[SI-LIST] Current Flow
- From: Doug Brooks <doug@xxxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Fri, 05 Aug 2005 14:12:24 -0700
(I have changed the subject line to better represent what I think we are
talking about.)
You raise an excellent example. Let me deal with the two points.
1. I'm not sure I understand what you are getting at here. The focus should
be at the point of the switch.
2. I have introduced the problem in some of my transmission line classes
that deal with point 2. Assume that (a) there is a propagation time for a
signal, (b) current (i.e. electrons) flows in a closed loop, (c) current is
constant everywhere in that loop ---- aren't these mutually exclusive
conditions? The answer is no! The current flows down the transmission line
from one side to the other through the distributed capacitance (as
suggested in Bogatin's Figure 7-19). This is a current flow (i.e. electron
flow) picture. If you want to call it a wave flow, well that's fine. But
you can also describe it as current (electrons) flowing to the point of the
distributed capacitance, repelling charge away from the other side of the
capacitance back to the beginning of the line, charging the capacitance up
(with electrons) along the way. At the steady state, current (electrons) is
flowing in the DC loop we would expect. If we don't have a transmission
line ---- well, we always have a transmission line of sorts. The question
is whether it's ideal or REAL crummy. There is always a characteristic
impedance, even if it is only that of air.
So the "fluid" analogy (I don't think that's the best descriptor) can deal
with this issue perfectly fine. Likewise, it can deal with the crosstalk
coupling issue equally as well. (I don't have a figure like Eric's in my
book, but there is a very detailed illustration of how crosstalk coupling
works in my book that doesn't need Maxwell and wave theory to understand.)
So I don't see the difficulty here.
Doug
At 01:18 PM 8/5/2005, steve weir wrote:
>Doug, in the fluid model, there are two misleading elements:
>
>1. The focus is on the source of EMF, sic the battery,
>2. It implies a time lag between the foward current starting from some
>point and the matching return current closing that path.
>
>If we take the switch example you offered, one might imagine a couple of
>different cases:
>
>a. The switch is located very close to one terminal of the battery and a
>say 300m wire connects it to the other through some load resistor.
>b. The switch is located at the end of two 300m wires back to the battery
>through some load resistor.
>
>Now, what will each the fluid analogy, and wave propagation tell us about
>each case? Where does each model show the propagation beginning and
>ending? How accurate is each? Which model can explain behavior from
>virtual DC to any frequency we like? I don't think it's the fluid analogy.
>
>On a PCB with switching I/Os the time and distance scales have changed but
>not the behavior. What we have is in essence case b from above. The wave
>emanates from the switches in our ICs, not from the power supply. The
>wave model makes this clear, as it does the propagation path. The wave
>model makes clear the critical point that the return and forward currents
>propagate together. The fluid analogy with its unidirectional emphasis
>fails us badly.
>
>Where has the fluid analogy brought us? How many times have you seen
>people talk about bone-headed ideas like the PCB planes or bypass
>capacitors supplying current to high speed edges, when the entire edge has
>completed long before the wave front through power pins can reach
>significant charge in the planes, much less even reach the PWB bypass
>caps? Yet this kind of junk mythology sadly makes its way into books and
>other publications on a regular basis. I don't like it one bit.
>
>For my money, I find the fluid analogy terribly misleading, and a source
>of much misunderstanding. One doesn't need to be able to derive Maxwell
>to understand wave propagation. I think that as Eric's book demonstrates,
>most SI concepts are not that difficult to understand. Even a dummy like
>me gets them from time to time.
>
>Regards,
>
>
>Steve.
>At 12:51 PM 8/5/2005 -0700, Doug Brooks wrote:
>>A couple of people have interpreted my statement re "flow" of electrons
>>as meaning electron drift. Let's kill that right now.
>>
>>One electron in = one electron out is the flow of electrons. One electron
>>in = SAME electron out is electron drift --- not at all the same thing.
>>
>>Certainly I don't argue against Maxwell's equations. But I don't argue
>>against the fundamental definition of one amp of current either --- the
>>flow of one coulomb of charge (6.25 x 10^18 electrons) across a surface
>>in one second. I spend a lot of time with engineers (and technicians) who
>>never took Maxwell's equations and didn't understand them if they did. My
>>goal has been to take our difficult SI concepts and explain them in terms
>>that these "poor" people can understand. To suggest that you can't
>>explain what happens during planar transitions without Maxwell's
>>equations (I believe) is simply wrong. To say that the classical
>>description of current can't explain the difference between DC and high
>>frequency is also (I believe) flat wrong. To say that one description is
>>"more accurate" than the other --- well I suggest that depends a lot on
>>whose working with them! And while people have been misled by seminar
>>leaders teaching without the benefit of Maxwell's equations, we all know
>>seminar leaders whose ability to mislead wasn't one bit hampered by a
>>thorough knowledge of Maxwell's equations!
