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

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.)

TK:

I don't think I have ever used a capacitance meter or an inductance meter.  It's funny I vaguely remember doing the engineering labs where you look at RC and L/R time constants to measure capacitance and inductance, "like a man."  So I kind of frown to myself when I hear about capacitance and inductance meters.

I think we are back to the always interesting issue of the limits for your measuring apparatus.  What I did on the bench with a scope had it's limits.  Similarly for both capacitance meters and inductance meters, eventually the values you are trying to look at get so low that either the measurement device was never designed to go that low, or the values that you are trying to measure fall under the "background noise level" and the capacitance and inductance of the leads of the instrument are larger than what you are actually trying to measure.

You mentioned that most inductance meters use about 900 Hz.  Right away I am thinking forget it, it will never be able to measure nanohenries, it was designed for milihenries and possibly microhenries.  Any lower than that forget it.  There is always "RTFM" but a lot of these commodity meters don't actually give you the real specifications.  That's a Marketing decision to increase sales (or to prevent reduced sales).  The Dark Side of Capitalism.  They know that the average Joe Blow might not buy and say, "What?  It can't measure picohenries??!!"

What I think PW may be suggesting is the following:  I assume that your signal generator has a 50-ohm output impedance.  Just solder your resistor array to a 1/2" length of coax connected to a male BNC connector and then jack it straight into your function generator.  Then sweep up the frequency and observe the voltage across the resistor array.  Assuming that the inductance dominates as you go up towards 1-2 MHz, you should see the AC voltage across the resistor array increase as you go higher in frequency.  Obviously at lower frequencies you have 50 ohms in series with 0.25 ohms, so you should meaure a very low output voltage.   But at 1.5 MHz if the impedance is 16 ohms you should see a higher AC voltage.

So I suppose that this method will work out to much higher frequencies.  But there are limits of course.  Eventually the inductance inside the function generator itself will start to come into play and the stray capacitance across the leads of the resistor array itself will come into play.  As a certain frequency you should see a roll-off and the AC voltage across the resistor array will start to decrease again.

Somebody might correct me here but I think I am in the ballpark.

MileHigh

TK,

If you have another resistor similar to the ones you used for your CSR, try placing it directly across the FG terminals with the FG set to 1.2MHz (I am assuming an FG with a 50R output).  Scope across the FG before and after connecting the resistor.

If they truly are 7uHy, and therefore close to 52R at 1.2MHz, you should see the FG output go down by 6dB (half).  If the output drops much more than this, they are not 7uHy.  You could sweep the freq to get an idea of what the resistor is "acting like" at higher frequencies as well.  There are a few issues that make this method unable to tell if the observed reactance is inductive or capacitive, but should suffice for the way the resistors are to be used.  If you have no spare, use the four in your circuit (you will have to free the connections at least on one end) and modify calculations/frequencies accordingly.  You can calculate the equivalent resistance based on the observed drop at a given frequency (with the Rgen 50R as part of the divider) and calculate an inductance from there.  There may be a capacitive component, so I would rather just say that at 1.5MHz, their ESR is the observed/calculated value

Possibly, the low resistance or some internal capacitance of the resistor are fooling the LCR meter.

The lower values your alternate inductance tests indicate are more inline with other values given.

I would want to know how the resistor performs at the fundamental and the second and third harmonic, as these are most prominent in the waveforms.

PW



 

TK,

If the values being used for the inductance/ESR are closer to the values your alternate measurements arrived at, you will have to crank up the FG output to get an accurate measurement of the drop with the FG's 50R.  If they are closer to 1R at 1.5MHz, with your FG at 40volts (amazing!), you should be able to read the divided voltage fairly well.

PW

TK,

With your new resonance measurements, I think you are in the ball park. I used all the stated inductance values in the sim, and the results are close to Rosemary's.

You are right about the CSR value, it is of little consequence when for starters one is obtaining a negative VV from the CSR and VBAT trace products.

Have you measured across ONE of your battery's terminals directly with the scope, while all other scope and FG leads are removed? If so, what did you see directly across the ONE battery?

The Interstate F43 is set to make a sine wave at 2.0 MHz (closer to the actual value of TB's osc freq) (actually 2001.86 kHz on the Philips counter) and 40.0 V p-p "no load". When I then hook a single 1 Ohm 10 Watt power resistor across the FG's output, it drops to 10.4 Volts p-p.

TK,

Is that with the resistor connected directly to the FG terminals or are you using a test lead?

You don't want to be measuring any test lead inductance, so go for right across the terminals.

PW