Resistor Grading - Page 1
Forum section: electronics / general

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2025, 04:47:52 pm
content: What kind of instrument should I be looking for to grade batches of precision resistors into matched groups?My highest resolution ohmmeters are only 4.5 digits which is marginal for 0.1% resistors. Alternatively I was thinking of designing a circuit to make resistance comparisons to higher resolution.

author: TimFox
author_rank: Super Contributor
timestamp: May 28, 2025, 04:53:01 pm
content: A Wheatstone bridge with fixed resistors and one leg adjustable can be set up easily to match resistors.If you need absolute accuracy, you need a couple of previously-calibrated standard resistors.

author: Gyro
author_rank: Super Contributor
timestamp: May 28, 2025, 07:59:03 pm
content: Agreed, definitely a case for roll your own Wheatstone bridge.Better than trimming one leg is to use two thermally coupled nominally equal (as close as you can get with your DMM) resistors with a low value wirewound pot between them, taking the output from the wiper.If you visualize the bridge as a pair of potential dividers, one the 'reference' divider, made up of the two nominally matched resistors and pot, and the other made up of the two resistors to be compared. Both driven from the same energizing source with the meter on lowest mV range between pot wiper and junction between resistor pair to be matched.Set the pot to the middle of its rotation, note to output of the bridge. Now reverse the energizer connections to the reference divider and note the reading. The resulting difference is a combination of the mismatch in the resistors under comparison and mismatch in the balance of the reference divider. It's then a matter of repeatedly reversing the connections to the reference slowly tweaking the pot position until you obtain the same mV reading (not zero) in both forward and reverse connection.What you have now achieved (at least in the short term) is an accurate 2:1 divider, without the need for calibrated standard resistors. Any remaining offset between the two dividers is then entirely due to mismatch between the resistors you are comparing. Obviously the whole thing becomes a lot easier if you put effort into construction, including a reversing switch, terminals etc.I took this approach to ridiculous, but successful, lengths when building my Hamon divider, by using a pair of 50k Vishay foil resistors and a 5R 10 turn pot, monitoring the output with a 1uV resolution meter, giving a theoretical 5ppm/turn. I have since added a reversing switch...https://www.eevblog.com/forum/metrology/anyone-else-built-a-hamon-divider/

author: Kosmic
author_rank: Super Contributor
timestamp: May 28, 2025, 10:53:44 pm
content: Hioki has some resistance meter (ex RM3545, 0.006% basic accuracy) where you can set your ref value and accepted deviation in absolute value or %. Just need to measure the resistor and you have normally a dedicated indicator on the front panel (Comp Hi-In-Lo in the video below).

author: trobbins
author_rank: Frequent Contributor
timestamp: May 29, 2025, 12:22:10 am
content: Along the lines of Gyro's suggestion, you may find Conrad Hoffman's articles on a Mini Metrology Lab informative for resistor grading - as it is discussed in the first and third series.https://www.conradhoffman.com/mini_metro_lab.html

author: David Hess
author_rank: Super Contributor
timestamp: May 29, 2025, 03:42:03 am
content: These suggestions are all helpful.I am doing more than strictly matching pairs of resistors; I am grading the resistors into groups of close values, as shown below. The absolute value of the resistors in each group is not important.The problem as I see it is that I am using the ohmmeters that I have to their last digit of precision, which seems questionable. The simple solution may be to get a better ohmmeter with higher resolution, and presumably stability. I was thinking of designing a circuit to make the equivalent to an expanded scale ohmmeter to get better resolution and stability.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 29, 2025, 06:33:34 am
content: I suggest that you are aiming to do pretty much exactly as per Conrad's method of sorting batches of 100x 1% resistors for use in his Kelvin-Varley 6-decade divider project.Strangely I have just embarked on such a divider, and will end up sorting 200x 10kohm, 100x 402ohm, and 100x 18ohm - all basic 1% tolerance and 50ppm spec. I'm aiming to use 5x parallel resistors to obtain the steps of the next adjacent decade, so only need to purchase 3x values. I'll initially sort with a 6-digit DVM, and then fine tune each decade set with a Wheatstone bridge (although luckily I have a nice ESI 230B on loan to help). But yes, this is a labour of love to sort and match, and has the caveat emptor of using basic metal film rather than high spec resistors.

