MiliOhm meter as an exercise for students - Page 1
Forum section: electronics / projects

author: firewalker
author_rank: Super Contributor
timestamp: July 13, 2025, 04:55:53 pm
content: I teach electronics in a vocational training institute. In the analog electronics class I was showing them the 4 point measurement method for milliohm range resistors (3 phase motor coils). Someone asked me if we could build a circuit to measure those low resistors.Such a circuit:Doesn't need to be really accurate. Just repeatable to compare the difference between the measured coils.Utilize through hole parts.Avoid expensive ICs (like INA106).Combine the lesson material.Is the attached circuit too much for automotive technicians learning analog electronics? My goal is to also design a pcb for an enclosure, kelvin clips and every students to keep it as memorabilia.An alternative circuit is using a transistor multivibrator instead of the 555.Alexander.

author: BennoG
author_rank: Frequent Contributor
timestamp: July 13, 2025, 06:57:33 pm
content: What I would do, is use a bench power supply and set it current limit to 1 A (or 10A if you want to measure really low resistance) put that DC current into the DUT and measure the voltage with a normal volt meter over the DUT (then you wil have a repeatable measurement of the MiliOhm (1 mV per MiliOhm, or 10mV per MiliOhm)then if you will put the volt meter over the output of the bench power supply you can explain and proof that you need 4 wire measurement for this task.Benno

author: Gyro
author_rank: Super Contributor
timestamp: July 13, 2025, 07:05:37 pm
content: Old Maxim AN106 (note, DC measurement despite thread title)...https://www.eevblog.com/forum/projects/design-review-ac-milliohm-meter/msg4589653/#msg4589653Edit:http://www.wentztech.com/radio/resources/Projects/AN106.pdf

author: firewalker
author_rank: Super Contributor
timestamp: July 13, 2025, 07:12:00 pm
content: Quote from: BennoG on July 13, 2025, 06:57:33 pmThis is the exact thing we did in the lab to measure the winding of an PMSM from a Volvo (bench power supply and a volt meter). Also used a somewhat expensive handheld instrument to compare.The question of the student was if we can make something our selves.Alexander.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 13, 2025, 07:26:16 pm
content: I don't think this circuit should be hard to assemble. I don't know what type of material you cover in the course, so I don't know how well this matches that.A couple of thoughts on the circuit:I would use a better opamp than the lm358. There are cheap opamps with better offset voltage.Make sure to specify precision resistors for the instrumentation amplifier.You shouldn't really need the offset trim. With careful part selection you can get the offset sufficiently low for this application. Also, you can use the zero cal feature of whatever DMM you use with this.This circuit can be made to work in a single supply configuration. But the charge pump demonstrates a useful concept and makes it easier to understand.I would add a momentary push button to enable the current source. That avoids draining the battery too much and also allows easier zeroing of your DMM if you find that necessary.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 13, 2025, 07:35:14 pm
content: I would make a simple MOSFET and opamp current sink, with a coin cell or two supplying the gate drive and an AA cell supplying the output current of 1A.

