Precision Resistors - General Thread

[MegaVolt]

What is the purpose of the two contacts are far behind the housing? Why are they so far?

[TiN]

C1 and C2 are current terminals for Kelvin connection?.

[leighcorrigall]

Why are they so far?

Although distance matters, I think that the standoffs are made of solid copper. The unusual feature may have been strategically placed as part of a grand design.

[TiN]

They are solid copper bars, yes. Typically standards of such designs with horns designed to support low-contact resistance mercury-wetted stands for various metrology applications.

This is not common today, but many of standards designed back then are working great today too, so no reason for change. Also it is convenient to use when you have some room for fingers to get those copper forks extra tight, so no ppb have chance to escape

[guenthert]

Why are they so far?

Although distance matters [..]

      In which way?

[leighcorrigall]

Why are they so far?

Although distance matters [..]

      In which way?

For instance, the temperature difference at the 4 junctions. Copper is a great thermal conductor so the effects are lower.

EDIT:
https://www.eevblog.com/forum/projects/4-wire-resistance-and-the-seebeck-effect/

[Kleinstein]

The thermal EMF is known problem with precision resistors. It was a major point going from konstantan to manganin.
For the modern low TC materials the thermal EMF relative to copper is an important point.
It not only effects the large standard resistors, but also more normal ones in low voltage circuits. As an example I see quite some thermal EMF with temperature gradients at PTF56 resistors.

The standard resistors are usually used at a relatively low power and in a isothermal environment.  I would consider the shape with the protruding connections also a practical solution for holding the resistors in a oil bath.

The thermal EMF effect seen in the video is likely to a large part from the clips at the test cables. The problem is a temperature gradient in non copper parts only.

[MegaVolt]

I would consider the shape with the protruding connections also a practical solution for holding the resistors in a oil bath.

This is a good version. Need to check whether all resistors with long contacts are designed to work in the bathroom with butter.

[alm]

They are solid copper bars, yes. Typically standards of such designs with horns designed to support low-contact resistance mercury-wetted stands for various metrology applications.

This is not common today, but many of standards designed back then are working great today too, so no reason for change. Also it is convenient to use when you have some room for fingers to get those copper forks extra tight, so no ppb have chance to escape

Except that in this case the bottom of the "horns" are insulated, and I don't think this insulation is designed to be removable.

Another reason I can imagine is easy series configuration as voltage divider or to compare two resistors with an equal current flowing through them, like how the Fluke 5450A is adjusted to an external standard resistor.

[Conrad Hoffman]

Wow, that HP divider just boggles the mind!

[miro123]

SR104 forward, JAN.10.2022

10000.00128 Ω ±0.13 ppm

0.316 mA

G9330-1k

3.16 mA

20 s

1000.03477 Ω

±0.03

More details and internal design of Guildline 9330's also available on my review/teardown article.

Thanks for sharing. As novice to this topic I  have few stupid questions. I have checked your site but I did not find the answers - https://xdevs.com/article/tcr_test/
 - How does it look like your setup?
 - What is the meaning of following parameters? - Rx, Ix, Rs, Is and   "Reversal time"

Thanks in advance

[TiN]

Nothing stupid in questions, we all started somewhere 

Block diagram of setup used is essentially same as this. Bridge is sourcing Ix current into connected resistor and then matching current Is multiplied by ratio sourced into second resistor. This is done using internal transformers and nanovolt detectors, and by keeping ratio very stable this setup essentially allow to calculate unknown resistance value from known calibrated resistance standard to <0.03ppm resolution. Also since currents are compared using switchable transformer windings, there is no resistance or voltage reference drift or temperature coefficients unlike in any DMM with resistance function.

This is typical setup for many top-level calibration labs and NMIs, outperformed only by CCC and QHR systems. Currents can be programmed to any value from <1mA to up to 250mA (or thousands amperes with high-power extenders for purposes of high-power shunt calibrations).



On block diagram you see Rx and Rref, on my earlier post Rs is considered Rref, same thing, different wording.
To cancel parasitic thermal EMFs (we talking single nanovolts importance to obtain such high resolution) bridge has reversal time that sets period of current polarity reversals. Smaller resistors work with 6-10 seconds reversal, while larger like 1k-10kOhm require bit longer times.

Hopefully this helps.

[TimFox]

In that beautifully-drawn diagram, does "Ground" on the DCC Bridge connect to anything, or is it internally connected to the PE of the power cord?
I'm well aware of the difference between Ground and Guard, but am often frustrated that TME often does not have a front-panel ground/chassis/panel connection to facilitate shielding (braid over test wire bundles) the wires between units.

