Testing the TK Tar Baby

[picowatt]

TK,

Why would you want any FG other than that beautiful Interstate.  I'd love to have it.  40volts into 50R!!  Nice...

How on earth are you switching on both Q1 and Q2 at the same time?  To turn on, Q1 needs the FG to go positive, Q2 needs the FG to go negative.  I can't get my picobrain around that...  Does that Interstate also perform magic? (if so, it's the perfect FG for the current task)

The RA circuit appears to only switch Q1 on very briefly followed by a several second duration wherein Q2 is biased on.

Reasons for staying with the IRFPG50:

1.  The much larger Ciss allows more AC current to pass when Q2 oscillates (Q2 is the "array", it is labeled as both Q1 and Q2 depending on which "paper" you read, I'll stick with Q2).  Draw in the Ciss (gate to source capacitances) and you will see that the Q1 and Q2 combined Ciss of 12,500pf or more is the AC path for current when Q2 is oscillating at HF (as well as the FG's 50R).

2.  The larger Ciss allows Q2 to have more AC gain

3.  I thought you were going to attempt a replication

Think about tacking a wire onto your Interstate prior to its 50R output resistor so you can measure across the 50R to get bias current, and, prove that current can indeed pass through an FG.....  Though I don't think much battery charging is going on...

PW

[picowatt]

TK,

Reasons for staying with the IRFPG50:

4.  They're magical??

PW

[MileHigh]

TK:

About the often repeated story about "18 months of continuous use."  I posted once before that all of this is purely anecdotal "evidence" and Rosemary should never have even stated it.  Then you have to ask yourself about what really happened and human nature.  Do you really think that every single day the students (or whomever) would say, "Time to set up the RAT circuit and stare at it again!"  What new insights could be learned by doing the same thing every day for months on end?  It just doesn't add up at all.  I would guess that the circuit was set up and running when Rosie was around and after the novelty wore off it collected dust when Rosie wasn't around.  I don't get the impression that she was there every day for 18 months.

It's all junk anecdotal data not even worth discussing.  I figure that she had somewhere between 10 and 20 megajoules to start with in her battery set, and when you average it out over 18 long months, the setup might have been on 10 to 15 hours per month.  Also, she has no idea whatsoever what the power consumption of the setup was because she measured "energy being returned to the batteries while the setup ran."  Without bothering to crunch the numbers, perhaps she burned off 5 or 6 megajoules during her testing.   And of course, we can't forget that it makes absolutely no sense to talk about draw-down test if you believe that the batteries are always recharging, none!

Moving on to more interesting stuff, Picowatt made a lot of interesting comments and really appears to know his stuff indeed.  I plumb forgot about an AC path at high frequency having less impedance back to the battery ground via the CSR as compared to the the path through 50 ohm resistor inside the function generator.  That may have saved Rosie's function generator resistor from burning out.  So indeed there are two paths for the power to return to ground, a mainly DC path through the function generator and an AC path through the various capacitances in the MOSFET array through the 0.25 ohm CSR.

It's a bit complicated and I acknowledge that when I talked about the "fake" voltage measured across the CSR possibly being from capacitive-inductive coupling, I honestly wasn't thinking about the capacitive coupling through the various MOSFET gate-source-drain AC paths  (I plumb forgot - a dumb mistake).  So there is very likely some real AC power flow there with real current, and not just a "fake induced tingle voltage" with no associated current.

I read somewhere that the various capacitances inside a MOSFET structure are also a function of bias voltage.  The width of the gate channels change as the voltage changes.  (I am outside the envelope of my direct experience here and can't even remember the proper terminology).  However, putting that aside for a second, let me mention a test that may help you get a feel for this.

You could try to use a second signal generator and a inject high-frequency sine wave into the common MOSFET drain node.  As you sweep the sine wave higher and higher in frequency, observe what's happening at the CSR.  I think Picowatt is dead-on and above a certain frequency you should see a very strong sine wave at the CSR.  Call it an "anecdotal" or "get a feel for it" experiment.

Anyways, don't let too many cooks spoil your soup and your fun.  The good news is that you can measure the AC power across the CSR and that is just one component of the "power pie."  As long as you know the total power the circuit is consuming, then the fun can be finding and measuring (or inferring) what all of the "power slices" are.  Picowatt astutely reminded us that some of those power slices are of the AC-only variety.

MileHigh

[TinselKoala]

@picowatt:

I'd like to use the 830a for several reasons. First, I can get them for $1.30 each locally, and the PG50 costs about 5 times as much. Second, I think it would really be nice if the 830a turned out to work "just like" the PG50 in TarBaby. But of course I'll be using the PG50s when they finally arrive.

The optoisolators work too well. They cause the mosfet(s) to switch cleanly and more precisely, if with a bit of slow turn-on. This allows a lot more power to get to the load; in oscillation mode my inline DMM reads about 170 -200 mA, but when I use the optoisolators and get a clean turnon on the drain signal the current goes up to 1.8 or 2.0 amps or more, and the load heats like crazy. This also happens when "tuning", trying to get the oscillations to appear, when a clean turnon is achieved. I am formulating a theory about the chain of events during a typical NERD test session, if there is such a thing.

Right now I'm powering the optoisolator output side with a 9v battery; the only connection to the DUT is by the gate drive leads coming off the isolator collectors. I'm using an LED and its resistor as a "pulldown" for the optoisolator's phototransistor stage, so I can also monitor the pulses visually when they are slow enough. Very nice switching. Next will be to hook up the optoisolator's power needs to the main board battery supply, to see if the oscillations will return.

The only time Q1 and Q2 are both on at the same time is when I really crank the FG's output to some stressful amplitudes and offsets. I can get them both to saturate and stay on and that lets 3 or 4 amps thru to the load. Or if I use a symmetrical pulse positive and negative, at very fast frequencies there is "shoot thru" when both are briefly on during the zerocrossing of the gate signal. But when I mentioned it earlier, I meant both could be turned on, but alternately, with a symmetrical gate drive signal. I think. It is also possible to get the oscillations in both phases of the drain signal and I think both sets of transistors are partially on during this time.

If I can't reliably get the oscillations with the FG isolated like this, then I'll have to look for some other control condition that will allow me to compare _with_ function generator current path and _without_ FG current path.

[MileHigh]

TK:

I did my last edit in the previous post I swear.  There is a code freeze on that posting.

I just wanted to say congrats on the big fat capacitor.  It can always be used as a low-pass filter in conjunction with your batteries to make a rock rock steady power consumption measurement for the entire circuit also.  In that case of course you lose the "purity" of powering of the circuit by a set of batteries and it is instead powered by the batteries transferring current into the capacitor first.  So in theory the circuit is being powered by the big fat capacitor.  You will still see the identical magic oscillations though.

MileHigh