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What is the best equipment to obtain the correct I and V of an RC Circ
3

What is the best equipment to obtain the correct I and V of an RC Circ

What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
This is an ongoing question I have for which I still have not found a satisfactory answer. This concerns measurements with oscilloscopes in general and to understand it I am considering one of the simplest model circuits -- the RC circuit. Suppose you have a 10Ohm resistor and a 100pF capacitor in this RC circuit and you apply 800kHz of 8ppV amplitude. I have asked a question regarding this circuit earlier in this forum and I remained with the impression that studying such model circuit even with the state of the art equipment is so unreliable that the I and V data obtained hardly reflect the real I and V that would be expected theoretically. Is that the case? Is it really true that there is no equipment at present that would provide true I and V? If it isn't then what exact equipment (oscilloscope and probes) would you recommend that would yield true current and voltage of the said sine curves measured across the capacitor and the resistor, with all parasitic effects (capacitances and inductances) at a negigible level?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

The usual garden-variety passive 'scope probe will impose capacitance on the order of XX pF (should be in the product spec.). This will obviously have an impact on circuits using 100pF capacitors; this almost goes without saying. One must *always* contemplate the impact of the measuring instrument on the circuit. Often it just takes a second or two to estimate that the impact should be negligible in most cases. But high speed digital, RF, and pF are warning signs to be careful.

In the real world, one would not often have cause to require the sort of data in your question. Prototyping an RC circuit would typically rely on modeling. Trouble-shooting would be looking for gross failures, perhaps by comparison to parallel-channel circuits. Is this for school?




 

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
No, not for school. This is a fundamental problem I'm encountering and I'd like to have it resolved first on the model circuit mentioned in order to proceed further. The voltage probes I have access to are Tektronix P2221 (the current probe is Hall effect TCP0030 which is the best on the market). It's true they have 100pF capacitance but, curiously, turned out to give better results compared to the Tektronix active probes I tried and returned because they were influencing the phase shift badly while P2221 keep it just fine. In addition, for those parameters the 10X turned out to be less appropriate because of its lower resolution. What is of great concern is the low input impedance of the probes, even the most expensive ones. Wish there could be something like a Keithley electrometer type of a probe. But there isn't, apparently.

As for modelling, I've dropped it altogether after I saw how sloppy PSpice (which is at the basis of all else having to do with modeling) is. So, at this point it seems that there is no reliable apparatus that can measure correctly even the simplest of circuits. Hard to believe.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

Quote: "it seems that there is no reliable apparatus that can measure correctly even the simplest of circuits. Hard to believe"

That's why EE:s are there to fix your problem. There's always a way. And it is usually easy to follow. But you seem to concentrate more on how difficult things are. Go find a qualified EE. He will sort this out for you.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
I need that particular phase shift or of similar order. Unfortunately, the problem persists for all measurements on the lower end of caps and that's unexpected for me. How come that simple of a circuit cannot be measured properly?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@Skoggsgurra,

Thing is, is it my problem or that's an inherent problem in measuring circuits. If it's the latter EE can't help, won't you agree?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@IRstuff,

Just curious. It appears that while theoretically a, say, RC circuit in its ideal state is well understood, experimentally it is quite a challenge to obtain the current and voltage which theory requires. This may have unforeseen ramifications, don't you agree?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

Can you attach a resistor to your test point without adding too much capacitance? Take your measurements (test point and reference) through 10k resistors.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

@pybb

Is this something personal? Why the whining?

Heisenberg told us long ago that you cannot observe without influencing the observed object. And resistive and capacitive loading effects on a circuit has been known for centuries and those effects can be handled with great success.

So, the answer is that an EE can help. Believe it or not. If you want to make a philosophical statement, just do so. But do it in some other forum. This is for practicing engineers.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

As above. In other words, one steps away from the 100pF RC circuit and finds another, higher up, method to answer the practical question.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

py,

Look at it another way... consider a circuit with a 1 ohm resistor and a 1F cap.  That scope probe isn't going to affect the measurement one whit.  Theory will match up with actual spot on.  As you approach your circuit with much lower capacitance, the measured will start to roll away from theoretical... but you know why it's rolling away, the effect of the measurement tool is getting larger.  Theory tells us how it will effect the measurement and to what degree, so a good scientist will account for those changes in his data (or at least know where the measurement limits are).  I don't see a problem with that.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
So, I take it as established that there is no apparatus at present, no matter how much state-of-the art it is, which can test the theory in the region of nF or fractions thereof and R on the order of Ohms. Supercaps are no substitute because of many other problems and most importantly because of the already mentioned insufficient phase shift.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

If that makes you happy: Yes. You cannot measure anything 100 percent correctly. There's always some influence and there are tolerances. There's also thermal noise and other things.

