Ferrite bead
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A ferrite bead at the end of a Mini USB cable A ferrite bead (also known as a ferrite block, ferrite core, ferrite ring, EMI filter, or ferrite choke) is a type of choke that suppresses high-frequency
electronic noise In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In particular, noise is inherent in physics, and central to the ...
in electronic circuits. Ferrite beads employ high-frequency current dissipation in a ferrite ceramic to build high-frequency noise suppression devices.


Use

A ferrite bead with its plastic shell removed Ferrite beads prevent electromagnetic interference (EMI) in two directions: ''from'' a device or ''to'' a device. A conductive cable acts as an antenna – if the device produces
radio-frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
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 EMI Group Limited (originally an initialism for Electric and Musical Industries, also referred to as EMI Records Ltd. or simply EMI) was a British Transnational corporation, transnational Conglomerate (company), conglomerate founded in March 1 ...
. 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. 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. On wires intended to be DC conductors, beads can block low level unintended radio frequency energy by acting as a low pass filter. On unbalanced
coax Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a ...
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
s (for instance, USB cables and video cables) the cable is designed to contain the signal, and beads can be used to block stray common mode current from using the cable as an antenna while not interfering with the signal carried inside the cable. In this use, the bead is a simple form of a
balun A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...
. 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: this type of ferrite core is usually designed so that the wire passes only once through it. If the fit is not snug enough, the core can be secured with
cable tie A cable tie (also known as a hose tie, zip tie, or tie wrap) is a type of fastener for holding items together, primarily electrical cables and wires. Because of their low cost, ease of use, and binding strength, cable ties are ubiquitous, findi ...
s or, if the center is large enough, the cabling can loop through one or more times. (Note, however, that although each loop increases the impedance to high frequencies, it also shifts the frequency of the highest impedance to a lower frequency.) Small ferrite beads can be slipped over component leads to suppress
parasitic oscillation Parasitic oscillation is an undesirable electronic oscillation (cyclic variation in output voltage or current) in an electronic or digital device. It is often caused by feedback in an amplifying device. The problem occurs notably in RF, audio, ...
. Surface-mount ferrite beads are available. These are soldered into a gap in the printed circuit board trace, just like any other surface-mount inductor. Inside the bead component, a coil of wire runs between layers of ferrite to form a multi-turn inductor around the high-permeability core.


Theory of operation

An RF inductor wound on a ferrite core (not a ferrite bead), and a PCB mount ferrite bead Ferrite beads are used as a
passive Passive may refer to: * Passive voice, a grammatical voice common in many languages, see also Pseudopassive * Passive language, a language from which an interpreter works * Passivity (behavior), the condition of submitting to the influence of o ...
low-pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
, by dissipating
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the ...
(RF) energy as heat, by design. Pure
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s on the other hand have no resistance and hence do not dissipate energy as heat. Pure 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 energy drawn) rather than dissipating that energy as heat (as done by the resistance in ferrite beads). Note that while an inductor's reactance may commonly be referred to simply as ''impedance'', impedance in general 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 EMI Group Limited (originally an initialism for Electric and Musical Industries, also referred to as EMI Records Ltd. or simply EMI) was a British Transnational corporation, transnational Conglomerate (company), conglomerate founded in March 1 ...
/ 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. 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, 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. 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.


See also

* Braid-breaker *
Balun A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...
* Electromagnetic interference * Magnetic core *
Toroidal inductors and transformers Toroidal inductors and transformers are inductors and transformers which use magnetic cores with a toroidal (ring or donut) shape. They are passive electronic components, consisting of a circular ring or donut shaped magnetic core of ferromag ...
* Unintentional radiator *
Decoupling (electronics) In electronics, decoupling is the prevention of undesired coupling between subsystems. A common example is connecting localized decoupling capacitors close to the power leads of integrated circuits to suppress coupling via the power supply conn ...


References


External links

{{Commons category, Ferrite beads
Ferrite bead inductor usage in electronic circuits
Electromagnetic radiation Wireless tuning and filtering Ferrites el:Συσκευή φερρίτη