author: firewalker
author_rank: Super Contributor
timestamp: July 13, 2025, 08:00:43 pm
content: All of those are part of the class.Voltage regulators.OpampsInstrumentation amplifier (part of the strain gauges for the way some pressure sensors work).Stable and astable multivibrators (simple flasher circuit).Zener diodes (half the charge pump circuit).Voltage peak detectors (half the charge pump circuit).Alexander.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 13, 2025, 08:07:50 pm
content: Quote from: firewalker on July 13, 2025, 04:55:53 pmThat has a high parts count.You could use a ICL7660 as a charge pump, or eliminate it entirely?Since everyone should have good multimeters, simpler would be a precision current design, as suggested above.A choice of 100mA, 1A and maybe 500mA for USB chargers ?Or, a SMPS can reduce the load seen at 5V to give you 1A / 400mV from 100mA/5V.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 13, 2025, 11:24:30 pm
content: Why THT only? Wired resistors are nice for breadboards, but for the rest they're just a nuisance. Drilling holes in a PCB, bending and cutting all those wires, rotating the PCB to solder them. It all adds up, especially on bigger projects. SMT resistors are much more user friendly. You just pick them up with some decent quality tweezers and solder them in their place (I like to put a bit of solder on one pad beforehand, so I can make a preleminary fixation without having to have a third hand (soldering iron, solder, Tweezers for SMT part). SMT parts of 0805 size are very easy to handle, 0603 is also still quite doable for most people, but smaller parts start to become cumbersome.You have also made a complicated schematic.First, you don't need the 555 and the charge pump. LM358 works down to a handful of mV for the inputs and outputs, and you can easily create an offset voltage which is used as the reference. Building an instrumentation amplifier out of LM358 is also questionable. It's got a quite high offset voltage, and more opamps equals more difficulties with offset voltages. If your input has an impedance in the milli ohm range, and the other inputs of the opamps in your in-amp have a 5k Ohm impedance, you also have extra offset voltages due to bias currents.You also don't need the LM7805. LM358 works perfectly from a raw battery voltage, and the current source around the LM317 also works just as well directly from the battery voltage.So, what I would keep is the current source of the LM317.1.) I would replace R17 with a simple diode to get a virtual GND of around 600mV (I.e, negative battery terminal becomes -600mV) This is plenty to make the LM358 happy.2). Ditch two of the three opamps of the in-amp, and make a simple differential amplifier with just 4 resistors. Your expected input impedance is in the milli-ohm, so the opamp does not have to have a particular high input impedance itself. You can also make it a differential amplifier, so both inputs see the same input impedance, which minimizes input offset due to bias current.3). Ditch the potentiometers. 0.1% resistors are 10ct these days (at least SMT versions in 20+ quantities) But still, having a trim pot to zero out opamp offset would not be too bad. Meh, maybe leave them both, so the students have something to tweak.There are two things that are not very clear to me. First, what is R16? Is it the resitor to be measured? Where is that and how is it connected?Second: How is the DMM connected? The DMM has it's own floating power supply, so you can connect it's "negative" wire to any arbitrary part in the system that you like.Also, have a look at the circuit below.It's got a whole article with background info in Circuitcellar #314, It uses a bunch of uC pins to create an alternating current (to compensate for thermocouple effects) and it does not have a steady voltage reference, but instead it uses a radiometric comparison with a known resistor.On CircuitCellar, you can also get the original source code of (nearly?) all projects. It's such a shame that their subscription more then doubled when they started "cooperating" with Elektor. That is when I did not renew anymore.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 14, 2025, 12:59:22 am
content: Quote from: firewalker on July 13, 2025, 08:00:43 pmMaybe the circuit is ok, if all that is covered, but it's still a high parts count.Likely you also cover current sense amplifiers, so you can use one of those INA180 class parts, sub 20c from lcsc, with typical 25uV VOS and 0.1% Gain specs.The common mode range of a current sense amp also allows you to use a low-cost 0.5% shunt-sink voltage reference + NMOS as a precision current sink reference.If you want precision currentsource, a LM4041-ADJ + PFET has the lowest parts count.

author: shabaz
author_rank: Frequent Contributor
timestamp: July 14, 2025, 01:40:28 am
content: Hi,You can make it dirt cheap and stable and accurate and (if desired) through-hole. Plus, less "intrusive", and longer battery life, with just a 10 mA or 1 mA current rather than 100mA.Here's a demo video (might want to set the speed to 1.5, since it's mostly real-time, I didn't have time to edit much) for a comparison against a calibrated meter :The circuit costs very little as mentioned (you can significantly reduce the components by only implementing one range, and (if desired) perhaps even just use a cheap multimeter as the display rather than a panel meter or ADC+microcontroller.The circuit snippet below shows the key values for the constant current part of the project, it is temperature compensated to aid with stable measurements.There's no need for the trimmer resistor shown in the circuit snippet, unless you wish to really fine-tune the stability. The fixed resistor values are already quite good enough, provided you use a TO-92 package 2N3904TA for Q1, from On Semi (which is what they are optimal for).The circuit will require one trimmer for the (approx) 100X gain op-amp amplifier adjustment, and a single known-value test resistor of about 3-4 ohms (could be shared around the class) for the alignment of that trimmer.For the constant current circuitry, it would be good to use stable resistors, but they don't really have to cost a lot to get the results demonstrated in the video.Probably the entire circuit could be tweaked for $10 electronic component BOM maybe, excluding Kelvin clips/enclosure and so on. To save costs, and construction time, students could hard-wire the Kelvin clip cables to the meter, banana plugs/sockets are not essential.I don't have a PCB design for this project, if you were interested, you'd have to make your own (hopefully more spacious than the cramped enclosure that I had). If you do make it, please share the PCB design, since I might be interested to assemble one more too.

