4 wire resistance measurement at smaller current - Page 1
Forum section: electronics / beginners

author: G-son
author_rank: Regular Contributor
timestamp: October 04, 2023, 11:29:23 pm
content: I have a LM317 100mA constant current source for quick n dirty 4 wire resistance measurements, together with a multimeter that measures 0.1mV that gives a resolution of 1m. Plenty good for my needs really, things like checking if a generator winding off a small two stroke engine actually is the normal 0.5 ohms or not, but you know that urge to improve things until they fail completely...There's a couple of things about it I don't quite like about the current setup, 100mA constant as long as it's connected is a bit much for running it off batteries, and that current is enough to heat things up so much so you see the resistance change.Icouldjust add a switch to push while measuring, so there's no current the rest of the time. Simple, effective, and no fun overkill at all, or I could add more electronics.I was thinking about using an op amp to amplify the voltage signal, I'm not sure how much gain could be suitable but 10 or 100 should end up with the same output voltages as now while using 10 or 1mA instead. Perhaps higher gain to get 0.1mresolution instead - I really don't need it but overkill can be fun. The problem is, such a circuit would be very sensitive to small voltage errors, and as I understand it, op amps can easily be a few mV out, and probably drift a little with temperature etc. too.What would be a good way to manage that? I wouldn't mind having a potentiometer to tune for a 0.0mV output after switching it on every time, as long as it remains pretty stable once adjusted.Other solutions for the same job are welcome too. As long as they're cheap and relatively simple. (Buying a proper 4 wire meter is not an option now. I don't use it often enough to motivate that.)

author: Benta
author_rank: Super Contributor
timestamp: October 04, 2023, 11:44:58 pm
content: Don't be so timid.Go all the way and use pulse measurement with a synchronized ADC system to go with it. Say, at a 1:1000 duty rate with 1 ms current pulses. That way, self-heating is eliminated. I have more ideas on how to make this a REALLY BIG project if you like.

author: G-son
author_rank: Regular Contributor
timestamp: October 04, 2023, 11:48:55 pm
content: I like the way you're thinking, although maybe it's better to make it a multi step process? Start small, then go bigger and bigger until it's boring!Oh, besides. I think 1ms pulses might not work when measuring something inductive, like a generator winding.

author: donlisms
author_rank: Frequent Contributor
timestamp: October 05, 2023, 01:07:53 am
content: Part of what you need to think about is what the various potential errors are, i.e. in how much current you're *really* using, and how much voltage you *really* have. Then, when you combine those, do you have enough accuracy to bring joy. Relative resolution is enough in some cases, but if the overall absolute accuracy has shifted too far, you end up where you can't trust any of your readings and always saying "huh."For the op-amp drifting thing, take a look at Auto-Zero and/or Zero-Drift amplifiers. You're not the first to be concerned, and enough have been concerned that Those Smart Folks make and sell amplifiers just to calm us down.

author: trinacria
author_rank: Contributor
timestamp: October 05, 2023, 02:37:28 am
content: You can make a decent 4-wire resistance measurement with a second multimeter.

author: G-son
author_rank: Regular Contributor
timestamp: October 05, 2023, 09:06:17 am
content: Yes, that's basically what I'm doing with the LM317 current source and one or two multimeters (depending on if I want to monitor the current all the time or just assume it's stable @ 100mA).Going down to 10mA with no other changes would give me 10mresolution on a 0.1mV multimeter. Still 10x better than the 0.1resolution measuring resistance with a regular 2 wire meter, but I'd like to keep the 1mresolution or possibly even improve it.Donlisms: You do have a point about accuracy. You need to be able to trust the results, relative to the accuracy you need for that particular job.

author: trinacria
author_rank: Contributor
timestamp: October 05, 2023, 03:35:47 pm
content: Resolution factors into the accuracy, but it's not the whole story.When you take the measurements, go to the meter datasheets and look up the accuracy specs for the ranges they're in. To improve the accuracy, adjust the current source until the meters are in their most accurate ranges and the readings are near the tops of those ranges. Keep the power dissipation in mind and find a balance that doesn't overheat the part so much that the readings change faster than you can write them down.It's better to connect both meters at the same time, and as shown in the picture so that the burden voltage of the ammeter doesn't matter.

