[sdiy] switched mode PSU design

[John L Marshall]

Best to use four terminal capacitors.

Take care,
John

www.sound-photo.com
47-30-39.02 N, 122-09-51.75 W, elev 440'
----- Original Message ----- 
From: "fmg" <eqys at iposadas.com.ar>
To: "Tim Parkhurst" <tim.parkhurst at gmail.com>; <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, August 10, 2005 8:54 PM
Subject: Re: [sdiy] switched mode PSU design


> Tim, (and list)
>
> Old-fashion pi filters (C-L-C) can do miracles... don't know why they
> aren't used anymore. (cost? space?..) Try a dual pi scheme, ie:
>
> _____C______   __L1__   _____C_____    __L2__   _____C______
> 100pF + 1nF ->  50uH -> 1nF + 0.1uF -> 150mH -> 0.1uF + 470uF
>
> L1 can be done with some copper and one 1/2W resistor, L2 can be done
> from the secondary of a dead wall-wart trafo (choose one with an open
> primary, discard those whose primary is burnt (melt))
>
> I've seen some designs using Rs instead Ls for the pi. This is bad.
> The inductor on the pi keeps Rsource low while having a hi_z for the ac
> content (including transients), something that doesn't happens with
> resistors. A Pi(c-r-c) and a Pi(c-l-c) are two completely diferent
> things.
>
> Of course, a good emi shield is another completely diferent thing too.
>
> As a side note: I always include L-C filtering in power rails of
> every pcb I make. It takes a bit more time and space (just a bit)
> but it extend noise isolation not only between the card and the PS
> but among the cards themselves. (and it doesn't hurt)
>
> Fabio.
>
>
> Tim Parkhurst wrote:
>>
>> You know, there has GOT to be a way to properly filter and/or shield a
>> switcher to keep it from goofing up audio circuits. Old PCs (with a
>> power supply) can be had for next to nothing, and someone really
>> clever (and with more spare time than I have) could take an old PC
>> case and make a really nifty little synth out of it. Where else could
>> you get a nice rugged sheet metal case AND power supply for $10 or so?
>> Maybe even use the motherboard as a 'backplane' to distribute signals
>> and to provide a nice mechanical mounting for synth modules. It sounds
>> to me like the toughest part would be figuring out how to quiet down
>> that pesky switching supply. Of course, you could just yank the
>> switcher, stuff in a linear supply and still use the case, but that
>> adds significant cost. Another method might be to 'remote mount' the
>> switcher supply in an external box, but there still could be noise on
>> the supply lines (and again, this makes the whole mess more costly and
>> bulky).
>>
>> Any thoughts?
>>
>> Tim (messy and costly, but not bulky) Servo
>> --
>> "Imagination is more important than knowledge." - Albert Einstein