[sdiy] "backwards" connected positive regulator (was: Problem with723 PSU)

[harry]

I'd not completely agree with this JH... ;^)

My experience is that the "two positive regulator" approach is inferior
in most respects.  It is/was cheaper at one time when negative regulators
were less available.

Disadvantage:
 Two Transformer windings (as you mentioned)

Ground is not truly "ground" it is actually the positive supply of the
"negative"
regulator. This is likely to be more noisy than the real "zero" voltage
point of
the pos/neg regulator pair.

Current dumped into ground will likely bounce the lower (negative)
regulator.
Transient response may suffer.  They really are not quite like batteries
in series...

Tracking more difficult (but so what... if you were using 78xx 79xx you
don't care
anyway).

Advantage: Cheap, good if its Sunday night, the stores are closed... and
you have
two 78xx regulators handy.

H^)  harry ('s $.02....   exchange rate unknown  ;^)

jhaible at t-online.de wrote:

> >
> > I'm still working on my Tiracon 6V synth and I've noticed that it
> uses the
> > method of a "backwards" connected positive voltage regulator (LM317)
> for
> > the -15V rail. I was and still am a little puzzled about how this
> method
> > exactly works.
>
> Nothing special here - it's just like connecting two batteries
> in series - the + output of one will become the "middle" (GND)
> connection of the stack.
> Just make sure that everything else is floating - you need two
> separate secondary windings on the transformer, NOT just a
> center tapped winding.
>
> This approach has one big advantage over +/- regulators:
> You have full control of defining your GND location on the
> PCB. (With center tapped winding and 78xx / 79xx you have
> one common trace from the big capacitors to regulators, and
> you'll have some hum on the GND if you don't choose the right
> point of this trace for GND distribution.
>
> Disadvantage: Tracking regulators might be more difficult.
>
> JH.