Ohm's Law Still Works!
by Darren Ashby

There I was. A lowly engineering student, just starting the professional program at Utah State University. I was arguing with my boss who had a MSEE degree. But he just wouldnęt believe me, neither would my lead engineer (who had a BSEE). I couldnęt bring myself to distrust Ohmęs Law however. Even in light of their ęsuperiorę knowledge. Ięd had less heated debates with rabid dogs.

So this was the problem. I was working as a technician, or associate engineer, (it depended on who was talking). Our department needed to measure the current of a DC motor it could range from 5A to 15A at any given time. But our multi-meters had a 10A fuse in the current measuring circuit. So using Ohmęs Law (which was fresh on my mind being a student and all) I designed a shunt to measure current. I wanted to get a good reading but disturb the circuit as little as possible, so I chose a 0.1 ohm resistor. I built a box to house it and installed banana jack plugs to provide an easy interface to a volt meter. The circuit looked something like Figure 1.

Figure 1: Designed Simple Current Measuring Curcuit

Everyone thought it was great idea, and I built up a couple of boxes. We started using them all the time. After a while, however, we noticed that they were not very accurate. Sometimes they would be off by as much as 50% to 60%. No one could figure out why so I sat down to analyze what I had created.

After a few minutes, I said to myself, "Well duhhh!" I realized that to make the assembly easy I had soldered the wires from the motor to the banana jacks and then soldered some short 14 gauge jumpers to the shunt resistor. My circuit really looked like Figure 2.

Figure 2: As-built Simple Current Measuring Curcuit

My voltmeter was measuring across a larger resistance value than 0.1 ohms. Wire has resistance too, even a couple of inches of 14 gauge wire has a few hundredths of an ohm. Now, V=I*R, if you increase R, you get more V, leading to errors we were seeing. I had made a simple mistake that fortunately was easy to correct. I redesigned the box on paper to look like Figure 3.

Figure 3: Redesigned Current Measuring Curcuit

I took this to my boss, (the one with the MSEE that could do math in his head that I would only attempt with MathCad and a cold drink). His reaction floored me. He reviewed it with the lead engineer and they came to the conclusion that I was completely wrong. They were talking about things like temperature coefficients and phase shifts in current and RMS and a bunch of other topics that were over my head at the time. Thus began the argument. I explained that two points on a schematic had to be connected by a wire and a wire had resistance. While it is often ignored, it was significant in this case.

As they hemmed and hawed over this, I learned that many times it is human nature to ignore what one learned long ago and try to apply more advance theories just because you know them. Also, all the knowledge in the world isnęt worth jack if it is incorrectly applied. I continued to press my point. I must have written Ohmęs Law on the white board fifty times by then.

They finally conceded. They agreed that the extra wire between the banana jack and the shunt was the cause of the error. But that was not the end of the disagreement. How in the world was my new design going to fix the problem by simply repositioning the wires? The resistance was still in the circuit, was it not? I wrote down Ohmęs Law another hundred times, and explained that the current through the meter was very small making the resistance in the wire insignificant again. My astonishment reached new levels as I observed the human ability to overlook the obvious. The first argument was nothing compared to this one. The fireworks started to really fly then.

What is the moral of this story? Well, Scott Adams, creator of Dilbert said, "Everyone has moments of stupidity" as he explained watching someone fix his "broken" pager by putting in a new battery. I have to agree with him. I rediscover Ohmęs Law about every 6 months. Always, always, always check the basics before you start looking for more complicated solutions! My father, a mechanic, tells a story of rewiring an entire car just to find a bad fuse. (It looked okay, but didnęt check out with a meter.) That was how he learned this lesson. Me, I just participated in 4 hours of the dumbest argument of my career.

How did the argument end you say? We never came to an agreement, so I went ahead and fixed boxes with the new design anyway (which they spent several weeks proving were working correctly.) I didnęt say another word, but transferred out of that group as soon as possible. The same design has been in use for about 6 years, and the documentation notes the need to wire it correctly to avoid inaccurate readings. I didnęt write that document, my old boss did. Funny how we didnęt argue Ohmęs Law anymore after that.

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