Winfield said:
Chris Carlen wrote...
The problem with this is the poor folks begin to think what spice
tells them must be right. I mean, they get answers to six-digits,
how can that be wrong? So we see a statement like, "I tested the
circuit," when really should have said, "I ran a spice simulation."
Meanwhile they're using the wrong model, or poorly-designed models,
or mis-using the model, and have failed to add any necessary real-
world parasitic elements, like an actual bench circuit will have.
Furthermore, they run the model with a fixed supply voltage with
zero-ohms impedance, and at the default temperature. Of course
they fail to test for any effects due to component tolerances.
Although the results may have little relation to reality, they go
forth with great confidence they've designed and tested a circuit.
In the case of basic circuit analysis 101, they use inductors with
no internal resistance or self capacitance, and have no idea what
magnetic saturation, eddy currents, or self-heating, or proximity-
effect losses are. In the end they learn just what the book taught,
once again, bearing little relation to the real world. <sigh>
Thanks,
- Win
Hi Win:
I would expect that astute students, especially those who have tinkered
with electronics before engaging in formal study, and even those who
have not but are serious about learning the subject at not just a
theoretical level but a practical one as well, would raise questions
during their study that point to these very issues. For instance, my
text on basic electric circuits goes into the effects of loading on
filters after presenting the basic filters. It doesn't take much to
realize once someone mentions the existance of internal resistance in
inductors and caps, and self-capacitance of inductors, non-linearities
of non-air cored coils, etc., that the real world situation is far more
complex than the basics. But the basics provide the necessary tools to
then begin to tackle the complexity. Any student having completed a
basic circuits class, and having been told verbally that an L is really
a (R+sL)||(1/sC) and a C is really an (R+1/sC+sL), and that if needed
even more levels of parasitics can be factored in, should be able to sit
down and write out what the circuit really looks like in the final
analysis. Faced with that pile of complexity, they can then begin to
grasp that there are times when this or that parasitic is relevant, and
other times it isn't.
Since most students will have labs with teachers and lab guides that
will be able to bring up the subjects of parasitics, etc., the students
should discover the important clues regarding the limitations of
idealized analysis during formal education. Of course, then that just
means a more detailed analysis. The road is a bit more rocky when one
forges a path alone. But the same effect can be realized. By filtering
through discussions on this group to which I never chimed in, I've
amassed a vast collection of tidbits, that slowly congeal into real
knowledge as I encounter design problems in which the theory must be
complemented by these bits of reality.
In my prior post, I tried to suggest a combination of experiences with
theory, the bench, and simulation as being necessary to get to the point
of being able to synthesize the various factoids into real productivity.
I also tried to make it clear that grounding (heh heh) in the basics of
pencil/paper analysis and bench experiments is the defense mechanism
against sheepishly believing a computer. The simulator and the bench
should complement one another for beginners. Heck, I am studying
Verilog right now, and I frequently burn the exercises into a real PLD
with real switches and LEDs, just to keep getting that feeling of the
real McCoy.
Of course, maybe I am being optimistic about students. I only know the
way I was in school, and now learning on my own. It isn't about doing
the assignment, it is about exploring all the questions that arise from
each assignment or factoid that leads to the development of maturity of
technical insight.
And once I got to that point with chemistry, I promptly got tired and
moved to electronics. ;-)
Win, I would be interested to here what you perceive to be the
statistics on how many students really go beyond the call of duty, and
how many just get by?
Good day!
--
____________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
[email protected]