When I Really learned Ohm’s Law

Doc Fry first taught me Ohm’s Law; V=IxR. As a high school senior I joined my younger sister Margaret in Introduction to Electronics. For labs we played with vacuum tubes, voltage sources, resistors. I recall the countless times we had a mess of wires on our breadboard and nothing worked. We quickly learned the first lesson in electronics debug–“Did you check that power and ground were connected?”bbb_breadboard1

Studying electrical engineering at the University of Maryland I had several more classes in which Ohm’s Law appeared. In physics, I learned about the electro-magnetic laws behind that simple equation. I sat through two semesters of Circuits and Systems and the professors taught us Kirchoff’s laws for analysis as well as DC and AC characteristics of circuits. I learned about voltage dividers to ratchet down a voltage source and how combinations of resistors with op-amps had useful properties. In our lab class we played with oscilloscopes to measure rectifying circuits and low pass filters. I received A’s in all of these classes.

In March 1985 I began my first full-time job with IBM, working at the East Fishkill campus. I accepted this position because of the opportunity to work in an electronics lab. While I’ll never claim to be a lab whiz, these experiences increased my confidence as an engineer. I will tell you an embarrassing story in which I learned some deeper lessons when Gate Array Illustration 1_chipdesignmagapplying Ohm’s Law. Rookies make mistakes, below is one of mine.

I worked in a group called Product Assurance, whose focus was checking the design quality of the circuit building blocks used in ASIC products (Application Specific Integrated Circuits–Gate Array style.) In our lab we measured the actual circuits against the provided design specifications. Scott, a lab technician, and I worked first on a circuit that had a voltage source of 5 volts and mirco-amps (10-6) current drawn. bench_3_633x327 wrote down the measurements we needed to take, Scott set it up in the lab, Scott took the measurements, I analyzed the data, and I reviewed with the designer. Everything matched the designer specifications. First project successfully accomplished: Check.

Next circuit used the same measurement set-up. This time the measurements were significantly off, by 10’s of millivolts. I went to the designer and pointed out the mismatch; boldohms-law-1_voip.southcars.com enough to claim that our measurements were right and his simulations were wrong. My lack of hubris eventually brought me down, because our measurements were incorrect. A 1 ohm resistance in the measurement set-up caused the mismatch. Now this same 1 ohm resistance existed when we measured the first circuit. The difference in the circuit design mattered: 2.5 volts for a voltage source and milliamps (10-3) current draw. Let’s apply Ohms Law and assume 10 microamps and 10 milliamps.

I, microamps R, ohms V=I x R, millivolts
10 1   0.01
10000 1 10

 

Busted! I revised the data measurements accounting for the voltage drop across that 1 ohm in the measurement set-up caused. Now simulations and data matched within the measurement tolerance.

With this humbling experience I learned about attention to detail when applying a law as basic as Ohm’s Law. This rookie mistake impacted my pride and the respect from the design engineer.
In high school when debugging our circuits on the bread-board I learned to ask “Are power and ground connected?” With my IBM job I learned you needed to check your full measurement set-up. In addition, one does not place blame when resolving an issue. I should have simply stated “Our results do not agree” and asked “Can we determine why together?” I most likely would have had more success working with the designer. So another rookie lesson relates to dealing with your peers: state the facts and make no judgments on who to blame. Solve the problem!

Dear Reader, please share your comments and stories that are sparked by this piece. For instance: share a rookie mistake that you made early in your career. See Contribute for how you can share a story at The Engineers’ Daughter.

Have a Productive Day,

Anne Meixner

4 Comments Add yours

  1. John says:

    Hi Anne

    I enjoyed the blog. But the editor in me noticed that the opening sentence says Ohm’s Law is “I=VxR”. The table later on in the post has the right formulation, V = I x R.

    You may have noticed my last name. Margaret Marcou, the former math teacher at Magruder, is my mom.

    Turns out I’m an engineer too. I work in the oil and gas business.

    Over the years I have learned you are never too old for a rookie mistake. I made a memorable one relatively recently when working on a liquefied natural gas (LNG) plant.

    (An LNG plant take natural gas and cools it to the point where it liquefies at near-atmospheric pressure.)

    To prepare for a meeting I had looked up some of the fundamental properties of LNG, including the temperature at which it liquefies.

    (OK, despite decades of experience, at that time I was a relative newcomer to that part of the oil and gas world, so maybe I was a rookie.)

    Later, during the meeting, I found myself in a heated (ha!) discussion about the liquefaction temperature with a fellow-engineer who was a seasoned pro at LNG.

    Afterwards, I went back to check on the matter and realized that I had incorrectly converted from the value my reference book provided, which was in degrees Fahrenheit, to degrees centigrade. Oops!

    Needless to say, there was much eating of crow and apologizing when next we met.

    Cheers,

    John

    Cheers,

    John

    1. Anne says:

      John,

      So good to meet your virtually. Your mom had a great influence on my high school studies.

      First, thanks for finding the typo in the post. I will go fix it right now.

      Appreciate you pointing out that we are never “too old” for a “rookie mistake,” by sharing your own experiences.

  2. BJ Reckman says:

    There is only one law of engineering: Fick’s Law:

    Flow = Driving Force / Resistance

    It does not matter whether it is electrical, chemical, mechanical, civil, or any other engineering discipline. When you understand this Law, you understand engineering!

    1. Anne Meixner says:

      Thanks for the education on Fick’s Law, I definitely see the similarity and makes a lot of sense.

      https://en.wikipedia.org/wiki/Fick%27s_laws_of_diffusion

      apologies for not replying sooner, I need to check on alerts for comments.

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