Spacing of two EMC suppression cores

[Circlotron]

So I have this old clunker laptop and the power supply box causes twitter noises on the AM broadcast band. I have several split ferrite cores that I am going to try out. Wurth 742-7113. Leftovers from work. Anyway, I'm am thinking would it make any difference if the cores are separated somewhat or butted right together?

[cdev]

if either one is big enough for you to loop the power cord through several times that works best.

If the noise source is the power brick, you may want to try putting one of them on the AC side if its acting as an antenna for the RFI.

If its a real problem you may want to buy some more.

[Circlotron]

I tried two on the AC side, both with the cord looped through twice and one on the DC side also with two passes. Didn't really help to any noticeable degree. Might just get a better brick.

[DaJMasta]

I don't think spacing matters so much, it's not like the capacitance of the cable in between is going to be that much to make much of a filter outside of just the effect from the number of ferrites and the number of turns you can get through them.


I think the difficulty is just that the AM band is pretty low frequency and since just using a ferrite with a couple turns of mains cable isn't a huge amount of extra inductance,  the real rolloff point is probably high above the normal AM broadcast band.


Is shielding the box or the cable a potential option?  May be simpler than just replacing the power supply, though replacing the supply or adding a nice choke on the board would probably be a better solution.  Actually, does the power supply have any sort of mains filtering built in?  Maybe you could get an IEC connector with built in filter (or something similar) and just splice it onto the mains side.  Should do a better job than just a ferrite on the cable.

[cdev]

(Magnetic) Materials used makes all the difference in RFI suppression for any given frequency.

[@rt]

If two cores are next to each other, butted together sideways, there’s a magnetic coupling so that a positive going curve produces a negative going curve on any adjacent cores, making it a rather inefficient, but perfectly valid transformer.
There are example Amiga computers where separate analogue RGB video signals are sent parallel through RFI suppressors, and they use a piece of heat shrink
on the centre core so that it can’t touch the outer two. They must have found it worthwhile to do for an analogue video signal.