Ferrite bead

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For other uses, see Bead (disambiguation).
A ferrite bead at the end of a Mini 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 that suppresses 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.[1]

Overview[edit]

Ferrite beads prevent interference in two directions: from a device or to a device.[1] 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.[1]

Large ferrite beads are commonly seen on external cabling. Various smaller ferrite beads are used internally in circuits—on conductors or around the pins of small circuit-board components, such as transistors, connectors and integrated circuits.

Ferrite beads are used as inductors to form a passive low-pass filter. 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.

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

A ferrite core or 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, increases 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.[2] This loss heats the ferrite, but normally it is a negligible amount of heat. While the signal level is large enough to cause interference, or undesirable effects in sensitive circuits, the energy blocked is typically quite 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 take into account specific circuit characteristics and the frequency range to block. Different ferrite materials have different properties with respect to frequency, and manufacturer's literature helps select the most effective material for the frequency range.[2]

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 five or seven times. Clamp-on cores are also available, which attach without wrapping the wire. Though 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 wire's self-inductance, and thus still absorbs energy from noise 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, the cabling can loop through one or more times. Small ferrite beads can be slipped over component leads to suppress parasitic oscillation.[2]

See also[edit]

References[edit]

  1. ^ a b c Vanhoenacker, Mark (November 1, 2012). "What Is That Little Cylinder on My Computer Wire?". Brow Beat. Slate. Retrieved 2012-11-03. 
  2. ^ a b c Joseph J. Carr RF Components and Circuits, Newnes, 2002 ISBN 978-0-7506-4844-8 pages 264-266

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