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Old 12th December 2011, 05:34 PM   #511
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Quote:
Originally Posted by Merlinb View Post
This was my feeling all along...
It is contest for Proper Application of Theorem.
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Old 12th December 2011, 05:49 PM   #512
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janneman, please take a look at the Thevenin theorem if you are not already familiar with it. A good presentation can be found at Thévenin's theorem - Wikipedia, the free encyclopedia In it, will see first that there is nothing special about ground - it's just another node. Second, you will see that the theorem is about measuring the impedance between a pair of nodes, not a trio of them (ground being one of the three.)

Any attempt to measure impedances by shorting three nodes together lacks theoretical underpinning and violates the Thevenin theorem.

Also, Thevenin will not allow you to place shorts between two pairs of nodes in a circuit. This is because the second short affects the impedance seen at the first, and the first short affects that seen at the second. You wouldn't think of applying a second short somewhere inside a circuit with just one output when you measure its output impedance - that would mess up your result drastically. So why would the mere fact that the circuit has a second output that just happens to be the negative of the first give you dispensation to short the second to ground when you measure the ground-referenced impedance of the first?

I know that there is an intuitive appeal to a number of people (SY chief among them) to subject the circuit to what he calls boundary conditions during testing. But a clear reading of the Thevenin theorem simply does not allow it - UNLESS the "boundary conditions" means that you can connect a single load from the P to the K when you are measuring the Zp-k impedance. That would be a valid two-node Thevenin impedance measurement.

The current source circuit should get you thinking. You'll note I can't get a response to it out of SY.
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Old 13th December 2011, 12:06 AM   #513
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But why can't he just test two Thevenin sources at the same time?
jan
I'd like to expand on and amplify Chris' reply to Jan.

If the Thevenin circuits are independent, i.e., represent two separate circuits, then there's no problem with testing both at the same time. They're independent; one doesn't affect the other at all.

But these Thevenin equivalent circuits in question are not independent, i.e., they are coupled.

Let's say that you wanted to find the Thevenin equivalent of some circuit and started by measuring the open-circuit voltage. You are ready to measure the short-circuit current but, before you do, you change the value of one of the resistors in the circuit and then measure the short-circuit current.

I imagine that you would instinctively know that you measured the open-circuit voltage of one circuit and the short-circuit current of another, different circuit and that you couldn't combine the two measurements in any meaningful way.

But look, if both loads are changed at the same time as SY does, the above is precisely what is happening; the change in the cathode load changes the circuit between the anode and ground nodes and vice versa. SY is changing the circuit between measurements. This makes it impossible to determine the actual Thevenin equivalents for the anode and cathode nodes.
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Old 13th December 2011, 12:18 AM   #514
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Actually, it's quite easy- you apply a signal, open both nodes and measure the voltage at the one you're interested in (or both simultaneously- physics allows you to do that). Then short both to ground and measure the current at the one you're interested in (or both simultaneously- physics allows you to do that). If you honor the boundary conditions, the coupling is irrelevant- the next stage is driven by two equal and opposite sources.

When you do that, honoring the boundary conditions, you find both Thevenin source impedances to be low and equal. Experiment is consistent with that prediction. I'm still waiting for the pair of loads which cause the simple model to make a wrong prediction. The crickets are getting hoarse from all that chirping...

Quote:
This makes it impossible to determine the actual Thevenin equivalents for the anode and cathode nodes.
Nope, it's trivially easy.
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Old 13th December 2011, 12:31 AM   #515
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Seriously SY, is that the best you can do? Is that all you've got?

Last edited by Alfred Centauri; 13th December 2011 at 12:35 AM.
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Old 13th December 2011, 12:41 AM   #516
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Yep. All I've got is a simple, intuitive, and useful model that gives correct predictions, and two unhappy people who can't come up with a single example that causes it to give the wrong answer.
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Old 13th December 2011, 12:54 AM   #517
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who can't come up with a single example that causes it to give the wrong answer.
In fact, we have. Ignoring that fact doesn't make it go away.
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Old 13th December 2011, 12:57 AM   #518
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OK, what's the pair of loads which cause this model to give the wrong answer?
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Old 13th December 2011, 01:04 AM   #519
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All real loads are the problem. Your assumption of correlation is not math.
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Old 13th December 2011, 01:09 AM   #520
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OK, what's the pair of loads which cause this model to give the wrong answer?
Any pair of loads.
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