DIY Milliohmmeter - how to solve grounding issue? - Page 1
Forum section: electronics / projects

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 24, 2022, 10:30:31 am
content: HiI'm completing this project and would like a tip on how I can solve a grounding issue. Note in the image that the current of 50mA, passing through the DUT, should completely cover the AEBF path. However, this current is divided by the AEDF path, adding the resistance of the reading cable to the final value. What would be the best technique used to resolve this issue? This question seems to be fundamental so that I can read a very low resistance value (1mR).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 24, 2022, 11:44:41 am
content: For the measurement side, do not use a grounded connection, but use a differential amplifier.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 24, 2022, 11:49:19 am
content: QuoteI can't change the project this way. These are the best opamps I have available. There has to be another output without changing the design too much.

author: Gyro
author_rank: Super Contributor
timestamp: June 24, 2022, 12:48:13 pm
content: That seems a very complicated implementation for what you are trying to achieve (it would be a different matter if you were doing AC measurement and synchronous detection). Try the attached app note (substituting one of your own opamp and reference). As long as 1A maximum measurement current isn't an issue you will achieve 1mV/mR, so at least one decimal place on a typical DMM.Completely breaking any internal links between force and sense leads is essential when Kelvin (4 wire) sensing.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 24, 2022, 06:59:01 pm
content: I really like the simplicity of the circuit described in Circuit Cellar of September 2016, issue 314.How it works:the microcontroller has 2x 4 pins parallel with a 30 Ohm resistor on each of its pins, and this creates a bidirectional test current.This test current goes through a 10 Ohm reference resistor and your DUT (Device Under Test).Then the ADS1115 is used to measure both the voltage over the reference resistor and over the DUT, then swaps the direction of the current though the DUT with the microcontroller, repeats the measurement in the other direction. It also repeats this whole process a bunch of times and does some averaging.It also uses 4 wire kelvin sensing. (Two "drive" wires, and two "sense" wires), as shown in the schematic.Go buy that back issue if you like it. Unlike Elektor, circuit cellar projects always come with all source code.. In this case it's a quite simple "arduino" contraption, but it is something to get you started and to improve upon.With the 1uF and 1k resistors there is some protection for the ADS1115, but I would add a bunch of extra diodes to short any voltages over 3Volt or so over the test leads.Also, because there are no extra opamps in this circuit, they can also not generate any errors, and it makes the circuit cheaper.I quite liked CC. They did not have many (interesting) projects, but they had the source code available for all of them and a subscription was relatively inexpensive. Then they had some kind of collaboration with elektor, the subscription doubled in price and I canceled (or better, did not renew) my subscription.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 25, 2022, 11:31:42 am
content: QuoteReally 1mR in this project can be quite challenging, as it will only generate 50µv at the input of the OPA277. But I think the OPA277 might surprise me. The main use of this project of mine will be to measure the RDson of the mosfets. I have Mosfet (IRFB3306) which reaches 3mR. I don't need great accuracy in this case, just that it reads within a certain acceptable range. For example, in this case I could be measuring between 4 and 2mR.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 25, 2022, 02:05:18 pm
content: HiAfter several failed attempts, I managed to come up with an arrangement that worked. I will take some pictures of the results obtained. And the rehearsal is taking place on a protobord.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 25, 2022, 02:09:34 pm
content: The ADS1115 I mentioned earlier can measure down to 8uV (rms noise) and a resolution of about 4uV.For measuring MOSfet RdsOn, I would use another approach. Just build a simple current source from an LM317 and a single resistor, and calibrate it to 1A. Then you can simply measure milli ohms with a small power supply, the 1A current source and a DMM in the mV range.Only calibrating an "around 1.25 Ohm" resistor that can handle 1A is a bit fiddly. I would start with something like a 1.2Ohm 10W resistor and then add some other resistors in series / parallel in a few iterations. 10W is oversized (it only dissipates around 1.25W, but this lowers it's temperature and temperature effects. Using a big bunch of 1/2W metal film resistors is also a good option.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 25, 2022, 05:05:30 pm
content: The circuit still does not look good. There is still no real 4 wire sensing. There is no real need for a precision OP-amp at the current source side and the extra chanrge pump is odd and only producing extra noise.Just for MOSFET testing and thus no high demand on the accuracy of the scale factor the LM317 typ current source should be good enough. With 1 A no amplification is needed.If amplification is wanted, just consider the low side sense as the ground for the amplifier and output and if really needed plan for current compensation (but this needs a negative supply so not that attractive).

