Half Ohm - milliohm adapter
Half Ohm - milliohm adapter

Half Ohm - milliohm adapter

Half Ohm milliohm adapter is ready! Multimeter adapter that transforms any multimeter into a precision milliohm meter and debugging device. Notice that your multimeter is useless when measuring small resistances, like connectors or tracks. So did I, so I built this: 1% precise, small and cheap adapter for you. Ultra useful for finding position of shorts on your board. I have already used it tens of times. Get one!

Milliohm meter in connected

Milliohm meter in connected

Usage:

  1. Connect the adapter to your multimeter and power both of them.
  2. Half Ohm should light up brightly.
  3. Switch multimeter to voltage range.
  4. When nothing is connected, you can read the battery voltage. Battery is 3V CR2032, so if the voltage is lower than 2.6V, consider changing the battery.
  5. When connected with a resistance smaller than 1Ω the voltage in mV shows measurement in mΩ. So 1mV = 1mΩ, 10mV = 10mΩ and so on.
  6. Connect your probes to adapter and connect them both with each other (press into one pad or something like that) the readout is more or less the resistance of your probes, connections and meter itself. Zero that with RELATIVE button or just memorise it.
  7. Measure the real thing, the difference between last and this measurement is your resistance.
The back side

The back side

Finding position of short circuits:

Connect and power everything. Press your probes to shorted tracks. Probe around. The direction where the resistance decreases is the direction of the short.

Calibration:

If you want to test or calibrate the adapter then you can use 100mΩ resistor that is included. On the bottom edge of the PCB there is big 1206 100mΩ 1% resistor that you can measure.

The sources:

Schematic and board in KiCad. My KiCad libraries! Schematic in PDF.

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Posted on 2012-06-06, 22:58 By
24 comments Categories: Half Ohm, KiCad

24 thoughts on “Half Ohm - milliohm adapter

  1. Probably just my noob status with Kicad...but the HalfOhm project doesn't find the opa335, the coin cell conn, etc. and I don't seem to find them your Kicad libs either.

    1. That was my bad. I had accidentally deleted some of the library components. Now I redrew the components.

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  2. 1mΩ to 1Ω. Under 10mΩ the pressure on the probes starts playing almost too much. This will probably improve with sharper and/or golden probes.

  3. Hi Jannus,
    Where is the resistor for calibration connected to?
    All the resistors used in the schematic are precision resistors?

    Thank you very much.

    Bimi.

  4. Calibration resistor is not connected to anything. You must use your multimeter leads to measure it.

    And all the resistors are 0.1% precision resistors.

  5. What part are you using for the banana plugs? I'm working on a project that I'd like to plug directly into a power supply with banana jacks, but can't find any form of SMT banana plug part.

  6. I'm using hobby RC 4mm banana connectors. They are not SMD, I made the shape of the PCB weird so it would hold the connectors in place.

  7. The lower the shunt current the better the op-amp has to be. At the moment the balance between currents and precision is the best I could do.

    It is possible to make the current 2x smaller and lose 2x of precision.

  8. Hello Jaanus,

    Greetings from Lithuania! Great job! I would like to ask what zener diode (U1) you use? And is it possible to expand measure range up to 2 ohms?

    Thanks for Your answer!
    Best regards

    1. I use precision voltage reference, not zener. Voltage reference is needed to get the claimed spec. It is possible to expand measurement range by changing resistors or using higher reference voltage. Both would have impact on absolute precision.

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