As you've seen, inductors store magnetic energy, creating reactance. Inductors are usually visualized as the classic winding of wire from one end to the other of a round form this winding shape is called solenoidal giving rise to the common term "coil." An inductor's core is whatever material the wire is wound around, even air. (An inductor whose core consists of air is called air-wound.)
Solenoidal coils make great figures in books, but are actually fairly uncommon in electronics. A winding of wire around a hollow form filled with nothing but air is a relatively inefficient way to store magnetic energy. A form made of magnetic material increases the storage of energy because it focuses the magnetic field created by the current in the surrounding winding. The stronger magnetic field increases the inductance of the inductor.
Inductance is determined by the number of turns of wire on the core and on the core material's permeability. Permeability refers to the strength of a magnetic field in the core as compared to the strength of the field with a core of air. Cores with higher permeability have more inductance for the same number of turns on the core. In other words, if you make two inductors with 10 turns around different core materials, the core with a higher permeability will have more inductance.
Manufacturers offer a wide variety of materials, or mixes, to provide cores that will perform well over a desired frequency range. Powdered-iron cores combine fine iron particles with magnetically-inert binding materials. Combining materials such as nickel-zinc and manganese-zinc compounds with the iron produces ceramic ferrite cores. The chemical names for iron compounds are based on the Latin word for iron,ferrum, so this is how these materials get the name ferrite. By careful selection of core material, it is possible to produce inductors that can be used from the audio range to UHF. Inductors with magnetic material cores are also called ferromagnetic inductors.
The choice of core materials for a particular inductor presents a compromise of features. Powdered-iron cores generally have better temperature stability. Ferrite cores generally have higher permeability values. however. so inductors made with ferrite cores require fewer turns to produce a given inductance value.
Core Shape
The shape of an inductor's core also affects how its magnetic field is contained. For a solenoidal core, the magnetic field exists not only in the core, but in the space around the inductor. This allows the magnetic field to interact with, or couple to, other nearby conductors. This coupling often creates unwanted signal paths and interactions between components, so external shields or other isolation methods must be used.
To reduce unwanted coupling, the donut-shaped toroid core is used. When wire is wound on such a core, a toroidal inductor is produced. Nearly all of a toroidal inductor's magnetic field is contained within the toroid core. Toroidal inductors are one of the most popular inductor types in RF circuits because they can be located close to each other on a circuit board with almost no interaction. See Figure 4-29 for a photo of a variety of toroidal inductors Toroidal inductors are used in circuits that involve frequencies from below 20 Hz around 300 MHz.
ข้อสอบที่เกี่ยวข้อง
What material property determines the inductance of a toroidal inductor with a 10-turn winding?
A. Core load current
B. Core resistance
C. Core reactivity
D. Core permeability
What is the usable frequency range of inductors that use toroidal cores, assuming a correct selection of core material for the frequency being used?
A. From a few kHz to no more than 30 MHz
B. From less than 20 Hz to approximately 300 MHz
C. From approximately 1000 Hz to no more than 3000 kHz
D. From about 100 kHz to at least 1000 GHz
What is one important reason for using powdered-iron toroids rather than ferrite toroids in an inductor?
A. Powdered-iron toroids generally have greater initial permeabilities
B. Powdered-iron toroids generally have better temperature stability
C. Powdered-iron toroids generally require fewer turns to produce a given inductance value
D. Powdered-iron toroids have the highest power handling capacity
What is a primary advantage of using a toroidal core instead of a solenoidal core in an inductor?
A. Toroidal cores contain most of the magnetic field within the core material
B. Toroidal cores make it easier to couple the magnetic energy into other components
C. Toroidal cores exhibit greater hysteresis
D. Toroidal cores have lower Q characteristics
What is one reason for using ferrite toroids rather than powdered-iron toroids in an inductor?
A. Ferrite toroids generally have lower initial permeabilities
B. Ferrite toroids generally have better temperature stability
C. Ferrite toroids generally require fewer turns to produce a given inductance value
D. Ferrite toroids are easier to use with surface mount technology
