Gas station without pumps

2015 October 6

Crawlspace ventilation—better low-voltage handling

Filed under: Uncategorized — gasstationwithoutpumps @ 10:10
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In Crawlspace ventilation again, I mentioned the problem that I was having with the fans whining if the power supply could not provide enough power:

With the 8Ω series resistance, the 4-fan load had a hard time getting started, and whined a bit before starting. Once one of the fans got up to speed and reduced its current draw, the others quickly came up to speed also. The regulated voltage was only 4.9V, and fluctuated a bit, rather than the constant 5.02V with smaller loads or less series resistance between the 9V power supply and the regulator.

With the 10Ω series resistance, the 4-fan load could not start at all, but just whined. The output of the regulator was only 2.2V while the fans were stalled. If a 4th fan was added while 3 fans were already running, the fans ran, but with a 4.11V regulator output. (The three fans had already reduced the regulator voltage to 4.99V).

I may have a little trouble with the fans each morning as the power comes up gradually—they may have trouble starting if the panel is not yet putting out enough power. If that turns out to be a problem, I may need to add some circuitry to detect low voltage on the regulator and turn off one or more fans.

The fans I’m using have brushless motors, which means that they include electronics to set the speed of the fan.  Based on the electrical noise I see on the power line, they appear to have a constant frequency independent of whether the fan is spinning or not. This simple design for the electronics is cheap, but not as good as a design that detects the speed of the motor and optimizes the phase and frequency of the rotating magnetic field to maximize torque or efficiency. The whining at low power when the fan is not turning seems to be at the frequency at which the fan is expected to spin with a 5V supply, but I’ve not used a microphone to check.

I tried two different approaches to handling the low-voltage problem:a power-on-reset chip and a simpler FET circuit of my own design.

The first design was to use a power-on-reset chip and an nFET to turn off the motors when the voltage was too low.  This was inspired by the standard Miller engine used in solar motor toys to harvest energy from low-current solar cells, store it in a capacitor, and discharge the capacitor through the motor when there was enough charge. This did not work well, as the voltage from the regulator quickly got up to 5V when there was no load, but dropped almost immediately when the fan was turned on, causing the reset chip to turn the fan off again. The chip I was using, MCP100-450DI/TO has only 50mV of hysteresis, so it would turn off again almost as soon as it turned on, and then wait for 0.3 s before turning on again.  The 50mV hysteresis meant that I’d need a large storage capacitor to get the fans past their initial high-current startup—I estimated around 15F, which would be an expensive supercapacitor.  Without a huge capacitor,  this circuit resulted in pulsed whining when the power supply was not capable of delivering the full 5V, which was even more annoying than the steady whine.

The second design I tried was just using the exponential turn-on of an nFET to provide full current when the voltage was high enough, but limit the current to very low levels when the voltage was insufficient:

The large capacitor on the gate keeps the transistor on for a while even after the voltage starts dropping.

The large capacitor on the gate keeps the transistor on for a while even after the voltage starts dropping.

One essential part of the circuit was the large electrolytic capacitor on the gate voltage.  Without it, the voltage on the gate would rise to a point where the motors were struggling to start and stay there (when the power supply was provided with too low an input voltage). With the capacitor in place, the fans would continue to get current even as the voltage dropped a bit, giving them enough time to spin up and reduce their current load.  Eventually the gate voltage would drop enough to start pinching off the current and the fans would hesitate and need to be spun up again. The result was that the fans would start even with a large series resistance between the 9.26V power supply and the input to the regulator (16Ω and sometimes even 32Ω), but not run smoothly unless there was enough power being delivered to the power supply for their continued running.

A smaller capacitor (220µF) worked also, but the fluctuation in speed happened more rapidly.  Much smaller (33µF) did not filter the power-supply fluctuation enough to hold the fans on long enough to start up cleanly.

Adding capacitance across the 5V terminals decreased the electrical noise, but did not seem to change the behavior of the circuit.

The flyback diode (the BAW62) is probably not necessary with these fans, since the brushless-motor controller built into the fans undoubtedly has its own flyback diodes.  I saw no evidence of inductive spikes when turning off the nFET.

So far I’ve only breadboarded the nFET control circuit, but I’ll probably solder it up later this week, when I get some time.

2015 October 3

AOI514 nFET I-vs-V

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 21:03
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I wasted a lot of time today trying to get a good current-vs-voltage plot for a diode-connected AOI514 nFET, which is the one I plan to have the students use this year.  When I started on it this morning, I thought I was working on my book, but in the end I decided not to include the I-vs-V plot in the book, so most of the time ended up being wasted.

