Author Topic: 10k precision resistor project  (Read 7777 times)

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Offline MisterDiodes

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Re: 10k precision resistor project
« Reply #25 on: February 25, 2018, 06:44:45 am »
Have you noticed this piece of information from accutrim datasheet: "through the wiper ± 50 ppm/°C"
Yep, I'm still taking the 15ppm because those TCRs are stated for -55 to +125 and I expect them to be much better around 25º and also I've decided to place the trim inside the oil box (trimmable from the outside)
And still, this is the best trim I have right now

Not quite.

The Vishay's will tend to have around 50+ppm TC any time you use the wiper - which is "always" (Otherwise why do you have a pot?).  Their datasheet is somewhat fantasy when you actually measure the pot's TC and long term stability.  If you're at DC, we stick with Bourns WW pots if possible - at least their datasheets are more realistic.  You get a little more stepped adjustment effect on the lower values, but on 1k and up values it's really not that noticeable.

On the upside if you use the pots correctly they contribute very little TC as long as the precision resistor they are trimming has a very overwhelming TC effect on the final trimmed value.
 
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Offline MisterDiodes

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Re: 10k precision resistor project
« Reply #26 on: February 25, 2018, 08:16:57 am »
...Remember that a "Standard" reference resistor doesn't necessarily mean "Low TC".  That's why SR-104's have a temperature compensation curve right on the cover.  L & N standard resistors are actually pretty good thermometers too.  It's up to you to maintain the proper lab temperature for best performance.

What you're talking about in a "standard" resistor is -low- long term drift @ a certain fixed temperature.  Normally you want that close to around 20 ~ 25C, 40C is going to probably cause accelerated drift depending on what resistor material you're using.  Devices with very low long term drift aren't that common in the commercial market.   

A "Precision" resistor might be something that you could trim up and null out on a resistance bridge to match a good resistor (like an SR-104) for a short term transfer;  the word "short" might be up to interpretation - depending on what you really need and at what uncertainty, since this is the metrology section.

A 3458a isn't really the best resistance transfer device - (or for voltage transfer, despite the claims of the HFL version) - for really accurate measures a resistance bridge + good reference resistor + good null meter are your best friends... Even when you're building you're own "precision" resistor, you'll want to know how well it really works, and how fast is it drifting (it will, guaranteed).

Or at least have some buddies with good Ohms-nut equipment :D





 
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Offline Vtile

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Re: 10k precision resistor project
« Reply #27 on: February 25, 2018, 08:52:45 am »
Have you noticed this piece of information from accutrim datasheet: "through the wiper ± 50 ppm/°C"
Yep, I'm still taking the 15ppm because those TCRs are stated for -55 to +125 and I expect them to be much better around 25º and also I've decided to place the trim inside the oil box (trimmable from the outside)
And still, this is the best trim I have right now

Not quite.

The Vishay's will tend to have around 50+ppm TC any time you use the wiper - which is "always" (Otherwise why do you have a pot?).  Their datasheet is somewhat fantasy when you actually measure the pot's TC and long term stability.  If you're at DC, we stick with Bourns WW pots if possible - at least their datasheets are more realistic.  You get a little more stepped adjustment effect on the lower values, but on 1k and up values it's really not that noticeable.

On the upside if you use the pots correctly they contribute very little TC as long as the precision resistor they are trimming has a very overwhelming TC effect on the final trimmed value.
It is nice when someone who knows backs up ones amateur free thinking.  ;D I think the TC of those Vishay resistors are more for "biasing" use case, not as much as trim resistor. Ie. in case you put one of those to output of voltage reference and the current consumption needs to be in 15 ppm and the output voltage of the wiper is buffered.   ... or something on those lines. 

