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

[Terry Michaels]

Hi Juergen:

I don't see anything wrong with two positive supplies either.  Many
suppliers of off-the-shelf linear multiple output pwer supplies (in the US,
Lambda, PowerOne and others) often build two exactly identical power
supplies that share a transformer core, a chassis, and nothing else.  You
can strap the output terminals any way you want to give you various
voltages.  I have used these types of power supplies many times with no
problems.  Here you have the most flexibility regarding bringing your load
return paths to a common point (star ground).   I was able to use a surplus
triple output (+12V, -12V and +5V) power supply to power Moog 900 series
modules, which require +12V and -6V, by re-strapping the +5V output to
produce a negative output, and readjusting it for 6 volts.   Beware though,
some power supplies do share some circuitry, and cannot be strapped
together like this.

OTOH, a dual output tracking supply can have some advantages.  This is
where you have one regulated supply, typically the positive one, and the
negative supply output voltage is made to be the same as the positive one,
but with opposite polarity, This is usually sensed by an opamp in the
negative regulator connected to a resistive divider between the outputs.  
One advantage here is both polarities come up to full voltage together
during power up.  This will eliminate potential latch up problems that can
occur with some devices tied to both supplies and ground.  Also, sometimes
with separated supplies, if you have a load from plus to minus, it is
possible for one supply to power up and pull the other one into a reversed
output situation, from which it won't start up.  

A tracking supply is also useful for those occasions where you use the
voltage sense input feature to eliminate voltage drops on your output
lines.  The better quality tracking regulators usually have voltage sense
inputs.   Two separate but identical supplies typically won't have voltage
sense inputs for both output voltages.

Terry Michaels