how to measure in meg-ohm voltage dividers?  - Page 1
Forum section: electronics / beginners

author: Pfriemler
author_rank: Regular Contributor
timestamp: January 01, 2023, 03:35:06 pm
content: Months ago I repaired a 36V Einhell battery BMS. It suffers from humidity and/or dust in the voltage dividers. (The BMS is a "discrete" one - lots of voltage dividers compared with op amps to decide which cells should be bypassed on charging (to be considered as full)). The dividers are made of 4M7 resistors.This was the first moment in 30 years hobbyist practice where I wanted to have a hi-z input multimeter (10 GOhms). I had the choice of unsolder and resolder all ~20 resistors or to measure their voltage in the circuit.I read a lot about 10 GOhm DMM meanwhile. They seem to be limited to +-10V. I had a cheap Amazon DMM on bench with a 400mVDC range input impedance of about 800 MOhm (obviously no voltage divider applied) (sent it back for other reasons).I read about high voltage dividers for DMM (> 1kV and more).I never found a hint how to measure voltages up to 40 V with an 10 mV resolution without dismatch such voltage dividers, leading to a BMS disfunction.As I am about to buy a new DMM in a sort of Bryman BM867/869 or a 121GW, I had the idea to look for some device which allows me to measure the stuff mentioned above under "live" conditions. After a few hours of researche, I did not found any considerable device at all. Am I asking for too much?Maybe there's a high impedance measure amplifier somewhere, maybe I should build a simple voltage divider with 1G/1,05M, leading to voltages of 3..40 mV with a 10µV resolution to measure - no of my DMMs will do that out of the box.How may I solve this little problem? It is a little one, I do not need this as a professional but twice a year, if ever. No option to buy an expensive (>1000US$) multimeter for that.What would you use? any trick will do. But I do not know. I feel like a beginner...sorry for my English...

author: jonpaul
author_rank: Super Contributor
timestamp: January 01, 2023, 03:48:15 pm
content: The pros use the classic GenRad 1862/1863 megohmmetes, with test V to 1000VRough indications with Electricians "meggers" really special high voltage Ohmmeters, called "insualtion testers"Most have a crank generator or batter powered inverter and test at 500 or 1000V.Range of a typical megger is 500k..50G OhmThe Gen Rad go to tera ohms.Below 100V the currents are so low that the meters cannot distinguish between the resistance DUT and noise or leakage.Jon

author: Pfriemler
author_rank: Regular Contributor
timestamp: January 01, 2023, 03:52:21 pm
content: You misunderstood me. I am not about measuring high impedances, but voltages in high ohm voltage dividers without faults.I have an UNI-T insulation tester, 5 GOhms@250V...

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2023, 03:58:07 pm
content: A possible trick is to use a kind of differential voltmeter like in the old analog days. So don't use the DMM for the whole voltage, but only for the difference to a first approximation. This could need to make / build a switchable reference of resonabel stabilty as an auxilitry tool. With a 400 mV range with reasonable high Z the voltage steps should be something like 300 mV or smaller. For a 20 V range this would be some 60 steps - so a bit too much for a simple multi-step switch or pot.One could consider a relatively simple Kelvin varlay type circuit, maybe with 2 steped switches or just 1 switch and 1 good pot. When measuring the actual voltage set, there is no need for high precision.Instead of switched, jumpers could be an alternative.Another option would be to build a relatively simple high impedance buffer amplifier. This could be as little as a single low drift low bias OP-amp (e.g. TL071H, OPA170 ) and a little portection (e.g. 100 K series resistor). For negative voltages or small voltage with a non single supply type one could shift the ground point (neg suppy to pos supply). For the start battery supply (e.g.2-3x9 V block) would OK. Depending on the requirements one could get higher performance (lower bias/ higher Z and less drift with better parts and a 2nd OP for bootstrapped supply) . So the DMM would than just measure the buffered output voltage.It still needs care with leakage (e.g. the cable isolation may be in the 100 Gohm range) and some patience.

