Author Topic: Ferrite Beads in Digital Circuits  (Read 871 times)

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Offline ZedRedLed

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Ferrite Beads in Digital Circuits
« on: March 29, 2018, 11:05:59 pm »
Hi,

I'm currently trying to repair a keyboard after a spillage, that is giving me pretty random crosstalk between keys.
I noticed what appeared to be a potentially damaged ferrite bead on the pcb. This leads me to the question:

What purpose do ferrite beads have in a digital circuit?

I read that their main use is things like radio applications or other things involving high frequencies to avoid EMI. I also read that they are used when powering an analog and digital circuit from the same power source. This might be the case here, as the keyboard also has audio jacks, but this doesn't fit its location on the pcb.

Would it be necessary to clean up the power source?
And could a broken ferrite bead in a keyboard pcb lead to this kind of strange behaviour?

In case it helps, I'll include a picture of the pcb with the ferrite bead marked in red, the circuitry from the audio jacks to the very left and the rest being hooked up to the LEDs and membranes

Thanks in advance!
 

Offline Chris-IP5

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Re: Ferrite Beads in Digital Circuits
« Reply #1 on: March 30, 2018, 04:28:30 am »
I think like you the beads are often used to prevent EMI.

So an electronic equipment designer might addferrite beads so that their product conforms to FCC standards.

If the keyboard is attached by a cable to the computer that cable could act like a transmitting antenna.

As for "crosstalk" theres no spill left on the PCB, key contacts or the legs of the ICs? And correct keyboard selected in Control Panel (or equivalent) ?


Additional - is it a laptop keyboard?
« Last Edit: March 30, 2018, 04:33:57 am by Chris-IP5 »
 

Offline ZedRedLed

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Re: Ferrite Beads in Digital Circuits
« Reply #2 on: March 30, 2018, 08:31:34 am »
It's not a laptop keyboard and I don't think there is any left. I cleaned everything with isopropyl alcohol and I even completely taped over buttons on the membrane that still ended up being pressed

The only other thing I can think of is that it might be related to the 8 diodes in the picture, all of which I had to replaced.
 

Offline Teun

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Re: Ferrite Beads in Digital Circuits
« Reply #3 on: March 30, 2018, 09:20:16 am »
The beads are likely used to clean up the DC-voltage. Since they filter out higher frequencies they dissipate the noise energy into heat.

http://www.analog.com/en/analog-dialogue/articles/ferrite-beads-demystified.html

You can try to "bake" the silicon membrane to get out the oily substances.

http://www.michaelshell.org/gadgetsandfixes/keypadsiliconeoil.html
 


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