I teach a linear electronics course for which I wrote and self-publish the
"textbook." Here is the opening paragraph from my chapter on DC power
supplies.
* * * *
Some people restrict "DC power supply" to mean a constant voltage supply.
To them, anything less than constant should not be called DC. But DC
literally means direct current, i.e. unidirectional flow, i.e. the voltage
does not change polarity. In this text we shall use the literal definition:
DC is unidirectional but not necessarily constant. We might choose to call
constant DC "good" and non-constant DC "less good" or even "poor." For one
application a fluctuating but unidirectional source may be completely
adequate whereas for another application it may be totally useless. The
only way to avoid confusion is to be specific about the DC fluctuations.
What is the source of the fluctuations? What is the magnitude of the
fluctuations? What is the time dependence of the fluctuations?
* * * *
I go on to define the following:
half-wave-rectified DC
full-wave-rectified DC
filtered DC with ripple (both half-wave and full-wave)
regulated DC, which may have any of the following
-- ripple (periodic fluctuations)
-- noise (random fluctuations)
-- drift
-- transients (intermittent noise)
All DC power sources will have some of these problems. For example, even
batteries have voltage drift as they discharge, or as the temperature
changes, and they certainly have voltage variations if the load changes.
Anyway, in electronics it does little good to quibble over what DC means.
The DC voltage levels of power supplies are changing all the time at some
level; often more than you would think. The important thing is to make it
clear what you have, or what you need.
* * * *
Further responses...
The capacitor discharge example Carl Mungan gives is interesting, but
essentially no different than a battery (except the time constant might be
different and the discharge curve might be a different function), but the
average person would certainly call a battery "DC."
I do not use the wording "quasi DC." This wording does not convey useful
information other than to say the DC is not perfectly steady... but then all
DC is quasi DC. If I feel it necessary to note the DC is not perfectly
steady, certainly it is worth the time to specify what type of unsteadiness
we have, using the kinds of wording I outlined above.
Yes, I use DC and AC to refer to voltage as well as current. "AC voltage"
is very common terminology. I would prefer to use the word "impedance"
instead of "AC resistance," but I believe AC resistance and DC resistance
are often used, especially DC resistance... which strikes me as redundant.
However, having just designed a hi-fi speaker system and having picked out
some drivers from a vendor, I was very happy to see the wording nominal AC
resistance and nominal DC resistance in the specifications. I sometimes see
"resistance" in speaker specifications and I don't know if the person meant
to say impedance or if they really meant resistance. Although not best, AC
resistance and DC resistance do communicate the proper intent, and that's
the purpose of language.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817