author: johansen
author_rank: Super Contributor
timestamp: July 14, 2025, 02:22:43 am
content: A milliohm meter can be made from any common strain gauge amplifierBut instead of a 750 ohm strain gauge bridge, you substitute two 100 ohm resistors and kelvin clips..Zero the amplifier by shorting the kelvin clips together. Move the clips to the dut.Its typical for 2 or 3 parts out of 1000 to be full scale (often 16 bits minimum)The 100 ohms (or whatever you need to make the conversion) is not a perfect current source but remember: the strain gauge amp doesnt have a fixed voltage reference in the adc. Its a fixed voltage ratio amplifier with the source voltage used as the ADC reference voltage! It has always been measuring a ratio of two resistances (4 really, in a bridge configuration)I was able to reliably measure down to 0.1 milliohms and 0.1 ohms full scale from a 1kg "drug" scale powered by 2 AAA batteries. You could reliably see 0.1 grams per 1/8” of an inch as the kelvin clips were slid from one end to the other of a 10 ga wire. I dont remember what resistors i was using but i remember being surprised i didnt burn out the switch that turns the strain gauge on. (A mosfet buried under the epoxy blob)I would start with 100 ohms or 75. Two of them, so the DUT is half way between the rails.. im sure its a chopper stabilized amplifier to get the offset down to less than 1 microvolt.

author: JohanH
author_rank: Frequent Contributor
timestamp: July 14, 2025, 05:24:34 am
content: Scullcom's videos are very educational:I built one, but followed a bit upgraded design by Barbouri:https://www.barbouri.com/2016/10/09/milliohm-meter-version-1-5/I recreated the schematic and PCB and made my own variant of it. I had issues with the panel meter that I used. The cheap three wire voltmeter panel meters have common ground with their power supply. When the meter is powered by the same power source as the constant current source, there will be a small measurement offset. I solved it by using an isolated DC-DC converter to power the voltmeter, which made the meter floating and eliminated the offset completely.

author: shabaz
author_rank: Frequent Contributor
timestamp: July 14, 2025, 05:55:34 am
content: Quote from: JohanH on July 14, 2025, 05:24:34 amThere's an easy solution to that, provided the meter is an AliExpress one with MCP3421 ADC which a lot of the accurate ones have. See the photos, first step is to pull off the plastic connector shroud as indicated, then cut the track indicated with a purple dot. Then, connect two wires as shown in the second image, this will be a differential input. This worked for me for the milliohm meter in my earlier post, I think it would work for that scullcom one too perhaps.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 14, 2025, 07:14:50 am
content: I would be tempted to build an ESR meter instead of a miliohmmeter, because that will also be useful to detect bad capacitors, or bad batteries. There are many types of ESR meters out there, this is one example (only 5 transistors):#247: Circuit Fun: 5 Transistor ESR Meter circuit by EEVBlog user Jay_Diddy_Bw2aewMore about the schematic in that video, from the schematic author:https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/Another simple ESR meter:#5: My ESR Meter project from 2006w2aewhttps://youtu.be/bmYAgat-sOQ

author: firewalker
author_rank: Super Contributor
timestamp: July 14, 2025, 09:20:12 am
content: Quote from: RoGeorge on July 14, 2025, 07:14:50 amWe did build an ESR meter (utilizing an old analog meter), and an injector tester and a voltage peak detector tester fpr motorcycle coils.