30 Jun
2008
30 Jun
'08
6:47 p.m.
I wrote:
They aren't independent. They share a common
ground reference.
Roy wrote:
They're independent, though, in the sense that
loads on one will not
affect the regulation of the other.
Ever heard of ground bounce? Calling one side of the supply "ground"
doesn't exempt it from Maxwell's equations. The fact that the +5V rails
are separate doesn't completely prevent noise from coupling between
the two, because there are signals crossing between the domains.
The point I was making wasn't that it isn't a good idea to avoid noise
coupling between the two +5V supplies, but that the machine isn't going
to burst into flames if the external +5VDC is derived from the same
transformer winding. There are various ways of reducing noise
coupling between two DC power supplies without requiring separate
windings. In practice, I expect that simply having reasonable
filter caps on the 5V regulator would be sufficient.
Eric