Electronics > Open Source Hardware

Scullcom MilliOhm Meter Software


Several years ago Louis Scullcom published circuit for a "quite good" milliohm meter. An associate then improved the pcb layout and it was made available from OSH Park. This circuit used a DPM or Multimeter to display the resistance, which is fine if that's all you want, but more is possible with a micro.

I had a need for such a meter to measure the winding resistances in 3ph motors I use in radio controlled aircraft. These values are quite small because the wires are very fat - my motors are 3 to 10kW. I make my own inverter/controllers and the published resistance figures are just a marketing fantasy making it difficult to tune the controllers.

So here it is for anyone who is interested. Commercial jobbies are available from China for around 140AUD. This one is not quite as pretty but performs better and has more creature features.

The pics are self explanatory. The instrument is auto-ranging and shows mV and uV. There is a protocol that defines when the reading is steady after which the filtered value and RMS deviation is displayed. The battery indicator is just a volt meter which works for Lithium chemistry. There is a USB charging port on the side. The battery is scavenged from a worn out laptop pack and so far I haven't charged it in a year of use.

The switch is an old-fashioned mechanical one that kills the power to the circuit - no chance of bleeding the battery down with this circuit. Put it on the shelf and leave it there for two years and it will still work when you need it to.

The software and hirez pics are on my github site https://github.com/jfitter/MilliOhmMeter

Nice Project!
I was working on same project & was looking at the same Kelvin probes available from China, I would like to know, how is the quality of the Kelvin Probes you are using?

I recognise Barbouri's (Greg) PCB creation.
Your LCD is way much better though.


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