Gas station without pumps

2014 October 15

LED board I-vs-V curve

Filed under: Data acquisition — gasstationwithoutpumps @ 21:27
Tags: , , , , , ,

I earlier did simple go/no-go testing for a bunch of the LED boards that I designed for lighting fixtures (see Summer project), but I thought it would be interesting to characterize at least one of the boards more thoroughly. So last night I wired up a little circuit to control the current through board to plot an I-vs-V curve:

The transistor is a 1W transistor with a DC current gain of about  120, so this circuit can sink up to about 300mA with Vss=9v.

The transistor is a 1W transistor with a DC current gain of about 120, so this circuit can sink up to about 300mA with Vss=9v (as long as the voltage drop across the LED board keeps the collector voltage down low enough).

I measured the voltage across the board and the current through it with multimeters (using the better multimeter for the current measurement). I was expecting a constant current when the voltage was high enough, then a linear decrease in current as the voltage was lowered down to a cutoff threshold. That is pretty much what I got:

The constant current was set at about 118mA (I had intended 130mA), and the linear region corresponded to 10.6Ω.

The constant current was set at about 118mA (I had intended 130mA), and the linear region corresponded to 10.6Ω. (Click to embiggen)

The constant-current region was not as crisply defined as I had expected, and the current was lower than I had intended.  The 10.6Ω impedance in the linear region initially came as a bit of surprise, but when I added together the resistance of my current sense resistor and the dynamic resistance of the LED in the relevant region it seems about right.

The measurements were hard to make, because the current did not remain constant, but tended to drop as I was measuring, particularly in the high-current regime. I believe that this droop is due to thermal effects—the current drops as the board warms up, and I did not wait for the board to reach equilibrium temperature.  The lower-than-expected  constant current is probably also due to thermal effects, since it was based on scaling up tests done at low currents, where there would have been no significant heating.

To test this hypothesis, I set up a different experiment this morning, connecting the board in series with  a 20Ω resistor and connecting both to a 12V power supply, monitoring the voltage across the resistor (and hence the current through the board) using PteroDAQ on the FRDM KL25Z board.

The initial current was close to the 130mA I had expected, based on scaling up of experiments I had done with a similar circuit that had provided 13mA.  But as teh board warmed up, the current dropped substantially, down to around 116mA.

The initial current was close to the 130mA I had expected, based on scaling up of experiments I had done with a similar circuit that had provided 13mA. But as the board warmed up, the current dropped substantially, down to around 116mA.

So my “constant-current” circuit isn’t really constant current—it is very temperature dependent. The change in brightness is about the same as I would get from a 13° change in the position of the control potentiometer for my dimmer. I can live with that in the design, but it is a much bigger temperature dependence than I had expected.

According to my infrared thermometer, the heatsink got up to about 60°C at the end of the run, with 9.75V across the board and 0.116A through it, for a power dissipation of about 1.13W. If the room was at about 20°C, that means a temperature gain of about 35.4°C/W.

The LEDs get derated to about 93% of their room-temperature efficiency at 60°C, so when combined with the current drop to 116mA, I expect about 75 lumens for each board when it is fully on. Maximum efficiency would be at the knee of the I-vs-V plot, where the voltage is about 6.67V, getting 75 lumens for 0.77W, or 97 lumen/W.  (The temperature may not get as high at that voltage, since the power dissipation is less—the terminal temperature would probably be only about 48°C, which means the current would drop less and efficiency would be slightly higher.)  At my design voltage of 9V, the efficiency is only about 72 lumens/W.  The LED boards seem to be able to run at 12V, where the power dissipation would be 1.4W/board and the efficiency only 54 lumens/W.

One surprise for me in the testing was how low a current would still produce light. I observed dim light down to about 4µA, giving the LED boards a dynamic range of about 32000 in brightness (4.5 decades), while my PWM circuit only has a dynamic range of about 43 (1.6 decades).  The range on the dimmer is adequate, since even a night light produces about 15 lumens, and my dimmer goes down to about 2 lumens. But it is clear that one could design for a much wider dynamic range.

Redoing the I-vs-V plot to have a logarithmic scale for the current, we can see the whole dynamic range.

Like many semiconductor devices and circuits, the boards have an exponential characteristic for subthreshold conduction.

Like many semiconductor devices and circuits, the boards have an exponential characteristic for subthreshold conduction.

The LED boards could be operated at as low as 5V, but the brightness is very low at that voltage (about 0.15 lumens)—not suitable for a room light or even a night light. A 7.4V Li-ion battery pack would be a good match to the LED boards.  A $14, 2200mAh battery should be able to power the LED board at full brightness for about 10 hours, and at reduced brightness for another 8–10 hours. I’m not currently planning any battery-operated lights, but it is nice to know that they are doable.

