ferrite bead

Ferrite beads are used (in a way similar to inductors) as a passive low-pass filter. The geometry and electromagnetic properties of coiled wire over the ferrite bead result in a high resistiveimpedance (resistance) for high-frequency signals, attenuating high frequency EMI/RFI electronic noise. The energy is either reflected back up the cable, or absorbed resistively within the ferrite core and dissipated as low level heat. Only in extreme cases will the heat be noticeable.
 
A pure inductor does not dissipate energy; it merely absorbs energy from the circuit and returns it at a later time. A ferrite bead, by design, filters out the high frequency noise in the circuit by dissipating it as heat. The ferrite bead is effectively an inductor with a very small Q factor. When electrical noise within the target frequency range travels in the signal cable a back-emf is induced in the ferrite bead because of its high inductance. The material used to construct the ferrite bead however, becomes highly resistive at the design frequency range (the magnetic field within the bead is unable to establish properly at that specific frequency range) and the induced current inside the bead is dissipated as heat instead of inducing an opposing current back in the signal cable. It is for this reason that the specific circuit characteristics as well as the frequency band of the noise need to be taken into account when the ferrite bead is installed as a noise filter.
 
Ferrite beads are one of the simplest and least expensive types of interference filters to install on preexisting electronic cabling. For a simple ferrite ring, the wire is simply wrapped around the core through the center typically 5 or 7 times. Clamp-on cores are also available, which can be attached without wrapping the wire at all. Although the wire is not coiled around the core for this type of ferrite beads, the introduction of the ferrite core around the wire increases the self-inductance of the wire thus still has the effect of absorbing energy from the noise traveling in the wire. If the fit is not snug enough, the core can be secured with cable ties, or if the center is large enough, have the cabling looped through one or more times.
 File:Cable end.JPG
 

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