude from the coil winding by at least 3-5mm.

If flanges are bonded to the core to form a bobbin, the above formula is only approximate, but inductance may increase substantially (sometimes to almost twice the value without flanges).

Two P-Core rods may be glued end to end if necessary to achieve the required specification. Suitable adhesives for joining rods or fixing flanges are: polyurethane, nitrile, epoxy resin, cyanoacrylate, silicon, hot-melt and the Scotch (3M) VHB Joining System.

Like other iron components, P-Cores may rust in the long term or if exposed to humidity. Rusting may be impeded by coating with lacquer.

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Magnetic Properties of P-Cores

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Sample Inductor Designs Using P-Cores

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