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 25, 2022, 06:35:20 pm
content: QuoteI'm considering it acceptable, given what I had initially. I don't have a differential here to implement.QuoteInitially I started testing with the MCP6002, with simple font, but it had a very strange behavior for my configuration. I also tested with the MCP6022 (smaller offset) and had the same bad result. Despite being Rail-to-Rail, they all had an expressive output voltage (headroom), adding to my AZ431. Using the OP189 the voltage added to the AZ431 accurately reflects the voltage of the DUT. I am very satisfied with the stability of the Current Source, using the OPA189.QuoteThe project is not just for testing mosfet.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 25, 2022, 09:35:20 pm
content: If you don't do some kind of differential measurement, then it's almost useless to use a 4 wire measurement in the first place.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 25, 2022, 11:05:37 pm
content: QuoteThank you so much for actively participating in my case.I need to understand why it would be Useless. Assuming each cable has a resistance of 50mR and I'm measuring a DUT of 1R. Ideally I would have 1v at the Opamp output, which would give me approximately 47.6µA (1v/21k) going through the feedback resistor. The voltage drop across the cable (Vc) would be approximately 2.38µv. The input voltage would be Vin = Vr(DUT) + Vc(Cable). I would have an addition of 47.6µV to my output, leaving Vout=1.000047v.Please correct me if I'm making a big mistake in my analysis.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 26, 2022, 02:05:33 am
content: Start by not making assumptions on your cable wiring.If you measure a 1 Ohm resistor, and you have 50mOhm cable resistance, then you already have a 5% error.Or:If you put 50mA though a cable with 50mOhm resistance then you have a 2.5mV over your cable.Your last picture is not complete. In earlier pictures you drew a star (Ring?) GND where both the "force" and the "sense" wires are connected on both sides, which means those wires are just put in parallel, and that would then be a (static) error of 25mOhm (half of 50mOhm).In you last picture you have drawn a triangle with a "G-20" in it.It's a completely bogus picture, but for simplicity sake I shall assume it's your opamp and it's configured for a gain of 20.But what signal does it amplify?There is no way of telling without showing a complete schematic, and this last picture is too incomplete for that.The proper way for accurate low-ohm measurement is:1. Generate a test current though the "force" wires. (This makes cable resistance irellevant).2. Do a differential measurement over the sense wires. Current though the sense wires is very low, voltage drop is negligible.Also, on your "force" side. You've got an OPA899 as a buffer, then you stack a TL431 and a LM317 on top of that but the pins of the LM317 are not labeled. You have two (presumably 1W) resistors on (probably) it's output. I think you want to create a current source there, but you can't create a current source that goes into your test resistor and at the same time control the voltage with the AZ431. (TL431).On the measurement side you have an OPA277 that may be configured to amplify 20x (too many resistors to count), And then after it you have some 7-segment display (with an ADC?), but that also measures the voltage drop caused by the 50mA though the wire on the "force" side.The overall design is so complex and convoluted that I don't even want to go into further details.Just curious, have you even taken a half look at the (Deceptively) simple schematic with the AMS1117 I posted?The schematic looks simple, but it's clever and they've done everything that I can think of right (Except maybe adding some extra protection on the wires).

