Current measuring resistor calibration

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The problem with that is that unless you can guarantee that the tempco
of the resistor will be within limits which you specify you won\'t know
be able to predict the current accurately by measuring the voltage drop
across the resistance.

lp.

Better is to go the whole hog and use an alternating current bridge; this lets you use transformer widings to define your bridge ratio's - a Blumlein bridge.

National standards laboratories use such bridges to compare and calibrate standard resistors,with one another and with quantum Hall effect primary standards of resistance.

Rayner and Kibble's "Coaxial Alternating Current Bridges" discusses this approach at some length

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The book was published in 1984, so it doesn't cover quantum Hall effect standards of resistance. Rayner and Kibble both worked at the British National Physical Laboratory (NPL), and they pretty much reported on what they were doing there then - the work is still going on

The book is out of print at the moment but you can find it in university libraries - for a long time the NPL would sell you a copy printed on demand, but they've now stopped doing that.

One of the approaches used to calibrate a resistor was to use a "quadrature" bridge to compare the resistor to the impedance of a known capacitance - eventually derived from a Thompson-Lampard calculable capacitor of around 0.15pF - at a well-defined frequency.

-- Bill Sloman, Nijmegen

A Fluke is a lot easier.

John

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That's not going to work because your reference current will split between your sensing resistor and the possibly unknown source impedance. IOW not all the reference current will pass through the sensing resistor.

---------------------------------------------------------------------------------------- Hi Bill, That was very interesting. I couldn't find enough information Online on how to build a calculable capacitor , so maybe I can find a proper book later but I think if I realize that this method is really practical and applicable, definitely it is very useful for me.

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2- When current is high (>1A), current measurement resistor becomes hot

------------------------------------------------------------------------------- Hi Linnix, That was also a very interesting idea. In theory what you say completely makes sense because by creating a look up table one can take it into account the effect of Tc and also it can minimize non-linearity related to CMRR of opamp. does anyone knows if this method of creating a Look up table for calibrating resistors is actually used in any commercial product?

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We are selling to end-user for a profit, so I guess it's a commerical product. The main problem we are facing is variations in components accuracy, with or without temperature changes. With a One-Time- Programmable micro, we build unique lookup table for each unit. Piror to programming the chip, we isolate the I/Os from the external circuit and test it with an external tester. Since we are only interested in linearizing an RC feed op-amp circuit, we can easily construct an inverted lookup table indexed by operating temperature. The lookup table is part of the micro's runtime code adjusted by current temperature.

It's preposterous. Buy a decent DVM that's certified calibrated.

John

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--------------------------------------------------------------------------------------------- John, I have more than 5 decent DVMs. calibration of all of them have expired of course. Meanwhile I have experienced different reading even with calibrated DVMs, one from Fluke and another from Hioki both very decent DVMs.

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2- When current is high (>1A), current measurement resistor becomes hot

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----------------------------------------------------------------------------------- Thanks linnix, your input was very useful for me. may I ask if you also had to overlap current values read with one resistor on other resistors? as I mentioned earlier our most annoying problem is that when current range changes i.e. current measuring resistor changes, they produce different values for the same current. we calibrate them of course but if we calibrate lower end to overlap on previous range then upper end values are invalid and if we calibrate upper end to overlap on next range then lower end values are invalid and produce gaps when range changes.maybe producing a look up table also can make this problem to go away.

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It's anything but preposterous. "Coaxial" AC-excited bridges built with properly constructed ratio transformers are more stable and precise than any DVM you can buy.

What do you think national standards labs use to certify the calibration of "decent" DVMs?

There are probably a few customers out there to whom you could sell this kind of technology, if you ever bothered to learn a bit about it.

-- Bill Sloman, Nijmegen

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It should be a definite over-kill for you application. Vishay will sell you four terminal resistors with a temperature coefficient of less than 1ppm/C for of the order of $100 each - or they would the last time I looked into it - and you can compare these with the resistors you want to calibrate to better than 1ppm with a "coaxial" AC bridge.

