thumb|A ferrite bead at the end of a [[Mini USB cable]]

A ferrite beadalso called a ferrite block, ferrite core, ferrite ring, EMI filter, or ferrite chokeis a type of choke that suppresses high-frequency electronic noise in electronic circuits.

Ferrite beads employ high-frequency current dissipation in a ferrite ceramic to build high-frequency noise suppression devices.

Use

thumb|A ferrite bead with its plastic shell removed

Ferrite beads prevent electromagnetic interference (EMI) in two directions: from a device or to a device.

Theory of operation

thumb|An RF inductor wound on a ferrite core (not a ferrite bead), and a PCB mount ferrite bead

thumb|A clamp-on ferrite bead heating up in operation

Ferrite beads are used as a passive low-pass filter by dissipating radio frequency (RF) energy as heat by design.

Ideal inductors, on the other hand, have no resistance and hence do not dissipate energy as heat. Ideal inductors only have inductive reactance, which reduces the flow of high-frequency signals by returning some of their energy back towards the signal source (possibly reducing the amount of power drawn) rather than dissipating that energy as heat (as done by the resistance in ferrite beads). While an inductor's reactance may commonly be referred to simply as impedance, impedance generally can be any combination of resistance and reactance.

The geometry and electromagnetic properties of coiled wire over the ferrite bead result in an 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.

thumb|left|A collection of snap-on/clamp-on ferrite beads

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 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, generally 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 relatively small. Depending on the application, the resistive loss characteristic of the ferrite may or may not be desired.

A design that uses a ferrite bead to improve noise filtering must consider specific circuit characteristics and the frequency range to block. Different ferrite materials have different properties concerning frequency, and the manufacturer's literature helps select the most effective material for the frequency range.

See also

  • Braid-breaker
  • Magnetic core
  • Toroidal inductors and transformers
  • Decoupling (electronics)
  • Fuse (electrical)
  • Zero-ohm resistor

References

  • Ferrite bead inductor usage in electronic circuits