Gas station without pumps

2016 April 29

Miswiring errors

Filed under: Circuits course — gasstationwithoutpumps @ 15:25
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In yesterday’s post, Revised microphone pre-amp lab too long, I wrote about problems in this week’s lab, and one of the items seems to have resonated with at least one other instructor:

a surprisingly large number connected both nodes for a resistor to the same end of a resistor, leaving the other end unconnected.  I’ve not seen that mistake before, so I don’t know what triggered it.

CCPhysicist commented

I’ve seen that error (connecting two wires to the same end of a resistor) before, more than once, but I also don’t have a clue why they do it. It is worst if the resistors are in a box where they can see the connectors but not the resistors (even when they see the resistor symbol between the connectors), but also happens with loose resistors. Now my students have the excuse that we start doing those labs before we get to DC circuits in lecture, so I assume it means they have no idea that current flows through things and that switches break a circuit, but I have no idea why they get to college without any experience related to the basic concept of electric current. Maybe whatever misconceptions they have about current are stubborn enough to survive a semester of physics.

As for why you got many instances of that error, I’d suspect “authoritative ignorance” syndrome. Others were following someone who talks a good game but doesn’t know the play. Can happen just by one person looking at what another is doing, without any actual bad mentoring taking place.

I don’t think that “authoritative ignorance” was the problem here, as the students making the error were in both sections and they made it in different places in the circuit. I responded with my best guess at what was happening:

My conjecture is that students aren’t using a misconception of current—they aren’t thinking about the function of the resistor at all. They just have the idea “connect up the resistor to A and B”. Having a wire between point A and the resistor and between point B and the resistor satisfies that objective, even though it doesn’t mean anything if the resistor is not between A and B

I discussed this with the class today, and suggested that they change their mental language and think of connecting a resistor between two nodes, rather than to two other components. I also talked about switching their thinking from “components connected by wires” to the dual graph, nodes connected by components, and assigning a color to each node.

Color coding each node makes it much easier to notice incorrect connections (two different colors connected together), though it doesn’t help with noticing missing connections.  For that, I recommend that students check each component to make sure every node is there, and every node to make sure it has the right number of components.

CCPhysicist commented

Perhaps I will work on introducing the concept that labs like most of our circuit labs are about discovering the function of everything we use (meters as well, because they are part of the circuit, and even the wires themselves), and discourage the use of words like “to” instead of “through”. After all, the two wires in your example actually do carry current “to” and “from” the resistor!

I insist in the weekly design reports that students not use “voltage through” or “current across”, but always “voltage across” and “current through” to talk about V and I for a component.  I don’t think that this help much with their understanding, though, as the misunderstandings about voltage always being a difference are still common, and students still routinely apply Ohm’s Law to voltages and currents measured in different places.

Any problem that involves a voltage, a current, and a resistance causes many of them to invoke V=IR, even when the voltage and current are unrelated or related in something other than a simple resistance.  (For example, when chosing a DC bias resistor for an electret microphone, we have a non-linear I-vs-V relationship for the mic, and generally have a voltage drop across the resistor that needs to be added to the voltage drop across the microphone to get the power-supply voltage, but students will take any of the voltages (the mic voltage, the voltage across the resistor, or the power-supply voltage) to get the resistance of the bias resistor, when only one of the voltages is appropriate.

My labs are not about “discovering the function of everything we use”, but about learning how to design circuits with imperfect parts. (That’s one difference between a physics lab and an engineering lab.) I’m trying to give the students tool skills: both mental tools and physical tools.  The notion of having multiple models for something and using the simplest one you can get away with is one of the skills I’m trying to get them to develop.  The extremely simple models used in intro physics courses are often not good enough for practical use and developing better models from first principles is too hard, so we do a lot of measuring and empirical fitting.  (The loudspeaker modeling lab is a good example, where we go through 4 different models of the loudspeaker: R, L+R, L+R+RLC, semi-inductor+R+RLC.  Sometimes the simple model of the loudspeaker as being 4Ω is adequate, and sometimes we’ll use the full complexity of the non-linear model.)

There are a lot of learning experiences that are generally unavailable with simulations (like the problem of measuring voltages in voltage dividers made of 4.7MΩ resistors when your meter has a 10MΩ input impedance, or the problem of clipping when using high gain in an op amp, because of input voltage offset errors).  Students are much more likely to remember to design around input offset voltages if they have observed an unexpected output voltage offset and tried to figure out what caused it, than if they are simply guided to do designs that have low gain without knowing why (or allowed to do large-gain designs without realizing that they wouldn’t work reliably, as I have often done myself, even though I theoretically know better).

 

5 Comments »

  1. ” … nodes connected by components, …” I like that a LOT!

