Multimeter add-on lets you measure tiny resistance values

This multimeter add-on is called the Half Ohm. It allows you to measure small resistance values, and can be used to track down shorts on a PCB.

The board acts as a pass-through for both probes. When your meter is set to measure voltage and nothing is connected to the probes the display will read out the level of the coin cell that powers the add-on. When you are probing, the value in millivolts is actually showing the resistance in milliohms. This works for any measurement less than one Ohm. Interestingly enough, it will help you zero in on a solder bridge. By probing the two shorted tracks you can find the issue by following the falling resistance values.

[Jaanus] published several posts leading up to the final version of the board. Check out this category link for his blog if you’re interested in reading through them.


  1. Dr. DFTBA says:

    Why don’t they give a schematic for it? Seems like it goes against the open-source philosophy of HaD.

  2. Richard says:

    In the link, there’s a schematic in KiCad format, and in the “category link”, there is some theory of operation.

    As for me, when I’ve needed to measure very low resistance, I’ve just hooked up a known current source, usually from a bench power supply, and then measured the voltage across the resistor with a known current going through it. Doing it this way, the probes to my voltmeter don’t carry significant current, so the probes don’t contribute to the resistance reading. That’s known as “Kelvin” or “4 wire” resistance measurement. If you don’t use the Kelvin method, then yes, it’s possible to zero out the resistance of the probe wires themselves, but it’s not so easy to eliminate the variable surface resistance of the contact between the probe tip and whatever it’s touching.

  3. Trav says:

    looks like a real useful tool.

  4. Mike says:

    It is very rare that I see a project on Hackaday that immediately makes me take my wallet out. TAKE MY MONEY ALREADY

  5. David says:
  6. vic says:

    The multimeter add-on idea is nice but the problem of this deign is that it also measures the resistance of the probes and probe contacts. That’s not negligible when measuring milliohms. A better design (but more complicated) would use 4 probes, 2 to supply current, and 2 to read the voltage drop.

  7. FartFac3 says:

    very astute comment vic. keep it up.


  8. craig says:

    exactly vic. the ultra low resistance meter i use can measure down into micro-ohms. it also needs to be calibrated often, leads cleaned and well maintauned.

  9. Ren says:

    Sorta like my GenRad Bug Hound…
    Useful in finding a shorted IC on a PCB by starting at the supply and following the rails to find the least resistance.

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