Electronics > Metrology

New Precision Resistors

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A while back I ordered (4) precision resistors from a member here Mr. Pettis, 100R0, 1K, 10K and 100K, all at 10PPM.  These took a while to get, but he was upfront about that when I ordered and his communication was great!  I'm very pleased.

Anyhow, goal is basically a standard, age tracking, general fun.  These will be installed in a sealed metal enclosure, Pomona Microvolt Banana jacks, PT100 and some kind of small resistive heater.

I've been on vacation so no initial measurements till the DMM7510 warms up for a day or two.  Picture of how they arrived, not sure why only the 100R0 was labeled..

Big thanks to Mr. Pettis.   :-+

About a year ago I did the same as you with the same intent. Except for the 100-100k I also ordered a 1Mohm (+ LTZ resistors). All of them arrived quite soon with measured values. The problem was just that my measured values was not the same. Also the 1Mohm that was about 100ppm high in my measurement soon got open circuit after just a few tests with kelvin clips. The others was also high and seemed to get worse over time. I quite soon suspected humidity problems. But the answer I got was that it was impossible and it must be my measurements (skill). After a while a I got a mail that others also have seen this and the problem was the epoxy. A cure would be to bake the resistors. As I have ordered two 10kohms I baked one and it went down to almost the same as delivered but soon started to go up again slowly. As can be seen in the attached graph (scale is ppm) the time constant is months so a long test is needed to see this. My resistors is also without markings.

I have been promised to get a replacement but so far have seen nothing.

So for me these resistors are in the same league as the 2ppm volt refs on Ebay.

I really hope you have better luck and see forward for your results.


About a year ago I ordered 26 resistors mainly for LTZ board. All of them has been measured for tempco. Tempco were exceeded on 5 pieces (10k and 100k ones). Replacement is going to be shipped soon.


--- Quote from: lars on October 20, 2016, 08:45:36 pm ---I really hope you have better luck and see forward for your results.

--- End quote ---


Well, I won't start tracking until in the enclosure, but tomorrow we'll be able to eval the as shipped against my 7510 and 2010, both caled in the past 2 years.

Edwin G. Pettis:
Yes, as I have told these gentlemen, the epoxy I had acquired was supposed to work quite well, it didn't and its use was very short lived.  Replacements have been shipped at my expense for the resistors they said were not up to my standards.  Yes I also agree that it took longer than I would have liked to find a more workable solution, it is difficult to find and test epoxies which consumes a lot of time on my part.  Essentially, all plastic (epoxy type) resistors absorb water molecules at some rate, this rate is often specified under a specific condition which typically shows very small percentages of absorption, in reality the absorption rate is much higher than the data sheet reveals, hence trials must be done to determine how suitable a given compound is.  In the case of these resistors, the epoxy exhibited no significant water absorption over the trial period under 'normal' use conditions.  Suffice it to say that under conditions which are not 'normal' here, the epoxy showed a real good sponge effect which essentially applied stress to the windings, hence the apparent drift, this is NOT a change in TCR, stress can change the resistance measured and when that stress is removed, the resistance returns to original value.

Since this unfortunate episode, I have taken steps to eliminate such problems, previous resistors that I manufactured before this epoxy was used did not exhibit this particular problem which I was able to replicate.  I have made many resistors since I removed this goofy epoxy and none exhibit the humidity problem.

I might add that if humidity is a problem with components, including some plastic ICs, keeping the circuits energised 24/7 will minimize or eliminate this problem, also if humidity is generally high most of the time, I would suggest coating the circuit board and components with paralene which is fairly common and relatively cheap, this will stop any humidity from getting into the wrong places and causing problems.


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