Testing the TK Tar Baby

[TinselKoala]

Please watch the video.

[picowatt]

Please watch the video.

TK,

I watched the video.  What is the test frequency on your meter?

I would agree that the resistors do measure 7uHy.  But, you could verify that they do indeed act like ca. 50R resistors at 1.2MHz with the FG. 

PW

[TinselKoala]

TK,

I have a few questions regarding that scope shot, particularly if the text is correct regarding the FG being at full negative offset.

In any event, I do not see how the CSR inductance can be estimated from that shot.  As this is critical to power calculations, I assume it will be addressed with greater clarity in the future, or at least a non-inductive resistor used.

As for now regarding your circuit, and not being certain what resistors RA used, I do not know what to tell you.  Possibly .99 has some suggestions.

I believe you said you checked your LCR meter against a "known".  Are you fairly confident in your inductance measurement of the shunt resistors?  Enough so that you do not feel it necessary to try an alternate meaurement of one of the ten watters with the FG, etc?

PW

You do see the mean "VV" figure, though, don't you? Is there something wrong with that value in the scope shot?

And I'll be happy to show an alternate measurement of the inductances in my circuit.... when RA does so for her inductances. Are you thinking that my figures are implausibly high? They don't seem so to me. How do you explain the readings I obtain on marked commercial inductors in the video, then?

Did you watch the video concerning my quick meter qualification and measurement of the resistor's inductance?

[TinselKoala]

TK,

I watched the video.  What is the test frequency on your meter?

I would agree that the resistors do measure 7uHy.  But, you could verify that they do indeed act like ca. 50R resistors at 1.2MHz with the FG. 

Did I argue or act surprised when you pointed out that the stack would actually be at around 16 ohms or so? Am I arguing about that now? There's no question in my  mind about the inductance and reactance of my resistors. I have questions about RA's resistors though and the only answer we ever get is "look at the papers".

The manual says "about 200 Hz." I seem to recall that most inductance meters use 900 Hz, or at least the last one I had did.

So at what value does this "shunt" inductance cause the current measurement to change sign?

And what is so special about my resistors that makes them 16 times more inductive than RA's that look just the same but with only one line of text on them? Can ink really add that much inductance?


The least you could do is hold those goalposts still for a few minutes.



(And these are shunts. The current viewing resistor in the RA circuit could be called a "shunt" I suppose. Sensing? I don't think it senses anything, because it does not change in any way (except heating and temp coefficient of resistance) due to the current through it. Unlike a thermistor, for example, which does "sense" by changing its resistance in response to temperature. We use the current viewing resistor like a lens: we look through it with a voltmeter and look at what the _voltmeter_ senses. The CVR itself is passive.)

[TinselKoala]

Well, I dunno.

Using the FG to set a frequency, the scope just to look at amplitude, and the Philips counter to read the frequency, I looked for resonance in the LC tank made by connecting a 10000 pF poly cap across the single resistor. Sweeping, I got a resonant peak at about 2252.9 kHz, and crunching the numbers gives me about 0.5 microHenry as the inductance. And when I did the same thing with a ceramic cap of measured 0.007 mF I got a resonance at 2698.7  kHz which also gives an inductance of about 0.5 microHenry.

L = 1/(C *(2*pi*f)^2)

So.... if that's the case.... then I don't understand why the meter reads differently on the resistor but gets the other inductor values right.

Also... if the inductance really is that low, that also takes the AC reactance down as well, to about 5 or 6 ohms at 1.5-2.0 MHz, or about 1.5 ohms for the stack.  Right? So now we are moving back up.

So if my meter is right, my power is small -- but still averages negative -- and Ainslie's impedance values seem too low. If my meter is wrong in spite of the known inductor measurements, and our resistors are in fact about 0.5 microHenry each, hers and mine, then her 110 nH number makes sense ... and then my power levels go back up again and look even more like what she's reported.

I'm perfectly happy to accept the 0.5 microHenry value for the single resistor. That means the stack is 125 nH, close to the RA figure. And if I measure the load inductance by the same method, I get 1.2 - 1.4  microHenry, compared to the 70 or so mH the meter tells me. So fine, my new meter is junk and my shunt and load inductances are very close to Ainslie's stated values. Fine.

(Imagine... all this trouble just to prove that my batteries do discharge.)