The author of Gas Station Without Pumps has posted this thought-provoking list of technician-level skills every engineer should have:
- Reading voltage, current, and resistance with a multimeter.
- Using an oscilloscope to view time-varying signals:
- Matching scope probe to input of scope.
- Adjusting time-base.
- Adjusting voltage scale.
- Using triggering.
- Reading approximate frequency from display.
- Measuring time (either pulse width or time between edges on different channels)
- Using a bench power supply.
- Using a signal generator to generate sine waves and square waves. Hmm, only the salinity conductance meter uses an AC signal so far—I may have to think of some other project-like labs that need the signal generator. Perhaps we should have them do some capacitance measurements with a bridge circuit before building a capacitance touch sensor.
- Using a microprocessor with A/D conversion to record data from sensors.
- Handling ICs without frying them through static electricity.
- Using a breadboard to prototype circuits.
- Soldering through-hole components to a PC board. (I think that surface-mount components are beyond the scope of the class, and freeform soldering without a board is too “arty” for an engineering class.)
I really like this course-design approach, and I think it will yield a very interesting, engaging course.
I started thinking out loud about the kinds of conceptual difficulties I’ve noticed and assessments I use. When I realized it was turning into yet anther one of my marathon comments, I thought I’d open up the conversation over here.
1. Using a Multimeter
When teaching students how to use meters, I’ve found it interesting and conceptually useful for them to use their meters to measure other meters. For example, use the ohmmeter to measure the input resistance of the voltmeter, or use the ammeter to measure the output current of the diode checking function. It gets students thinking about what the meters do, helps them get a sense for the differences between meters (especially if you have a number of makes and models available), and can help them build their judgement about when, for example, a current-sense resistor’s contribution to a series circuit can no longer be ignored.
It makes for useful test questions as well: draw a meter measuring another meter, and have students justify their predictions of what each meter will read.
2. Using an Oscilloscope
The trigger function is difficult for a lot of my students to make sense of. This becomes evident when they make a measurement on channel 1, then make another measurement on channel 1, then infer the phase relationship between two signals that were not measured simultaneously. This also makes a useful test question — describe this scenatio, and ask students to explain specifically why the conclusion is not valid.
I’ll also be curious to know if the students are able to relate the techniques for vector addition to the reality of phase shift in the time domain, including the apparently illogical concept that in a series RC circuit, the resistor’s voltage can lead the supply’s. (Where did the resistor get that voltage before the supply turned on? would be the type of frustrated question my students would be upset about.) Although introducing the concept of start-up transients seems like it should increase cognitive load, I find that my students welcome it as a way to resolve this apparent contradiction. This is easier, of course, if you have storage scopes or (better yet) simulation software.
In case it’s useful to anyone to have an electronic copy of an “oscilloscope grid” (for including in test questions, etc.), here’s one I made. (Whoops, upload problems. Will add it here as soon as the upload succeeds).
When we start making a lot of use of the oscilloscope, that’s when the headaches start to flare up about “what ground is exactly, anyway.” Lots of fruitful discussions are possible; what does the scope’s ground clip mean if the the scope is plugged into an isolation transformer? (Note, some isolation transformers isolate the grounding conductor, others don’t.) What happens when two probes have their ground clips in different places? (This is another favourite test question of mine: what is the voltage across component X, where X is shorted out by scope ground clips).
What does AC coupling do, exactly? Why would you use it — why not just adjust the volts per division? Asking them to measure the magnitude of the ripple on a DC supply can help them make sense of this. My students also often have trouble being confident of the difference between moving the display level on the scope and adding DC offset on the signal generator.
3. Using a Bench Power Supply
This is fairly straightforward, except for current limiting (especially on a supply where the current limit knob is not graduated, or maybe even labelled in any way). I find it useful for students to be able to choose a replacement fuse (and shop for it on a supplier’s website). This apparently simple procedure can help students grapple with the meaning of the distinction between voltage and current. For beginners, it is counter-intuitive to imagine that there is voltage across an open fuse, even though there is no current.
4. Using a Signal Generator
Measuring things in a bridge circuit is another conceptually useful experience; I use it to motivate Thevenin’s theorem, since a bridge circuit has no components in series nor in parallel, making it resistant to simple circuit-solving strategies.
Other uses of a signal generator: if applicable, you could have your students perform a frequency sweep of something. This can yield interesting insights, like noticing that, due to stray capacitance, high-pass filters are actually band-pass filters.
Soldering well, and accurately inspecting soldering, are great skills to have. Surface-mount components might not be out of the question; if you want to introduce them, it’s not much harder to solder a 1206 chip resistor than a through-hole component, and can reasonably be done with a regular iron. Knowing the difference between lead and lead-free solder might be useful too, especially as it relates to reliability and disposability.
I go back and forth about using perf-board. On one hand it’s great for cheap soldering practice. On the other hand, the lack of solder mask makes it very difficult for beginners to make tidy joints, with solder running down the lengths of the traces.
I’ll probably keep using this post as a catalogue of common difficulties. If anyone can think of others (or has suggestions of other technician-level skills that engineers should have), I’d be curious to hear them.