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KF5JRV > TECH 12.10.16 13:40l 67 Lines 4158 Bytes #999 (0) @ WW
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Subj: Ferrite Applications
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Sent: 161012/1133Z 3479@KF5JRV.#NWAR.AR.USA.NA BPQK6.0.13
Ferrite Applications
One of the first applications of ferrite vividly demonstrates the immediate
advantage gained through its use. In 1944 soft ferrite was developed by
J.L. Snoek and permitted a quantum leap in the efficiency of telephony
filter coils. Lower losses and higher permeabilities over a period of 30
years have reduced the size of the typical filter coil to 0.6% of the volume
of the earliest version! These coils were used in multiplexing telephone
circuitry. Here, the pot core was developed to provide a high Q, precise
inductor to allow 12 simultaneous transmissions on a single line. The
earliest versions of this type of core were complex structures consisting
of multiple pieces of ferrite rings, discs, and slugs. The now familiar
designs came into wide use in the late 1950’s.
As telephone technology progressed, the same design was used in early touch
tone telephones and the pot core evolved into a bobbin that employed terminals
to be inserted in printed circuit boards. This led to a tremendous growth
for ferrites. While the use of ferrite in this market has been displaced by
semiconductor tone generators, a new market emerged that continued to expand
the use of ferrites.
In the TV industry, ferrites helped reduce the size of yokes which were
monstrous and employed no core to concentrate the magnetic field that guided
the electron beam. The many developments which created the present circuitry
first increased greatly the amount of ferrite components in a TV set, in
width and linearity controls, separate color convergence yokes and control
coils, and then drastically reduced the use of these ferrites through the
inception of precision yokes and the employment of integrated semiconductor
circuitry.
Applications of ferrites in the role of antennas are now apt to be part of a
resonant circuit incorporating perhaps only a capacitor, but often also a
semiconductor chip to provide encoding in addition to discrimination by the
resonant frequency of the circuit. For example, one application involves a
small wound ferrite rod that is implanted in animals to help with tracking.
The use of ferrite cores in sensors has resulted in many unique applications.
Coin vending machine no longer depends upon only mechanical sizing and simple
magnetic devices to discriminate between coins of different values or slugs.
Using half a pot core, a coil is wound on a bobbin and placed in the pot core.
A signal is imposed on the coil, and a metallic object (in this case a coin)
passing through the magnetic field at the open end of the pot core will change
the Q and the inductance of the coil. This information is used to determine
the characteristics of the coin and thus its value.
Ferrites are also used for filtering power supplies. For differential mode, slug
or bobbins are typically used; for common mode, a toroidal or ungapped “Eö core
is utilized. A unique application for the latter is the common mode filter used
to isolate a vehicle’s rear window defroster from the 12 volt power
supply – allowing the defroster to be used as the antenna element instead of
the whip antenna or one that is retractable.
Ferrites are also used high frequency power supplies such as pulse transformers,
Switched-Mode Power Supplies (SMPS), switching regulators, and output chokes.
Transforming energy at higher frequencies allows for smaller form factors and
improved efficiency of the device. Many electronics today, from personal
computers to LCD displays, incorporate these types of power supplies into
their designs.
With the increase of digital electronics comes the need to suppress electromagnetic
interference (EMI) caused by fast switching circuits. Legislation from the FCC
has put limits on the amount of EMI a certain device can radiate and also how much
EMI a device can tolerate as outlined in FCC Part 15. With this comes the need to
mitigate this noise, and ferrite has been one solution. From chip beads to cable
cores, the application of ferrites into devices either on cables or at the board
level has allowed various electronic devices meet these standards.
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