Gas station without pumps

2016 July 27

Measuring a high-voltage nFET

Filed under: Circuits course — gasstationwithoutpumps @ 21:07
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I got the nFETs today that we’re planning to use for the LED theater lights, and decided to characterize them, to make sure that they would work as we expected.  These are IPU50R950CEAKMA1 500V 4.3A nFETs from Infineon.

The gate voltage at which these nFETs turn on is a fairly high one (traditional, rather than low-threshold for logic-level input), so I had to use a voltage divider to measure Vgs.  I tried measuring with the 3.3V power from the Teensy board and with a 9V power supply—I needed to use voltage dividers to measure Vds and Vdd with the 9V power supply.  (Note: the power supply was a Mean Well GS60A09-P1J power supply—it delivered 9.24V without load and 9.14V with a 790mA load.)

The gate voltage at which the nFET turns on drops as the nFET gets warm:

The flat current at higher voltages is an artifact of the load resistor being used—the load resistor acts as a current-limiting resistor.

The flat current at higher voltages is an artifact of the load resistor being used—the load resistor acts as a current-limiting resistor.

The specs give a Vgs for a current of 0.1mA of 2.5V to 3.5V, and so this device seems to be in spec (I have to guesstimate the shift for the higher currents).

The current is limited by the load resistor, but the maximum current here is close to what we expect to use for the LEDs, and the gate voltage is supposed to be at least 7.25V for the driver we are using (HV9910B), so the characteristics here are probably a bit conservative.

I tried using a smaller resistor (1.8Ω) with the 9V power supply to try to characterize at higher currents. The power supply and the 50W resistor had no problems with this, but the nFET got very hot very fast, so I terminated the test before I could damage the nFET. I think that the nFET was trying to dissipate about 8W of power, and at 63°C/W that would exceed the 150°C max junction temperature long before reaching equilibrium.

I also tried running with the 10Ω resistor for a few minutes, to get closer to an equilibrium condition for the nFET, and then recorded for about 87 seconds. I used this recording to plot power dissipated in the nFET versus Vgs, in order to figure out what the steady-state power dissipation would be at the currents we expect to use.

The power peaks at 2.11W when the effective drain-to-source resistance matches the load resistance (around Vgs=4.4V), but even with the transistor fully on, we're dissipating about 1.26W.

The power peaks at 2.11W when the effective drain-to-source resistance matches the load resistance (around Vgs=4.4V), but even with the transistor fully on, we’re dissipating about 1.26W.

The 1.26W at 63°C/W would give a junction temperature around 105°C, which is well below the 150°C limit. Of course, that is assuming an ambient temperature of 25°C, and the interior of the lighting can will be warmer than that. We’ll have to run fans to keep things cool enough.

I would not want to use this nFET for class-D amplifier lab in the circuits course, because it has such a high threshold voltage, though 35¢ price in hundreds is not bad.  It does look like it will work well for the theater lights, though.

2016 July 25

Hearing aids

Filed under: Uncategorized — gasstationwithoutpumps @ 22:20
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Because I’m gradually going deaf (I’ve lost a lot of my hearing over 2kHz already), I’ve been learning about hearing aids—I’ve even been thinking of doing “thrift-store science” with them, with a plan for making my own test box and 2cc coupler.  I have not bought hearing aids yet, but will probably need to do so by next year.

So far, I’ve read two books to bring myself up to speed on modern hearing-aid design:

Digital Hearing Aids James M. Kates [Plural Publishing 2008]
A fairly easy read, with a good intro to signal-processing basics, but a bit dated.
Hearing Aids, 2nd ed. Harvey Dillon [Thieme 2012]
A much more thorough look at the acoustical physics and prescription of modern hearing aids, but surprisingly little on the signal processing or algorithms needed to make them work. The author is also way too fond of acronyms—he lists over 140 of them in the index in the back. Many of them are very similar to each other (REAG, RECD, REDD, REIG, REOG, REOIG, RESR, RETSPL, REUG, RIC, RITA, RITC, RITE, RSETS), , and he generally uses them without expanding them except the first time, which makes reading the book unnecessarily tedious.

