LysVolt MicroOhm - 4-wire precision ohmmeter

After a few teasers, here’s the first fully functional version of my USB micro-ohmmeter.

Originally designed to test contact resistances in EV charging connectors, it’s now proving to be a precise and repeatable measurement tool for shunts, cables, and low-resistance connectors of all kinds.

Key Specs

• Measurement range: 0–100 mΩ (more to be tested)
• Resolution: < 2 µΩ
• Calibrated: Using precision resistors (100 µΩ, 1 mΩ, 10 mΩ)
• Interface: USB-C with SCPI command set

The first measurement results are even better than expected — especially in terms of stability and effective resolution.
For example, the ADC output shows about 88 counts when measuring a 100 µΩ shunt, which translates to an estimated resolution of ~1.14 µΩ.

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Looking for Early Testers

I’m now looking for early testers interested in trying out this first version. If you work with low-resistance measurements and would like to give feedback on performance, usability, or possible improvements, please leave a comment or send me a message.

Your input will directly shape the next revision — which I plan to release later on Tindie.

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What’s Next

In the next project log, I’ll run a stability analysis and share some thoughts on calibration methods for milliohm measurements.

Before designing my own precision micro-ohmmeter, I started by studying an excellent open-source project by Dennis Vollrath, published in Servo Magazine:

https://www.servomagazine.com/magazine/article/build-a-simple-micro-ohmmeter

It’s a compact, well-thought-out DIY design with solid explanations — a perfect starting point for anyone exploring low-resistance measurements.

Simplified schematic of Dennis Vollrath’s design

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What Makes Dennis’ Design Great

This project achieves surprisingly high performance for something you can build at home thank's to:

• High test current — around 1.2 A
• 18-bit ADC ENoB ! (17 bits in unipolar mode)
• Programmable differential amplifier with gain up to ×8 (in the ADC)

That translates into an estimated measurement resolution of:

Resolution = ADC voltage step / Current = (0.256 / 2^17) / 1.2 = 1.6 µOhm

For a device you can build for under €50, that’s remarkably good — especially with such a short BOM and a straightforward assembly.

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Why I Had to Go Further

While Dennis’s design is excellent for hobbyists, my goal was to build something industrial-grade for integration into automated test benches. That required several key changes:

1. Professional Hardware

I redesigned the PCB to meet professional manufacturing standards — compact, robust, and ready to fit inside a proper enclosure with DIN rail mounting and standard connectors.

2. Power Supply Upgrade

The original design used batteries and a 5 Ω resistor to limit the current (Power loss = R*I² = 7,2 W in heat). My version had to work directly from the client’s 24 V DC supply, so I redesigned the power stage to efficiently deliver a stable, high test current. I also added circuitry to pulse the current only during measurements to avoid unnecessary heating.

3. USB-Only Interface

Since the device is part of an automated test bench, it doesn’t need a screen. I switched to a USB-only interface and rewrote the firmware accordingly to communicate directly with the client’s software.

4. Improved Analog Front-End

I added analog filtering on the ADC inputs and carefully optimized the PCB layout to minimize crosstalk and noise pickup — critical when you’re measuring signals in the microvolt range.

5. Calibration for Accuracy

Finally, I wanted each unit to be factory-calibrated against precision reference resistors to guarantee performance and reproducibility. I’ll share more about the calibration process in an upcoming log.

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What’s Next

Dennis Vollrath’s project was a fantastic foundation, and my version takes the concept further — from a DIY project to a precision lab instrument.

In the next log, I’ll unveil my custom micro-ohmmeter design — optimized for accuracy, reliability, and industrial integration.

Stay tuned!

Measuring small resistances value isn’t new to most engineers — but once you get down to the milli-ohm or micro-ohm range, small details start to dominate. Wiring and connectors can add more resistance than the part itself. Contact resistance can change between measurements. High test currents cause self-heating, altering the value you’re trying to capture. And even tiny temperature differences between metals can generate microvolts that mask your real signal.

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The Main Challenges

• Parasitic Resistance
Wires, connectors, PCB traces…  …all add unwanted resistance.

• Self-Heating
High current improves resolution, but even a few °C rise can cause a 1 % error.

• Noise & Resolution Limits
Measuring 60 µΩ at 1 A means resolving 60 µV — far beyond the 10-bit ADC of an Arduino UNO.

• Thermoelectric Voltages
  
Temperature differences between metals create microvolt offsets.

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The Reliable Fix: 4-Wire Measurement

The only robust method is the Kelvin (4-wire) connection:

Two wires inject a known current through the resistor. Two separate wires measure the voltage drop.

This bypasses wiring resistance and lets you use a high-impedance voltmeter or precision ADC for clean readings.

Limit heating by applying current only during measurement. Your resolution will depend on voltmeter precision, range, and source current quality.

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DIY 4-wire setup

You can try this right now with:

• A power supply
• Two multimeters
• Four probes

One meter measures current, the other measures voltage. Simple, but dramatically more accurate than a 2-wire setup.

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Measuring micro-ohms isn’t magic — it’s about eliminating every source of error.

In the next log, I’ll share the open-source design that inspired me to build my own compact, “low cost”, precision micro-ohmmeter.

This whole project started with a phone call from a friend who designs custom industrial machines:

- Hey Dorian, I need to measure how the electrical resistance of automotive connectors changes as they get mated and unmated thousands of times. Is that something easy to do?

- Well… that depends on what resistance values we’re talking about.

- Around 60 micro-ohms.

- And what’s your budget?

- Under €1000 ... for the whole electronics part of the test bench... and everything has to fit in something the size of a shoebox.

At that point, I couldn’t help but laugh.

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When I started looking into it, I realized I couldn’t find any instrument that could:

• Resolve a few micro-ohms reliably,
• Connect to a PC over USB,
• Be compact and robust enough for industrial use,
• And still cost under €500 (most commercial units are well above €1000).

The closest solution I found was to cobble something together using a programmable lab power supply and a bench multimeter in a 4-wire setup, all controlled by a PC.

With decent Siglent gear that would have cost around:

• SDM3045X 4½-digit multimeter – €369 (excl. VAT)
• SPD1168X programmable power supply – €229 (excl. VAT)

So about €600 per measurement channel, and that’s just for the instruments — not exactly compact or budget-friendly.

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And that’s when I decided to build my own tool.

In the next logs, I’ll dive into why measuring such low resistances is tricky, and show the open‑source design that kick‑started this project.