[sdiy] Ferrite beads on module power inputs

[Harry Bissell]

Low ohm resistors can have too much voltage drop, that varies with module current use.
I do use that technique on single chips (preamps). Ferrite beads do stop RF from entering or
exiting the PCB. I also use larger 'beads' that are really small inductors, these can reduce a lot
of power supply hash (if you have any) or radiation from a module. I once had a VCF that impolitely
oscillated at an ultrasonic frequency under certain conditions, which got back through the power
supply lines to the reset comparator of  VCO causing all sorts of strange behavior. Better decouplind (including
beads) uhhh... decoupled the modules. Which VCO and VCF designs are left as an exercise for the reader :^)

H^) harry




----- Original Message -----
From: David <dingebre at 3dphysics.net>
To: synth-diy at dropmix.xs4all.nl
Sent: Tue, 22 Mar 2011 21:53:51 -0400 (EDT)
Subject: Re: [sdiy] Ferrite beads on module power inputs

Now for a more naïve, but not totally uninformed answer.

As I've been learning and re-learning electronics and studying the designs
of those who make their designs available (untold thanks to you), I've
noticed a couple of things.

1. It doesn't take a lot of board space to add the spot for a ferrite bead.
2. The space can also be used for a thermal fuse, like John Blacet uses.
3. They are probably unnecessary if you have any kind of a new design,
reasonable power supply.
4. They have to filter some high frequency rail noise, maybe kill a spike or
two.

So, following the likes of Synthtech and others, I put them on the PCBs I'm
laying out and I put them in because they might help and I haven't seen any
compelling reason for not using them.

David

David Ingebretsen

David M. Ingebretsen, M.S., M.E
Collision Forensics & Engineering, Inc.
2469 E. Ft. Union Blvd., Ste 114
Salt Lake City, UT 84121

dingebre at CFandE.com
dingebre at 3dphysics.net
801 733 5458



-- -----Original Message-----
-- From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
-- bounces at dropmix.xs4all.nl] On Behalf Of David G. Dixon
-- Sent: Tuesday, March 22, 2011 5:46 PM
-- To: 'Neil Johnson'; synth-diy at dropmix.xs4all.nl
-- Subject: Re: [sdiy] Ferrite beads on module power inputs
-- 
-- > 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.
-- 
-- Never use 'em, never miss 'em.
-- 
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-- 
Harry Bissell & Nora Abdullah 4eva