Ferrite beads are used to suppress unwanted signals that can interfere with electrical devices such as DC supplies, transmission lines and cables. They provide attenuation of selected frequency bands. The physical shape of ferrite beads is similar to a toroidal inductor, but the beads have a greater length to diameter ratio and usually a greater outside to inside diameter ratio than most toroid cores. Different size / shape beads of the same material have different degrees of suppression. The type of ferrite material used to manufacture the bead determines the range of frequencies for suppression purposes, and the physical size and shape of the bead determines the amount of attenuation. In general the impedance is directly proportional to the length of the ferrite beads.
Important general specifications to consider when searching for ferrite beads include technology, form factor, beads in package, and packing method. Choices for technology type include solenoidal (standard) core, toroidal, bobbin core, wirewound, multilayer, and laminated. Solenoidal cores are rod-shaped cores. Toroid inductors look exactly like doughnuts and come in various diameters, thickness, permeability and types depending upon the frequency range of interest. They have a high inductance for the physical space occupied. A bobbin core is an inductor with a core with the shape of a bobbin or spool. Bobbin cores are available with and without leads and in the axial and radial form. A wirewound inductor is an inductor with a core made of wound wires. Multilayer inductors are inductors constructed by layering the coil between the layers of core material. The coil normally consists of a bare metal material (no insulation). This technology is normally referred to as "non-wirewound". The inductance value can be made larger by adding additional layers for a giving spiral pattern. Laminated inductors are ferrite beads with a core that is constructed by stacking multiple laminations on top of each other. The laminations can be of a variety of materials and thicknesses. Form factor choices include rod, bead on leads, bar, plate, tube, and chip. The number of ferrite beads in the package can be 1, 2, or 4. Choices for packing method include tape reel, tray, tube, and bulk pack.
Important performance specifications to consider when searching for ferrite beads include impedance range, impedance tolerance, impedance testing frequency, DCR, rated DC current, and operating temperature. The impedance is the total resistance to the flow of current, including AC and DC components. The DC component is simply the DC resistance of the device. The AC component includes the bead reactance. The impedance tolerance is the allowed amount of variation from the nominal value as specified by the manufacturer. The impedance testing frequency is the frequency at which the bead impedance is tested. Direct current resistance (DCR) is the resistance of the inductor winding measured using DC current. The DCR is most often minimized in the design of an inductor and specified as a maximum rating. The rated DC current (IDC) is the level of continuous direct current that can be passed through the inductor with no damage. The DC current level is based on a maximum temperature rise at the maximum rated ambient temperature. The rated current is related to the inductor's ability to minimize the power losses in the winding by having a low DC resistance. For low frequency currents the RMS current can be substituted for the DC rated current. The operating temperature for ferrite beads is the full required range of ambient operating temperature.
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