Ferrite beads are used as a passive low-pass filter, by converting RF (radio frequency) energy to heat, by design. (Contrast this with inductors, which by design minimise conversion of RF energy to heat, offering a high impedance to RF.)

The geometry and electromagnetic properties of coiled wire over the ferrite bead result in impedance for high-frequency signals, attenuating high-frequency EMI/RFI electronic noise. The energy is either reflected back up the cable, or dissipated as low-level heat. Only in extreme cases is the heat noticeable.

A pure inductor does not dissipate energy but it produces reactance that impedes the flow of higher frequency signals. This reactance is commonly referred to simply as impedance, although impedance can be any combination of resistance and reactance.

A ferrite bead can be added to an inductor to improve, in two ways, its ability to block unwanted high-frequency noise. First, the ferrite concentrates the magnetic field, increasing inductance and therefore reactance, which impedes or ‘filters out’ the noise. Second, if the ferrite is so designed, it can produce an additional loss in the form of resistance in the ferrite itself. The ferrite creates an inductor with a very low Q factor. This loss heats the ferrite, normally by a negligible amount. While the signal level is large enough to cause interference or undesirable effects in sensitive circuits, the energy blocked is typically quite small.

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