Ferrite bead

For other uses, see Bead (disambiguation).

A ferrite bead at the end of a USB cable

A ferrite bead with its plastic shell removed

An RF inductor wound on a ferrite bead, and a PCB mount ferrite bead.

A ferrite bead is a passive electric component used to suppress high frequency noise in electronic circuits. It is a specific type of electronic choke. Ferrite beads employ the dissipation of high frequency currents in a ferrite ceramic to build high frequency noise suppression devices. Ferrite beads may also be called blocks, cores, rings, EMI filters, or chokes.

Overview

Ferrite beads are used to prevent interference in two directions: from a device or to a device. A conductive cable acts as an antenna – if the device produces radio frequency energy, this can be transmitted through the cable, which acts as an unintentional radiator. In this case the bead is required for regulatory compliance, to reduce EMI. Conversely, if there are other sources of EMI, such as household appliances, the bead prevents the cable from acting as an antenna and receiving interference from these other devices. This is particularly common on data cables and on medical equipment.

Ferrite beads are used in inductors to form a passive low-pass filter. The geometry and electromagnetic properties of coiled wire over the ferrite bead result in a impedance 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 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. Different bead materials have different properties with respect to frequency and the manufacturer's literature will show the frequency ranges where dissipation is highest.

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 bead, the introduction of the ferrite core around the wire increases the self-inductance of the wire and 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. Small ferrite beads may be slipped over component leads to suppress parasitic oscillation.

See also

• Balun
• Electromagnetic interference
• Magnetic core
• Toroidal inductors and transformers
• Unintentional radiator

External links

• An explanation of the physics behind ferrite beads
• What are the bumps at the end of computer cables?
• Understanding Ferrite Bead Inductors, by Murata Manufacturing
• How to use ferrites for EMI suppression by Fair-Rite
• Tekzilla Daily Episode 13, short video podcast with explanation
• Ferrite bead inductor usage in electronic circuits