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2016 July 24

More thoughts on measuring FETs

Filed under: Circuits course — gasstationwithoutpumps @ 21:29
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I have spent some time this week thinking about a lab for characterizing nFETs (and pFETs) for the applied electronics course, because I was not very happy with the preliminary work in Possible new FET lab for electronics course and pFET Ron_vs_Vgs. There were a couple of things that bothered me:

  • Should we be looking at equivalent resistance or current? which is more consistent across different test conditions? which is more useful for continued design?
  • Does it matter what circuit we use for making the measurement? What are the advantages or disadvantages of choosing different power supply values, different load resistors, or different transistor configurations?

The most basic choice is between a common-source or common-drain amplifier setup, both of which are special cases of the same circuit:

For a common-source configuration, the output is Vd, the load resistor is Rd, and Rs is set to 0Ω. For a common-drain configuration, Vs is the output, Rs is the load resistor, and Rd is set to 0Ω.

For a common-source configuration, the output is Vd, the load resistor is Rd, and Rs is set to 0Ω. For a common-drain configuration, Vs is the output, Rs is the load resistor, and Rd is set to 0Ω.

The testing I did before was with a common-source circuit. The output voltage range is from very close to 0V up to Vdd, which means that either Vdd has to be limited to the range of the analog-to-digital converter or a voltage divider is needed to divide down Vd. The current we are interested in is I_d = (V_{dd}-V_{d})/R_d. Taking the difference of two measurements increases the noise of the measurement, and if Vdd is large enough that voltage dividers are needed, then the current through the voltage divider also goes through the load resistor, making measuring low drain currents (when Vgs is small) difficult.

The common-drain circuit only requires measuring Vg and Vs, both of which can be within the analog-to-digital converter range even when Vdd is large, assuming that the load resistor (Rs in this case) is chosen sufficiently small. Determining Vgs requires a subtraction, but Id does not. Avoiding voltage dividers (with their current stealing and the extra trouble of measuring the divider ratio) seemed like a good idea.

I tried making measurements with the common-drain configuration today, and found it surprisingly difficult. I kept getting huge amounts of noise on the Vs plot, whenever the FET was on. It appeared to be 120Hz interference, but I have no idea where the interference was coming from—the power supply did not show significant 120Hz ripple and large bypass capacitors on the power supply lines did not help. I finally managed to get rid of the problem by putting a 10µF capacitor between the source and drain, providing a negative feedback path that eliminated the problem. (The capacitor appeared to make no difference to the plots for common-source configurations.)

The common-source circuits, however, provided much cleaner plots than the common-drain circuits. The common-source circuits also allowed me to measure much higher drain currents, because I could go to a higher Vgs value without exceed the ADC range when Vs was constrained to be 0V.

For the range that I could measure, I got essentially the same current whether I measured drain current or source current, and pretty much independent of the voltage and load resistance I used. To first approximation, it looks like Id is a function of Vgs in a way that the equivalent resistance that plotted in Possible new FET lab for electronics course is not.

Using a small load resistor allows measuring up to a fairly high current (Vdd/R), as long as the power supply can deliver the current and the resistor doesn't overheat. Using a large load resistor allows measuring to a fairly low current.

Using a small load resistor allows measuring up to a fairly high current (Vdd/R), as long as the power supply can deliver the current and the resistor doesn’t overheat. Using a large load resistor allows measuring to a fairly low current.

2016 July 22

Modeling bicycle balance—a disappointing Nature article

Filed under: Uncategorized — gasstationwithoutpumps @ 17:38
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The bicycle problem that nearly broke mathematics in Nature News & Comment is a badly titled (click-bait) article that talks about one person who contributed to the development of  the differential equations that accurately describe bicycle balancing (which has been incorrectly or incompletely described many times in the physics and engineering literature).

The one-line summary of the article is pretty accurate:

Jim Papadopoulos has spent a lifetime pondering the maths of bikes in motion. Now his work has found fresh momentum.

There is nothing in the article giving any indication that the equations Papadopoulos derived provided any stress to mathematics.  The problem, as in many physics problems, is all in deciding what needs to be included in the model to get the best compromise between the tractability of the model and its accuracy.  So far as I can tell from the vague descriptions in the article, the equations themselves are pretty much standard PDEs.

Unfortunately, the article does not give the equations themselves, so this article is particularly disappointing.  It is People article, not a science article.

The article did give one prediction from the equations that showed their worth: it is possible to design a rideable bike with no gyroscopic balancing and negative trail, which would be inherently unstable in previous, simpler models. The trick is to move the center of gravity far enough forward to be ahead of the steering axis. Supposedly, such a bike has been built [Kooijman, J. D., G. Meijaard, J. P., Papadopoulos, J. M., Ruina, A., Schwab, A. L. A Bicycle Can Be Self-Stable Without Gyroscopic or Caster Effects Science 3(32), 339–342 (2011)], but that article is hidden behind the Science paywall, so you’ll need to go to a university library to access it.