[TiN]

In my setup ground is connected to PE and internally to digital/analog grounds. Typical resistance standards do not have mains power, so they are true floating devices and there is no need for additional grounding to standards here. Different story for high resistance stuff, but DCC bridges typically operate only up to 10-100kOhm max. High resistance story is for another thread.

[alm]

Wow, that HP divider just boggles the mind!

Indeed! It's from the 1966-04 issue of HP Journal. They were trying to sell the HP 11102A/11103A/11104A/11105A standard resistor that had as distinguishing feature that it had a small adjustment range so you could closely match them. A voltage divider is a possible application, though this looks very expensive. The resistors were only in the catalog for one year. I'm guessing they either didn't sell well or didn't perform as well as they hoped.

[1audio]

I bought into the idea of using resistance wire to trim the resistors. And bought a roll of Evanohm from eBay. Now that I have it I'm looking at #38 awg bare wire. . . that is 800 Ohms per foot. Now to figure out how to work with this stuff. Any suggestions?

[alm]

Termination will probably be your main challenge. From what I've read, the only reliable way is to weld it to a compatible material (not copper). This has been discussed on this forum a couple of times. Especially pay attention to the posts by Edwin G. Pettis, who has many years experience working with Evanohm. This thread contains some useful info: https://www.eevblog.com/forum/metrology/manufacturing-custom-high-precision-low-tc-resistors/

[1audio]

It seems I'll be looking for something else for this project. The Evenohm has too much overhead to be useable. Is anyone interested in a full still packaged roll of EvenOhm thats marked =/- 5 ppm TCR?

Edit- I just checked and I have 12 100 Ohm 15ppm TCR Vishay foil trimmers. I should have checked before buying the wire. The trimmers will be the easy solution. I'll make up a little PCB for all the connections.

[ramon]

Is anyone interested in a full still packaged roll of EvenOhm thats marked =/- 5 ppm TCR?

wire diameter?

[1audio]

It says .0095  I think that's 9.5 thousandths of an inch. Its very fine wire. Pay for the shipping and its yours. PM me.

[miro123]

I have few general questions.
1. What are the pre-condition before starting resistor performance test. For example – X % humidity, T=Y C  for at least Z days/months/years
2. Is baking of Vishay precision resistors a good practice? If yes what is the temperature profile?
3. Should I ask for it the Vishay application engineers or somebody already asked?


I’m asking those questions because I’ve ordered 50pcs of Vishay RC02 resistors

[leighcorrigall]

I have few general questions.
1. What are the pre-condition before starting resistor performance test. For example – X % humidity, T=Y C  for at least Z days/months/years
2. Is baking of Vishay precision resistors a good practice? If yes what is the temperature profile?
3. Should I ask for it the Vishay application engineers or somebody already asked?


I’m asking those questions because I’ve ordered 50pcs of Vishay RC02 resistors

Hi Mirco123.

What is your intended use for these resistors? Vishay resistors are excellent, well characterized, and their specifications are met or exceeded. Why not purchase resistors that are already made for your intended design? I suggest Z foil for long-term stability.

Do you mean Vishay's Post Manufacturing Operations (PMO)? If so, what do you expect the PMO to do? If you cannot answer the question, you may want to reconsider asking Vishay to perform additional operations.

[ramon]

It says .0095  I think that's 9.5 thousandths of an inch. Its very fine wire. Pay for the shipping and its yours. PM me.

Thank you, but that wire is waaaay too thin. I learned that on my first spool.

I will pass for the 2nd time, as in fact I already saw that spool on summer last year and was tempted to buy it. But was somewhat able to control my TEA * WSA syndrome. So I only bought him the spools with the diameters (or materials) that I didn't have before. Like a PDAG spool, Nickel (Alloy 200), Constatan (Advance), and a Nickel spool with TCR 5000 for making 5000 PPM/K RTD (not on that picture, that one is in another box).

That seller got the spools from a closed Clarostat manufacturer facility, and there are many nice and special alloys, he even got some tooling for resistor making. The only wire material that he lacks is Manganin but I already got that covered from Europe source (thanks to a noble man who was a precision resistor maker - many years ago - in Italy).


*Wire Spool Anonymous

[miro123]

Do you have history of resistors, e.g. measured value, Alpha, Beta from the past?

[bsw_m]

Teardown of a МРХ wirewound precision resistors.
From up to down, resistive elements from:
1. МРХ 100 MegaOhm 0.05% tolerance.
2. МРХ 100 MegaOhm 0.1% tolerance.
3. МРХ 300 MegaOhm 0.05% tolerance.

The second picture shows the resistors themselves in the same order. For comparison, there is a 100M 1% resistor manufactured by TE: https://www.te.com/usa-en/product-3-1625959-3.html

P.S. these resistors are very stable.
After 30 years from the date of manufacture, all fit into the tolerance with a margin, many with a 2-fold margin.