Precise and accurate measurement of high value resistors - Page 1
Forum section: electronics / general

author: stevebyan
author_rank: Contributor
timestamp: July 14, 2024, 03:12:51 am
content: As an example, let’s say I want to restore an old HP-410B VTVM. It has two 50 Megohm 1% resistors in its input voltage divider. I need to measure them to within 0.33% or even better to within 0.1% in order to determine whether they are within tolerance or if instead they must be replaced. What instruments can I use to make this measurement? The only choices I know of are the Guildline 9520 Teraohmmeter and its successors, the General Radio 1666 DC Resistance Bridge, and the Guildline 9920 Current Comparator Bridge and its successors.I’m particularly interested in instruments that are affordable by a hobbyist. Are there other possibilities? I’d like to cover up to a couple hundred Megohms with 0.1% accuracy.

author: jbb
author_rank: Super Contributor
timestamp: July 14, 2024, 07:05:59 am
content: I could be horribly mistaken, but would a Source Measure Unit (SMU) work? They can apply a pretty high test voltage in combination with a sensitive current range.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 14, 2024, 07:15:21 am
content: The high value resistors are used in the higher voltage ranges. It should be enough to check if these range work OK and thus the divider. This is testing the resistor ratio for the divider, but this may be even better than just a resistor measurement.

author: stevebyan
author_rank: Contributor
timestamp: July 14, 2024, 12:23:13 pm
content: So I then need an electrometer, something with an input impedance of 5 Gohms or more, with 0.1% accuracy, right? So a Fluke differential voltmeter, I guess.Edit: looks like my HP 3456A could do it if I stick to less than 10 Volts input.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 14, 2024, 12:24:24 pm
content: Is it more important that the 50 Meg Ohm resistors be entirely accurate or more important that they are the same value?Do you have equipment to measure One Meg?

author: mawyatt
author_rank: Super Contributor
timestamp: July 14, 2024, 12:35:29 pm
content: Since the resistors are for a voltage divider why not use the Ratio Function of the HP34401A or other DMMs? Set a convenient Ref Voltage and measure this and divider output, thus the voltage division ratio as shown in HP34401A manual.See page 44 of HP34401A users manual.https://engineering.purdue.edu/~aae520/hp34401manual.pdfBest,

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 14, 2024, 12:56:11 pm
content: It depends on the value. Up to 100M even a decent 6.5 digit multimeter, like Keysight 34465A, will give you 0.3% (1 year) accuracy and if you have a reference resistor with, say, 0.1% tolerance, you could improve on it. Above 100M things became increasingly difficult with the value increase. In my home lab (using Keithley 263) I can measure up to 100M with better than 0.2% accuracy, up to 1G better than 0.3% and up to 100G better than 1% (and can compare resistors to a much better accuracy, however my reference resistors for high values are accurate to 0.07% for 100M, 0.1% for 1G, 0.25% for 10G and 0.4% for 100G). My Keithley 617 electrometer can measure up to 200M with 0.3% accuracy (1 year) and up to 100 TOhm with about 3% accuracy (using V/I method).CheersAlexP.S. - I've just tried a direct measurement of 100M from K263 by HP3456A, the displayed value is 98.84M for the 99.007M reference , so about -0.25% error (specifications for 100M range is 2%) . Might be OK for comparative measurements though, however for my HP3456A, I know that the input leakage current is under 10pA, might not be the case with a different unit.

author: stevebyan
author_rank: Contributor
timestamp: July 14, 2024, 01:09:10 pm
content: Quote from: Wallace Gasiewicz on July 14, 2024, 12:24:24 pmThey need to be accurate, as they are in series with other values that make up the range divider.Yes, my HP 3456A should be adequately accurate for measurements below one megohm.Edit to add: I do have a working GR 1666 that as near as I can tell is still accurate, and a pair of Guildline 9520’s that I hope to get working. But I’m interested in other options.

author: stevebyan
author_rank: Contributor
timestamp: July 14, 2024, 01:13:47 pm
content: I wonder how HP did it back in the 1950’s, without a 34465A or 34401A? And without the GR 1666 or Guildline 9520 or 9920?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 14, 2024, 01:17:24 pm
content: I think you meant to say that the 3456 is accurate enough below One Gig Ohm.It should be fine for this measurement.Otherwise put a One Meg Ohm resistor in parallel and use one Meg scale.

