What is the purpose of the ferrite bead on DC Adaptors?
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.







simulate this circuit &ndash; Schematic created using CircuitLab




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


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