Happy now?

Why supercaps? They are in an entirely different capacity range. Some ten billions or more higher capacitance value. I really cannot see your problem. And why you are so persistent. Are you designing power supplies for guitar amplifiers? smile

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

"...test the theory..."

We're getting close to a hundred years past the point where testing the (R-C) theory is necessary, or even interesting.


I'll give you a couple of typical situations where one could tune capacitors to pF precision without even raising a sweat.

Let say you had several channels in a circuit that all had to be matched as identical as possible (hey, I've got this T-shirt). One can sort through the components, measuring them out-of-circuit using a $10,000 4-wire instrument with lots of digits, and bin them into exact measured value values. Then build the circuit (carefully), and all three channels WILL BE exactly the same (unless you've done something wrong).

Another example. Presumably the real-world circuit has an output. Presumably these components have some real-world measureable effect on the output in some circumstances (otherwise, why not remove them). So one can arrange for the circumstances to be true (bench test), and then measure the husky output to see if it's what you want.

 

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@VE1BLL, yes, that approach is one way to go but it is an indirect way of proving theory experimentally. Thus, you would rely on the fact that because the signal is periodical the errors will even out.The nagging question will remain, though -- where's the direct experimental proof of theory. By the way theory is always subject to testing, a hundred years or not.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

Quote:

...reliable experimental evidence to support theory as to how a 100pF...

There's no science to be discovered here; only the frustration of uncontrolled strays.

You might wish to look-up 4- and 5-wire measurement techniques. That's exactly where you're headed if you are serious.

 

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@VE1BLL,  tell me more about the 4- and 5-wire measurement techniques and how they can solve the problem at hand.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

Yeah, Google terms like:
4-wire OR 5-wire measurement technique

I even see 6-wire technique mentioned in the results, something I've not done.

We have 4-wire Wayne Kerr RCL meter here that's fantastic. It can reliably measure components with very small values. I think it even gives the equivalent circuit for parallel or series mixed circuits (R and C, or R and L).
 

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@MikeHalloran, so how does this book answer the problem at hand, regarding correctly measuring the current and voltage of a 100pF-10Ohm RC circuit?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@VE1BLL, you're suggesting methods to measure the values of individual components, right? This is not the question at hand. The question at hand is how do you correctly determine the values of current and voltage when you apply 800kHz signal on resistors and caps with relatively small known values, stated above. So nether the book cited above no the multiple wire techniques help answer that paticular question.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

Mike,

You _would_ pick the book we used as a text for Aero lab class! :)  Though I'm pretty sure it would tell how to build an op amp circuit to pick up the signal desired.  I could even sell my copy to the OP, but that would just guarantee that I'd need to wire an op-amp the day after I posted it.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
Maybe I should put the question differently. It has already been established that there is no known system capable of correctly measuring the voltage and current of an RC group consisting of a 10Ohm resistor and a 100pF cap when a signal of 800kHz is applied to it. What are the lowest values of the capacitance and resistance that would allow a correct measurement of I and V at 800kHz with an 8-bit scope and a garden variety of a passive probe (meaning a passive probe having 100pF capacitance and 1KOhm resistance)?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

Is there a reason for picking 800kHz? Why not 20kHz, 100 kHz or 20 MHz? Holding the voltage constant and sweeping through various frequencies may enlighten.

Is it safe to assume that the test cap is capacitance, the whole capacitance and nothing but capacitance? Same goes for the test resistor.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

May I turn the question around?


How do you proposed to directly measure the current?

...And do so without affecting the voltage?

 

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@Thealanator, like I said, the phase shift between I and V should be as large as possible.That' the main requirement, I guess. Also, the measured current and voltage should be within reasonable bounds to maintain the accuracy with the 8-bit scope. Why 800kHz? Just because it happened I started the question that way. Could be any hundreds of kHz. Let's stay with 800kHz for now, though.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

I was hoping the book would educate the OP about the existence of parasitic components even in test gear, and some of the techniques used to compensate for their presence.  

