Testing the TK Tar Baby

[Rosemary Ainslie]

The LeCroy is always supposed to be DC coupled for all of your signals, and it's critical for the current sensing to be DC coupled.  So your point doesn't make sense.  The reasons for the missing current flow can be speculated upon, as was done by PW and others.

Not actually.  Each channel can be coupled as required.  As we'll demonstrate.  In due course.  I never realised this.  I just always set everything to DC.  But then I'm an amateur - is my excuse.

I know that you had some kind of strange fight over AC vs. DC coupling and it's another one of those cases where there is no issue.  I don't remember the details because I was 'filtering' myself.   

'filtering'?  'strange fight?'.  Strange terms you're using there MilesOffThePoint.  No idea what filtering means and I have never had a 'strange fight' with anyone.  And the case was certainly NOT devoid of issue.  It was, as ever IGNORED.  That's par for the course.

Rosie Pose

[MileHigh]

More thoughts on this:

The simple DC current flow measurement showing clockwise current is allowing you to infer that the batteries are discharging.

I am being 'nice' here by using the term 'inference' because of the DSO data.

However, thinking about it a bit more you in fact can conclusively state that the batteries are discharging because of the measured current flow.  It goes back to the chemical reactions inside the batteries.  The 'potential pump' in the batteries is pushing electrons out of the negative terminal and into the positive terminal.  This current flow is caused by a net one-directional chemical reaction.  Therefore net current flow in one direction means net chemical reactions in one direction - the two things are tied together.  So net chemical reactions in one direction by definition means that the batteries are discharging.

MileHigh

[TinselKoala]

I know, beyond a shadow of a doubt (maybe I read it in WIKI, even)  that if it is raining and I look outside, the sidewalk in front of my house will be wet and there will be droplets of water falling from the sky.

Right? I mean, this is the very definition of rainfall and its result on the environment. Isn't it?

So... I'm getting to go walk the wild beasts and I take a glance out my soiled, dirty tiny window, and I see that the tiny corner of my sidewalk is indeed wet, and there sure are droplets of water falling from the sky.

Therefore, I can reliably conclude that it is raining........... RIGHT?

So I proclaim to all the world on my blog that I have discovered that it is RAINING in South Texas for the first time in DAYS... and I run and get my umbrella and galoshes and put the canidae in a big plastic sack and go outside..... where I notice that the sun is shining brilliantly from a cloudless sky and the next door neighbor's kid is watering their lawn. And my sidewalk. And she is squirting water up to see how high she can get it.


Post hoc, non propter hoc. And the distinction between Modus ponens and Modus tollens.

If A, then B. Observe A.... then conclude B. Duh. If the premise is true the conclusion is true because this is a tautology.
Observe NOT A... maybe B, maybe not. Something else besides A might be able to do B. To conclude NOT B from NOT A is a fallacy.

If A, then B. Observe B.... maybe A, maybe not. Something else might be doing the B that you observe. To conclude A from observing B is a fallacy.
Observe NOT B..... then you can be CERTAIN... NOT A. Of course your observations have to be not only Precise... but also ACCURATE.

Note carefully: the only thing you can be sure of is either a NEGATIVE or a TAUTOLOGY.

[MileHigh]

TK:

Now, for the negative bias situation, the FG or other bias source has effectively  switched polarity and is supplying a (relatively) Positive voltage to the BLACK input point (which is really held at "ground" potential, but voltages are always relative to some reference.) As in the second diagram below.  Right?
The _relative_ positive voltage at the BLACK input point causes the RED input point to become "even more negative" than the negative pole of the main battery supply, thus biasing Q2 (Festus) into the linear response region and passing some DC current and some AC current too. Oscillations happen. But.... look at the zener in Q1 (Moses), with respect to the polarity of the Bias Supply. I don't know if there is any other "end" to this current path or if it contributes to load heating, but I can think of some easy ways to find out.

Indeed, you can see the same potential for current to leak through the "opposite" MOSFET (Moses) in negative bias mode.  So a very similar phenomenon could be taking place.

This current path will not contribute to load heating.  It's another independent current loop that will not circulate though the load resistor.  It just happens to overlap with the main oscillation current loop during part of its path.

Here is a funny one:  It would now appear that if you substituted the function generator for a nice beefy bench power supply, there is a chance that the "opposite" MOSFET for both positive and negative bias voltages would go "Boom!"   

So it appears that Team NERD was "saved by the 50-ohm resistor!"  (Again?)

MileHigh

[MileHigh]

Rosemary:

There is no reason that I can think of to set your channels to AC coupling in this circuit and there isn't really anything to discuss.  You select the channel coupling as you want depending on what you want to do.

'Filtering' in my head - I filter out some of the stuff I read.

MileHigh