Ferrites are polycrystalline compounds of oxidized iron and other metals with strong magnetic properties used to suppress electromagnetic and radio frequency interference in electrical circuits. They come in two categories: soft and hard ferrites, with various trade names for different applications. Ferrites are easy and cheap to produce and are effective noise filters in both AC and DC. Ferrite cores are designed to be as thick and long as possible and are often enclosed in plastic or heat shrink sheaths to prevent breaking. They are used to shield 100 Base-T cabling in computer networks.
Ferrites are a class of compounds composed of oxidized iron and other metals in a brittle ceramic state. They are polycrystalline, meaning they are composed of large amounts of tiny crystals and exhibit strong magnetic properties. A common use of ferrites is in the suppression of electromagnetic interference (EMI) and radio frequency (RF) interference in electrical circuits, where they are often called magnetic insulators.
There are two general categories for the composition of ferrites. Soft ferrite compounds are a blend of iron and light metals such as nickel, aluminum or manganese where they are used in electrical transformers and other devices that require the ability to easily reverse the magnetic field. Hard ferrite compounds are composed of iron and harder metals, such as cobalt, barium and strontium. Barium ferrite compounds have uses as magnetic insulators and where permanent magnets are required in consumer applications, such as magnetic door catches.
The use of ferrite materials is widespread, as they are easy and cheap to produce. Their main attraction is that they exhibit large magnetic flux densities compared to the small magnetizing forces applied to them. Their frequent use leads to various trade names for different applications, with EMI suppressing ferrites often called magic beads, due to the lump-like appearance they can have when connected to electrical wiring.
At ambient and RF electrical signal suppression, ferrites are most effective at bandwidth levels above 100 megahertz, where they replace decoupling capacitors that begin to exhibit circuit resonance problems in filtering noise above 75 megahertz. They can also be designed to prevent low frequencies below 10 megahertz. This makes ferrites useful as noise filters in both alternating current (AC) and direct current (DC).
Ferrite cores are designed to be as thick and long as is practical for the cabling or device in which they are embedded. This requires that, when used on electrical wire, they are enclosed in a plastic or heat shrink sheath that prevents them from breaking under stress due to their brittle ceramic nature. The ferrites used for EMI suppression also tend to be of the hard type, making them more prone to cracking than their soft counterparts. A ferrite core is often employed to shield 100 Base-T cabling used in computer networks, which can be subjected to a significant amount of shock during installation and maintenance. Small amounts of damage to a ferrite screen, however, will not reduce its ability to filter noise.
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