Gas station without pumps

2019 May 10

Inductive spikes

Filed under: Circuits course — gasstationwithoutpumps @ 22:04
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One of the labs in my textbook Applied Analog Electronics asks students to look at the inductive spikes created by switching a nFET on and off with a loudspeaker as a load:

A 5V pulse signal to Gn will turn the nFET on.

My students were very confused when they tried the experiment, because they got a different result:

What the students got at the nFET drain went a little above 5V, but did not have the enormous inductive spike they expected.

Of course, I lied to you a little about what their circuit was—they were working with half-H-bridge boards that they had soldered:

The half H-bridge boards have a pFET and capacitor on them, as well as an nFET.

The pFET was left unconnected, so the circuit was really the following:

The gate and pFET source were left floating in the student setups.

So what difference does the pFET make? Well, with the gate floating and staying near 0V, the pFET turns on when the pFET source voltage gets high enough, allowing the capacitor to charge.

The pFET source gets up to about 7.2–7.3V, and the time constants for the capacitor and loudspeaker are long enough that the capacitor looks like a power supply (not changing voltage much on this time scale), so that the body diode of the pFET snubs the inductive spike at about a diode drop above the pFET source voltage.

So how did I miss this problem when I did my testing before including the lab in the book? One possibility is that I left out the bypass capacitor—without it you get the expected spike. But I know I had included the capacitor on my half-H-bridge boards—I had to solder up a board without the bypass capacitor specially last night, in order to get the “expected” plot in the first plot of this post.  I think what happened is that when I had done my tests, I had always connected the pFET gate to the pFET source, to ensure that the pFET stayed off, but when I wrote the book, I forgot that in the instructions. Here are the plots of the board with the pFET gate and source tied together (both floating), both floating separately, and with the them both tied to 5V:

With the pFET gate and source tied together, the circuit behaves as expected, with large inductive spikes if the pFET source is floating, but snubbed to a diode drop above 5V if the source is tied to 5V.

The pFET source voltage gets quite high when the pFET gate and source are tied together to keep the FET off, but they are not tied to the power rail:

Because the pFET never turns on, the body diode and capacitor acts as a peak detector, and the capacitor charges until the leakage compensates for the charge deposited on each cycle, around 33.7V, snubbing the inductive spike at about 37V (more than a diode drop above, but the duration is short).

This summer and fall, when I’ll be working on the next edition of the book, I’ll be sure to improve the instructions for the FET lab!

2019 May 8

UCSC principles of community

This year UCSC has had banners up on the main roads of campus touting the campuses “priniciples of community”:

We strive to be:

  • Diverse: We embrace diversity in all its forms and we strive for an inclusive community that fosters an open, enlightened and productive environment.
  • Open: We believe free exchange of ideas requires mutual respect and consideration for our differences.
  • Purposeful: We are a participatory community united by shared commitments to: service to society; preservation and advancement of knowledge; and innovative teaching and learning. 
  • Caring: We promote mutual respect, trust and support to foster bonds that strengthen the community.
  • Just: We are committed to due process, respect for individual dignity and equitable access to resources, recognition and rewards.
  • Disciplined: We seek to advance common goals through reasonable and realistic practices, procedures and expectations.
  • Celebrative: We celebrate the heritage, achievements and diversity of the community and the uniqueness and contributions of our members.


These principles are nice, if rather vague, principles for the community to support, and the banners are nicely designed graphically:


(Sorry about “be caring” being backwards—I didn’t hang the banners, but just photographed them on my way to work today.)

I have one major objection to the banners:  they are all paired with “because actions speak louder than words”, but none of them demand specific actions!  They are asking for states of being, rather than calling for action.  The only exception is “embrace diversity”, probably because “be diverse” is something that the community aspires to, not something an individual can do much about.  But even “embrace diversity” is not a very clear action.

About the only one of the slogans that can be easily turned into an action is “be celebrative”, which should be “Celebrate!”

I think that the designers of the banners did not really think through the meanings of the words they were putting on the banners, or they would have chosen a phrase to pair with that did not have this semantic dissonance of exhorting states of being, while claiming that actions are better.

Readers, what common phrase would you have put on the banners, to pair with all the “be” phrases?

2019 May 1

Eighteen months later

Filed under: Uncategorized — gasstationwithoutpumps @ 11:33
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Those of you who’ve been following my blog for a while will remember that last year I had my whole head shaved for the St. Baldrick’s cancer-research fund-raiser.  I’ve done a couple of posts since then on the regrowth of the beard and head hair: Regrowth: a return to normal and oneOne year later.

Since the shaving, I’ve let my beard grow back (except for trimming my mustache and shaving my cheeks) but I’ve had one haircut.  This is how long my beard gets in 18 months:

The beard is getting a bit scraggly, so I’ll start trimming the ends.