I wasn't going to attempt elucidation re Heisenberg.

pybb3ezv, if you wish for someone to point you to a particular line of a particular paragraph of a particular book to answer your particular question as phrased, how much money are you willing to pay for that information?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
Mike, a simple suggestion will do. Money is not under discussion in this forum. Probably you should look for a job-related forum.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

2
The simple suggestion that comes to mind would violate local rules for decorum, even on Usenet.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
No, it won't, as long as it pertains to the question. Suggestions outside of what pertains to the question are not asked for and, like I said, you may address them in other forums to spare the bandwidth here.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

I find it interesting how some people want to tell the old timers how the site ought to be used.  Why the heck do they come here if they won't accept any answers?  If they have all the answers already, what's the point.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
David, I do accept the answers but where are they? The only thing I learned so far is that there is a lower limi, never mentioned in standard texts,t concering reasonable values of capacitance and resistance beyond which the electrical theory hasn't been and cannot be tested experimentally. If that's a nutty statement prove me wrong, cite a peer-reviewed paper that shows evidence to the contrary or make a suggestion as to how this difficulty can be overcome. I asked a question about the lower limits of C and R which would allow correct measurement with the available apparatus. Don't see an answer to that question. Do you?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

The limit is mostly about money, time and skill. That's why it's fuzzy.

"...cite a peer-reviewed paper..." Such a demand implies that it matters to the regulars on this forum. It doesn't.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

You guys obviously find this "scientist" entertaining. So I refrain from RF.

Why is he discussing ohms and pF? There are techniques to measure microohms and femtofarads - and less than that. I see no point whatsoever in discussing a trivial case like his.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@Skog, like I said, the question is not about measuring precisely the values of resistors or caps. These are known with the highest precision (or accuracy, if you wish). The question is how to measure correctly the current and voltage of an RC circuit consisting of these elements with precisely known small but not unusual values, when a 800kHz signal is applied to the circuit.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

I am concerned about my own mortality. I demand a precise and accurate answer as to how I can overcome this problem.

Oh this is a work related engineering problem. I intend manufacturing and marketing the solution and I am not a student nor am I preparing a thesis about a theoretical problem that in practice does not really exist as workarounds are common practice and effective.

I reject the forum rules about not hijacking threads so don't bother criticising me in that regard either.

Oh I have been kicked off here before for cheating on my homework, so now I make sure my homework umm job is work related before I post.

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: What is the best equipment to obtain the correct I and V of an RC Circ


At these frequencies nothing is simple. I doubt that you have a pure 10 nF or a pure 10 ohm device.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

When the value of any parameter of a test device becomes significant in relation to the same parameter of the device under test, significant errors are often the result.
Take it to the extreme. I suspect that internal corrosion may be affecting the resistance of a piece of 500 MCM cable ten feet long.
I have been testing it with my $25 multimeter but have not been able to determine whether the cable is good or faulty.
Similar problem at the other end of the scale.
Why am I wasting my time?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: What is the best equipment to obtain the correct I and V of an RC Circ

I dispute that it's not possible to make this measurement.  For a measley $500K, I will make this measurement.  Whether it's worth $500K for you to make this measurement is up to you.

I mean, we're talking about 0.8 AMPs of current, for crying out loud.  People have been able to make even more difficult measurements involving 100 AMPs, with even less headroom.

TTFN

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RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@IRstuff, can you give reference to support this statement: "People have been able to make even more difficult measurements involving 100 AMPs, with even less headroom"?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
@IRstuff, evidently you cannot provide evidence to sustain your claim that people have been able to make even more difficult measurements. I don't doubt that in all other aspects people have made all kinds of difficult measurements. However, in that particular aspect we're discussing here there's no evidence that they have.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

Well, that's your opinion alone.  I don't have to give you proprietary information just to satisfy your apparent whim. I've already given the basis for my statement, which, because you are not an engineer, you've completely failed to recognize the significance thereof.  That's on you, not me.

TTFN

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RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
That's for others to judge. What I see is that I'm not getting a satisfactory answer to my question other that such measurement is impossible to be made at present and therefore that part of the theory has never been confirmed experimentally.

RE: What is the best equipment to obtain the correct I and V of an RC Circ

One last attempt.  The answer is that you're in the range in which simple instruments can't make sufficiently detailed measurements to confirm a nice neat theory.  The theory is well proven and accepted.  At larger values and lower frequencies, you can get there with simple instruments, but not where you want to be.  At the values you want to work at, there is no such thing as a capacitor that exhibits only capacitance, nor is there such a thing as a resistor that exhibits only resistance.  Theory and real world aren't the same thing.  Measurements affect that which is being measured.  Accept the theory, use it, but also accept all the "fudges" necessary to get theoretical results using real components.

If the theory didn't work, there would be many devices that do work that wouldn't.  Can't you accept that?

Why not worry about chemical problems instead of trying to reprove well established electrical principals?

RE: What is the best equipment to obtain the correct I and V of an RC Circ

(OP)
Thanks.

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