author: Pfriemler
author_rank: Regular Contributor
timestamp: January 01, 2023, 04:13:01 pm
content: Thanks, these are good hints. In my use case, it's not about some ten volts with a high source impedance, only not to load the source at all. Just after clicking on "POST" I had a similar idea: If I use a good linear PSU with low ripple, I can just apply it to the divider with my DMM in series and adjust the psu 'till the DMM shows Zero. The psu output voltage is my value now I need.This will work for stable DC. In the BMS mentioned above, the cell voltages were varying by some mV (pulsed charging current). Therefore, a real high-Z meter will be better. With a fast bargraph, I will have an idea how big the steps are. By now, I regret that I sent back the meter

author: bdunham7
author_rank: Super Contributor
timestamp: January 01, 2023, 04:25:13 pm
content: Do you have measuring access to each of the resistors? If so, then you would use a Hi-Z voltmeter to measure the voltage across each individual resistor, not in aggregate. That way the voltage you would be measuring would only be a few volts. Hi-Z DMM inputs are typically limited to ~12V or so, although some models go to ~20V and a few others are limited to much lower voltages. Using passive division methods will probably not get you a very good result--a 1000X HV probe is only 1G, and even that loading on a 4M7 resistor would be noticeable.The least expensive options I can think of with Hi-Z are going to be very old models like the HP 3478A (up to 3V) and the Fluke 8800A (up to 20V). There are others. If you can find someone who will sell you one of those in good condition, they go for a few hundred dollars at most.One other point about Hi-Z input DMMS--the spec isgreater than10G, not 10G. It is typically actually nearly infinite plus some bias current and a bit of leakage, so not really resistive. So you can't put a 10G resistor in series with it to divide the voltage in half.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 01, 2023, 06:01:40 pm
content: Doing a null measurement with a variable power supply and a regular 10 MOhm voltmeter is a good solution with no specialty hardware needed. That's how people had to do this before ultra-high impedance meters were common. It takes more effort to set up and do a lot of measurements compared to a plug and play meter but if this is for occasional and non-professional use it's certainly fine.

author: Peabody
author_rank: Super Contributor
timestamp: January 01, 2023, 07:37:42 pm
content: +1 on doing a null measurement. You could use a 10-turn pot across a stable power rail, and adjust the wiper until there is zero difference between the wiper and the resistor junction you're measuring. Then measure the wiper with respect to ground.You'll be on the lowest range of your voltmeter, which is usually also the highest impedance.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 01, 2023, 10:19:44 pm
content: There are plenty of FET based opamps with very low bias and offset currents, but their supply (and therefore input) voltages are limited to a value usually to 36 or 44volts.It would be quite easy to add a known offset to that. For example just using 5 18650 cells in series would have very low drift over the time of your measurements, so you can just subtract this as a known offset. You can of course also make a fancier reference voltage for the extra offset.If you can tolerate 10mV error over over a 10Mega Ohm resistor, you need an opamp with less then 1nA of offset current, and opamps with one or two orders of magnitude are widely available and cheap. You could use a very low offset and accurate chopper opamp, but you could also add a decent ADC (16-bit ADC's are quite cheap these days) and a uC, characterize offsets and non linearities with some reference meter, and then put some corrections in the firmware of the uC.Edit:Which is quite similar to what Kleinstein wrote earlier...

author: David Hess
author_rank: Super Contributor
timestamp: January 02, 2023, 07:18:09 am
content: The modern instrument that you are looking for is called an "electrometer", and many can make a high impedance voltage measurements to hundreds of volts. An old Keithley 614 can only do it to 20 volts, but maybe that is enough. A 6517B can do it to 200 volts.Alternatively a low input current operational amplifier can be configured to "bootstrap" its own supply voltages from its output to buffer an input at higher voltages, and then the measurement can be made with a common voltmeter.

author: Mahagam
author_rank: Regular Contributor
timestamp: January 02, 2023, 07:49:14 am
content: You can take into consideration the resistance of your DMM and recalculate readed values into actual ones.

author: srb1954
author_rank: Super Contributor
timestamp: January 02, 2023, 08:00:14 am
content: Quote from: Pfriemler on January 01, 2023, 04:13:01 pmOr you could buy a real differential voltmeter that will do all this for you with greater ease of use and better measurement stability.Search for Fluke differential voltmeter on eBay. There seems to be a few available the moment.

