LYCOS RETRIEVER 
Ferrite: Ferrite Beads
built 630 days ago
A Ferrite bead is a dowel-like device which has a center holes and is composed of ferromagnetic material. When placed onto a current carrying conductor it acts as an RF choke. It offers a convenient, inexpensive, yet a very effective means of RF shielding, parasitic suppression and RF decoupling.
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Ferrite beads work by reducing the RFI (radio-frequency interference) created by these cables. Without the beads, these cables can act as nice, long antennae for the signals they carry. The signals they broadcast can interfere with radios and TVs, causing the sound and/or picture to go fuzzy. The cables can ... receive signals and transmit them into the computer, where they cause problems. A ferrite bead eliminates these "broadcast signals." Essentially, it "chokes off" the RFI transmission at that point on the cable -- this is why you find the beads at the ends of the cables.
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Ferrite beads are usually quite small and as a result only one pass, or a small number of turns are possible. On the other hand, a toroidal core usually has a much larger inner diameter and will accept a greater number of turns. The greater number of turns can be an advantage in some cases where a large amount of impedance is required. The increase in impedance is proportional to the square of the number of turns.
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Ferrite beads are similar to inductors in some ways but work especially in an area that is parasitic for general purpose inductors. They essentially act as a high impedance, or "resistor" to high frequency EMI/RFI electronic noise. The absorbed energy is converted to heat and dissipated by the ferrite, but only in extreme cases will the heat be noticeable.
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The number of turns on a single hole Ferrite bead or a toroidal core is identified by the number of times the conductor passes through the center hole. To physically complete one turn it would be necessary to cause the wires to meet on the outside of the device... the bead or core does not care about the termination of each end of wire and considers each pass through the center hole as one turn.
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Ferrite Beads