Tom Donaly wrote:
Cecil Moore wrote:
Roy Lewallen wrote:
I don't recall seeing anyone actually use a Thevenin or Norton
equivalent circuit in illustrating a point on this newsgroup. I don't
believe I ever have.

Some of your example sources walk and quack like
a Thevenin equivalent circuit. :-)

While some of yours just walk and quack.

I need to address this, because Cecil has frequently declared all
combinations of an ideal voltage source and resistor as a "Thevenin
equivalent" as I see he's implied here once again. He has claimed this
relieves him of the nagging problem of accounting for such things as
power dissipation in a source resistance. I'll explain how this
characterization and claim are false.

I often use an ideal voltage source in series with a resistance for
illustration of transmission line phenomena. This very simple circuit
allows complete analysis without the unnecessary clutter of more
complicated circuits. You'll find this technique very widely used in
elementary electrical circuits texts for the same reason, and well
before the introduction of Thevenin or Norton equivalent circuits.

But these aren't Thevenin equivalent circuits. Let's review what a
Thevenin equivalent is. I'll quote here from Pearson and Maler,
_Introductory Circuit Analysis_, but you can find an equivalent
definition in any elementary circuit analysis text.

"A theorem named after Leon Thevenin is often useful in reducing a
complex circuit to a simpler one. This theorem, which is proved in
Appendix E, may be stated as follows.

"Any one terminal pair (one port) network which is linear and which may
have any number of independent and dependent transform sources (as long
as the dependent sources are not functions of quantities outside the
network) may be replaced by a transform voltage source in series with a
transform impedance. The transform voltage source is the voltage across
the terminal pair when these are open circuited and the transform
impedance is the ratio of this transform voltage to the transform
current which flows between these terminals when short circuited."

Pay particular attention to the first sentence of the quote. A Thevenin
equivalent circuit is a reduction of a circuit to a simpler one. If you
have a complex circuit containing multiple components and reduce it to a
Thevenin equivalent, the theory says that the equivalent circuit looks
just the same to the outside world as the original. Because the Thevenin
equivalent could represent any number of very different original
circuits, you can't determine anything at all about the internal
workings of the original, such as power dissipation, by looking at the
Thevenin equivalent. That's a completely valid statement which has
frequently been misapplied.

A circuit consisting of a perfect voltage source and an impedance isn't
a Thevenin equivalent circuit unless it's used to replace a more complex
circuit. If it's used simply to represent those two circuit elements and
none others, then all the conclusions we draw from the circuit,
including dissipation in the source and impedance, are and must be
valid. For that matter, the circuit analysis for a Thevenin equivalent
must obey all rules and laws, including source and impedance voltage,
current, and power. We only have to realize that any quantities within
an equivalent circuit aren't necessarily the same as those of the
circuit being replaced by the equivalent.

Declaring all circuits consisting of an ideal voltage or current source
and impedance to be a Thevenin or Norton equivalent is wrong. Declaring
that Thevenin or Norton equivalent circuits don't have to obey
fundamental rules of circuit analysis is also wrong. Continuing to do so
after more than ample evidence has been presented to the contrary is
dishonest.

Roy Lewallen, W7EL