Powdered Iron Cores

As frequency increases above a few thousand cycles, gap loss becomes predominantly large. At such frequencies, cores of powdered iron are preferable for large Q. Powdered-iron cores are made from several grades of iron and nickel-iron alloys. Proportions of insulating bond and iron powder are varied to obtain permeabilities ranging from 10 to 125. Permeability in such cores is only apparent; it is far less than the inherent permeability of the iron used because of the many small gaps throughout the core structure. Finely divided iron has low eddy-current loss and virtually zero gap loss. Equation 85 indicates how Q varies with frequency; that is, low-permeability cores should be used to reduce inductance and maintain large Q at high frequencies. At frequencies higher than audio, coil eddy-current losses make stranded wire necessary. This is discussed further in Higher-Frequency Transformers.

One of the problems of filter design is the maintenance of cut-off and attenuation frequencies under conditions of varying temperature. This may be so important as to dictate the choice of core material. Powdered cores are available which have very low temperature coefficients. Usually these cores have less than the maximum Q for a given kind of iron powder. With low-temperature-coefficient cores, attention also must be paid to filter capacitors in order to obtain the requisite overall frequency stability.

Table XIV. Shapes of Powdered-iron Cores

Core Shape	Use
Shell	Low-voltage r-f transformers and inductors.
Cup	Adjustable low and medium r-f inductors.
Slug	Adjustable r-f inductors. Also used to adjust cup-core inductance.
Toroid	Audio and low r-f inductors.
C	High-voltage audio and r-f transformers and inductors.

Powdered cores are made in several forms. Table XIV indicates the main areas of usefulness of such forms.

Fig. 144. Powdered iron-core shapes.

Figure 144 illustrates the core shapes in this table. A study of available molds and materials is worthy of the designer's time.