The big question is whether I should keep the long patriarchal beard (which makes me look a bit like an etching of my great-grandfather), or trim back to a more fashionable young man’s beard, which would be more work to maintain.

Readers, what do you think? Long beard or short?

2019 April 20

Update on son’s job search

Filed under: Uncategorized — gasstationwithoutpumps @ 10:47

In MS > PhD I posted

I have once again seen that in engineering fields, a Master of Science is a more valuable degree than a Doctor of Philosophy.  My son (who just finished his M.S .in computer science) has been on the job market for a few weeks and has just gotten his first job offer.  The salary is larger than the salary offered to a new engineering faculty member at UCSC who has a Ph.D. and 3 or 4 years of postdoc training (in fairness, his is a 12-month salary offer while the faculty member’s is a 9-month salary, which could be supplemented another 22% if the faculty member gets grants to fund it).

For that matter, his starting salary would be over three-quarters of my salary as a full professor with a Ph.D. in computer science and 37 years of experience.  It is easy to see why academia has a hard time hanging onto engineering faculty, when industry is willing to pay so much more for shorter hours.

I’ve no idea whether my son will accept the job offer. He has had serious interviews at 4 companies, so may be getting more offers soon—he is down in Santa Barbara for a 2-day interview right now.

As it turned out, my son got three job offers in the past week, each more lucrative than the one before.  He had to make his decision yesterday, which was rather stressful for him, as each of the job offers had its own strong points. One was in San Francisco, near enough to BART, MUNI, and Caltrain that he could live anywhere is a large area and commute to work by public transit (they even pay a commuter allowance). The one that paid the most was in Santa Clara, which has a huge concentration of tech firms, but is a bit short on housing for the tech workers—he would have had to do a long bike commute or taken the light rail for about 45 minutes from Mountain View or San Jose.  The company with the widest variety of different contracts and clients and probably the most stability was in Santa Barbara, where he could get housing in walking distance of the office.

All the job offers paid more than  enough for him to live on, even if he joins the FIRE (financial independence, retire early) crowd and saves half his income for early retirement. They all had decent benefits (health, dental, 401k, stock grants/options, … ), though the details varied.

This was a difficult decision for him—choosing between three highly paid jobs that were well suited to his interests (first-world problems, right?).

In the end he went with the lowest offer, not the highest, because it seemed to be the most exciting work and the best location—the job is near transit in San Francisco, and he is thinking of living in Berkeley.  It was also the smallest company, being a 40-person startup, so he will probably get a variety of different tasks and relatively rapid promotion.  The stock options could become either extremely valuable or worthless, depending what happens to the company in the next two or three years. Berkeley seems to have a few community theater groups, which means he may be able to continue acting, even as he works his day job.

2019 April 17

Running hiatus

Filed under: Uncategorized — gasstationwithoutpumps @ 10:48
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I am going to stop running for a few weeks and reassess my goals.  What triggered this change was my annual physical, where I got some of my various aches diagnosed.  There were three main outcomes:

  • It is worth trying ezetimibe in addition to my current rosuvastatin to see if that controls my cholesterol better—it is probably less of a risk than increasing the statin dose.  Interestingly, my insurance requires prior authorization for ezetimibe (a $9/month generic), which makes no sense at all.  I’ll probably be paying for the prescription without insurance, since the full price will be less than the insurance co-pay would be.  (They don’t pay for the $6/month rosuvastatin either.)
  • My elbow injury is probably tennis elbow (lateral epicondylitis), for which I’ve been prescribed stretching and strengthening exercises.  I’ll also start wearing one of the tennis-elbow braces that wraps around the forearm.  I’ve had elbow inflammation before (due mainly to bad keyboarding posture), so I know more or less what to do about it.
  • The ache in my hip that I’ve had since last August is osteoarthritis, as I suspected (confirmed by X-ray). This is bad luck, but not unusually bad luck—the incidence of hip arthritis among men my age is about 12%, if I’m reading the literature correctly.  Experts seem to disagree about whether running is safe for people with hip arthritis, but even those who recommend running agree that one needs to change to low-impact running styles and do other exercises to stretch and strengthen muscles to avoid further injury to the joint.  Almost all recommend working with a sports medicine specialist and physical therapist trained in running and arthritis to determine exactly what is reasonable.  I’ll probably be doing that when I have some spare time (this summer?).

I was just feeling good about having done a 5km run last weekend, albeit at a slow pace (8:35 mile pace), and now it looks like I won’t be increasing my distance to 15km this summer as I had planned.  I may never run a marathon—I should have tried 20 years ago, when my body was better able to tolerate abuse and recover from it.

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