>>
>>Don't sell these more basic principles short when it comes to
>>understanding what is happening on circuit boards. They can very
>>effectively explain what is happening, and why one design approach may be
>>more effective than another depending on the important design
>>considerations. Especially for all those board designers who have no
>>knowledge of Maxwell and wave theory.
>>
>>Doug
>>
>>
>>
>>
>>At 12:01 PM 8/5/2005, steve weir wrote:
>>>Doug, well I am going to argue vehemently that until someone repeals
>>>Maxwell that the wave description is fundamentally more accurate than the
>>>fluid analogy. The E/M fields cause the electron drift in those
>>>wires. From the time you closed the switch the changing E/M field that
>>>resulted propagated outward. Marconi found a useful purpose for that
>>>phenomenon.
>>>
>>>The fluid analogy is certainly easy to understand, but what is the point
>>>when it is so misleading? I can't tell you how many times otherwise
>>>intelligent engineers that I have known have been thrown off understanding
>>>PCB wave guides, because they were intent on following the DC current loop
>>>of the fluid analogy.
>>>
>>>Teaching the fluid analogy requires that we later break that teaching when
>>>we want to explain what happens at significant frequencies. Consider for
>>>instance visualization of return current ( which is the original subject
>>>matter ) when we transition planes in a PCB. If we think about it as a
>>>fluid model we are easily misled into searching out a conduction path. For
>>>ready examples of this mass confusion, just look at some of the discussions
>>>on splitting-up grounds in the wrong ways for the wrong reasons, with the
>>>wrong results. But if we simply consider waves to begin with, then the
>>>behavior is easy enough to intuit out.
>>>
>>>Eric does a very nice job in his book explaining signal propagation that
>>>does not rely on the fluid analogy. I think his approach is very
>>>accessible.
>>>
>>>Regards,
>>>
>>>
>>>Steve
>>>At 11:21 AM 8/5/2005 -0700, Doug Brooks wrote:
>>> >With all due respect, Steve, if I have a battery connected to a transistor
>>> >through a switch, I can turn the transistor "on" and "off" with the
>>> >switch. That is easy to explain using the electron flow concept (which I
>>> >hesitate to call an analogy, it in fact describes the physics involved).
>>> >
>>> >Is your description more complete AND also easier to understand?
>>> >
>>> >And if it is the frequency with which I "flip" the switch that bothers
>>> >you, that simply means that some of the parameters that were not an issue
>>> >with slow "flipping" (inductance and capacitance, for example) start
>>> >becoming more of an issue with faster "flipping!" But the basic nature of
>>> >what is happening (in particular where the electrons are flowing) is not
>>> >changing, just speeding up. (How the electrons are flowing is speeding up,
>>> >the electrons themselves, of course, don't change speed!)
>>> >
>>> >Doug
>>> >
>>> >
>>> >
>>> >At 10:45 AM 8/5/2005, steve weir wrote:
>>> >>Doug, I have some real heartburn with some of those representations,
>>> >>particularly the fluid analogy that speaks of current as the flow of
>>> >>electrons. When I grew up current was defined as time variation of
>>> >>electric flux. When an E/M field impinges a chunk of metal the
>>> resulting
>>> >>interaction concentrates the field forming a wave guide. All practical
>>> >>wave guides leak, be they a microstrip over a plane, a stripline, or
>>> >>whatever. Some, like a good semirigid coax leak only a little tiny
>>> >>bit. When they leak too much creating excessive disturbance in
>>> nearby wave
>>> >>guides, we have cross talk problems. I hope that this is what you were
>>> >>trying to convey.
>>> >>
>>> >>Regards,
>>> >>
>>> >>
>>> >>Steve.
>>> >>
>>> >>A
>>> >
>>> >Check out UltraCAD's new presentation videos and new skin effect
>>> >calculator at http://www.ultracad.com
>>> >
>>>
>>>
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>>
>>____________________________________________________________________________-
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>
>
>
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