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2023, 12:44:00 am
content: Even non-chopper precision operational amplifiers like the OP-07 are good enough for what you want to do, but a chopper stabilized part will work fine and will be easier to use. I would use a gain of 1000 just to make things easy, which might result in 100 microvolts of output offset, but this is still only 1 count on your 100 microvolt resolution meter. It is easy to add a trimmer to the circuit to remove any offset error.

author: trinacria
author_rank: Contributor
timestamp: October 06, 2023, 04:15:23 pm
content: The gain should be selectable so you can get the reading into the accurate part of an accurate range. In my example picture, a 1000x voltage gain would improve the accuracy. But if instead the reading was 61mV, amplifying it to 6.1V would destroy the accuracy. It's hit and miss, which is why you need to choose whether to amplify, and which preset amplification to choose. Options up to 1000x would be good because it's enough to change between meter ranges.

author: G-son
author_rank: Regular Contributor
timestamp: October 06, 2023, 09:55:44 pm
content: Makes sense. I'll have a look in the parts boxes and see if there's anything that seem suitable available, or if I need to go shopping.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 06, 2023, 10:35:47 pm
content: 1) buy a DMM with one or two digits more (0.01 mV or 0.001 mV). It can be helpfull with other tasks too.2) buy miliohmmeter, e.g. YR1035+. It is very convenient and usefull.

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2023, 11:04:47 pm
content: Quote from: trinacria on October 06, 2023, 04:15:23 pmInstead of changing the gain of the amplifier, change the excitation current.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 08, 2023, 02:57:16 am
content: I have Aneng AN870 DMM which has 0.000 mV resolution (yes, 1 uV). This is very convenient. I bought it exactly for this possibility to measure low DC voltages.Then I bought YR1035+ miliohmmeter and now I have wider capabilities to directly measure low resistanse, but having microvolt resolution on my DMM still helps sometimes.

author: trinacria
author_rank: Contributor
timestamp: October 08, 2023, 04:25:31 am
content: Quote from: David Hess on October 07, 2023, 11:04:47 pmYes, I already suggested adjusting the current. I misunderstood your suggestion as either/or. Fixed gain with adjustable current is the way to go.

author: G-son
author_rank: Regular Contributor
timestamp: October 08, 2023, 09:56:03 am
content: Quote from: Vovk_Z on October 08, 2023, 02:57:16 amLooks like they can be found for not very much money. I'll take a closer look at them later.

author: tszaboo
author_rank: Super Contributor
timestamp: October 08, 2023, 10:12:48 am
content: I would ditch the LM317 and replace it with an LTC3082 or an LTC3092. You can select the current with that, and it's going to be much more accurate.OP07 or it's more advanced versions are accurate enough to amplify the DC signal from the DUT.Design it on a board, order it from China for 5 dollaridos. Then assemble a few versions with different currents and gains. Less current, more gain for example.

author: G-son
author_rank: Regular Contributor
timestamp: October 08, 2023, 04:54:09 pm
content: I need to read up on those components, but so far I like the idea!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2023, 05:29:12 pm
content: A more sensitive DMM is probably easier, at least for a factor of 10 more. Below 10 µV one also has to worry about thermal EMF at the conectors and maybe resistors to test.For the amplifier an OP07 with a zero trim would be good enough, but it takes quite some power and votlage. So it is not a good choice for battery operation. Some modern AZ amplifiers may be easier, though often SMD only.If the current source can get away with AA cells, 100 mA could be reasonable OK - so maybe a more stable and lower voltage current source for the 100 mA may be an option too.For best results one would need to look at the difference without current to with current anyway to at least get of much of the thermal EMF.Building an amplifier to work with the same supply as the current source adds a little complication, but is not impossible.

author: TimFox
author_rank: Super Contributor
timestamp: October 08, 2023, 07:46:51 pm
content: Another possibility is to use low-frequency AC to drive the four-wire circuit, since there will be no offset voltage in the AC amplifier.Phase-sensitive detection allows a narrow bandwidth to reduce noise.

author: alligatorblues
author_rank: Regular Contributor
timestamp: October 08, 2023, 11:21:36 pm
content: Quote from: G-son on October 04, 2023, 11:29:23 pmAll resistance measurements ahould use 4-wire. But if you just measure the resistance of the leads, and subtract that from your measurement, that's essentially what 4-wire does. It neutralizes the lead resistance.

author: coppercone2
author_rank: Super Contributor
timestamp: October 08, 2023, 11:33:19 pm
content: the pulse measurement will by far be the most accurate. use a simple switchthat is what real mili ohm meters do.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2023, 09:13:17 am
content: Quote from: Kleinstein on October 08, 2023, 05:29:12 pmI suggested the OP07 as a place to start because it is cheap and readily available. There are more modern low and micropower alternatives to the OP07 and some low and micropower chopper stabilized parts as well.Quote from: TimFox on October 08, 2023, 07:46:51 pmIt also removes thermocouple offsets which become significant at these signal levels in this sort of application.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 10, 2023, 12:59:43 pm
content: I have not build it myself yet, but I find the milli ohm meter in CircuitCellar from Semptember 2016 quite appealing. Just a small uC, ADS1115, a display and some passives.It uses radiometric measurement, so most of it's stability will be from the quality of the reference resistor, and it uses AC measurement and synchronous sampling to counter thermocople effects.

author: G-son
author_rank: Regular Contributor
timestamp: October 11, 2023, 08:01:59 am
content: Cool, but arduino solutions are a bit out of my wheel house. I'm sticking with analog solutions that I understand.