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

[Synthusiast]

My experiences with the two positive regulators for the +15V and -15V rail
in my Tiracon 6V is, that the regulator for -15V tends to run quite hot.
(Originally the +15V and -15V regulators were placed about 1 cm distance of
the synth's voice boards. Not the best way to make your synth stay in
tune... So, someone in the past replaced all regulators on a metal plate
connected under the front side of the keyboard and wired them to the
regulator board.)

Further more, I experienced a problem with the fuseholder of the -15V rail:
it was corroded in such a way, that it no longer made any contact with the
fuse itself. The other rails only had some minor corrosion.

To me, these facts seem to be all related to the use of the "negative"
regulator, especially after reading Harry's story (current being pumped into
the "gnd" of the positive regulator). As I'm working on repairing the
Tiracon so that it can run perfectly for 10 years or more, I'm going to
change the "negative" regulator for a normal one soon.

The designers of the Tiracon 6V must have made the PSU protype on a Sunday
evening ;)

Thanks to all of you who responded to this subject.

Martijn
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Harry wrote:

> 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  ;^)
>

JH wrote:

> > 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.

Synthusiast 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.