The supplementary material for the Science article is where the equations are presented and explained.

2016 July 19

Americans for the Arts poll

Filed under: Uncategorized — gasstationwithoutpumps @ 10:04
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Americans for the Arts  Public Opinion Poll Overview has recently published a summary of their opinion poll about the arts. It does not come as a surprise to me that people are broadly in favor of the arts and participate at a moderately high rate—the questions are “motherhood-and-apple-pie” questions that would be difficult to disagree with. Some numbers are a bit lower than I would hope to see—only  68% of adults attended an arts event in the past year, and some are higher than I would expect—27% donated to an arts organization.

What Americans Believe About the Arts

The American public is more broadly engaged in the arts than previously understood—believing that the arts not only play a vital role in personal well-being and healthier communities, but that the arts are also core to a well-rounded education.

1. “The arts provide meaning to our lives.” 63 percent of the population believe the arts “lift me up beyond everyday experiences,” 64 percent feel the arts give them “pure pleasure to experience and participate in,” and 73 percent say the arts are a “positive experience in a troubled world.”

2. “Most of us seek out arts experiences.” Seven in 10 American adults (68 percent) attended an arts event in the past year, like going to the theater, museum, zoo, or a musical performance.

3. “We often experience the arts in unexpected places.” An even greater proportion of Americans (77 percent) say they experienced the arts in a “non-arts” venue such as a park, hospital, shopping mall, or airport.

4. “Across demographic groups, the arts are part of our lives.” People of color were more likely to attend an arts event than their white counterparts (71 percent vs. 66 percent). Higher rates of attendance for people of color were noted for multiple art forms, including dance, museums, and theater.

5. “Arts institutions add value to our communities.” Regardless of whether people engage with the arts or not, 87 percent believe they are important to quality of life, and 82 percent believe they are important to local businesses and the economy.

6. “We donate to the arts.” 27 percent of the population (more than 1 in 4 Americans) made a donation to an arts, culture, or public broadcasting organization within the past year. Donors were typically younger and had higher incomes and education.

7. “We will support candidates who want to increase arts funding.” Americans are more than twice as likely to vote in favor of a candidate who increases arts spending from 45 cents to $1 per person than to vote against them (37 percent in favor, 16 percent against).

8. “We believe the arts are part of a well-rounded education.” Nine in ten American adults (88 percent) agree that the arts are part of a well-rounded K-12 education.

9. “We believe the arts should be taught in grades K–12.” 90 percent believe students should receive an education in the arts in elementary school, middle school, and high school. 82 percent say the arts should also be taught outside of the classroom in the community.

10. “We are making art in our personal time.” Half of all Americans are personally involved in artistic activities (49 percent) such as painting, singing in a choir, making crafts, writing poetry, or playing music.

11. “We engage in the arts because it makes us feel creative.” Among those who are personally involved in making art or displaying art in their home, 60 percent say that “arts and music outside of the home” makes them feel more creative—a rate that jumps to 70 percent for Millennials.

12. “Social media increases our exposure to the arts.” 53 percent of social media users say that they are more exposed to the arts thanks to connecting online. 59 percent agree that art created on social media is a legitimate form of art.

13. “Yes! Tattoos are art.” 27 percent of Americans boast a tattoo (12 percent have more than one). Three-quarters believe that tattoos are a form of art (73 percent).

14. “The arts unify our communities.” The personal benefits of the arts extend beyond the individual and to the community. 67 percent of Americans believe “the arts unify our communities regardless of age, race, and ethnicity” and 62 percent agree that the arts “helps me understand other cultures better.”

15. “Despite the benefits the arts provide, not everyone in our communities has equal access to the arts.” Despite the individual and community benefits, just 45 percent believe that “everyone in their community has equal access to the arts.”

Source: Americans Speak Out About The Arts, Americans for the Arts. 2016.

*The 3,020 respondents self-identified by race and Hispanic ethnicity. For the report, the “white” category is non-Hispanic whites. Included in the “people of color” category are blacks, Asians, all Hispanics, and others.

I’ll have to dive into the full report or even the supplementary data tables to see exactly what questions were asked and what biases there were in the survey. One that they note is that the survey was done online, and that the non-white subset of the sample skewed somewhat higher on education and wealth than the non-white population as a whole.

The higher attendance by non-whites coupled with the perception of unequal access is a little disturbing—particularly given the emphasis on appeals to elderly white people by so many of our major cultural institutions. Of course, there is an obvious reason for the the appeals to old white people—the same reason that people rob banks: because that’s where the money is. But younger generations are more interested in the arts, and so more should be done to incorporate them into the life of our arts institutions.

