Powdered-iron and ferrite cores

Chapter Powdered-iron and ferrite cores

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • almost unlimited current-carrying capacity, just by using heavy-gauge wire and makingthe radius of the coil large. Air does not dissipate much energy in the form of heat; it isalmost lossless. For these reasons, air-core coils can be made highly efficient.Powdered-iron and ferrite coresFerromagnetic substances can be crushed into dust and then bound into variousshapes, providing core materials that greatly increase the inductance of a coil having agiven number of turns. Depending on the mixture used, the increase in flux density canrange from a factor of a few times, up through hundreds, thousands, and even millionsof times. A small coil can thus be made to have a large inductance.Powdered-iron cores are common at radio frequencies. Ferrite has a higher per-meability than powdered iron, causing a greater concentration of magnetic flux lineswithin the coil. Ferrite is used at lower radio frequencies and at audio frequencies, aswell as at medium and high radio frequencies.The main trouble with ferromagnetic cores is that, if the coil carries more than acertain amount of current, the core will saturate. This means that the ferromagneticmaterial is holding as much flux as it possibly can. Any further increase in coil currentwill not produce a corresponding increase in the magnetic flux in the core. The result isthat the inductance changes, decreasing with coil currents that are more than the crit-ical value.In extreme cases, ferromagnetic cores can waste considerable power as heat. If acore gets hot enough, it might fracture. This will permanently change the inductance ofthe coil, and will also reduce its current-handling ability.Permeability tuningSolenoidal, or cylindrical, coils can be made to have variable inductance by sliding fer-romagnetic cores in and out of them. This is a common practice in radio communica-tions. The frequency of a radio circuit can be adjusted in this way, as you’ll learn later inthis book.Because moving the core in and out changes the effective permeability within a coilof wire, this method of tuning is called permeability tuning. The in/out motion can beprecisely controlled by attaching the core to a screw shaft, and anchoring a nut at oneend of the coil (Fig. 10-7). As the screw shaft is rotated clockwise, the core enters thecoil, so that the inductance increases. As the screw shaft is rotated counterclockwise,the core moves out of the coil and the inductance decreases.ToroidsInductor coils do not have to be wound on cylindrical forms, or on cylindrical ferromag-netic cores. In recent years, a new form of coil has become increasingly common. Thisis the toroid. It gets its name from the donut shape of the ferromagnetic core. The coilis wound over a core having this shape (Fig. 10-8).190 Inductance