Edit. Removed unrelated late addition.
« Last Edit: February 26, 2018, 03:04:50 am by Vtile »
 

Offline Inverted18650

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Re: 10k precision resistor project
« Reply #28 on: February 25, 2018, 12:11:29 pm »
I recently scored these and will donate a set to any/all of you for testing. If interested, just send an offer for $1 (eBay min) and I will accept( or PM me to set it up). I am currently working on getting my K2000 DMM's to work with LabView and then I plan on trying a few experiments of my own based on forum suggestions. I quickly tested a handful and they seem to be within tolerance. Even at just 5pmm, we may be able to create something cool with a combination of them.

here's the eBay link:  https://www.ebay.com/itm/253445474469 (edit: all lots are gone)

here's my basic test:
« Last Edit: February 26, 2018, 06:16:35 am by Inverted18650 »
 
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Online ArthurDent

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Re: 10k precision resistor project
« Reply #29 on: February 25, 2018, 01:10:11 pm »
I have 2 HP 11104A 10K standard resistors I picked up some years ago that are nice. They are adjustable and quite similar to the one described in the OP. You can read the technical data and see the innards (starting on page 8 ) at:

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-04.pdf
« Last Edit: February 25, 2018, 01:11:57 pm by ArthurDent »
 
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Offline ramon

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Re: 10k precision resistor project
« Reply #30 on: February 25, 2018, 02:57:55 pm »
And please can anyone provide detailed information about the thin material layer too. As I don't believe it is polyester (as stated in the HP journals) due to it's high coefficient of linear expansion. Or does it has some glass reinforcement? When I was looking information about the lowest CTE plastics (without glass reinforce) the only one I found was Kapton (Polyimide). And it's CTE is not that low (20) compared to 14 for Evanohm.

High-spec-(unobtanium) space materials (like Novastrat 905 datasheet) says 0 ppm/k between -120 and 20C. Does anyone has a sheet of this? I can provide 0 ppm/k nikrothal lx (evanohm) wire.
 

Offline ramon

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Re: 10k precision resistor project
« Reply #31 on: February 25, 2018, 03:45:04 pm »
That's correct. Evanohm (-R and -S) datasheet says 7.22 x10^-6 in/F (12.996 x10^-6 ppm/k). And Nikrothal LX says 14 x10^-6 ppm/K (between 20-300C)
 

Online Gyro

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Re: 10k precision resistor project
« Reply #32 on: February 25, 2018, 09:44:13 pm »
...Remember that a "Standard" reference resistor doesn't necessarily mean "Low TC".  That's why SR-104's have a temperature compensation curve right on the cover.  L & N standard resistors are actually pretty good thermometers too.  It's up to you to maintain the proper lab temperature for best performance.

What you're talking about in a "standard" resistor is -low- long term drift @ a certain fixed temperature.  Normally you want that close to around 20 ~ 25C, 40C is going to probably cause accelerated drift depending on what resistor material you're using.  Devices with very low long term drift aren't that common in the commercial market.   

A "Precision" resistor might be something that you could trim up and null out on a resistance bridge to match a good resistor (like an SR-104) for a short term transfer;  the word "short" might be up to interpretation - depending on what you really need and at what uncertainty, since this is the metrology section.

...

Just an off-the-cuff comment... Doesn't that open the option of Copper, at least for some lower value resistors. Granted you need a decent length but it certainly helps with termination and thermal EMFs.

I would have thought that, not being an alloy, it would be very long-term stable and measurable at a specified temperature.

An ovened copper resistor?
« Last Edit: February 25, 2018, 09:48:06 pm by Gyro »
Chris

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Offline Echo88

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Re: 10k precision resistor project
« Reply #33 on: February 25, 2018, 10:35:09 pm »
Copper has a coefficient of 4250ppm/K (according to zlymex), so in order to hold the value stable (<= 1ppm) with temperature regulation you would need an absolutely amazing oven to stabilize the resistor within <0,2mK.

 

Online Gyro

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Re: 10k precision resistor project
« Reply #34 on: February 25, 2018, 10:44:09 pm »
Well, that's maybe a small issue (I didn't realize off-hand that Copper was that high!) :D
Chris

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Offline ramon

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Re: 10k precision resistor project
« Reply #35 on: February 25, 2018, 11:40:21 pm »
If I have a 100k wirewound resistor with -4.25 ppm/K, then I just need a 100 ohms copper resistor with 4250 ppm/K to match 0 TCR.
 
