What is this small cylidrical thing on most power adapters., choke , ferrite ... can any one please explain .?

Question

why are they on the secondary / dc side

Answer 1

it is a filter. it is made from a stack of torroidial cores.

Torroids are made from a ferrous material, ferrite.

They are inductors.

AC voltages are suppressed by inductors, the torroids on the end of the cable limit radiation from the device and limit interference to the device.

Answer 2

The noise comes from the device that you are powering with the adapter, and it IS an AC signal.  The circuits in the device produce various high-frequency signals, and some of those change the current  being drawn from the adapter, so there are AC signals superimposed on the DC.  The filter helps to keep these from being transmitted from the power lead, or from being transferred back to the AC power line.

Answer 3

It is called a Ferrite Bead.  Used mainly to reduce RF or EM interferrence and to obtain EMC Compliance. (eg FCC, CE mark or C Tick). The purpose is to suppress Common Mode Noise.

In laymans terms:

The cables connecting the devices to a computer or other modern equipment are a source of noise (Radio Frequency Interferrence). These cables act as nice, long antennae for the signals they carry. They broadcast the signals quite efficiently. The signals they broadcast can interfere with radios, TVs and telecommunications equipment. The cables can also receive signals and transmit them into the case, where they cause problems. A ferrite bead has the property of eliminating the broadcast signals. Essentially, it "chokes" the RFI transmission at that point on the cable -- this is why you find the beads at the ends of the cables. Instead of traveling down the cable and transmitting, the RFI signals turn into heat in the bead.

Here are some resources:

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

http://www.butlerwinding.com/inductors/c...

Answer 4

They are placed on many cables (sometimes at both ends) leading from adapters to peripheral devices to reduce RF interference from nearby voltage sources.