I breadboarded three different test jigs for measuring the current of the nFET:

I started with the simplest test jig, and worked my way up to higher currents.

I started with the simplest test jig, and worked my way up to higher currents.

The simplest test jig just uses the function generator to provide the power for the measurements and a Teensy board running PteroDAQ to make the measurements.

The second jig allows higher voltages on the function generator, hence somewhat higher currents (limited mainly by the 50Ω output impedance of the function generator, but also by the current limits of the function generator).

The most complicated jig uses an external power supply with the function generator controlling the current by changing the gate voltage of an extra nFET.

The 10µF capacitors for removing noise on A10-A11 are almost certainly too big, introducing bias into the measurements, but the 22nF capacitor works well.

By changing test jigs and current-sense resistors, I was able to span a wide range of currents (almost 7 decades).

By changing test jigs and current-sense resistors, I was able to span a wide range of currents (almost 7 decades).

The weird plots at high currents show the effect of temperature changes on the FET characteristics. At 3A the transistor got warm, but as the current dropped it cooled off a little, getting a bit warmer on each 5 second cycle.

At the other end, I had some difficulty measuring currents less than 1µA—current-sense resistors large enough to give sufficiently large voltages would be too high impedance to handle the noise injected by the sampling circuitry on the Teensy 3.1. I also went to 60Hz sampling, to alias out 60Hz interference from capacitive coupling to the breadboard. I still don’t trust the measurements below 500nA.

I decided that the I-vs-V curve here is too messy to put in the book, so instead of working on my book all day, I’ve wasted my time getting nothing more out of it than this blog post and a reminder that even “simple” concepts like I-vs-V plots are not so simple in the real world.

2015 October 2

What is probability?

Filed under: Uncategorized — gasstationwithoutpumps @ 17:43
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Today’s class in Bioinformatics: Models and Algorithms went fairly well.

I started by collecting the first assignment and asking if there were any questions, about anything in the class.  I used a bit longer wait time than I usually use for that prompt, and was rewarded by having a reasonable question asked by someone who had clearly been hesitant to ask.  I’ve been finding that “Wait Time” is one of the most powerful techniques in Teach Like a Champion.

The first part of class was just a quick summary of DNA sequencing techniques, with an emphasis on the effect the different technologies had on the sequence data collected (read lengths, number of reads, error models).  Many of the students had already had much more extensive coverage of DNA sequencing elsewhere (there is an undergraduate course about half of which is sequencing technology), and several students were able to volunteer more up-to-date information about library preparation than I have (since they worked directly with the stuff in wet labs).

I reserved the last 15 minutes of the class for a simple question that I asked the students “What is probability?”

I managed to elicit many concepts related to probability, which I affirmed and talked about briefly, even though they weren’t directly part of the definition of probability.  This included things like “frequency”, “observable data”, and “randomness”. One volunteered concept that I put off for later was “conditional probability”—we need to get a definition of probability before we can deal with conditional probability.

Somewhat surprising this year, as that the first concept that was volunteered was that we needed an “event space”.  That is usually the hardest concept to lead students to, so I was surprised that it came up first.  It took a while to get someone to bring up the concept of “number”—that probabilities are numeric.  Someone also came up with the idea “the event space equals 1”, which I pressed them to make more precise and rigorous, which quickly became that the sum of probabilities of of events is 1.  I snuck in that probabilities of events meant a function (I usually press the students to come up with the word “function”, but we were running low on time), and got them to give me the range of the function.

Someone in the class volunteered that summation only worked for discrete event spaces and that integration was needed for continuous ones (the same person who had brought up event spaces initially—clearly someone who had paid attention in a probability class—possibly as a math major, since people who just regard probability as a tool to apply rarely remember or think about the definitions).

So by the end of the class we had a definition of probability that is good enough for this course:

  • A function from an event space to [0,1]
  • that sums (or integrates) to 1.

I did not have time to point out that this definition does not inherently have any notion of frequency, observable data, or randomness.  Those all come up in applications of probability, not in the underlying mathematical definition.  One of my main purposes in asking a simple definitional question (material that should have been coming from prerequisite courses) was to get broad student participation, setting the expectation that everyone contributes.  I think I got about 50% of the students saying something in class today, and I’ll try to get the ones who didn’t speak to say something on Monday.  Unfortunately, I only know about 2 names out of the 19 students, and it takes me forever to learn names, so I may have to resort to random cold calling from the class list.

