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DC Adaptor with Ferrite Bead near the plug

According to WikiPedia: Ferrite bead suppress high frequency noise.

In the case of wall power adaptors, are they trying to protect the device being powered or are they protecting the PSU from high frequency noise?

Why is it placed on the plug end rather than on the adaptor end?

What would be the consequence were the ferrite bead not installed?

Thank you!

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These are similar to all the ones found on VGA cables. The DC power also conducts broadband spectrum on unbalanced impedance wires from the switched current transients. The purpose of this Ferrite, Folding clamp, split clamshell, ungapped component is to act as a BALUN or to BALance UNbalanced lines at RF frequencies. The other functional name is a Common Mode (CM) Choke. Since the AC-DC converters carry lots of harmonics up to 10MHz this ferrite raises the impedance of both lines DC+/- such that they become balanced radiators and thus cancel each other out for far-field EMI emissions.

schematic

simulate this circuit – Schematic created using CircuitLab

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Two main bands exist:

MnZn ≤ 10MHz(AM band range) (most common for DC power cords)
NiZn ≤ 300MHz(FM band range) options available

•Split construction , precision mating surfaces enter image description here

ref

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It removes noise, specifically high frequencies.

See first paragraph from Ferrite Beads Demystified:

An effective method for filtering high frequency power supply noise and cleanly sharing similar voltage supply rails (that is, analog and digital rails for mixed-signal ICs) while preserving high frequency isolation between the shared rails is the use of ferrite beads.

Also see the related question Ferrite Bead: How much impedance do I need?

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Firstly, it’s not a ferrite bead; it is much bigger than what is generally regarded as a ferrite bead and, as such, it will have a noise reducing effect in the low hundreds of kHz to probably over 10 MHz. Generally, ferrite beads are only useful above 10 MHz.

I suspect that the ferrite core is placed on the wire to ensure that the wall wart complies with emitted noise regulations. Which regulation? There are many but the device markings might give a clue.

However, given that it is on the barrel plug end of the cable, one can be suspicious that it is conducted emission regulations that were the problem. If the non compliance were radiated emissions then it’s more likely that the ferrite core would be much closer to the body of the wall wart.

But who can really say? You can’t rule out that someone in the manufacturing side of the product was leant on by someone from sales (to move the core) after many devices were returned due to the core being broken by people smashing their vacuum cleaners into it when plugged into a low wall socket.

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As you stated, the ferrite is installed to suppress high frequency noise. As you have also pointed out, the location of it is close to the plug end that goes into the device it is powering. This may indicate that its purpose is to suppress noise generated by the end device from getting onto the cable. Cables can act as antennas, and any high frequency noise on the cable can radiate, potentially beyond limits imposed by governing agencies. If the end device requires EMC compliance testing, it may be certified with a specific power supply PN, and they are certified together. The ferrite this present to provide the best chance of passing emissions requirements. If it is removed, the emissions may change rendering the certification invalid.

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The permeability of the ferrite bead increases the inductance of the wire going through it. This increased inductance increases the impedance of the wire at higher frequencies.

The increased impedance reduces the amount of RF common mode power that can flow in either direction:

  • reducing harmonics from a power supply's DC-DC converter switching frequency from entering the device as RF noise,

  • and reducing any digital RF noise created by the device from using the power supply cord (and building's AC wiring) as an antenna that can radiate EMI into the environment (potentially exceeding EMC regulations, such as CFR 47 Part 15 in the U.S.)

Both of the above are especially important with any radio equipment in the room, where one want to reduce the RF noise from interfering with signals of interest. (radiated and conducted EMI, etc.)

To hear this RF noise, take an old analog AM radio, tune it between stations, and hold it next a cheap LED light or USB power supply. You often will hear a very nasty buzzing. You might want to reduce the amount of this power supply noise that gets conducted into a radio receiver or other circuits or equipment that might be sensitive to RF noise.

A poorly filtered power supply can increase a radio's RF noise floor and completely obliterate amateur radio weak DX signals. Often a single ferrite bead is not enough, so one might add multiple clamp-on ferrite beads to the cord(s), or wind multiple turns of the cord(s) through a ferrite toroid of a proper mix to increase the impedance at an RF frequency range of interest. Amateur radio operators are known to do this, not only for power supply cords, but for every network, USB, and video cable in the room.

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