• Prototype Reveal: USB Micro-Ohmmeter for Sub-Milliohm Measurements

    Dorian Coves • 10/12/2025 at 14:06 • 0 comments

    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.

    Photo of the prototype measuring a precision shunt (100 µΩ, 0.25%).

    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.

    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.

    What’s Next

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

  • The Open-Source Project Behind My Micro-Ohmmeter

    Dorian Coves • 08/28/2025 at 15:15 • 0 comments

    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

    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.

    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.

    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 Micro-Ohms: The Art of Fighting Physics

    Dorian Coves • 08/13/2025 at 11:02 • 0 comments

    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.

    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.

    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.

    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.

    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.

  • Why Build a Micro-Ohmmeter?

    Dorian Coves • 07/30/2025 at 15:22 • 0 comments

    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.

    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.

    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.