# Gas station without pumps

## 2016 May 22

### Disappointing class-D amplifier reports

Filed under: Circuits course — gasstationwithoutpumps @ 15:59
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I have a huge stack of grading to do this weekend (about 20 new design reports and about 30 redone reports from earlier labs). I was rather disappointed with the reports for the class-D amplifiers, not because the amplifiers didn’t work (they mostly did), but because about 80% of the class is getting a REDO for errors in their schematics, which means I’ll be having to grade the reports again.  I consider 20% REDO an acceptable level, but not 80%.

The most common error in the schematics was one that I had seen a fair amount in previous years: getting the source and drain of the pFETs swapped.  It is a fairly serious error, as putting the transistors in backwards would cause a lot of shoot-through current from the body diode conducting.  Students were wiring their transistors correctly (after some false starts), but documenting their designs wrong.  A number of students also used the depletion-mode instead of enhancement-mode FET symbol, but I consider this a much less serious error (as long as they included the right part number), and would not have triggered an automatic redo for that mistake.

I warned the students about the source and drain orientation repeatedly, both as a class and (in many cases) individually.  I was very careful to point out the convention for the source and drain notation in class and in the book, and they had it on their data sheets as well.  I don’t know what else I can do, other than instituting in-class quizzes, which I may have to do next year.

There were a number of other documentation problems in the reports this week:

• Using their loudspeaker models, but not including the model in the report.  In many cases, it was clear from their plots that they had screwed up the model somehow, but without any formulas or parameter values, it was impossible to figure out what they had screwed up.
• Oscilloscope pictures that did not say what the probes were connected to, or had incorrect labeling of the probes. This was mainly really bad lab technique, where they failed to write down what they were doing, and couldn’t reconstruct it from their memories.  That is one aspect of the labs that I’ve not put much emphasis on this year—writing down what they are doing as they do it.  I may have to emphasize that more next year, especially since the labs will be broken up into 4 95-minute sections instead of 2 3-hour sections, which will make memory even more unreliable.
• Not reporting the PWM frequency.
• Not remembering to include their bypass capacitors in their schematics. Some students may not have had bypass capacitors, though it was very difficult to get the amplifiers to work without them, as the H-bridge dumps a lot of high-frequency energy into the power lines, which gets coupled back to the comparator and the preamp.
• Generally bad copy editing.  The spelling and grammar problems in some reports are just common non-native problems with articles, plurals, and verb tense, but a lot of the reports had huge numbers of spelling errors, duplicate words, missing words, comma problems, and incomplete sentences.  I’ll be addressing this problem next year by giving students a bit more time to complete the reports (though that didn’t seem to help on the one report that students had more time on this year).

## 2016 May 20

### 2016 T-shirts ordered

Filed under: Circuits course — gasstationwithoutpumps @ 20:53
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I’ve just placed my T-shirt order from my Applied Electronics course:

The 2016 shirt is an exclusive class-only shirt.

The shirt this year has a couple of differences from previous years’ shirts:

1. The slug now as a beard, at the request of the class.
2. The shirt is exclusive to the class this year, with “I survived” added at the beginning of the text, again at the request of the class.  In previous years I was willing to let anyone buy a shirt, to lower the prices for the students in the class, by amortizing the setup fees.

I’m also dealing with a different T-shirt company this year, at the suggestion of one of the students.  I’m using BroPrints, a Santa Cruz company since 1994 (or 2000, depending on whether you count the start of the business or their opening a storefront). Their prices are considerably lower than The Print Gallery, who I used last year. I’m hoping the quality is good (the student who recommended them said that she had used them for several different orders and had good durability of the printing).  I stopped using Sports Design after two sets of shirts (one in 2011 and one in 2014) had bad cracking of the printing, and an order was miscounted in 2014.

Broprints has lower setup fees and lower per-shirt charges than the other companies I’ve dealt with, and the order is a little larger this year (35 shirts), so the per-shirt price should be the lowest yet (about \$12 a shirt, and \$15 for long sleeve).

I’ve now got the slug design as an SVG file as a master, but what I communicate to the printer is a layered Photoshop file (created with Photoshop Elements from an Inkscape png output), with each silk screen mask on a separate layer.  The SVG file is only 85,868 bytes, while the layered Photoshop file is 11,075,976 bytes.  Obviously the vector format of the SVG file is going to be smaller than a 600dpi raster image, but I’ve found that T-shirt companies can’t deal with Inkscape-created SVG files—especially not when I use non-standard fonts like Optima. The photoshop file is quite inefficiently stored even for a raster image, as a PNG file with essentially the same information (except for splitting the different colors into different layers) created by Photoshop Elements is only 201,677 bytes.

## 2016 May 11

### Don’t put pulse monitor first

Filed under: Circuits course — gasstationwithoutpumps @ 21:25
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I think that the soldering lab should not be the first op-amp lab, but I still like the idea of the students having to solder up their microphone preamps. So I’ll have to do a major reorganization of the book this summer, to move a different lab into the first position.

Currently, I’m thinking that the transimpedance amplifier and pulse monitor lab would be a good choice as the first op-amp lab.

After a rather rough start in the first half of the transimpedance-amplifier lab yesterday, I no longer think that is a good idea. The instrumentation-amp lab went much smoother, so may be a better first choice.  Among other advantages, the instrumentation-amp lab with the pressure sensors has no analog filtering and simpler sensor sensitivity calculations.  One disadvantage of moving away from the mic preamp is that the microphone and loudspeaker characterization in the first half of the class would be separated from the audio amplifier design in the second half—not a problem in a single quarter class, but potentially a bigger problem in a two-quarter sequence.