The decade ratio transformers you'd use to build the bridge can be constructed to give stable and predictable ratios at the one part in ten million level without using a machine shop. Setting up the metal screening boxes to totally enclose the components - that's what "coaxial" means in this context - does require competent metal bashers with some kind of gear for folding thin sheet metal and soldering up long seams, but it is a lot less like hard work than putting together a Thompsonp-Lampard calculable cross-capacitor, which is a symmetrical arrangement of at least four parallel circular metal rods, typically centimeter diameter rods a few hundred milimetres long.

-- Bill Sloman, Nijmegen

help.

And, literally, a hundred thousand times more tedious.

How many megabucks and man-centuries do you think they spend on it?

But they don't use capacitors to calibrate DC standards. They use Josephson junctions and quantum Hall effect.

Sure, he could divert development of a product that he could sell, for a few years while he played the primary-standards game. Of course, he'd be doing a very, very difficult set of operations to substitute mechanical precision (ie, machining the capacitors to sub-micron precision, out of invar or something) for electrical precision, which he can buy, calibrated, for under $1000. And nobody would accept his current cals as traceable.

Too few. I know.

I have designed and sold flux-balance 2nd harmonic DCCTs, which can match current ratios to PPM levels. It's not a game for amateurs.

John

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Spent on it. The money has been spent, the man years have been put in and the knowldege is available to anybody who can wander into a university library and read about it, as I've done.

These are the primary standards. They use them to calibrate secondary standards - which are essentialy Vishays top of the line sub-ppm/C four-lead resistors, in Petkovic's case, and subsequently compare them with other resistors using Blumlein bridges built with ratio transformers.

He's strictly in the secondary standards business, and it shouldn't take him more than a month to get hold of, or put together, the kind of bridge that he'd need. You'd be aware of this if you had anyt clear idea of what you were talking about

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There's nothing all that difficult about making a Thompson-Lampard calculable capacitor - it takes a machine shop, but the joy of the Thompson-Lampard theorem is that the capacitance of the calculable capacitor depends only on its length, which you measure with a laser interferometer. Of course, Petkovic would be nuts to go to that kind of trouble.

If he has his own Thompson Lampard capacitor, they wouldn't need to - it's a primary standard in its own right.

Since you don't seem to have any kind of grasp of the technology involved, you knowledge doesn't seem to be well-based.

Direct current current transducers are quite another ball game.

-- Bill Sloman, Nijmegen

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------------------------------------------------------------------------- Hi Bill, your posts are very useful for me, Thanks a lot. Yes, I also think Vishay's resistors are more practical for short term at least. In this thread, Linnix, suggested that I can produce a look up table for each resistor to make it more linear.

As I explained in the original post, I have 3 different ranges and I want to read the same value for a current when range changes. Till now using DVMs I could not do that.

My calibration procedure for each range is:

1-When there is no current passing through resistor, I read voltage(v1) across the resistor(It includes Opamp's Offset voltage). 2- I pass a current(i1) near the maximum range of that resistor and read voltage(v2) across the resistor(It includes Opamp's Gain Error). 3- Now to read a current, I read voltage(V) across that resistor and calculate current(I) from this I= (V - v1) * (v2/i1)

Do you also think that Linnix's suggestion of producing a look up table can solve the problem of reading different values for the same current when range changes?

Thank you in advance.

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A look-up table won't make the resistor any more linear, bu ti might help you document the non-linearity

It you are measuring a direct current through the resistor, the voltage you read after a range change will include any adventitious thermocouple voltages around the measruing loop, not to mention ampliifier offset voltages

Probably not.

-- Bill Sloman, Nijmegen

help.

If you build it exactly right, dimensions and all. Nobody who buys precision electronics would trust or buy from an amateur who makes his own standards. They would think he's crazy, and rightly so.

Your skill set seems to be centered on reading papers and fantasizing about building stuff. You do the OP a disservice when you make wild, impractical suggestions like this. He should buy a calibrated digital multimeter, which has lots of other uses. A couple of good lab-type shunts might be handy too. They can be had on ebay and sent to a local cal lab for certification.

Get real.

John

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Have you tried the method I suggested earlier?

------------------------------------------------------------------------------ John, you said "The right way is to start with resistors/shunts which have a temperature coefficient of resistance which will allow you the precision you require over the temperature range which they will experience."

I have used 0.1% metal film resistors and there was this problem. while I haven't used precise 4 terminal resistors that suggested elsewhere but I think 0.1% metal film resistors should be considered low Tc.

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