[sdiy] Isolating a +-15v/+5v PSU?

[Ken Stone]

in line

>I'm working on PSU schematic for my 808 clone today. <snip>
>
>"Here is a simple idea that works really well to isolate noise
>generating sections of a circuit from sections that need clean power -
>it uses just 3 diodes and 1 extra capacitor. In the common leg of a 3
>terminal regulator you add a diode that raises the regulator's output
>voltage by the drop across the diode. You then fit 2 identical diodes
>to the output from the regulator to drop the voltage back down to
>where it would have been. Separate smoothing capacitors are then
>fitted to the 2 separated outputs."

Not really that practical. it will prevent positive spikes over 5 volts
going back to the other circuit by some extent, but a) power diodes are
slow, so it wouldn't be great, and more importantly b) it is very unlikely
such spikes would be generated.

The main noise issue is where one circuit suddenly draws more power than the
other, in which case a spike in the downward direction will be generated.
This is what decoupling capacitors are there for.


>Seems logical enough, but I still have a few questions.  First can the
>7805 really handle +15DC at Vi? 

It is the heat dissipation that is the real issue here. If you can get rid
of the heat, yes. Possible ways to reduce what the regulator has to drop are -
 1) a power resistor before it. Low ohms. I don't like doing this, as I've
seen it create a lot of noise when current demand gets too high, if too high
a value resistor is chosen. It can also cause a slow power rise, resulting
some weird lockups!
 2) string a few poser diodes in series before the input. Each will drop the
voltage by about 0.6 volts. As such, the diodes will be dissipating part of
the waste heat instead of the regulator.

>Will I get better isolation by giving the digial PSU it's own
>rectifier, or will I be fine using the same rectification for both?

That depends entirely on the load. For a few CMOS chips, the regulator may
as well come directly off the same rectifier as the +15 volts. Maybe add a
low ohm resistor (1 ohm?) between the rail and the +5 regulator, and give
the +5V rail another smoothing capacitor after this resistor.
_______________________________________________________________________
Ken Stone   sasami at hotkey.net.au or sasami at cgs.synth.net
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