author: Pfriemler
author_rank: Regular Contributor
timestamp: January 02, 2023, 10:16:31 am
content: Yeah, in general this seems to be not that easy (and not that inexpensive) as I thought. There are no such (inexpensive) instruments on the market, so I found none. Nevertheless, a 10V Hi-Z multimeter will be better than nothing. As said: If I will need this all the day, I will go for an "electrometer" or build anything like that as suggested - a kind of "automatic compensator", adjusting the voltage compared to for a zero balance, in a few ms, sounds realistic. If not in hardware, then in a simple script using a readable DMM and an programmable PSU...I just wondered why there are no such things (like the no-burden nano current amplifier here in the EEVBlog shop for measuring low(est) currents of). A (maybe) 100V tolerating high-z impedance "divider" (1:10 for safety), powered by a low voltage supply, with a few hundreds of Ohm output resistance good for any kind of multimeter you want. Ready, tested, for 50$ or so...A few question to be answered:QuoteYes. But for the upper stages of the battery pack, these voltages are near 20V (half of 40V).Quoteand not suitable for this case, as it lowers the voltage to be measured down to the noise... (not with a better DMM, ok).Quotewaaaay to expensive for me.QuoteJust because the Hi-Z input impedance ist not known, any resistor in series will lead to unpredictable results ...?lately, I was lucky to identify some of the faulty resistors: It turns out that most of them are used simply in 1:1 dividers or arrangements with the same current through. Even if the measurement detunes the devider, the measured voltages on both resistors should not differ. In case where they did, I replaced the resistor. In the end, a simple resoldering solved most problems. Maybe only the coating on the layer was humid and the heat dried it. This helped to solve the problem, but not to understand the circuit. Nevertheless: The battery pack then reached 80% again of its capacity (>10 years old). Even a complete new BMS did not lead to better results. Mission accomplished.

author: Ian.M
author_rank: Super Contributor
timestamp: January 02, 2023, 10:22:58 am
content: For under 15 euro, you can get a CMOS input OPAMP that will run at over 100V between its supply pins, with an input bias current of only tens of picoamps. Configure it as a unit gain follower and supply it with a bit more than the full BMS battery voltage e.g. by adding a pair pf PP3 9V batteries, one in series with each main battery terminal. Its CMOS so will need input protection - low leakage clamping diodes to its supply rails and a series input resistor should be sufficient. Depending on the clamping diodes and OPAMP, the effective input impedance may be up in the tens of giga-ohms region, so the error caused by a 10K input protection resistor is negligible for all except precision metrology.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 02, 2023, 11:54:23 am
content: The eact input impedance is not known. This means one can not simply correct for the effect of loading. As the loading is small, the error is still smaller than with a meter with 10 M input resistance and doing the correction. 10 Gohm input impedance has an effect like only 0.1% unctainty of 10 Mohm input impedance with correction.An amplifier up to some 35 V is easy and such amplifiers can be cheap (e.g. TL071H used with a slightly lifted ground for low voltages). Depending on the amplifier one does not need extra diode if the series resistor at the input is in the 100 K range. If used the extra diodes for protection should get bootstrapping so that they don't see much voltage - with a buffer this is very little effort (just 2 more diodes and a resistor). If an input impedance way high than 10 Gohms is wanted one can use a bootstrapped supply also for the input amplifier - this circuit is a little more effort, but still not too complicated.When using the differential voltmeter method, the DMM does not have to have such a high impedance if the measurend voltage is low. The troublesome current is measured voltage divided by input impedance and thus low current at low voltage. It only needs a better adjustment of the compensating voltage.

author: bdunham7
author_rank: Super Contributor
timestamp: January 02, 2023, 04:20:13 pm
content: Quote from: Pfriemler on January 02, 2023, 10:16:31 amI'm not clear about this then--can you draw a quick sketch showing how the resistors are arranged? There is 20V across an individual resistor?

author: Pfriemler
author_rank: Regular Contributor
timestamp: January 02, 2023, 05:23:51 pm
content: Quote from: bdunham7 on January 02, 2023, 04:20:13 pmSee pic attached. 10,8,9 on the left side are the battery pack stages (10 = + GND is -, 9,8 connected between cells - balancer). The OPA are used to "move" the cell voltages down to GND, afaics - OPA's output to GND is then the difference between 10 and 9.10 has 41V to ground (if battery is full), 9 ~37V, 8 ~33V and so on.The whole story:https://www.mikrocontroller.net/topic/538595#new(in German)

author: Kleinstein
author_rank: Super Contributor
timestamp: January 02, 2023, 05:48:10 pm
content: The circuit looks like a series of relatively high impedance differential amplifiers. The defects could be with some of the resistors, but it could as well be some of the OP-amps getting increased input bias current. Another possible problem is contamination of the PCB causing leakage currents.It looks like one could check individual amplifer stages: measure input voltage over the cell they work on and measure the output voltage. Chances are if it works OK in this test the CMRR is also not very bad. There is quite some common mode voltage, but not very much change in the voltage.If a bad difference amplifier is found a in circuit measurement it would still be hard to distinguish between bad resistors and a bad OP-amp in circuit. Just a voltage measurement at the OP-amp inputs alone may not be enough, though it can narrow down the error.

author: EPAIII
author_rank: Super Contributor
timestamp: January 03, 2023, 06:52:07 am
content: A Wheatstone Bridge should do the trick.