Top 50 Colleges for Hispanic Students

UCSC recently got some good news, being top-ranked in BestColleges.com’s list of the Top 50 Colleges for Hispanic Students:

In 2012, 49% of Hispanic high school graduates enrolled at a postsecondary, public institution. This percentage surpassed that of white students for the first time, and Hispanic enrollment in colleges and universities, which has increased 240% since 1996, is expected to continue to grow. Many Hispanic students are the first in their families to attend college, so it is important for them to find a support system that will help them navigate degrees, financial aid, and their school and social obligations.

To make the transition from high school to college, many students may be looking for “Hispanic-friendly” schools. These are schools with a high concentration of Hispanic students already in attendance, or they have a cultural center that focuses on Latino/a, Chicano/a or Hispanic heritages.

Students may also look for a school that will protect their rights and ensure they receive the same quality education as non-Hispanic students. The Hispanic Association of Colleges and Universities (HACU) is an organization that strives to protect the educational rights of Hispanic students. It was instrumental in increasing funding from Title V of the Higher Education Act for Hispanic-Serving Institutions (HSIs). For the 2014 academic year, HACU convinced Congress to give $98 million to HSI undergraduate programs.

To create our rankings, we relied upon our normal methodology to find schools that rank well for academics. Our team then compared that list to the 242 HACU member schools in the U.S. to find the best schools for Hispanic, Latino/a and Chicano/a students. We included the percentage of Hispanic students currently enrolled at each college, along with in- and out-of-state tuitions to add more weight to our rankings. Each school on our list boasts a cultural center, degree programs, or scholarships dedicated to enhancing the experiences of Hispanic students.

The Schools

  1. University of California-Santa Cruz – Santa Cruz, CA
    Hispanic Students as Percent of Total Enrollees: 26%
    Graduation Rate: 91%
    Retention Rate: 74%
    Admissions Rate: 60%
    Tuition and Fees: $13,398 (in-state) and $36,276 (out-of-state)This public research university, located alongside the redwood forests and just under 10 miles from the coast [actually under 2½ miles from the campus entrance to the beach], offers 60 majors in 30 fields. Because of the network of UC campuses, students have a wealth of opportunities that extend beyond UC Santa Cruz. For Hispanic students, the Chicano Latino Resource Center, more commonly known as El Centro, offers a number of programs and resources to support and bolster the on-campus Hispanic community, including academic support, scholarships and financial guidance and social events geared towards unification and integration.
  2. San Diego State University – San Diego, CA
  3. University of California-Riverside – Riverside, CA
  4. Whittier College – Whittier, CA
  5. St. Edward’s University – Austin, TX
  6. California State Polytechnic University-Pomona – Pomona, CA
  7. University of La Verne – La Verne, CA
  8. University of Houston – Houston, TX
  9. Florida International University – Miami, FL
  10. California State University-Long Beach – Long Beach, CA
  11. University of California-Merced – Merced, CA
  12. University of St. Thomas – Houston, TX
  13. Woodbury University – Burbank, CA
  14. California State University-Fullerton – Fullerton, CA
  15. St. Mary’s University – San Antonio, TX
  16. University of New Mexico-Main Campus – Albuquerque, NM
  17. Texas State University – San Marcos, TX
  18. Fresno Pacific University – Fresno, CA
  19. California State University-Channel Islands – Camarillo, CA
  20. California State University-San Marcos – San Marcos, CA
  21. Cuny City College – New York, NY
  22. Mount St. Mary’s College – Los Angeles, CA
  23. California State University-Fresno – Fresno, CA
  24. Texas Lutheran University – Seguin, TX
  25. California State University-Stanislaus – Turlock, CA
  26. La Sierra University – Riverside, CA
  27. California State University-Monterey Bay – Seaside, CA
  28. New Mexico State University – Las Cruces, NM
  29. College of Mount Saint Vincent – Riverdale, NY
  30. California State University-Northridge – Northridge, CA
  31. California State University-San Bernardino – San Bernardino, CA
  32. Schreiner University – Kerrville, TX
  33. Cuny Lehman College – Bronx, NY
  34. Saint Peter’s University – Jersey City, NJ
  35. University of Texas-Pan American – Edinburg, TX
  36. University Of Texas-San Antonio – San Antonio, TX
  37. Texas A&M University-Corpus Christi – Corpus Christi, TX
  38. California State University-Bakersfield – Bakersfield, CA
  39. California State University-Los Angeles – Los Angeles, CA
  40. University of Texas at El Paso – El Paso, TX
  41. Texas A&M International University – Laredo, TX
  42. Eastern New Mexico University – Portales, NM
  43. St. Thomas University – Miami Gardens, FL
  44. Angelo State University – San Angelo, TX
  45. California State University-Dominguez Hills – Carson, CA
  46. Adams State University – Alamosa, CO
  47. Texas A&M University-Kingsville – Kingsville, TX
  48. The University of Texas Health Science Center at San Antonio – San Antonio, TX
  49. Boricua College – New York, NY
  50. Our Lady of the Lake University – San Antonio, TX