author: stevebyan
author_rank: Contributor
timestamp: July 14, 2024, 01:44:31 pm
content: Quote from: Wallace Gasiewicz on July 14, 2024, 01:17:24 pmNo, at 90 days the accuracy spec for the HP 3456A on the 100 Mohm range is 1.8% plus one count. The 1 Gohm range is 16% plus one count :-(100 Megohms in parallel with 1 Megohm on the 1 Megohm range might get me there, I’ll have to run the numbers.

author: Vgkid
author_rank: Super Contributor
timestamp: July 14, 2024, 01:53:20 pm
content: Quote from: stevebyan on July 14, 2024, 01:13:47 pmThey would have used a guarded wheatstone bridge. That is what everyone was using.

author: EC8010
author_rank: Frequent Contributor
timestamp: July 14, 2024, 03:00:38 pm
content: I've hit this one before. The problem with any electrometer is that it uses high value resistors internally, so they have to be thick film, and that means they have a significant voltage coefficient, yet (in a Keithley) they might have any where from 1V to 200V across them. In short, an electrometer is only accurate to about 1%. I needed some accurate 100M resistors, so I measured a load of 10M 1% metal film resistors individually on a 34410A, added the numbers together and added a trim resistor to get an accurate 100M. Yes, eleven resistors in series. I looked, but 0.1% tolerance isn't available in 10M unless you go to metal foil, in which case you can have 0.02%. At a price. If you don't mind buying 10M 0.02% metal foil, you could use that as a way of measuring the 10M 1%, either by making a bridge or using the ratio function of a meter. Whatever you do, it will be tricky and huarding will probably be necessary. And screening the measurement to minimise hum.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 14, 2024, 04:17:02 pm
content: Quote from: EC8010 on July 14, 2024, 03:00:38 pmThat is not quite correct, the maximum voltage on the measuring resistor in Keithley 617 is20V, so a possible additional error from the VC even as high as 20ppm/V , would be less than 400ppm or 0.04%.CheersAlex

author: stevebyan
author_rank: Contributor
timestamp: July 14, 2024, 05:41:25 pm
content: Quote from: Vgkid on July 14, 2024, 01:53:20 pmWhich guarded Wheatstone bridge? The General Radio 544-B Megohm bridge is only accurate to 4% at high resistances, and the newer 1644-A Megohm bridge is only accurate to 1% in my range of interest.Did Leeds and Northrup make something suitable?

author: EC8010
author_rank: Frequent Contributor
timestamp: July 14, 2024, 06:12:04 pm
content: Quote from: Alex Nikitin on July 14, 2024, 04:17:02 pmBut if you use a 6517, itcango to 200V, so (by your calculation) it introduces a far less tolerable 0.4% error. When I first encountered it, I thought that extending the electrometer's voltage swing to +/-200V was a very clever solution to the range switching problem. Now, I'm not so sure.Back to the OP's problem, a very specific measurement needs to be made. Make a bridge from scratch and ensure that its terminals are guarded. That would be achieved by using an old-fashioned tag strip having its terminals on ceramic insulators fitted in a steel strip.

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2024, 06:12:24 pm
content: The long-gone Keithley 515B bridge was rated for 0.05% accuracy from 100 k\$\Omega\$ to 100 M\$\Omega\$ and 0.1% accuracy from 100 M\$\Omega\$ to 1 G\$\Omega\$.https://download.tek.com/manual/515B(Model515).pdf

author: EC8010
author_rank: Frequent Contributor
timestamp: July 14, 2024, 06:17:32 pm
content: I've just looked up the Keithley 515. Wow. On the plus side, with a size like that it should be cheap if one ever turns up. But post and packing won't be.

author: stevebyan
author_rank: Contributor
timestamp: July 14, 2024, 07:36:14 pm
content: Quote from: TimFox on July 14, 2024, 06:12:24 pmAwesome! Thanks! I'll keep an eye out for one of those!

