Computing stuff tied to the physical world

TK – Measuring milli-ohms

In Hardware on Jun 28, 2012 at 00:01

Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.

Today, I’d like to present a nifty new arrival here at JeeLabs, called the Half Ohm:

DSC 3348

It’s a brilliant little tool (with a cute name). What it does is convert milliohm to millivolt, and it works roughly in the range 0 .. 500 mΩ. Hence the name. On the back is a coin cell, and there’s a tiny on-off switch. Luckily, it’s hard to forget to turn the thing off because there’s a bright red LED while it’s powered up.

Milliohms are tricky. Fortunately, Jaanus Kalde, who created this tool has it all explained on his website.

So what can you do with a milliohm meter? Well, measure the resistance of your test leads, for one:

DSC 3349

That’s 22.8 mΩ, i.e. 0.0228 Ω. Not surprising, because (almost) every conductor has some resistance.

Being able to measure such low resistances can be extremely useful to find shorts. For example, when using longer test leads, I can see that their own resistance is 74 mΩ. And with that knowledge, we can measure PCB traces:

DSC 3350

Turns out that in this case I got about 112 mΩ, which means that this little 5 cm stretch of copper on the PCB has 37 mΩ resistance. And sure enough, the other ground pins have less resistance when when the path is shorter, and more when the path is longer. This is very logical – note also that between the GND pins I’ll be measuring relatively low values because of the relatively “fat” ground plane, which reduces overall DC resistance.

To find shorts, we simply measure the resistance between any two points (with no power connected, of course). If the measured value is close to 75 mΩ, then it’s a short. If it’s well above say 150 mΩ, then it’s definitely not a short.

To locate a short circuit, we can now simply move probes towards the direction of lowest resistance.

Looks like the Half Ohm will be a great help for this type of hard-to-isolate problems!

PS. No need to do the subtraction in your head if you have a multimeter which supports relative measurements.

  1. Hehe… I just knew you’d like that when I saw it… And here’s something I liked when I saw it:

    Your Raspberry Pi has now been shipped from our warehouse.

    Farnell Ref No: xxxxxxxx Your Order number: yyyyyyyyyy

    :-D

  2. I like the hard-to-isolate part.

  3. Have you got any idea, what sort of contribution the contact resistance has? Or are you not concerned by it? I suppose it doesn’t matter much if you’re only interested in finding shorts.

    • I don’t know… but using test probes with sharp pins it should not be much – a few mΩ at most, I expect. Same for probing in solder joints.

  4. I’m confused. Why can’t you test for shorts with a regular multi-/ohmmeter?

    • I’ve updated the post to explain how to locate shorts with this tool. Thanks for pointing this out.

  5. @georg – sure can. But if you are trying to find where along the track the bridge is hiding, mΩ resolution helps determine “closer”.

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