author: Gyro
author_rank: Super Contributor
timestamp: May 29, 2025, 08:19:50 am
content: Quote from: David Hess on May 29, 2025, 03:42:03 amI'm a little bit puzzled by you not wanting to know absolute values (comparing what you say against your image of the drawer fronts), however that does make things way easier.Remember that a Wheatstone bridge doesn't have to be used at null balance (exactly matching two resistors), it provides equally useful information when unbalanced. Again, visualize it as a pair of potential dividers, one perfect 2:1 (1:1 resistor values), as I described previously and one 'imperfect' made up of samples of your batch of resistors. From the offset voltage produced by each pair, and knowing the supply voltage, you can fairly accurately determine their relative values and group them.Start with the lowest value, as indicated by your DMM, and then use the bridge circuit to compare this with the other resistors, choose the granularity that you wish in terms of what defines a group.It you have a 4 1/2 digit meter, then hopefully it has a [Edit: mV range] least significant digit of 10uV, hopefully good enough. You didn't mention the value of the batches of resistors. The higher the excitation voltage to the bridge, the higher the resolution you will achieve but this needs to be done with a careful eye on their dissipation (unless you are interested in the direction of their TC too.Sorry if I am misunderstanding you, but this does seem a more sensitive method of grouping similar value resistors.

author: zlymex
author_rank: Frequent Contributor
timestamp: May 29, 2025, 10:29:27 am
content: Matching for value is the basic, matching for TCR is the next step, and more advanced matching is the long term stability.Fluke used to built a lot of 732As to test and match their resistor pair R7/R8 for long term stability.

author: bdunham7
author_rank: Super Contributor
timestamp: May 29, 2025, 01:59:06 pm
content: Quote from: David Hess on May 28, 2025, 04:47:52 pmPerhaps I'm unimaginative, but my method would be a better DMM. IDK what the market is like nowadays, but once upon a time you could get a decent condition Fluke 8842A for <$200 and that gives you on the order of 100ppm absolute accuracy and even better transfer/comparison precision. This just seems much quicker and more convenient albeit not a clever as bridge or divider solutions. For example, the 49k resistors you are showing would be measured to five full digits, 49.xxx with an absolute error of no more than 5-6 counts with stability and comparison precision of 1-2 counts at most.Granted, a used model from wherever may not have guaranteed absolute calibration, but that doesn't change much in your case. And extensive experience shows that the absolute calibration on these will likely be very good even on very old instruments.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2025, 01:01:04 am
content: Quote from: Gyro on May 29, 2025, 08:19:50 amI am matching 100s of resistors for identical values for use in dividers, so the absolute value is not important.Quote from: bdunham7 on May 29, 2025, 01:59:06 pmA better multimeter is probably the best solution, but I thought there might be a better alternative that I was missing.One concern that I have is that after sorting the first batch, later measurements do not return the same values, but the resistors still match, so I suspect my multimeters drifted that much.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 30, 2025, 02:43:20 am
content: Or the tempco of resistors, or meter, was an influence ? Do the resistors have a tempco spec?

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2025, 03:15:37 am
content: Quote from: trobbins on May 30, 2025, 02:43:20 am25ppm/C I believe, but I suspect it was the multimeter. The change was not great, but it was several counts at the resolution I was grading them into.