I’ve not included the information for the colleges other than UCSC—you’ll have to click through to the original web page to get that information. I note that 23 of the 50 colleges are in California, and 17 of them are public universities. The next biggest group is 16 colleges from Texas. The “name” universities (UCB, UCLA, Stanford,…) don’t appear on the list, because too few of their students are Hispanic—they serve mainly white and Asian students.  UCSC has been aggressively recruiting Hispanic students and has only recently gotten over the 25% enrollment threshold to become an official Hispanic-serving institution (officially given HSI status in 2013), but the figures here are a little old, as we were up to 30% Hispanic by Fall 2013, and are probably above 31% now (Fall 2014 figures aren’t available yet).

Correction 2014 Oct 15: UCSC is not officially an HCI by the US government definition—according to http://officeofresearch.ucsc.edu/broader-impacts/resources/diversity/index.html:

  • The Hispanic Association of Colleges and Universities (HACU) designated UCSC as a HSI member in 2013 because UCSC has >25% Latino undergraduate enrollment). [Of UC campuses] Only UC Merced, UC Riverside, and UC Santa Cruz are members of HACU, as listed on the HACU website.  
  • UCSC is planning to submit a Title V Part A (Developing HSI) grant application in 2015. This is one of EVC Galloway’s “Five by 2015” priorities. We are now conducting an in-depth self-study in preparation for the application—the steering committee in charge of this self-study includes top administrators and senior faculty at UCSC as this is very much a UCSC priority. Once we receive a Title V Part A grant, we will be officially an HSI according to the Dept of Education.

2014 October 13

Say this, not that

Filed under: Uncategorized — gasstationwithoutpumps @ 17:00
Tags: , , , , ,

This summer I bought my son a book to prepare him for college: Say This, NOT That to Your Professor: 36 Talking Tips for College Success. He read most of it, and found it to be reasonably well-written, somewhat poorly copy edited, and worth reading once. Most of the advice in the book he felt was just common sense, but that only means that he has been raised in an academic culture.  What the child of a professor sees as common sense in dealing with professors may seem arcane for someone coming from a different culture—perhaps the first in their family to go to college.

For the past 3 years, over half of our admitted students are first in their family to go to college. So what my son finds “common sense” may be the cultural knowledge of academia that many of the students at UCSC are missing.

After my son left for college, I decided to read the book for myself, to see if it was worth recommending to students at UCSC.

The author, Ellen Bremen, apparently teaches communication at a two-year college (Highline Community College in Des Moines, WA, about an hour and a half south of University of Washington by public transit), and some of the advice she gives seems to be more directed at two-year college students than research university students.  For example, she provides no advice on how to ask a faculty member if you can join their research group, because most 2-year college faculty have no time to do research, but she provides a lot of information about what to do when you miss half a quarter’s classes.

Her example students also seem to be a bit more clueless than the students I see at the University of California.  Perhaps this is because of the stricter admission criteria to UC, or perhaps she has selected the most extreme cases to use as illustrations. Or maybe I just haven’t dealt with enough freshmen—I generally see students in their sophomore through senior years, after they’ve had a chance to get acculturated to academia.

About 3/4 of Bremen’s book is dedicated to what students do wrong, and the last quarter to how students can deal with professors who screw up—about the right ratio for a book like this. Although the actual incidence of student mistakes and faculty mistakes is a larger ratio (more like 10:1 or 20:1), the student mistakes tend to fall into the same sorts of things over and over, with only the players changing names, so a 3:1 ratio is reasonable.

The advice she gives is generally good, though she recognizes only the teaching role for faculty, and assumes that all faculty have as much time and desire to meet one-on-one with students as she does.  At UC, many of the professors see their research role as more important than their teaching role (and the promotion process, summer salary, and publicity about faculty activity clearly favor this belief), so faculty are a little less willing to dedicate 10 hours a week to office hours or meet with students at random times outside office hours. I’m doing a lot of additional appointments this quarter, and it really does break up the day so that I can’t carve out a chunk of time for writing papers or programming.  In previous years I’ve kept one day a week free for working from home, free from student interruptions and meetings all over campus, but this quarter I’ve not been able to do that, so my research time and book-writing time has dropped to almost nothing.  Just coping with the pile of email from students every few hours eats up my day.  I find that a lot of student requests can be handled more efficiently by e-mail than by scheduling meetings—the extra non-verbal communication that Ellen Bremen is so fond of often gets in the way of the actual business that needs to be transacted.