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 26, 2022, 07:41:53 am
content: QuoteYes, I can make a guess, since you couldn't convince me that it's useless?QuoteYes, 2.5mv on the Drive cable and what is the value for the Sense cable?QuoteIs the complete and updated schema in Reply #6?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 26, 2022, 08:15:03 am
content: The upgrades shematics is still a bit confusing, but not that bad as it looks at first.The 10 Ohms resistor in the link the sense low is essentially eliminating that link, as long as the cable resistance is way small than those 10 Ohms. I consider the 10 Ohms more like a links for the case that no DUT is connected. A diode or 2 diodes back to back may be even better here. A considertably larger resistor would make it more obvious, that the 10 Ohms stand for a relatively large resistor were the exact value does not really matter.So the upgraded version kind of did use the low side sense as the ground for the output circuit. The more logical path for the ground of the display would be the sense line, at least if there is no much current flowing (shared power ground).Quite a few of the resistors (the 2x100 ohms and 1 K trimmer) are just there to adjust the gain and are a bit confusing to the reader, but otherwise OK.The current source is still a bit strange and not really good: the LM317 is a first crude current source and the OP deverts the the current that is higher than set by the power resistors and the 431 ref. to the reference. This is a rather complicated circuit for relatively poor performance and the need to trim the current from the LM317 quite well.The real test of the 4 wire ohms sense is working OK is not 1 Ohms DUT and 50 mOhms for the wire, but more the other way around. In this example the last circuit would have some 2.3 mV at the 10 Ohms resistor and as additional output voltage or about an 50 mOhms (or 100%) error to the result. The suppression of the wire restance is only about as high as the amplifier gain in the upgraded circuit. That is better than the inital circuit with about a 2 fold suppression from using both wires, but still not good.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 26, 2022, 12:47:03 pm
content: QuoteAlthough strange, in bench tests it has shown excellent stability. So I see no reason to make changes.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 26, 2022, 03:00:12 pm
content: Some tests:1 - IRLZ44 (Datasheet RDS=22mR)2 - IRFB3306 (Datasheet RDS= 3.3mR)3- Precision resistor of 1.744R4 - 1R resistor

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 28, 2022, 08:09:34 am
content: At the moment you just have a simple resistor to open the FET's and you connect it to "+B".Rdson of MOSfets change with Ugs and adding a potentiometer may be interesting (Or a simple DAC?)Most MOSfets open somewhere between 3V and 5V, but Rdson typically halves when the gate voltage is changed from 6V to 12V (and still changes but slower above that).Rdson values are also very temperature dependent.You can test linearity of your meter by first testing a bunch of resistors (for example cheap 1% metal film resistors) and then measuring series and parallel combinations of those same resistors.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 28, 2022, 08:26:59 am
content: QuoteMy main intention with testing mosfets is to find fake or damaged parts.Yesterday I tested some P-channel mosfets and found a fake unit.1 - IRF6218 - false unit on the right2 - IRF6218 (datasheet RDS=120mR ty / 150mR max) - OK3 - IRF6218 (rds three times higher than normal) - Fake4- IRF5210 (datasheet RDS=60mR) - OK

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 28, 2022, 08:48:43 am
content: QuoteI've done several tests in this direction and the linearity is good. All I have to do is keep the cables short and good gauge. But I'll take your advice and add a differentiator. I'm going to buy the INA143 la from LCSC (the only place I can make purchases).

author: Kleinstein
author_rank: Super Contributor
timestamp: June 28, 2022, 10:52:54 am
content: One can already get quite far with making the amplifier a difference amplifier. The difference in the circuit is not that large: just a slight divider for the input signal toward ground. One can check the adjustment by doing a test would a simulated 0 ohms and extra resistance in the low side ground connection. It is not as good as most INAs, but should still quite an improvement over the current circuit.An alternative (not very critical) would be having an additional difference stage (could be gain 1) between the amplifier and display.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: July 12, 2022, 12:47:00 pm
content: HiWaiting for the arrival of INA143 and some resistors (50mR, 30mR, 10mR and 2mR), for a new round of tests.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: July 17, 2022, 12:29:19 pm
content: HiI made two small changes to the design. 1) replaces OPA189 on the output with OP07. 2) I replaced the four 47R/0.25w resistors with a 10w wire resistor. The 1/4w ones were taking a while to stabilize the current. I didn't have the 1w ones, so I put a 10w one.1- 50mA current source with very good stability.2- Precision resistor 1R7443- 1R resistor4- 50mR resistor5- 30mR resistor

