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Ferrite/Choke question

I have a weird question. I recently bought a second Linksys BEFSX41 VPN router to test out some new firmware before deploying it elsewhere. I noticed that when it was on, there was a very audible buzzy/hissing- like noise coming from it. I ended up opening it up and tracing it to two chokes on the 12VDC input line. They look like Fig D in the following link:

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By moving them around, I could sort-of make the noise worse or better, but couldn't eliminate it. As a temporary workaround, I simply shorted them out with a solder bridge on the solder-side of the board.

On the other router, you can barely hear the hissing only when your ear is pretty much presed up against it.

My question is: What is causing the hissing, and why is it doing it on DC? Any fixes to stop the hissing without losing the EMI suppression?

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Reply to
Andrew Rossmann
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these days manufacturers do not include parts that are not needed.

glue the ferrite in place. a little glob of RTV silicon caulk should work.

Reply to
TimPerry

My guess would be that these chokes filter ripple current from a high frequency switch mode PSU, the best way to stop them ringing is with fast setting araldite epoxy - allow it to run into the holes through the ferrite where the wires run through.

Reply to
I.F.

The power supply is your basic plug-in brick. I tried both the original and a RadioShack that is heavy enough to NOT be any form of switching supply. Both are standard 120V in, 12VDC/1A out.

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Reply to
Andrew Rossmann

I have several old modems laying about that use 230V to 12V wall-wart adaptors and SMPSU circuits on the modem board to provide the variety of voltage rails required by the modem - if an old modem has this level of technology, don't you think it could be found on a modern router?

Reply to
I.F.

Andrew Rossmann hath wroth:

The wall wart just supplies unregulated DC power to BEFSX41. The switching supply inside the BEFSX41 takes the 12VDC down to 3.3VDC to run the router. I couldn't find the FCC ID to display the inside photos from the FCC web pile. The various WRT54G wireless routers are similar inside. Note the photos at:

formatting link
In the upper left corner of the board, you'll see the same ferrite beads. The switching regulator is nearby. The ferrite beads keep the RFI from leaking out of the box via the 12VDC power cable.

Incidentally, I use hot melt glue instead of epoxy for vibration and noise reduction. It makes less of a mess. If desperate, I use RTV (silicon rubber goo).

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Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
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Jeff Liebermann

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