[sdiy] Powerline LC filters etc.

[Harry Bissell]

I'm not sure I understood the original question. Are we talking about AC Power Line Filters,
or common mode filters used on DC power system rails ?

In either case, the usual use for these filters is to reduce the passage of EMI (RF) signals in or out
of the system. Usually this protection exists for frequencies from the 10MHz and higher range, its rare
to get down as low as 1MHz with any efficiency.

If I got the original point, the use of an inductor (as part of a common mode choke) in the ground has
several purposes. In the DC case, the return current to the supply cancels out the current from the supply, 
resulting in a nearly zero flux in the coil (so it will not saturate easily). 

This does have the downside that there is impedance in the ground line... OK if the circuit is an "island"
but maybe not so good if the circuit needs to be connected to other items in use (such as other modules, effects,
amplifiers etc).

Its a very complex issue... what is good for one unit may totally screw up the rest of a larger system.

Tom... what is the filter supposed to do for you ?  (then we can dive deeper :^)


----- Original Message -----
From: Jeff Brown <guitaricon at verizon.net>
To: 'Tom Bugs' <admin at bugbrand.co.uk>, 'Synth-DIY DIY' <Synth-diy at dropmix.xs4all.nl>
Sent: Tue, 28 Feb 2012 20:15:01 -0500 (EST)
Subject: Re: [sdiy] Powerline LC filters etc.

Try searching for "power line filter".  One that I found:

http://www.ecoca.ro/papers/09_ARtim991.pdf

shows a basic LC filter on the 3rd page.  You can also just buy these - even
ones built into a chassis mount female receptacle.
They're also called "EMI filters".  Most medical electronic equipment that
is mains powered has them.

-Jeff

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom Bugs
Sent: Tuesday, February 28, 2012 5:40 AM
To: Synth-DIY DIY
Subject: [sdiy] Powerline LC filters etc.

I would like no learn more about using LC filters for power-line noise 
suppression. On this front, therefore, a couple of questions::

- can anyone recommend a good source of info - a book or a particular 
site - google has got me a bit confused/overloaded
- I've been looking at the Murata BNX devices - 
http://www.mouser.com/catalog/specsheets/Murata%20BNX%20Series.pdf --if 
you look at the BNX002-01, you see a schematic diagram (similar is used 
on several other devices).
I am curious about i) having an inductor in the ground line and ii) how 
you may calculate suitable values for the caps & inductors (as I say, 
linkage to a book/site with info on these areas would be fantastic too).
Of course, these devices seem to be designed with some specific areas of 
importance (http://www.rapidonline.com/pdf/26-6010e.pdf mentions 
everything being aligned in the same direction) so perhaps it is best to 
use these parts 'off the shelf' rather than trying to DIY - but, still, 
I would love to understand better.

Cheers for any pointers,
Tom


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Harry Bissell & Nora Abdullah 4eva