Inductors, coils and chokes are passive devices that are designed to resist changes in current and store energy in the form of a magnetic field. In their simplest form, inductors consist of a wire loop or coil. The inductance is directly proportional to the number of turns in the coil.
Inductance also depends on the radius of the coil and on the type of material around which the coil is wound. With inductors, coils and chokes, air cores result in the least inductance for a given coil radius and number of terms.
Materials and Configurations
Selecting inductors, coils and chokes requires an analysis of material properties and core designs. Dielectric materials such as wood, glass, and plastic are essentially the same as air for the purpose of inductor winding. Ferromagnetic substances such as iron, laminated iron, and powdered iron increase the inductance obtainable with a coil having a given number of turns. In some cases, this increase is on the order of thousands of times.
The shape of the core for inductors, coils and chokes is also significant. Toroidal cores offer more inductance, for a given core material and number of turns, than solenoidal cores. Toroids look resemble doughnuts and differ in terms of diameter, thickness, permeability and type, depending upon the frequency range of interest. Toroidal cores also have a high level of inductance for the physical space occupied.
What to Consider When Choosing
Important specifications to consider when searching for inductors, coils and chokes include mounting options, core materials, lead types, and inductance types. Mounting options include through-hole and surface-mount technologies. Coils can be wound on various core materials.
The most popular are iron (or iron alloys, laminations, or powder); and ferrite, a black, nonconductive, brittle magnetic material. The use of these materials is designed to multiply the inductance of a given coil by the "permeability" of the core material. Other core materials include air, ceramic, and phenolic.
Lead types can be axial, radial, flying, no leads (SMT), tab, gull wing, and J-leads. With inductors, coils and chokes, the inductance can be fixed or variable.
Important electrical specifications to consider when searching for inductors, coils and chokes include inductance range, inductance tolerance, maximum DC resistance, and operating current range.
Applications for inductors, chokes, coils may be designated as common-mode choke, general-purpose, high current, high frequency, power choke, and RF choke.
Common-mode choke coils are useful in a wide range of prevention of electromagnetic interference (EMI) and radio frequency interference (RFI) from power supply lines and to prevent various types of electronic equipment from malfunctions. Operating temperature is another important specification to consider when selecting inductors, coils and chokes.
BS 5347 - Specification for silicon-iron strip-wound cors for use in transformers and inductors for telecommunications and electronic equipment.
DEF STAN-22-2 - Specification for chokes and transformers used in fluorescent fittings.
IEC 60723-4 - Inductor and transformer cores for telecommunications - part 4: sectional specification: magnetic oxide cores for transformers andchokes for power applications.
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