author: stevebyan
author_rank: Contributor
timestamp: July 14, 2024, 07:47:03 pm
content: Quote from: Alex Nikitin on July 14, 2024, 12:56:11 pmAlas the Keysight 34465A and the Keithley 263 and 617 are out of my bottom-feeding price range. A broken Keithley 417 might be affordable, I'll go see if the specs are good enough.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 14, 2024, 09:19:30 pm
content: I know the GR 716-C capacitance bridge can also measure high value resistors between 300 kohms and 80 kMohms at 1 kHz. No idea of the accuracy and it's probably more of a comparison, but just FYI. See page 6 and 7 of the manual under Special Measurements.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 14, 2024, 09:40:07 pm
content: You can always build an accurate MOhm Meter. You would need one 10M 0.01% resistor (Mouser p/n 684-USF340-10M) and an accurate voltmeter (say, your HP3456A) to measure the input and output voltages. The ADA4530 is also available from Mouser and can be replaced by a cheap LMC662, however you would lose some accuracy unless you add an offset adjustment. With a 10M reference resistor and a stable 1V input you should be able to measure resistors from 1M to 100M with better than 0.05% accuracy without any adjustments. You would need to use a correct build method, lifting the inverting input node (Rx1) into the air or on a PTFE support (a good PTFE isolated BNC socket will do the job), and using a very low leakage capacitor (TDK NP0 ceramic 1nF or Polystyrene) for C1 .CheersAlexP.S. even the input voltage needs to be stable but doesn't need to be accurate, if you use the Ratio mode on the HP3456A for the input and the output voltages. If you reduce the input voltage to 100mV, the range would extend up to 1Gohm with a somewhat reduced accuracy.

author: trobbins
author_rank: Frequent Contributor
timestamp: July 15, 2024, 12:27:24 am
content: I went the multiple series resistor path to configure 900M and 9G calibration resistors for full-scale analog meter setting of a 1960's megohmmeter. I had a box of 68M 5% VR68, so sorted them using the same meter, although I did stop short of preparing a 90G calibration resistor.The accuracy of the summed reference resistor should align with the measurement accuracy of the base parts, so that then returns to how confident you can make a measurement of a base part, and how well you can construct a string of parts with low leakage, and make measurements at a known temperature. Then compare measurements of the 50M parts with the reference.

author: Vgkid
author_rank: Super Contributor
timestamp: July 15, 2024, 01:38:07 am
content: Quote from: stevebyan on July 14, 2024, 05:41:25 pmSo L&N made the 4232 Guarded wheatstone bridgeshttps://www.eevblog.com/forum/metrology/ln-4232-wheatstone-bridge/Here is the 4737https://www.eevblog.com/forum/metrology/leeds-northrup-4737-a20-tear-down/new/#newIn addition to the Keithley 515 , L&N also made a megohm bridge as early as the mid 60's...Since I found this as a neat topic I will post a bunch of documents.Also add Bob Pease "What's all this Tee Network stuff, anyhow?"and Guildline 9336/9337 manuals / IET Labs SRC-100T manualThere is another one I'm missing as well.

author: ch_scr
author_rank: Frequent Contributor
timestamp: July 15, 2024, 08:54:27 am
content: Quote from: EC8010 on July 14, 2024, 03:00:38 pmI've made a python script for just this application. It takes the list of your measured samples and the target value,and it'll automagically tell you which ones will make a string that's exactly on point. Or rather, how close it gets. It takes surprisingly little overhead to find a "real close" combination.The idea is, if the stock has a Gaussian spread, therightcombination of "odd values" ends up stacking exactly right. The script tries them all for you.Functions:Code:string_optimizer.py -husage: string_optimizer.py [-h] [-i] [-p [R_P [R_P ...]]] Target S/P N R [R ...]Tool to optimize resistor series string or parallel connection for lowest initial errorpositional arguments:Target target value of the series/parallel connection to be optimized for (e.g., 10.198523)S/P choose between (S)eries or (P)arallel connectionN number of resistors to be selected for connectionR resistor values, space separatedoptional arguments:-h, --help show this help message and exit-i, --iter show each iteration that finds a new best value-p [R_P [R_P ...]], --prio [R_P [R_P ...]]specify resistor(s) values that will be assumed fixedExample for a target of 100 Meg, series connection, 10 resistors to be selected and 12 measured valuesCode:string_optimizer.py 100.00 S 10 9.999 10.010 10.123 10.001 9.890 9.780 9.991 10.1 9.8999 10.01 9.987 9.897Residual error 80.00000 ppmWith these 10 in series:(9.999, 10.01, 10.123, 10.001, 9.89, 9.991, 10.1, 10.01, 9.987, 9.897)

