[sdiy] Ferrite beads on module power inputs

[Tom Wiltshire]

I'd use them if I was designing a commercial circuit (which I mostly don't do) because I would likely be using microprocessors, and that *does* include 
the sort of MHz noise that you're talking about.

I suppose if your filter is going into a system which includes some uP modules, it might make sense too, but if it's entirely analog, I think I'd rather see a resistor and cap with a cutoff down around sub-audio to remove any audio noise from the power supply.


But mostly people just think "Well, it won't do any *harm*, will it?" and stick 'em in. I would. You can always use a link instead if you change your mind.

T.


On 22 Mar 2011, at 22:01, Neil Johnson wrote:

> Hi all,
> 
> A private discussion today on a module design raised the issue of the
> use of ferrite beads on module power inputs.  Some designers use them,
> while some don't, either opting for low-value resistors (10-22 ohms
> typically), or just a direct connection from the input to the main
> smoothing electrolytic (perhaps via polyfuse and reverse Schottky
> diode :-)
> 
> Looking at the datasheets for the sorts of ferrite beads used in
> modules it is clear that at audio frequencies they have zero effect -
> their impedance only becomes useful (10s to 100s of ohms) when you're
> in the MHz range and up. In effect you'd get just as much noise
> filtering from a short length of tinned copper wire.
> 
> So I'm curious to hear from those who do use them as to why, and any
> evidence or calculations which show that they do have some measurable
> and useful effect in synth module supply circuits.
> 
> Or is it simply that EMC (radiated and received) really is that much
> of a problem?
> 
> Cheers,
> Neil
> -- 
> http://www.njohnson.co.uk
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