    It is a nice version of the “where do you put the light bulb if you want it to light up” problem.

    Agree on the difference between discovery and design, but I emphasize discovery because I hope that helps make what they are supposed to be learning stick with them long enough (as a good mental model) that they can design when they get to an engineering circuits lab. You might gain some insight by visiting the physics lab they take and any engineering lab they take between that and your class.

    Good point about their reports. I’m not sure what my rubric says, but will check to see if I remind myself and the others who teach that lab to watch specfically for “through” and “across”.

    Comment by CCPhysicist — 2016 April 30 @ 12:57 | Reply

    • Some of them are currently in the Physics E&M lab (I had to reduce the prereqs to get students to take the Applied Electronics early enough). Their comments are that the physics labs are trivial (at least in comparison with the applied electronics labs). Most of the students have no other engineering labs (just chemistry and biochemistry) until their senior design project.

      Some of the students have had the EE circuits course and lab, but I can’t tell from performance on the design reports which students they are. I plan to check at the end of the quarter to see if the EE circuits course had any predictive effect on grades.

      Comment by gasstationwithoutpumps — 2016 April 30 @ 13:05 | Reply

      • That mix of prerequisite skills presents you with a really big challenge. They are correct that their physics lab is trivial by comparison. I would be shocked if their physics class even mentions complex impedance. (Adding a non-ohmic device to one of my course’s labs is pretty unusual, AFAIK. Those devices are not in the lecture part at all.) In contrast, the basic EE course has the physics class and lab as a prerequisite, and they would be dealing with complex impedance by about this point in a “term” calendar like you use. That is a huge spectrum of skills with “nothing” being your default requirement. Does your book have a section covering “what you haven’t or didn’t learn in physics” chapter?

        I’m learning a lot about your degree program as well as what some of my students should learn. Nearby Wannabe Flagship has a biomedical degree that is closely coupled to chemical engineering and pre-med requirements (some of my former students are in that major), but has nothing about electronics or mechanical systems. It does not even include the basic EE course taken by regular ChemE majors and all other non-EE majors. (That would seem limiting to me.) Former students with that interest (prosthetics, for example) go into mechanical engineering and diversify via senior design projects.

        PS – I really like the story and observation about theory and practice from John below!

        Comment by CCPhysicist — 2016 May 1 @ 11:10 | Reply

        • I don’t expect physics classes to cover complex impedance (that’s an EE modeling concept, not a physics one). I do expect Physics E&M labs to cover RC charging curves (but I’ve been told that they don’t here). I don’t expect EE circuits classes to cover non-linear devices (like FETs and loudspeakers), but I’ve found it useful to use them for modeling exercises in the applied electronics classes.

          I do expect students to pick up a huge spectrum of skills—not just EE theory, but a lot of lab skills and modeling/graphing skills. I also try to teach them about lots of tools, and not just electronic ones: I had them measure the thickness of packing tape with a micrometer, and I’ll be hauling my drill press up the hill in a bike trailer on Tuesday so that they can drill 2mm holes in PVC elbows to make breath-pressure measurement devices.

          I do have half a chapter on what students should know before starting the class, and I have to beef that up this summer (probably to a full chapter).

          Most of our bioengineering students are in biomolecular engineering, which requires a lot of biochemistry and molecular biology. I added the electronics course to the curriculum so that they would get an engineering experience with a technology that has faster turnaround for debugging than molecular biology does—each iteration there may take a day, a week, or a month, which makes debugging very slow. Our second most popular concentration is bioelectronics, and I’m hoping that we can grow that cohort, since I think that their job market is much more favorable.

          Comment by gasstationwithoutpumps — 2016 May 1 @ 15:18 | Reply

  2. At least, they had two wires. About a half century ago, I did some time as a post-doc in an EE dept. (I’m a p-chemist by training, but I was also a radio amateur.) We were working on BIG gas phase lasers – 10 to 20 ft long, 4 in. ID glass pipe – with correspondingly impressive high voltage power supplies.

    One of the (EE, remember) grad students came in one day, and said that he couldn’t get his laser to fire, and he kept getting shocks. I went out to the lab for a look. He had connected one end of his laser to the positive high voltage … but hadn’t hooked up a ground return. If he’d happened to touch the wrong place on the laser, it would have instantly fried him.

    This was a guy who was probably 6 years into his EE studies. He was not stupid – he knew Kirchhoff’s Laws and all that – but there was some kind of disconnect between … what? Theory and practice? I don’t know, but I’ve seen a lot of similar, if less spectacular, examples over the years. For many people, it’s a very difficult connection (sorry) to make.

    Comment by John — 2016 April 30 @ 13:24 | Reply


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