Quote from: poynt99 on April 25, 2012, 09:28:40 AM
GL,

Your second paragraph in the diagram is not correct.

There will be no power added to the circuit in the scenario shown. What you have there is simply an isolated VGS bias that turns ON the right MOSFET. There isn't even a return path for the FG source, so it can't provide any power to the circuit.

.99,

I think GL's simplified schematics provide a better tool to help Rosemary understand what the FG is doing.  Particularly the first drawing posted in reply #732 as it regards to her Q2 array.

As for GL's second paragraph, I spotted that too but did not at the moment want to complicate things.  The body diode on the left will, however, conduct and clamp the right side gate voltage to a couple volts or so above the drain voltage.  When the right side MOSFET is on, the drain voltage will be dependent on the battery voltage, the load resistor drop, and the MOSFET on resistance.  With a 72volt supply battery, the gate voltage would be clamped at around 14 volts and with a 48 volt supply battery, the gate voltage would be clamped at closer to 9 volts.  Any voltage applied to the gate by the FG (or a pwr supply) that exceeds this "gate clamping voltage" will be dropped across and dissipated by the 50R.  At the supply battery voltages typically used by Rosemary, and with a typical FG, this dissipation from gate voltage clamping would be minimal. (if any at all, depending on the FG settings)

Again, I really like GL's first drawing and was hoping it would help Rosemary understand how her Q2 array is being turned on and off.  I was then going to attempt to help her understand the current limiting/regulation that occurs regarding the voltage applied to the 50R and the threshold voltage of the MOSFET, that limits the Q2 DC current to around 200ma or less (thought I would discuss only DC conditions for a while).

Then, in response to her wondering why current flow at the CSR is not observed, hope to make her see that the 200ma of bias current will make the CSR only 50 millivolts positive, which would be barely visible at her scope settings.

It sounds like you have covered all this ground in the past.  Apparently it has yet to "sink in".

PW

PW:

QuoteIt sounds like you have covered all this ground in the past.  Apparently it has yet to "sink in".

Indeed.  For what it's worth, I probably have more technical prowess than I appear to have.  I went though the whole thing last year and when I got into this thread I posted a disclaimer to TK saying that I might not be getting into all of the nitty-gritty details again.  I also did the "COP 17" saga about three years ago (also TK and Poynt) which had just as much drama associated with it.  I never really played with MOSFETs but I certainly understand how they work.  So I am suffering from a form of burnout about this whole deal.  I am not engaging fully on the lowest level.

I certainly can qualify and to see you discuss the nuances of MOSFET design is very impressive.  When I was doing digital design the transition to sticking everything into a gate array was just starting to really take off.  I am more of a PAL and GAL kind of guy.  Then I jumped ship into sales anyway and it was a relief to not have to worry about timing diagrams or to learn how to "design hardware" with VHDL.

The fun part is that most of the stuff around here draws on the "engineering foundation" education that is still taught in school to this day.  We did lots and lots of labs with transistors, but only did MOSFETs in the classroom, perhaps covering them in two classes or something.

It's fun to watch the real experts do their thing!

MileHigh

Quote from: TinselKoala on April 25, 2012, 10:55:06 AM
@.99:
Your diagram appears to show a DC current path that puts both the main battery and the bias battery in series and provides a current pathway that does NOT flow through the CVR at all when the mosfet is conducting.

Am I right about this? (just sharpening the point)

According to the diagram, yes that would be correct. The location of the CVR/CSR was a point of contention for a while, as you may have read in the discussion I linked to. The diagram was made according to the video demonstration as-built apparatus.

Quote
I'm also wondering about something else, which also jives with my experience on the live circuit. If the FG has its own equivalent 50R in the output, why is the series 2R also required at the circuit itself? I think I understand why it's needed if the bias supply is a battery, but I don't think I  understand why it needs to be there if the FG or other source with a moderate impedance like the 555 timer circuit is used.

I've mentioned this before, but perhaps did not "take" well. In the simulations, if the FG (or DC source) resistance is made anything close to 50 Ohms, the oscillations either stop or look considerably different. Even the NERD's simulation uses a 22 Ohm instead of 50 Ohm. It worked best for me with a 2 Ohm.

Quote from: poynt99 on April 25, 2012, 02:23:01 PM
According to the diagram, yes that would be correct. The location of the CVR/CSR was a point of contention for a while, as you may have read in the discussion I linked to. The diagram was made according to the as-built apparatus.
I've mentioned this before, but perhaps did not "take" well. In the simulations, if the FG (or DC source) resistance is made anything close to 50 Ohms, the oscillations either stop or look considerably different. Even the NERD's simulation uses a 22 Ohm instead of 50 Ohm. It worked best for me with a 2 Ohm.

.99,

Are you saying the NERD circuit FG did not have an Rgen of 50R, or that the sim would not work with 50R?

PW

The sim would not work properly with 50 Ohms.

Of course the FG has a 50 Ohm output.