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resistance testing resistance testing:


I like to test what I've got, in addition to just simply going by the tables and charts. (trust but verify?)
I've been especially interested in testing the affects of wire gauge and resistance over distance at higher
current levels, mostly due to being the one managing the power grid at Field Day.

Testing resistance of wire is tricky because it's very low, and if you try to just measure the resistance of
the wire with a multimeter, you're probably not going to get a very accurate measurement. There are just too
many things that can significantly affect the results. So to get a good measure of low resistance, you need
to find something else to measure that you can get with a high degree of accuracy (that won't be affected by
the measuring process) which can be used to calculate the resistance of the wire.

The usual want to measure this is called the Kelvin ("four wire") method. Ohm's Law says if you know the
voltage and the current, you can calculate the resistance. Four Wire passes a known (or accurately measurable)
current across a conductor, and then you use a meter to measure the voltage drop across the conductor. I can
measure current pretty accurately, set my electronic load to a specific current, or set/measure the current on
my power supply, plus my Agilent can measure millivolts with high precision. So that's how I do it.

As an example, I connect my power supply (set to 50.00 volts) to the electronic load (set to constant current,
2.000 amps) using a known length of wire (25.0 feet), and measure the voltage drop over the wire. Then ohms
equals volts divided by amps. So we do the math and get ohms = 0.05334 / 2.000 = 0.02667 ohms over the entire
length of wire tested. Divide that by 25.0 (feet) to get a final measure of 0.00106 ohms per foot.
Don't run the test for too long or you may heat up the wire and raise its resistance.

There are other ways to go about this, but Kelvin is pretty easy and high precision.

If you were just going to try to measure the resistance of that 25ft of wire directly with a meter, you'd need
a very expensive meter to get a precise measurement like 0.02667 ohms, not to mention the trouble you'd have
trying to eliminate the small resistances of things like your test leads.

Bonus facts: the load "sees" 49.34 volts. Having lost 60mV (at 2.000 amps) and we know the wire dropped 53.34mV
(twice), so there was a total of 55.3320mV dropped by power supply wires, test leads, and connectors. (277mOhm)
This is FIVE TIMES the resistance of the wire under test. That's why it's impractical to directly measure small
resistances like this.

100' + 4.25" 8.6 (?) lbs


  1 prep (precision power supply and electronic load are ready)  
   2 prep (will set the PS to 50 volts)  
   3 prep (electronic load is set to draw exactly 2.000 amps)  
   4 test (there's exactly 25.0 feet of wire between the test leads)  
   5 test (power supply turned on - test leads are INSIDE the power supply wires)  
   6 test (Agilent measuring the millivolts at high precision, watching for drift)  
  1 prep (precision power supply and electronic load are ready)     2 prep (will set the PS to 50 volts)     3 prep (electronic load is set to draw exactly 2.000 amps)     4 test (there's exactly 25.0 feet of wire between the test leads)     5 test (power supply turned on - test leads are INSIDE the power supply wires)     6 test (Agilent measuring the millivolts at high precision, watching for drift)  
  7 verify (verifying the electronic load is drawing exactly 2.000 amps)  
   8 verify (power supply agrees with 2 amps)  
  7 verify (verifying the electronic load is drawing exactly 2.000 amps)     8 verify (power supply agrees with 2 amps)  


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