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Measuring Small Resistances in Live Circuits

Dear Reader,

Brian Benchoff just posted about this demo on Hackaday (Thanks!), and we thought we should add some commentary.

Usually to measure resistance, one must remove the resistive element from the circuit and connect the meter across it. Sometimes that’s impractical or impossible.

If you know the voltage across an element and the current through it, you can calculate its resistance. If your circuit has large enough currents, you can use a DC current clamp to measure the current through an element within a few percent (+-3% for my clamp), and a voltage channel on the Mooshimeter to measure the voltage across the element.

Since the Mooshimeter has an extremely high precision input, I decided to take the demo a step farther and measure a very small resistance – about 185µΩ – of a grounding cable in a car.  You’ve seen us use the ground cable as a current shunt in previous demos, but we had not accurately measured its resistance until now.

You can do this without a current clamp if there is a known resistance in series with the unknown resistance – in that case, you can attach one of the Mooshimeter’s voltage channels across the known resistance and the other across the unknown resistance.  In that scenario, the slope of the line in the XY graph will be the ratio of the two resistances.

Why Small Resistances Matter

185µΩ may not seem like a lot of resistance, but these sorts of numbers really matter when you get in to the realm of hundreds of amps.  When you’re working with that much current, you have to worry not just about the resistance of the wire but the resistance of your crimps and contacts as well, which are usually in the µΩ to mΩ range.

In a recent thread on ChiefDelphi there was a debate about the stock FIRST robotics battery connectors and a rather dramatic photo of an overheated connector:

melted_connector

Photo Credit Andrew Palardy

Though the connector was working and passing current, one side of the connector seems to have overheated and melted the casing.  The fact that it only happened on one side of the symmetrical connector would seem to indicate an assembly defect, possibly a shallow crimp, on one side of the connector, but it’s impossible to say for sure from here.  The resistance was probably still in the mΩ, but FIRST robots can easily draw over a hundred amps in competition.  A 100A current through a 5mΩ contact resistance dissipates 50W of heat.  Yikes.

Bottom line – small resistances matter in the power path!

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