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Offline cellularmitosis

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Online Edwin G. Pettis

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Re: 10k precision resistor project
« Reply #37 on: February 26, 2018, 04:27:08 am »
Actually, copper's TCR is +3900 PPM/°C, ±5% for 25°C to +125°C and the TCR is +3600PPM/°C. ±5% for +25°C to -55°C, obviously there is no abrupt change in TCR at the 25°C reference point.  One problem with using copper as a compensation resistor is the end product is very non-linear in overall TCR, at and near the reference temperature it will produce a fairly close approximation of a nearly flat TCR curve but it will be very much hyperbolic like Manganin.  So if you're expecting a nice linear compensation curve, it isn't going to happen.  You may get a fairly low TCR over a few degrees but if you're expecting zero TCR, you've got great expectations and little more.

The idea of using copper for compensation is old, been around for decades and in certain circumstances it provides a neat solution but not usually for high precision applications where the temperature is going to be moving around some and you're looking for very low TCR.

As I've indicated before, there are no free lunches, there is always a trade off that has to be made, no way around it.  Don't put too much weight on simulation, those are only approximate at best when it comes to resistance and many other components as well.  I'm not trying to throw water on ideas here but I'm talking from many years of experience, I'm not talking into my hat.
 
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Offline MasterTech

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Re: 10k precision resistor project
« Reply #38 on: March 05, 2018, 01:48:02 am »
Let’s go with an update of the project. Eventually I went with a slightly different approach and components. This is the final schematic:



Resistor enclosure
The container is made of copper sheet and the sides will be terminated with precut PCBs. Tin solder is used to avoid leaks. Eventually the enclosure will be covered with a thermal insulation sheet.



Heating element and temperature sensing.
I decided that I’d need a heated device cause I can’t control very well the temp everywhere at my lab over the year and because I’m more interested in short term accuracy rather than long term drift. At the lower end of the oil tank I’ve placed an aluminum housed power resistor and a RTD sensor at the top. The RTD sensor is a PPG102A1. The resistor terminals cross the copper wall with a couple of Tusonix feed throughs model 4101-002, small but rated for 10A. Still haven’t decided the RTD amplifier to use. The heating resistor and the 10k adjustable resistor will be separated by a metallic mesh for EM shielding.



Series adjusting element.
Eventually I went with the Vishay acccutrim trim for its smooth travel, was tempted to use a bourns wire wound though. The 1ohm resistors are wire wound from Welwyn AS21R00FTR. The trim needs an internal support so I made that with my mill in bakelite. The adjusting shaft was made in the lathe with aluminum rod and features 2 o-rings. This shaft is contained in a methacrylate cylindrical shell.



The adjustment range is as follows:

this design: 0.416 ohms
HP 11104A: claims +-30ppm, so 0.6ohms
SRX-10k: 0.666

Main resistor
I chose here a well trusted and very stable wire wound from the HSP series of Ohmite, model HS520A10K00B and manufactured 1 year ago. Internally it is made of 6 sections similar to other resistors seen on this forum (standard resistors thread).



This is a 0.1% resistor so depending on the measured value I’ll have to select an appropriate ‘compensation’ resistor. With the 3458A working at a stable TEMP? 34.5º, the resistor at 19º and after proper configuration (ACAL, OHMF, APER 1, DELAY 1, OCOMP ON, NDIG 8, it reads a very stable 9999.55. After heating the resistor at 40º (planned heating temp) it goes down to around 9998.4, so this equals about 6ppm/C for this range. After cooling of the resistor and another ACAL the original value is read back again although it takes nearly an hour to do so.


Adjusting resistor
Given the value obtained after heating the 10k resistor I need to use a 1ohm series resistor, same type as before.


To be continued...
« Last Edit: March 05, 2018, 04:45:25 am by MasterTech »
 
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Offline amspire

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Re: 10k precision resistor project
« Reply #39 on: March 05, 2018, 02:21:13 am »
This is a 0.1% resistor so depending on the measured value I’ll have to select an appropriate ‘compensation’ resistor. With the 3458A working at a stable TEMP? 34.5º, the resistor at 19º and after proper configuration (ACAL, OHMF, APER 1, DELAY 1, OCOMP ON, NDIG 8, it reads a very stable 9999.55. After heating the resistor at 40º (planned heating temp) it goes down to around 9998.4, so this equals about 0.06ppm/C for this range.
Don't you mean 6ppm/C ?
 

