What's this?

[Geoff45]

What are these cylindrical thingys that you see on many DC power cords?

[Waxing_Gibbous]

Surge protectors / phase buffers, I think. If you're like us and have jumpy two-phase power, I believe they sort of even out the flow of 'leccy and to some degree protect against spikes.
'Course I'm probably wrong, but that's what I've always THOUGHT they were!

[Scorpius51]

They are ferrite cylinders that are used to suppress high frequency interference. They behave as an in-line inductor on the conductor wires and present a high impedance to the high frequencies of much noise. They just slip over the cable and are coated with a protective plastic casing. Quite effective - that's why you see them so often these days. The particular ferrite material is frequency dependent and the compostion varies over the different ranges.

Cheers
John

[kustard]

They are a ferrite bead usually used in supressing RF noise. They're appearing more and more now on USB cables now that USB2.0 (and soon USB 3.0) have become mainstream for data transmission.

http://en.wikipedia.org/wiki/Ferrite_bead



*** Beat me to it John :) ***

[gary]

And just to add to John and Simon's responses ...

... they will also form part of the equipment manufacturer's overall strategy to keep
radiated emissions below levels that are required by law.

Different jurisdictions around the world have differing and often very complex (http://www.acma.gov.au/WEB/STANDARD/pc=PC_310707)
requirements with regards RF emissions and the compliance labeling on the product
itself will have logos for the various compliance regulatory bodies whose regulations
it claims to meet.

In Australia, it is illegal to import, buy, sell and use electronic products that
require approval that have not been tested and approved to be compliant with
the requirements of the Australian Communications & Media Authority (ACMA).
Products that claim to be compliant will come with the C-Tick logo (see below).
Telecommunications equipment will have what is called the A-tick mark which is
a similar logo, but instead of being a circle, the tick is enclosed in a triangle.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au