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: July 17, 2022, 12:30:47 pm
content: 1- 10mR resistor2- 2mR resistor

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: July 17, 2022, 02:23:16 pm
content: Now testing some Mosfets.1 - IRF6218 ( datasheet RDS ty = 120mR)2 - IRF5210 (datasheet RDS = 60mR)3 - IRF1404 (datasheet RDS = 3.5mR)4 - IRFB3306 (datasheet RDS = 3.3mR)

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: July 17, 2022, 04:24:56 pm
content: HiI found another batch of fake mosfets. Now it was the IRF3205.1 - IRF3205PBF - (datasheet RDS = 8mR) - OK2 - IRF3205PBF - (datasheet RDS = 8mR) - FAKE (RDS=27mR)

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: July 17, 2022, 06:08:58 pm
content: I went to measure some transformers and got a scare with the totally wrong readings. INA143 did not like the presence of the inducers at all. The addition of a 100nF capacitor in the driver line resolved the issue.1- wrong reading2- wrong readingAfter inserting the 100nF capacitor:3- correct reading4- correct reading5- correct reading

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2022, 06:24:22 pm
content: The revised circuit looks OK (though still a bit confusing) for the voltage measurement side and seems to work.The detection side should not care very much about inductance. If needed it is relatively easy to slow a down with some capacitors (e.g. 1-100 nF range) in parallel to the 100 K.The current source can be tricky with a highly inductive load and this is more tricky to fix. A possible fix could be an RC sereies combination in parallel to the DUT to dampen a possible resonance. The rather complicated current source makes it hard to get a good guess. on the suitable valaue. Also just a series resistor at the current source side can help.

author: johansen
author_rank: Super Contributor
timestamp: July 17, 2022, 11:09:53 pm
content: I had relatively good success building a micro ohm meter using a 7$ 1kilogram scale.What you do is remove the strain gauge and run the 4 leads out to kelvin clips... the positive and negative from the battery (one of which isn't directly from the battery but is rather switched on through the circuit board) are sent through 2 resistors. they don't have to be the same value each, but you want approximately the same value so that the amplifier is biased half way between the 3 volt battery and ground.i recall something like 115 ohms or something for full scale being .01 ohms.anyhow the way it works is that strain gauge amplifiers reliably amplify the .1uV difference between the bridge when the load is .1 grams... 1 to 3mv is frequently full scale.when you use a strain gauge as a microohm meter, you aren't directly reading ohms but rather are reading the ratio between the known resistors sending current through the kelvin clips and the unknown resistance.you have to clip the leads to one terminal of the unknown resistor, tare the scale, then clip the leads to the other end. i have found that 2% accuracy is relatively easy to achieve. the resistors used to load the unknown resistance need to have low thermal temperature coefficients, especially if you want to send 100ma through the unknown resistance with only two 1/4watt resistors . also you may want to just directly connect them to the battery rather than relying on the mosfet on the circuit board, which has a resistance that is probably highly temperature variable.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: July 18, 2022, 11:44:21 am
content: QuoteI continued performing various tests with inductors and everything is working as expected, after I added the 100nF capacitor.It was a great challenge and satisfaction for me to obtain the results achieved, using only 50mA as a current source. The entire circuit consumes 87mA.The percentage of accuracy and precision achieved was far beyond my expectations.Kleinstein, thank you so much for being here sharing your knowledge.Here's the updated schema.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: July 21, 2022, 01:03:08 pm
content: HiProject finished and thank you very much for the support of the participants.