Brads Electronic Projects Forum

Basics of Ferrite Beads for Filters, EMI Suppression, Parasitic oscillation suppression ...

This video discusses the basics of ferrite beads, and their uses for basic filtering applications. It discusses and demonstrates how ferrites act as resistors at high frequencies, and how this is used for EMI/RFI Suppression, improved power supply filtering, parasitic oscillation suppression, and others. The affect of the frequency dependent resistive characteristic of the ferrite beads is demonstrated by showing its effect on a swept RF signal, its effect on the rise/fall time of a digital signal, and its effectiveness of eliminating parasitic oscillation. Here are some links to the other videos mentioned in this video:
Harmonic content of square waves:

See the youtube page for the other links.

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One I would add is that putting a pair of wires through a bead (through the same hole) effectively balances them. It tries to force them to have equal and opposite currents, so you can reduce high-frequency radiation and pickup without reducing edge rates. In this situation, the magnetic field from one wire collapses on the other instead of building up in the ferrite, so there's no loss, even at high frequencies. (Reducing radiation without ruining edge rates is why there was a big bead on the mouse cord he showed in the video. It keeps the cord from acting as an antenna.) When there are two wires going through the same hole, one of them is commonly a ground wire; but you want the ground return current to flow right next to the signal wire, not elsewhere. I do this on my workbench computer especially for the A/D and D/A converters and their long-ish connections, to keep digital noise out of the analog. It is very effective and I have no problem with electrical noise even though the computer is wire-wrapped and lacks a ground plane.



You can see the dark-gray drums that are just over 1/4" long and 1/4" diameter.

The main place I use ferrite beads in our commercial products is at the input and output of switching power supplies, to take care of high-frequency electrical noise that gets through the other inductors because of the capacitance between windings.

Thanks for the info Chuck and Garth, great video and some very informative info on your wire-wrap boards Garth