Ferrite Bead - Overview

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.

While large ferrite beads are commonly seen on external cabling, there also exist various sizes of smaller ferrite beads which 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 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 dissipated as low level heat. Only in extreme cases will the heat be noticeable.

A pure inductor does not dissipate energy; but produces reactance which 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 frequencies which are considered to be noise. First, the ferrite will concentrate the magnetic field, increase the inductance and therefore the reactance which will impede, or ‘filter out’ this noise. Second, if the ferrite is so designed, it can also add an additional loss in the form of resistance that occurs because of loss in the ferrite itself. The ferrite creates an inductor with a very low Q factor . This loss heats the ferrite, but normally it is an infinitesimal amount of heat. While the signal level is large enough to cause interference, or undesirable effects in sensitive circuits, the energy being blocked is typically quite small. Depending on the application, the resistive loss characteristic of the ferrite may or may not be desired. When a ferrite bead is to be used in a design to improve noise filtering, specific circuit characteristics and the frequency range to be blocked must be taken into account. Different ferrite materials have different properties with respect to frequency and manufacturer's literature is used to help select the most effective material for the frequency ranges involved.

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.