author: Hedrix
author_rank: Contributor
timestamp: May 30, 2025, 08:51:12 am
content: Could be a multitude of reasons.Could be thermal EMFs generated by the test setup, depending on lead materials and whatever voltage/current range the Multimeter uses.Also for non-hermetic resistors, Humidity tends to have a large-ish influence.Getting a better multimeter would be my suggestion here (preferably one that has a correction function for thermal EMFs, though that is somewhat rare below 7.5 digits i think).If the only thing that matters is relative matching, then tempco also is / can be an important parameter (depending on your requirements for matching), although tedious to measure unless you are willing to invest/build an automated setup. Resistors from one batch should, but are not guaranteed to, behave the same over temperature. The cheapest option would be to stabilize (or at least measure) the Temperature of your test setup so that you are measuring all resistors under identical conditions.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 30, 2025, 12:22:47 pm
content: Quote from: David Hess on May 30, 2025, 01:01:04 amThat's a usual thing. A short term drift. It happens with all the DMM measurements (especially R, C, L).If you had 6-8 digit DMM you still have a drift in last digit.

author: guenthert
author_rank: Frequent Contributor
timestamp: May 30, 2025, 01:18:25 pm
content: Quote from: Hedrix on May 30, 2025, 08:51:12 am-hp- 3456A, 3457A, 34410A (but curiously not 34401A afaict) do offer offset compensation (and the first two tend to be available on the secondary market in the USA at IMHO reasonable prices). That might be an investment which pays off eventually, but if the task is just to match resistors, then I'd think the Wheatstone (or for better precision with low value resistors, the Kelvin) bridge would be the obvious choice. If a lock-in amplifier and low-frequency sine generator & amplifier are available, that could be used to achieve pretty good precision while bypassing the thermal EMF concerns.

author: tooki
author_rank: Super Contributor
timestamp: May 30, 2025, 01:34:39 pm
content: Quote from: David Hess on May 28, 2025, 04:47:52 pmSeems to me like a great opportunity to be “forced” to buy a nice bench multimeter!Remember that good ones, like halfway recent Keithley and models, are not only super precise, but super fast. You can measure resistance with them faster than your eyes can read the values. There’s basically no settling time — the meter goes as fast as you do. It’s not like the 1/5…1 second a handheld multimeter might need to range and settle. This could be a substantial time savings if you have a lot of resistors to sort.On the Keithley DMM6500, you can also write your own GUI apps for it, such that you could theoretically write one that tells you right away which bin a resistor goes in. (And I know it has a high/low limit alarm you can set without any extra or custom software.)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 31, 2025, 07:42:19 pm
content: Quote from: David Hess on May 30, 2025, 01:01:04 amHave you read the mini metrology lab article from Conrad Hoffman linked to above?It really is a very good article about making highly accurate comparisons with moderate quality equipment. It's also relevant regardless of the quality of your equipment. You can get more significant digits, but the principles to get results that exceed the limits of your measurement equipment are the same.

author: Smokey
author_rank: Super Contributor
timestamp: May 31, 2025, 08:03:59 pm
content: Get a meter with a computer connection (GPIB, RS232, USB, Ethernet, whatever). Write some scripts to help sort all that stuff. Then you can record the data in a way that you can analyze later too.

author: Harfner
author_rank: Contributor
timestamp: June 07, 2025, 10:44:18 am
content: Quote from: trobbins on May 29, 2025, 06:33:34 amWouldn't it be better to use 5x serial in the first decade and simple Rs in the second?The first decade is 10 times more critical. Building that out of several resistors gives more flexibilty to achieve equal values and distributes power (= heat) better. It also would probably even out TC statistically.If you can live with shunting the second decade, make it 2 serial in the first and 2 parallel in the second decade. You would need 44 instead of 66 good resistors.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 09, 2025, 06:46:06 am
content: Harfner, I found it a benefit to start with 200x 10k on a bandolier. They were MFR25S, and I was very happy to see them closely distributed in absolute value, but more luckily to have a tempco that appeared to be <5ppm/C in the 20-27C range (from a sample of about 16 units), so that gave me confidence to soldier on down into the diy ppm weeds.I was aiming for 100V input across a decade, so settled on 3x // and 3x series, to keep 1st decade operating dissipation at just a few % of rated. I agree that doing it all again I'd likely do your proposed 5x series for each first decade step - and use a 0.5W rated part (as I mistakenly chose the smaller version of 0.25W part), and cross my fingers that any such batch had similarly low tempco. Perhaps it is just modern manufactured parts from the same batch - but I very happy with the tempco !Sorting was helped by having a HP 34401A and HPIB interface and logging software to quickly indicate resistance for 1 ohm binning. Yes it was tedious, but I've ended up with a first decade Kelvin-Varley divider with <50ppm span from average. I have just finished the 2nd decade of 5x // for the 2k steps, which I will pad each step with circa 1ohm to bring steps <50ppm span, and the total decade to same 20k as first decade double step (or just a titch over 20k so I can final trim to 20k).The HP 34401A values correlate well with a wheatstone bridged assessment, although I'll set up a temp controlled and screened bench top environment to try and get bridge comparisons <50ppm between 1st decade steps and with the 2nd decade. My bridge uses an Aneng AN8009 detector with 10x gain AZ opamp preamp to get to <1uV floor, which is fine so far to easily discriminate a difference, and opposite polarity switching is giving good balance, so with a 12V battery the test jig has no mains connections.It can be easy to chase one's tail when trying to make diy ppm level tweaks, given slight temperature changes of parts and leads, especially if a tweaking resistor has been recently soldered into place. I recall a comment about vintage tweaking of manganin parts requiring overnight to let them relax.