I’m not sure why the transimpedance amplifier lab went poorly yesterday, and I’m hoping the second half will go smoother tomorrow.  It may be that prelab was not a good match for the lab this time.  Or it may be that over a third of the class didn’t do the prelab this time.  (I’ve threatened the class with a quiz worth as much as a design report if they don’t shape up by next week—I’m carrying around enough redone reports to grade that I really don’t want to do more grading, but I’ll follow through on the threat if so many students continue slacking.)

It turns out that I had several errors in the draft of the book that the students were using for the prelab exercises for the optical pulse monitor.  I’d decided in the summer or fall to switch to a new 700nm LED, but I’d only updated about half the scaffolding for the sensor sensitivity, so there were still a number of things that were only accurate for a 623nm LED.  Also, I’d been using a datasheet for the WP3DP3BT that I’d gotten when I first started using the part, but Kingbright has improved their datasheet a lot between V3 (which I had) and V5 (the current one), so there is now a spectral sensitivity curve on the datasheet, and the spectral sensitivity is quite different from what I was expecting.

I’ve started editing the book to correct the errors, but even after I fixed everything, the estimates for the current from the phototransistor were about 5% of what students were measuring in the lab. The model I had created, which worked fairly well for the previous LEDs, does not seem to work for the longer wavelength of the new LED.

I’m considering simplifying the model by eliminating the modeling of scattering, to see how well that works.  I should check the model with at least 3 different LEDs: the current 700nm one, the shorter wavelength ones I used to use, and an IR emitter.  If I can get the estimates to be within a factor of 10 of measured values for all the LEDs I have, then the model is good enough to include in the book.

I might also want to consider switching phototransistors to one with a wider spectral sensitivity, so that the estimation is not thrown off as much by the filter that blocks so much of visible light.  That would allow me to try a green LED as well.

I’m still thinking about doing a log-transimpedance amplifier as the first stage (not for the class, just for a demo unit) so that the pulse monitor can work in varying light levels up to full sunlight.  The fluctuation in light from the pulse seems to be about 1%, which should be a variation of about 850µV out of the logarithmic amplifier (based on the 9.8mV/dB I measured in Logarithmic amplifier again).  That’s a somewhat smaller signal than I’ve been getting with well-chosen gain resistors, but it may be worth it to get greater independence from the overall light level.

## 2016 May 3

### Revised pressure sensor lab went very well

Filed under: Circuits course — gasstationwithoutpumps @ 21:20
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Today I ran a revised version of the pressure sensor lab (see
Pressure-sensor lab went well, Class-D lab revision didn’t work, Blood pressure monitor, Blood pressure lab, and Blood pressure lab part 2 for descriptions of the old labs).

The revised lab included both blood pressure cuffs and breath pressure using the simplified breath pressure apparatus of Simplified breath pressure apparatus:

The ½” elbow is small enough that I can put my lips around the opening, which would have been a bit difficult with the 1″ tee.

To make the apparatus, the students had to drill 2mm holes in PVC elbows, so I packed up my drill press last night and hauled up the hill in my bike trailer this morning.  For those unfamiliar with Santa Cruz, that is a 3-mile ride with a fairly steady 4% slope, resulting in a climb of about 715′ (218m).  Needless to say, I went slower than usual uphill!  There is a drill press only about 150′ from the lab the students were working in, but the bureaucracy for getting the students access to the drill press is incredible (I tried, and failed, to get a dozen students access last quarter). So it was easier for me to haul my own drill press up the hill on my bike than to deal with the dysfunctional bureaucracy at UCSC to use the drill press supposedly there for student use.

I explained to each pair of students how to use a drill press, including basic safety precautions, and had them drill a 2mm diameter hole in their PVC elbows.  There were no problems with this, and I plan to do the same for the lab in future.

Each pair of students designed an instrumentation amplifier with an INA126P chip as a first stage and an op-amp as a second stage, wired it up on breadboards, checked the calibration, and recorded both breath pressure and blood cuff pressure.  A few students used extra time to play around with some toys I brought in: a hand vacuum pump, a Lego pneumatics pump, and an aquarium air pump.  One group even tried using the pressure sensor as a microphone, using a loudspeaker with a 300Hz sine wave for input (the pressure sensor could detect the 300Hz input without problems, though I suspect that it was not registering the full pressure fluctuation, as I think that the sensor has about a 200Hz bandwidth).

Most groups were done with this week’s lab in the 3 hours of today’s lab, so Thursday’s lab will consist mostly of students doing make-up work to redo old labs, with a few finishing up this week’s lab.  I expect to spend most of Thursday’s lab time grading design reports (I’m about 2 days behind—I got the design reports that were turned in a week ago done and returned yesterday, and I got the prelabs turned in yesterday done and returned today, but I haven’t started yet on the reports turned in last Friday, nor the stack of redone work turned in last week).

The instrumentation-amp lab went surprisingly well this year, despite adding the drill press.  I think that the big advantages over previous years are that they did not have to solder the inst amps this year and that they had already done a multi-stage amplifier for the microphone amp.

I think that I should rewrite the book to introduce multi-stage amplifiers as the default design (since every amplifier they do in the course is now multi-stage), and talk about how to choose the gain for each stage in general, before getting into individual labs.

One minor problem in lab today—students didn’t have the short pieces of tubing to connect up their breath-pressure apparatus.  This turned out to be my fault—I hadn’t included them on the parts list for this year!  Luckily the BELS staff had some pieces leftover from last year, and I had about 20 feet of my own tubing in the box of stuff I’d packed for the lab, so we had enough for everyone to get 6″.

## 2016 April 29

### Miswiring errors

Filed under: Circuits course — gasstationwithoutpumps @ 15:25
Tags: ,

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).

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