A ferrite bead or ferrite choke is a passive electrical component that suppresses high-frequency noise in electronic circuits. This is a special kind of electronic choke. The ferrite beads use high frequency current dissipation in ferrite ceramics to build high frequency noise suppression devices. Ferrite beads can also be called blocks, nuclei, rings, EMI filters, or chokes.
Video Ferrite bead
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The ferrite beads prevent electromagnetic interference (EMI) in two directions: from a device or to the device. The conductive cable acts as an antenna - if the device generates radio frequency energy, it can be transmitted over a cable, acting as an accidental radiator. In this case beads are required for regulatory compliance, to reduce EMI. Conversely, if there are other EMI sources, such as household appliances, the bead prevents the cable from acting as an antenna and receives interference from this other device. This is very common in data cables and medical devices.
Large ferrite beads are commonly seen on external cables. A variety of smaller ferrite beads are used internally in the circuit - on the conductor or around the pins of small circuit board components, such as transistors, connectors and integrated circuits.
The ferrite beads are used as passive low pass filters, by converting RF energy into heat, by design. (Compare this to the inductor, which by design does not convert RF energy into heat, but rather offers high impedance to RF.)
The geometry and electromagnetic properties of the circular wire above the ferrite bead produce an impedance for the high-frequency signal, attenuating the high-frequency electronic EMI/RFI sound. Energy is reflected back to the cable, or dissipated as low heat. Only in the extreme case is the visible heat.
The pure inductor does not remove energy but produces reactance which inhibits the higher frequency signal flow. This reactance is usually referred to simply as impedance , although the impedance may be a combination of resistance and reactance.
The ferrite core or bead can be added to the inductor to improve, in two ways, its ability to block undesired high frequency noise. First, the ferrite concentrates the magnetic field, increasing the inductance and therefore the reactance, which blocks or 'filters' the sound. Secondly, if the ferrite is designed in such a way, it can generate additional losses in the form of resistance in the ferrite itself. Ferrite creates an inductor with a very low Q factor. This loss heats the ferrite, but usually it is the amount of heat that can be ignored. While signal levels are large enough to cause disturbance, or unwanted effects on sensitive circuits, the energy that is blocked is usually quite small. Depending on the application, the resistive loss characteristics of the ferrite may or may not be desirable.
Designs that use ferrite beads to improve noise filtering should take into consideration certain circuit characteristics and frequency ranges to block. Different ferrite materials have different properties with respect to frequency, and the manufacturer's literature helps select the most effective material for the frequency range.
The ferrite beads are one of the simplest and cheapest types of interference filters to install on pre-existing electronic cables. For simple ferrite rings, the wire just wraps around the core through the center, usually five or seven times. Clamp-on cores are also available, which install without wrapping the wire: this type of ferrite core is usually designed so that the wire passes only once. If the match is not good enough, the core can be secured with cable ties or, if the center is large enough, the cable can rotate through one or more times. Small ferrite beads can slip on top of the component that leads to suppress the parasitic oscillations.
The surface-mounted ferrite beads are available. These are soldered into a gap in printed circuit board traces, just like any other surface mount inductor. In the bead component, the wire coil runs between the ferrite layers to form a multi-turn inductor around the high permeability core.
Maps Ferrite bead
See also
- Balun
- Electromagnetic Interference
- Magnetic core
- Toroidal inductor and transformer
- The radiator is accidental
References
External links
- Use of ferrite bead inductor in electronic circuit
Source of the article : Wikipedia
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