author: TUMEMBER
author_rank: Regular Contributor
timestamp: June 09, 2025, 10:34:32 am
content: Quote from: David Hess on May 30, 2025, 01:01:04 amIf you want to skip the whole layer of learning programming, creating "tools to create a tool" (Wheatson bridges) then use this Keithley 2700 meter, which already has the function you are looking for built in. You don't have to buy a model with scanner cards, a "bare" DMM will do. I don't know about the 2000 model (check the documentation), whether it has this function and it would probably be cheaper, but with "availability of this function may vary" they didn't design different models so that you as a buyer would have everything. Corporations have to make money on everything.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 09, 2025, 12:22:37 pm
content: Quote from: David Hess on May 30, 2025, 01:01:04 amDefinitely a better multimeter.A good, used 34401A is cheap these days and will give you 6 1/2 digits.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 11, 2025, 04:08:36 am
content: Quote from: David Hess on May 28, 2025, 04:47:52 pmIf you just need matching, and not absolute precision, I'd suggest a variant of a bridge that is a SPDT analog switch.You switch that into a shared load resistor, at some AC compatible rate, and the AC signal is the miss-match.The DC supply and load resistor do not need to be high precision.You could add a DPDT mechanical switch to swap sides, in case you are worried about the Ron arm miss-match of the analog switch giving an offset.An amplifier / low pass on the switch output could make this more tolerant of AC-lead pickup. 0.5mV AC on a 5V load, is 0.01%

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2025, 04:58:08 am
content: Quote from: PCB.Wiz on June 11, 2025, 04:08:36 amBased on the first lot of 100 resistors, I am grading the resistors into 7 bins and the largest group has 26 matched resistors.I should test how well my ESI impedance bridge would work at this level of precision. It might be more stable than my multimeters if not more accurate.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2025, 05:01:30 am
content: Quote from: HighVoltage on June 09, 2025, 12:22:37 pmThey do look like a good deal, especially since a full service manual is available. I was thinking that an older model would be easier to service.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 11, 2025, 06:24:18 am
content: I am a bit curious about the distribution of these resistors.In the old days it was common for 5% resistors, to always have a tolerance that is greater then 1%, because all <1% resistors were binned by the factory and sold as 1% resistors. But as far as I know that is not done anymore because production processes have improved to the point that they just manufacture what they need, but maybe <0.1% is still sorted this way.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 11, 2025, 08:57:50 am
content: It looks to me like quality control of processes, on a manufacturing batch or session basis, provides quite a tight distribution around some varying mean. Sort of akin to the control process making a slight adjustment after some quanta of parts made. And that would infer that when a batch or session has resulted in say 0.5% then the whole batch is then 'binned', rather than individual parts. The span of values I hand measured was 92% within +/-0.25% for 100x 402R 1% bandolier, and 96% within +/-0.18% for 200x 10k 1% bandolier. That relates to part value variation and not an absolute value or tolerance spec, but seems reasonable for binning say parts to a +/-0.5% tolerance, when the same part has 1%, 2% and 5% spec rating options.My initial effort on tempco similarly indicated that a statistically significant sample of tempco measurements on such parts then likely holds for all the parts on a bandolier - ie. a close distribution around a mean, rather than a scatter of pos and neg tempcos over the full range of the tempco spec. I haven't done a sampling of tempco of the 402R parts, but they seem to be well within their +/-50ppm/C spec, albeit noticeably higher than observed for the 10k parts I just got.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 11, 2025, 09:08:54 am
content: For finding matching parts it can make sense to record the actual values and than only sort out the best matching parts afterwards.For really accurate measurements one may have to wait a few 10 seconds or so for the temperature after handling stabilizeses. So the measurement may take some time.An impedance bridge may well have more resolution / stability than a 4.5 digit meter in ohms mode. It can also depend on how the values fits to the ranges.Quite often resistors from the same batch already match quite a bit better than there nominal tolerance.There are different way how resistor values are made / checked. Especially tighter tolerance tend to be individually trimmed (e.g. via laser) and the time spend there sets the accuracy / tolerace class. Lower grades may be produced in batches and only sample checked for adjusting the process for the next batch. Here batches further out may end up as high tolerance (or different value) parts.

