A ferrite bead is 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
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 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.
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 transmission lines (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.
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 ties 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.
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.
