[sdiy] Switching power supply transformers

[Batz Goodfortune]

Y-ellow Karl and Roy.
         Yup. What Roy said below. Couldn't have put it better.
At 04:11 PM 1/13/05 -0500, Roy J. Tellason wrote:
>On Thursday 13 January 2005 11:28 am, Karl Ekdahl wrote:
>
> > This is one of the issues i was thinking of. I'm doing a analog 
> drummachine
> > with a digital sequencer and the only signal i need to connect from the
> > digital part to the analog is the triggers (8 of them). How do i realize
> > this without mixing the grounds? Optocouplers? Or is there any even 
> simpler
> > method?
>
>You can connect the two grounds together,  just do it at *one* point to avoid
>ground loops.  The main thing with that is not so much that you don't want
>them connected as that you don't want the sort of nonsense that digital
>signals can do (including "ground lift",  where transient currents actually
>raise "ground" by a bit) getting into your analog circuitry.  Just thinking a
>bit about where that current is flowing should make it clearer.

In High-qual amplifier design the "audiophiles" (and I'll use that term 
loosely) made much of what they called "Central point earthing." Which 
worked really well until they actually hooked their systems up and found 
their cassette deck caused a hum loop with the earth strap on their 
turntable. Etc Etc. But it's the same principal. The A-Ground and D-ground 
returns are tied together at one central point. Back near the power supply 
usually.


> > What i found is a wallwart transformer on a flee market. I bent it open 
> and
> > it's shielded and everything so i'll probably just glue it back 
> together and
> > use it as it is. If 20% of usage is the requirement, then out of 2A i have
> > to use 200mA, right? My application is allready consuming well over 
> 500ma so
> > that should be O.K. i guess..

Yeah. That should not be a problem. As Roy said, throw a pilot lamp in and 
you're in business. Although I have to point out that bulbs aren't entirely 
reliable and were you relying on one as your current dump and it were to 
blow, the consequences are going to be, (As my good friend Dr. Panda often 
puts it.) "outside of manufacturer's specifications."

If your logic draws half an amp then you're likely to be in the sweet spot. 
(Specifications may be subject to change without notice.)


[quote from Roy:]
>The 20% figure is a guess,  some of the real early stuff needed closer to 
>full
>load,  some supplies can go as low as 5% without any trouble.

I was lucky enough to come across a PC PSU some time back that already had 
a bleed resistor on all supply rails for this purpose. It was as high as 
2k2 or 4k7 as I recall. That's all it took. Of course the supply was 
tricked out for this. Gave a rock stable output loaded or unloaded but the 
trick was that all these resistors did beyond the obvious was allow the 
SMPS chip (and associated comparator circuits) to control the output 
without overshoot. Even if no external current was to be drawn.

In this case however, go with the flow. (as it were) And of course, as 
always, cross your fingers, stick them in your ears and nudge the power 
switch with your nose.

Be absolutely Icebox.

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