author: EC8010
author_rank: Frequent Contributor
timestamp: June 11, 2025, 11:50:00 am
content: Quote from: Doctorandus_P on June 11, 2025, 06:24:18 amI can answer that one. I measured one hundred 1M 0.1% metal film resistors and saw a Gaussian distribution - centre values were not missing. What I also found was that standard deviation was 0.032%; they were defining their tolerance at 3 sigma. But they did not define what their tolerance meant. In fact, barring a comment I saw many decades ago in a Mullard book, nobody defines what their tolerance means.Measuring VTO for Supertex DN2540N5 depletion MOSFET, the centre valuesaremissing.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 11, 2025, 12:40:46 pm
content: Quote from: Kleinstein on June 11, 2025, 09:08:54 amI use fabric gloves and try not to touch resistors, only probes (it still transfers some heat if you hold these continuously, so there is a noticeable drift for a few seconds for resistors with larger TC). For resistor measurements I find my oldest meter - HP3456A - is the best, even the Keysight 34465A is not quite as convenient. 1ppm resolution is sufficient for all but the very best resistors measurements and Offset Compensated Ohms are useful so for this application I would recommend the HP3456A over 34401A, for example. On the photo is how I grade resistors on tape, in this case the RN55C is 50ppm/C grade, so numbers are in 10s of ppm, i.e. +14 means 10.0014K measured and -14 is 9.9986K. For better grades (i.e. <5ppm/C TC) the numbers could be in ppm.CheersAlex

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2025, 01:09:40 pm
content: Quote from: Doctorandus_P on June 11, 2025, 06:24:18 amBased on just looking at the number of resistors in each bin, it looks close to a normal distribution.QuoteI have only read about that happening and never encountered it myself.I am suspicious about some capacitors because the same series of capacitors can be available in 3 different tolerances.Quote from: Kleinstein on June 11, 2025, 09:08:54 amMy manual impedance bridge would be too inconvenient if I had to measure different resistors, but in this case the values are all close together so only two dials would need to be adjusted at most, so maybe it is worth trying it.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 11, 2025, 03:15:35 pm
content: For just looking at matiching resistors one can often keep the bridge resistors fixed and just use the reading of the null-detector. The reading is somewhat nonlinear and the scale may not be well known. For the small relevant range to reading of the deviation is usually still OK.

author: TimFox
author_rank: Super Contributor
timestamp: June 11, 2025, 09:56:14 pm
content: One can certainly calibrate the null meter for deviation from nominal value for a given expected value.General Radio used to make such bridges with detectors to evaluate deviations.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2025, 04:46:14 am
content: Quote from: TimFox on June 11, 2025, 09:56:14 pmIf it came to that, I could make a linear null meter amplifier for my impedance bridge to drive a multimeter.