On 6/14/05 2:28 AM, in article , "Floyd L.
Davidson" wrote:

Don Bowey wrote:
On 6/13/05 10:01 PM, in article ,
"The Phantom" wrote:

DC, of course, cannot exist at all ever. Because it would have to be
unvarying through infinite time.

Horse pucky. I think I can afford to miss the tutorials on your website.

Pure DC, or something close to it, is actually pretty rare
stuff.

Even on battery power plants, which are extremely good filters,
there is some AC on the leads of just about anything powered
from the battery unless either the battery or the load is all
but embedded in the other.

For example, the 48 volt battery plants that telephone companies
have, use some rather large cables to supply voltage to
equipment. Yet a filter is required at every fuse bay to
decouple the AC noise on the supply cable from the equipment in
the bay. Even then, the supply lines have an astounding amount
of AC noise on them.

That was particularly true back in the days of mechanical
switches, when a telco switch was filled with "DC" switched
lines that had mechanical contacts, and most of the loads being
switched were inductive.

There is even more of the same going on in modern digital
switching systems, minus the inductive kick, but those are
filtered much more effectively because unlike the old mechanical
monster, these new ones will malfunction themselves if the noise
isn't filtered out.

I agree with your examples of DC power supplies and AC noise. Been there,
done that.

Defining how may angels can dance on a DC power cable without having to
redefine it is pointless, however. Everyone I knew in the telco industry
had good, workable terms for the cause of the need for filters, not only at
the FB, but at the equipment rack too; it was noise, trash, crap.
spikes....., but the 48V and 130V "power" were always DC and we knew the
noise had to be dealt with as AC riding the DC. No other esoteric, mindless
definitions are needed even though the terms AC and DC may be misnomers.
They are historic and work very well.

Don