author: EC8010
author_rank: Frequent Contributor
timestamp: July 15, 2024, 09:09:21 am
content: That looks useful; I could have solved the problem with ten resistors rather than ten 10M plus trimming resistor (making eleven). I've never used Python, mind.

author: ch_scr
author_rank: Frequent Contributor
timestamp: July 15, 2024, 09:31:40 am
content: All the "UI" is the command line parameters (like the batch scripts of old). All you need is forpythonto be installed (and make sure to have the "add to path" option checked when installing), on windows.

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 15, 2024, 03:08:01 pm
content: Measuring high valued resistors used in HV dividers usually does not make much sense, since it’s the ratio that matters. So having a precise high voltage source would be more interesting and allows to precisely set the division ratio, by either adjusting the lower or higher arm, and measure the VC too.For example I have some ceramic 80kV dividers whose ratio is laser adjusted to be better than 1%, but the overall R has like 5..10% tolerance

author: floobydust
author_rank: Super Contributor
timestamp: July 15, 2024, 06:14:15 pm
content: What I have done is apply a known high voltage say 100VDC to the resistor or divider, and measure (output) current as nA to GND, or divider output voltage if the voltmeter's input resistance is known.But there is also the voltage-coefficient of resistance so at high operating voltages, resistance changes so I'm not sure how much accuracy can be achieved.

author: stevebyan
author_rank: Contributor
timestamp: July 16, 2024, 03:36:32 am
content: Quote from: PartialDischarge on July 15, 2024, 03:08:01 pmThe HP-410B VTVM range divider covers seven ranges, so there’s more than one divider ratio that has to be accurate.

author: ivo
author_rank: Regular Contributor
timestamp: July 16, 2024, 04:14:04 pm
content: If you have two good meters, you can use voltage divider effect to measure the high resistor. Measure first meter's impedance in its volts mode, probably 10M nominal but of course you want to know that exactly. Then measure some very stable voltage exactly. Then you can put first meter underneath the 50M resistor in series connection with this voltage. With a 10V source you would get in the neighbourhood of 1.67V reading by the voltage divider formed. Work backwards to determine the exact 50M value. You can test with other smaller resistors to get a sense of the accuracy or determine viability of method.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 20, 2024, 12:43:35 am
content: Just in case if someone needs a set of reasonably accurate and stable resistors from 10K to 10GOr you can just use the idea and build your own, I use such a box for many years and find it very useful.CheersAlex

author: MarkT
author_rank: Frequent Contributor
timestamp: July 20, 2024, 12:19:12 pm
content: Up in the 10G range I would worry about leakage currents - you need proper quality coax and connectors (polythene or PTFE insulation), and low humidity. Often a faraday cage is useful to keep out stray electric fields.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 20, 2024, 02:52:11 pm
content: We had a similar discussion here:https://www.eevblog.com/forum/metrology/my-triax-cable-shielded-box-project-for-keithley-instruments/It is not that easy to measure high value resistors.

author: KK6IL
author_rank: Regular Contributor
timestamp: July 21, 2024, 09:18:31 am
content: Back in the 1970's, I was measuring insulation resistances up to 10^12 by putting the unknown R in series with a 1M 0.01% R and the pair put across an accurate 1000V from a Cohu 324A voltage calibrator. Leakage current created a voltage across the 1M resistor, which was measured with a Wavetek 704 0.01% differential voltage. A lot more accurate than I needed to test insulators but that's what I had to work with.There's a limit on what one would be willing to spend to work on an old VOM, but an old Fluke differential, even if long out of cal would likely be accurate enough to implement the above test.BTW, mounting a copper wire in an aluminum block does not create an insulated terminal but instead, a battery.John KK6IL