In retrospect, I wish I had spent 5–10 minutes less time on DNA sequencing, so that there was a bit more time to go into probability, but it won’t hurt to review the definition of probability on Monday.

2015 October 1

Ninth weight progress report

Filed under: Uncategorized — gasstationwithoutpumps @ 07:36
Tags: , ,

This post continues the series of weight progress reports from the previous one.

My weight jumped startlingly in September:

I don't have a good explanation for the sudden jump in my weight on Sept 18.

I don’t have a good explanation for the sudden jump in my weight on Sept 18.

The jump in weight on Sept 18 is a mystery to me—the only explanation I’ve come up with for such a large, sudden change is that the scale was moved to weigh my son’s luggage, and that moving the scale changed its calibration.  Somehow that sounds to me like rationalizing away the weight change, and even if it is true, I don’t know whether the old readings or the new readings are more correct, so I’m treating it as a real weight change. The train trip down to Santa Barbara to help my son move into his new apartment at UCSB was also a low-exercise/fattening-food time, which probably prolonged the spike in weight.

I have gone back onto my strict raw-fruits-and-vegetables-for-lunch diet. I’m still allowing myself my normal breakfast of cereal and soy milk, plus whatever dinner we’re having in the evening, but I’m trying to reduce my portion sizes.

My weight dropped back into the target range fairly quickly, but it will probably take another 2–3 weeks on the diet to get back to the middle of the range.  I plan to stay on the diet until my weight has been below 157lbs for a week (or I hit 155lbs, which is less likely).

My exercise was  low for September (only averaging 2.9 miles/day of bicycling, slightly up from the 2.25 miles/day in August, but down from my normal year-round average of just over 4 miles/day).  We had a lot of hot weather in September, keeping me indoors most days until the quarter started on September 21. My exercise should be fairly high for the rest of this year, as I’ll be cycling to campus 5 days a week.  Even with no other bicycling, that would get my average back up to 4.5 miles/day, as my commute is about 6.25 miles round-trip.


2015 September 29

What makes college alumni appreciate their college

Filed under: Uncategorized — gasstationwithoutpumps @ 09:26
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I’ve just been reading the Gallup-Purdue Index 2015 Reportwhich analyzes a survey of about 30,000 college graduates to figure out whether they felt college was worth the cost, and what college experiences lead to higher satisfaction. Here are a few highlights:

  • Recent graduates who strongly agree with any of three items measuring supportive relationships with professors or mentors are almost twice as likely to strongly agree that their education was worth the cost. These relationships hold even when controlling for personality characteristics and other variables such as student loan debt and employment status that could also be related to graduates’ perceptions that college was worth it. 
  • If recent graduates strongly agree that they had any of three experiential learning opportunities—an internship related to their studies, active involvement in extracurricular activities or a project that took a semester or more to complete—their odds that they strongly agree that their education was worth the cost increase by 1.5 times.
  • However, whether recent graduates participated in a research project with a professor or faculty member is unrelated to their opinion that their education was worth the investment. This finding suggests that it is important to assess the quality of faculty members’ interactions with students—and the benefits students derived from them—rather than simply tracking participation in such projects. 

I found the third point above particularly worrisome, as we don’t have ways for really checking the quality of the interactions in things like thesis projects—we count on the goodwill of the faculty involved to make the experience a good one. We are also short on internships, though most of the BSoE majors now have multi-quarter projects for the capstones.

The “three items measuring supportive relationships with professors” were

  • My professors at [University Name] cared about me as a person
  • I had a mentor who encouraged me to pursue my goals.
  • I had at least one professor at [University Name] who made me excited about learning.

The first and third of those are hard to do anything about institutionally, and even on an individual level there is no general way to be successful, but we could be doing more about mentoring. One suggestion they had that may be worth following up:

… programs such as those that recruit alumni as mentors do not need to be costly, but they can make a powerful difference in more effectively engaging both students and alumni.

They did note that modest amounts of debt (up to about $25,000) did not seem to reduce alumni satisfaction, but larger amounts of debt seriously reduced whether alumni thought their college experience was worth the price.  There wasn’t much difference between public and private, non-profit colleges, but the for-profit colleges were much less appreciated by alumni. Also in-state or out-of-state public university did not seem to result in different distributions of satisfaction with the value of the education (despite the fairly large difference in price), and research universities followed the same distribution as public and non-profit private colleges.  Only the for-profits stood out as distinctly different (possibly related to the high debt load—I didn’t see an analysis of the for-profits that controlled for debt—maybe there were too few low-debt students at the for-profits to be statistically significant).



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