[sdiy] Powerline LC filters etc.

[Tom Bugs]

Cheers Harry,

Sorry - lack of clarity due to lack of clarity in my mind.
I was referring to DC power systems.

I'm looking at these areas for two ongoing projects -- first power for 
standalone processing boxes and then also for investigations into DC-DC 
converter based power for the modular.
The troubles I've been coming across may be down to power-line noise or 
(perhaps more likely) due to other fundamental flaws in my 
quasi-engineering approaches////

For the standalones - 9v DC operated boxes powered by wall-warts - now 
that many wall-warts are switchers.
- but I have a feeling that audio noise I was experiencing is actually 
down to my circuit approach rather than due to power-line born noise.
(Douglas Self's book 'Small Signal Audio Design' is useful for these 
areas - very good readability too)

For the modular - I've been trying DC-wall-wart to DC-DC converters to 
local LDO V-regs - the recent discussion on SMPSs gave me some pointers 
for lowering switching noise, but I still feel a bit out of my depth. My 
first build was something of a failure, but that may have been down to 
the choice of DC-DC converter - I have changed type and this seems to 
have improved things greatly. Again, problems may be down to my circuit 
approaches - I was finding that modules would pick-up other module 
signals when un-connected - something I'd never had with linear 
supplies. (at least now I'm learning to put local voltage references in 
for critical parts)

Here's a quick diagram of the approach I'm trying - do let me know if it 
is full of foolish holes...
http://bugbrand.co.uk/images/PowerPlan.jpg - first half is a module with 
DC-DC converters which then has output headers to go to distro boards 
which each have LDO Linear VRegs
- external wall-wart supplying 24V DC
- ? input DC filter ? Murata BNX016-01
- two 20W DC-DC converters (Cincon) with trims to take the output 
voltage up to c.15.5V
- output DC filters - note that the negative side one is connected the 
other way around as detailed in the Murata datasheet
- system ground banana connection comes post DC filters
- headers to take the +/- 15.5V to the distros

- distros then have a pair of LDO Linear V-Regs (a positive and a 
negative type) to regulate down to +/-15V

Does this sound like a sound approach? I'm keen to try to make DC-DC 
converters work, but have had my doubts so far.. They would bring many 
benefits (compactness, worldwide voltages, no mains worries, etc) but 
obviously I don't want to be opening a can of worms.

Cheers!
Tom

On 29/02/2012 14:33, Harry Bissell wrote:
> 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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