Overall, I think that Bremen’s book is a good one, even if some of the advice is slightly different from I would give.  I think that she would do well to work with a second author (from a research university) for a subsequent edition, to cover those situations that don’t come up much at 2-year colleges.  Despite those holes, I still recommend the book for UC students, particularly first-in-family students.

 

 

Practice, teaching, or genetics

Filed under: Uncategorized — gasstationwithoutpumps @ 10:04
Tags: , , ,

Mark Guzdial, in The 10K Hour Rule: Deliberate Practice leads to Expertise, and Teaching can trump Genetics | Computing Education Blog, responds to a Slate article claiming that genetics is more important than practice:

Here’s my argument summarized. The Slate authors and Macnamara et al. dismiss the 10K hour rule too lightly, and their explanation of genetic/innate basis for expertise is too simple. Practice is not the same as deliberate practice, or practice with a teacher. Expertise is learned, and we start learning at birth with expertise developing sometimes in ways not directly connected to the later activity. The important part is that we are able to learn to overcome some genetic/innate disparities with good teaching. We shouldn’t be giving up on developing expertise because we don’t have the genes. We should be thinking about how we can teach in order to develop expertise.

Mark’s blog is read (or at least commented on) mainly by teachers of computer science, so he is largely preaching to the choir here. I would like to believe that my teaching makes a difference—I spend almost all my time teaching, grading, or preparing to teach.

I do believe that most students in my classes leave the class with better skills than they came in with.  Whether that is due to my teaching or just to the students being forced to practice is somewhat difficult to determine—to a large extent my teaching style consists of forcing students to practice skills that they’ve generally ignored in the past (like in-program documentation) and providing them detailed feedback on their practice.  I’d like to believe that the feedback (both individual and group) matters, since I give up my weekends to provide the feedback.  If only the practice matters, then I could do as many of my colleagues do and just do I/O testing or delegate the feedback to untrained undergraduate graders.

So I have a bias towards believing Mark’s claim that teaching matters, and that there is a difference between different sorts of practice by students.

But the outcomes for individual students seem to depend more on the students coming in than on what I do.  Those students who come in better prepared or “innately” smarter progress faster than those who come in behind, so the end result of the teaching is that differences among the students are amplified, not reduced. Whether the differences in the students coming in are due to prior practice, prior teaching, or genetics is not really knowable, but also not really relevant.

Mark claims that “Genetics/innate starts at birth, no later”, which is somewhat of a simplification.  Although innate differences are present at birth (by definition), they may not be expressed until much later, either due to the developmental program that coordinates gene expression or due to environmental triggers.  So phenotypic differences may not appear until much later (genes for patterns of facial hair among men generally make no difference until puberty, for example).

He claims that

If you’re going to make the genetics/innate argument, you have to start tracking participants at birth. Otherwise, there’s an awful lot that might add to expertise that’s not going to get counted in any practice logs.

I’ve only had one child that I have taught from birth on (and lots of others also taught him), and we all know the uselessness of sample size=1, so it is not possible for me (and probably for anyone) to track participants from birth for a significant sample size.  But there are certainly ways to estimate the heritability of talent without tracking all activity since birth—the twin studies that he dismisses attempt to do precisely that.  (Some of the twin studies are well done and some are useless anecdotal reports—but there is substantial evidence that some talents have a substantial heritable component.)

Of course, it is always hard to pick apart whether “nature” or “nurture” is responsible for a particular difference in talents, since there is a large feedback loop.  Small differences in initial results can result in differences in how much pleasure practice provides and how much support is given, which can in turn affect how much practice is done and how valuable the practice is.  So small differences in “innate” talent can be amplified to large differences in outcomes.

I’d like to believe Mark’s claim that “Hours spent in practice with a good teacher are going to contribute more to expertise than hours spent without a teacher,” and that I’m a good enough teacher to make that difference.  But I fear that there is a lot of confirmation bias here—I want to believe that what I do matters, so I accept articles and studies that confirm that belief.