I am pleased that our local museum, Santa Cruz Museum of Art and History, makes a point of reaching out to the whole community and attempting to bridge divides. I think that they have done an excellent job of including young folk (high-school and college age) in their events and planning, as well as a moderately good job of including Mexican culture (the main non-white culture in our area). I think that there is more to be done in incorporating Mexican and local Mexican-American art into the museum.  They did recently have a very good display of the Kinsey African-American Art and History Collection, even though the African-American population in Santa Cruz County is quite small—about 1.4% according to the US Census.  The Hispanic population is about 33.3%.

I was a little surprised that the poll found that 27% of the population have tattoos—in Santa Cruz, I would find an even larger number credible, but in the Midwest the numbers are likely much smaller. I wonder whether this number indicates a sampling bias in the survey, which would call all the numbers into question, or if tattoos really have become so mainstream.

I’m also a little surprised that MAH has not done a tattoo art exhibit yet (or did I miss one?), since tattoo art has been a big thing in Santa Cruz for a long time.  For those of you who care, I don’t have any tattoos—not from any philosophical, religious, or æsthetic reason, but because I’ve never been able to think of any artwork that I’d be happy to have on my body permanently (also, I dislike pain).

I was interested in seeing what “arts and culture” events were the most popular (in terms of attendance in the previous year):

  • Zoo, aquarium, or botanical garden 36%
  • Historic site 30%
  • Musical performance (Classical or popular) 29%
  • Museum of history or science (including children’s museums) 25%
  • Theater performance 24%
  • Museum of art 23%
  • Visual arts, crafts exhibition, art gallery 22%
  • Opera/musical theater 13%
  • Dance performance 13%
  • Art or film festival 12%
  • Literary event 8%
  • Other 3%
  • None 32%

I’m surprised that they did not include a category for arts and crafts fairs, antiques fairs, maker fairs, Renaissance fairs, and so forth—many people attend such events, but would probably not think of them in the context of this survey.

I also wonder how much of the attendance is “for the children’s sake” rather than personal interest—the heavy emphasis on zoos, aquaria, historic sites, history and science museums suggests that there may be some deliberate educational component for kids, rather than personal enjoyment.  (I go to science museums and aquaria for fun when I travel, but many people do it only with kids.)

I note that theater minus musical theater is still at 11%, almost as big as opera/musical theater alone, which is pleasing but surprising—musical theater seems to get a lot more advertising and get performed in much larger venues than non-musical theater.

2016 July 18

Common-anode RGB LED I-vs-V

Filed under: Data acquisition — gasstationwithoutpumps @ 00:08
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My son and I got interested in what the I-vs-V curves looked like for RGB LEDs—in particular, the common-anode ones that were used for the first fabrication run of the Kinetic microlights. At least, we think that the package of LEDs was the same as those used for the Kinetics—they weren’t labeled and record-keeping for prototyping parts has not been particularly good. In any event, the LEDs should be fairly typical of super-cheap RGB LEDs from China.

I used a 47Ω resistor from a cathode (R, G, or B) to ground, and hooked up my FG085 function generator to the anode and ground (with a 470µF capacitor in parallel, to smooth out the steps).  In order to measure the anode voltage, I had to divide it down by a factor of 2, using a pair of 6.8kΩ resistors.

The red and blue curves are nearly parallel, just shifted by about a volt, but the current increases much more slowly with voltage for the green LED.

The red and blue curves are nearly parallel, just shifted by about a volt, but the current increases much more slowly with voltage for the green LED.

Each plot was done for several cycles of a triangle wave having a period of 22s. There is pretty good consistency from one cycle to the next, but substantial hysteresis in the green LED. The red and blue LEDs also have hysteresis (visible for both red and blue when I zoom in using gnuplot, but barely visible for blue in the PNG file).

The hysteresis is almost certainly a thermal effect—the threshold shifts when the LED is warm. The green LED when up the right-hand curve, and down the left-hand curve, suggesting that warming the LED lowers the forward voltage for a given current.

2016 July 17

Online bike registration

Filed under: Uncategorized — gasstationwithoutpumps @ 16:19
Tags: ,

You can now register your bike online in the City of Santa Cruz (and get mailed the sticker) for free. This greatly reduces the hassle of bike registration, and is well worth the couple of minutes it takes.

You do need to know your bike’s serial number.  The form is a bit weird, because they repurposed their incident report form software for the purpose:

I highly recommend that Santa Cruz residents register their bikes now, and that residents of other jurisdictions push their police departments, city councils, or whoever has the power locally to register bikes to also set up online bike registration.

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