John Midgley wrote:
>
> >When you want both supplies to live independant, you have to feed them
> >independant.
> >Try this: connect the middle of the transformer windings to ground, use 2
> >buffer elco's in series, the middle to ground. Your supply will work
> >perfect, up to 15V+-
>
> Sorry to be ignorant, but what exactly *is* a 'buffer elco'?
>
> >I believe your problem is the bridge recitifiers.  A bridge rectfier circuit
> >produces  floating outputs.  What you need to use is either a half or full
 wave
> >rectifier circuit with the ground attached directly to the transformer
 winding.
> >This will keep the ground from floating and keep your power supplies from
> >tracking each other.   If you can arrange your transformer secondaries into a
> >center tap configuration, then use this center tap as the ground.  Once this
> >grounded center tap is in place, use full wave rectifiers to produce your
> >positive and negative supplies.
>
> I think both these replies are saying similar things, but before I devastate
> North Norfolk with a huge explosion - can I treat a transformer with dual
> secondaries wired in series as a centre tapped transformer, and earth the
> 'middle' of the coil. If that makes any sense.
>
> Many thanks
>
> John Midgley

That's the trick.

Something like this:

      * | *               +----------+
  ----* | *----(1)--------+ ~        |
      * | *               |         +|--------->  +15V
 ~    * | *----(2)--> GND |Rectifier |
110 V * | *               |         -|--------->  -15V
  ----* | *----(3)--------+ ~        |
      * | *               +----------+




The center tap(2) is ground, Hook your favourite rectifier (coulnd't
draw one any better in ascii ;) ) up with the
two other wires on the secondary side.

To make a nice DC - voltage (to get rid of the big ripple)
connect two buffer-elco's in series between +15 and-15V (watch polarity)
The bigger the better.. If you take 35V types youll surely be safe.

Hans.