Looking back over my own education, I had a few teachers who helped me progress, and a few who probably delayed my learning by convincing me that the subject they were teaching was unutterably tedious, but a lot of my learning was on my own without a teacher. Sometimes the initial learning was with a teacher (often my Dad, when I was child, see Thanks, Dad), but subsequent learning was pretty much entirely from books and solo practice.  It is hard to say whether I would have achieved more expertise with teachers—some of the stuff I learned was esoteric enough that there were no teachers and I had to teach myself.  Other material was more commonly available, but I came at it from an unusual direction, so that the conventional ways of teaching the material would have been a very bad match for me.

Having an expert mentor around can make difference, and structured practice (such as I assign to my students) can make a difference—even just having an externally imposed reading schedule can make a difference.  But most of my learning in the past couple of decades has been without a teacher and without an externally imposed course structure.

So my own experience is that teachers are not the secret sauce to developing expertise.  Good teaching helps, but good learning can take place even in the absence of teachers.

Mark wrote

Look back at that definition of “deliberate practice”—who’s going to pick the activities that most address your needs or provide the immediate feedback? The definition of deliberate practice almost assumes that there’s going to be teacher in the loop.

I think Mark is wrong here.  For example, when I was teaching myself electronics design, I picked the activities based on what I wanted to design.  The feedback came from building and testing the circuits—from the real world, not from the opinions of teachers. I found that some of the simplified models used in the text books and religiously repeated in intro courses were not very useful, while others were very handy and gave good results.  Having a teacher steering me would have probably resulted in less learning, because I would not have been as invested in the examples (so less willing to explore) and the examples would have been chosen to give the conventional results, rather than showing where the conventional models break down.

For example, my post Capacitance depends on DC bias in ceramic capacitors explains how I found out about how ceramic capacitors change their capacitance with DC bias.  The knowledge was out there in various industrial application notes, but it is not generally taught in beginning electronics courses—capacitors are treated as ideal devices.  A teacher would probably have led me to a circuit that did not have a large DC bias on the capacitors, so that they would have acted much like the ideal devices, and I would not have learned a very important (and often overlooked) flaw in the models.  I may be less expert in the conventional models than someone who spent the same amount of time studying electronics with a teacher, but I have picked up odd bits of learning that I would have missed with most teachers.

Similarly, my posts Diode-connected nFET characteristics, More mess in the FET modeling lab, and Mic modeling rethought showed my learning about the characteristics of nFET transistors, where I ended up with a different model from the textbook ones.  Teachers would have almost certainly directed me to learn the conventional model first, and then much more complicated models to patch the conventional model (that’s all I could find in any of the textbooks).  Not having a teacher let me find a useful simple model for the I-vs-V curve that models the entire curve fairly well, without having to switch between models.  (Incidentally, I never did come up with an explanation for the negative resistance in the first nFETs measured in the “more mess” post—that part has been discontinued and other nFETs I’ve measured don’t exhibit the phenomenon.)

Mark might argue that I had good teachers in the past, which allowed me to develop more expertise at self-teaching.  I won’t dispute that, but I think his main point “the definition of deliberate practice almost assumes that there’s going to be teacher in the loop” is refuted by self-teaching with real-world feedback.

2014 October 10

Reference list for women in science

Filed under: Uncategorized — gasstationwithoutpumps @ 21:45
Tags: , ,

Every year I spend part or all of one of the classes in my “how to be a grad student” course on talking about women in science—more specifically about women in computational fields.  For the last couple of years, I’ve been fortunate to have one of the more senior female grad students lead the discussion, but she plans to finish her thesis this year, so I’ve asked her to try to spread the expertise around so that someone else could take over next year.  She has put together a panel for the class consisting of herself, a female researcher in the field (and alumna of our program), a female faculty member from another department who has done published research into ways to increase female participation in computer science, and an advising staff member with yet another valuable view-point. All three of the other panel members are likely to be here for several years to come, and they could easily incorporate a grad student onto their panel, should some other grad student wish to step up in future.  So this seems like a good way to create an institutional continuity even as grad students come and go.

I am looking forward to how the panel works, since we’ve not had a panel before.  We also have 6 women and 7 men in the course, which is as close as we can come to gender parity with an odd number of students.  That should help with the discussions (though last year went ok, despite having an all-male incoming group of grad students).

Earlier this week I came across an excellent list of resources on women in tech fields on the Slow Searching blog.  I recognized a few of the articles as good ones and the rest look promising, though I’ve not had time to read them yet.  Even more recently on the same blog, there was a pointer to Project Implicit at Harvard, which lets people explore their unconscious biases.  I’ve not had time to follow up on Project Implicit either.  Perhaps if I get the grading done this weekend I’ll have a little time left to do some reading.

« Previous PageNext Page »

The Rubric Theme. Blog at WordPress.com.

Follow

Get every new post delivered to your Inbox.

Join 276 other followers

%d bloggers like this: