Testing the TK Tar Baby

[TinselKoala]

TK,

I would not even consider visiting that site without a ten foot or longer pole...

PW

Maybe a giant pickle would work better. It's what I use, since Ainslie is so fascinated with them.

Meanwhile.... Testing Kontinues.

Capacitors.... how do they WORK, anyway? And what are they good for, huh?
In the next little video that no one will watch, I illustrate several things of interest: how a function generator's output can be placed in series with a 12-volt lead acid battery and the combination used to light up a tungsten bulb to white-heat, through a capacitor with just the FG's contribution or directly with both the battery and the FG contributing power to the bulb. That is, I illustrate the capacitor filtering out a 12 volt DC offset while simultaneously passing current that is going through a battery and coming FROM a function generator to light up a light bulb with considerable power.
In other words, I once again soundly refute an inane and asinine claim of willfully ignorant overweeningly arrogant Rosemary Ainslie.

And... in more than passing, at the end.... there is a little topical surprise, that some may have already anticipated.

(just for reference I here include the specific quote from Ainslie):

What you are trying to do is to get me to believe that a function generator is able to pass current from a battery supply source via its terminal to its probe. Since I KNOW that is is impossible I'm afraid I'm not receptive to you trying to teach me or anyone else.  So NO.  I spare me your 'lessons'.

(sic)


http://www.youtube.com/watch?v=udAfK3WxMoo

[poynt99]

If you now look at the NERD schematic with these two caps drawn in, you will see that both caps are in parallel with the FG terminals, allowing AC current to flow directly to the CSR via the MOSFET capacitances, which total 10,000pF in this instance, and possibly as much as 12.5nF or even a bit more).


PW

(And no, this is not a new position, I stated this way back in the locked thread.  But then, as now, it fell on her deaf ears.)

Back when I got the simulation producing the correct wave forms after the cross-wiring discovery, the only remaining "problem" was that my FG Rout had to be relatively small. It worked ideally at 2 or 4 Ohms, not 50.

I knew there was a low-impedance path missing somewhere, and I want to thank picowatt for bringing this missing AC path to light.

I had suspected the MOSFET models were perhaps the cause, and when I checked yesterday, it surely does appear they are. Here is a quote right from the text in the pspice model for the IRFPG50:

* The voltage-dependent capacitances are
* not included.

From the datasheet, we can see that Ciss is minimum (2800pF) at slightly over 20V V_DS. Any lower than 20V and Ciss increases. C_GS (Gate-to-Source Capacitance) makes up about 95% of Ciss.

.99

[TinselKoala]

Hah.... good find .99.
So I think I can guess what you are doing right now....
putting in the capacitances as discrete components to see the result in the sim, right ?

[poynt99]

Did it yesterday, 2800pF each side. Rgen=50 Ohm.

Works now, but the turn ON and OFF of the burst envelope are much slower, so I have to lengthen the run time.

[picowatt]

Back when I got the simulation producing the correct wave forms after the cross-wiring discovery, the only remaining "problem" was that my FG Rout had to be relatively small. It worked ideally at 2 or 4 Ohms, not 50.

I knew there was a low-impedance path missing somewhere, and I want to thank picowatt for bringing this missing AC path to light.

I had suspected the MOSFET models were perhaps the cause, and when I checked yesterday, it surely does appear they are. Here is a quote right from the text in the pspice model for the IRFPG50:

From the datasheet, we can see that Ciss is minimum (2800pF) at slightly over 20V V_DS. Any lower than 20V and Ciss increases. C_GS (Gate-to-Source Capacitance) makes up about 95% of Ciss.

.99

.99,

That is amazing that you found that line regarding Cgs in your Spice description, and that you actually looked/new where to look for it.  Great sim skills!

It would indeed be quite interesting to see if adding discrete caps as per TK's comments would allow you to acheive osc with Rgen=50R in your sims.

I was being generous when using 2nF per MOSFET, using 2.6 to 2.8nF is probably a bit more realistic.  Using 2.7nF per MOSFET, the five in parallel would have a reactance at 1.5MHz of 7.86R.  That 7.86R in parallel with the Rgen=50R would provide a 6.8 ohm path for an AC sine at the fundamental of 1.5MHz.

But again, it must be kept in mind that the oscillation is not a pure sine so harmonics will see an even lower impedance thru Cgs.  Half, third, fourth, fifth, etc.

If you are going to try the sim with the caps added, please keep us updated.  I always enjoy your sim work!

Can you do FFT's of waveforms in sim?

PW



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