Offline SirAlucard

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Re: 10k precision resistor project
« Reply #40 on: March 05, 2018, 04:07:03 am »
So what would you do with such a highly accurate resistor?
 

Online Edwin G. Pettis

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Re: 10k precision resistor project
« Reply #41 on: March 05, 2018, 04:28:08 am »
MasterTech,

Your TCR calculation is way off, using your measurements and a 21°C temperature variance, that comes out to 5.476PPM/°C not .06PPM/°C.  Not a bad TCR for an Ohmite resistor.
 

Offline MasterTech

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Re: 10k precision resistor project
« Reply #42 on: March 05, 2018, 04:40:52 am »
True. Should not be working on sundays... That makes me think because that resistor is 3ppm from -10 to 80deg. Will have to revise the setup
 

Online Gyro

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Re: 10k precision resistor project
« Reply #43 on: March 05, 2018, 04:46:44 am »
Good idea on the sealed copper shield, if you want to avoid indeterminate metallic junctions, you can get hollow solderable hermetic feedthroughs too... https://www.ebay.co.uk/sch/i.html?_odkw=glass+hermetic&LH_PrefLoc=2&_sop=10&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xglass+hermetic+feedthrough.TRS0&_nkw=glass+hermetic+feedthrough&_sacat=0

Chris

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Offline BNElecEng

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Re: 10k precision resistor project
« Reply #44 on: March 05, 2018, 05:29:54 am »
Hi everyone, pardon the intrusion. I've been following this topic with interest. Since you mention hermetic feedthroughs, does that imply that a vacuum would be useful or is it purely to stop airflow around the resistor?

Thanks in advance
 

Offline MasterTech

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Re: 10k precision resistor project
« Reply #45 on: March 05, 2018, 05:31:40 am »
Hi,

because the can will be filled with oil (Shell Diala), that's why the adjusting shaft for the trim has o-rings also.
 

Offline BNElecEng

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Re: 10k precision resistor project
« Reply #46 on: March 05, 2018, 06:34:25 am »
Got it, thanks. I missed the part about it being filled with oil.
 

Online Edwin G. Pettis

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Re: 10k precision resistor project
« Reply #47 on: March 05, 2018, 06:42:28 am »
By the way, that resistor is actually made by Ultronix and labeled Ohmite.....Ohmite bought Ultronix from Vishay a tad over 10 years ago.  Ohmite was strickly a power resistor house before then.  They wanted the rheostat manufacturer that Vishay was wanting to sell and had to take Ultronix as part of the bargain.
 
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Offline MisterDiodes

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Re: 10k precision resistor project
« Reply #48 on: March 06, 2018, 04:09:11 am »
MasterTech,
Just a suggestion:  Watch your current flow on that pot wiper, especially at max rated power dissipation of your box (max voltage input).  You don't want a dry wiper contact, nor do you want over around 200uA.  Otherwise you'll lose stability at that wiper, no matter what pot you use.  "Some" wiper current flow as some 10's uA is always desired during operation, and just keep an eye on that when you turn the pot to its max/min stops.

As a rule of thumb, that resistor just to the right of your pot would normally be -at least- 10X the pot value, or around 100 ohms as a bare minimum value - but that depends on the trim adjustment range you think you need for the resistors you have on hand. 

In other words - keep the adjustment range as small as possible.

« Last Edit: March 06, 2018, 04:11:39 am by MisterDiodes »
 
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Offline MasterTech

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Re: 10k precision resistor project
« Reply #49 on: March 06, 2018, 05:08:15 am »
True, I also realized that by using those values Im pushing things a little bit, however for the intended use it should be fine. But maybe I should consider changing those 1s to 5s or 10s at least...
« Last Edit: March 06, 2018, 06:08:31 am by MasterTech »
 


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