(Actually, I’m only about 2/3 of the way through this book—I’m finding it rather dull in places.)

Both of these books are aimed at clinicians prescribing hearing aids, not engineers interested in designing or testing them, and so they have had more information than I need about how hearing aids are prescribed, and less than I would like about how they are designed and programmed (much of which is likely to be trade-secret coding).

Recently, the New York Times had an article about PSAPs (Personal Sound Amplification Products), which are hearing aids sold under a different product name, because of US regulations that require hearing aids to be certified by the FDA.  That article points to research by Nicholas Reed, a Johns Hopkins audiologist, into how well these non-prescription devices work.

Note: the research article in American Journal of Medicine has Reed as the second author,  with Sara Mamo as the first author—why was the female first author overlooked by NY Times? Perhaps there is another publication coming out with the authors in a different order—maybe the cited [Reed NS, Betz J, Lin FR, Mamo SK. Electroacoustic analysis of direct-to-consumer amplification devices. In preparation.]?

The NY Times article claims that the authors were impressed by three products:

  • Soundhawk, which works with a smartphone (about $400) and is quite large (looks like you’re on the phone with a Bluetooth headset, and only designed to be used in one ear, I think).
  • CS50+, made by Soundworld Solutions (about $350, $700 for a pair) and looking like cross between a conventional behind-the-ear model and an in-the-ear model (so combining the disadvantages of each). Customization can be done with either a computer or a smartphone (a plus for me, since I still have not bought even a dumb cell phone).Based on the specs and the documentation for their customizer app, this device appears to be a 16-channel amplifier with 16-channel noise suppression, with feedback cancellation, moderate (not adjustable) compression, and switchable omnidirectional or unidirectional mic, and the left and right ears can be separately adjusted. The American Journal of Medicine article mentions good signal-to-noise ratio, noise reduction, and speech-enhancement software.
  • Bean T-coil, made by Etymotic (about $350, $600 for a pair), which looks like a conventional in-the-ear hearing aid.  It is not as adjustable as the other two—they even claim “No adjustments needed; no controls to adjust” as if everyone had the same needs.  According to the specs, it is a 15dB analog amplifier with wide dynamic range compression. The frequency response and compression are fixed, not adjustable. This is not a modern hearing-aid design, but one from the late 1980s!  It would be a good deal at $30, but  not at $350. The American Journal of Medicine article did not indicate that they were impressed with the Bean T-coil: too much low-frequency gain, only adequate signal-to-noise ratio, no noise reduction or low internal noise.

These are not low prices, but much cheaper than the medically priced ones, which average $1400 each (according to Wikipedia)—a ridiculously high price for what they contain—even the $350 ones have a huge markup, as the components probably cost in the $10–$50 range.

The Bluetooth-enabled Soundhawk and CS50+ are huge devices, which probably helps them put in all the components needed for the Bluetooth connectivity, in addition to the sound-processing components. The Companion hearing aid seems to be the same as the CS50+, but in a smaller, traditional behind-the-ear, receiver-in-the-ear aid ($450 for one, $735 for two). They claim a longer battery life, despite a smaller battery, so there are probably some compromises on the design (possibly a lower sampling rate).

You can get very cheap sound amplifiers from China ($5–$20 each), but these are often just 1-transistor amplifiers with awful distortion—equivalent to 1970s-era designs or earlier.  I’ve been wondering whether it would be possible to design my own PC board to fit into a behind-the-ear case and make a hearing aid that is comparable to the Bean T-coil, but designed around my hearing losses. I don’t think that I have the patience this summer to design a full digital hearing aid like the Companion, CS50+, or Soundhawk—there is a lot of code tweaking needed to make the signal processing work right at low enough power. If I can get good answers from their customer support, I may just buy myself a pair of the Companion hearing aids.

2016 July 24

More thoughts on measuring FETs

Filed under: Circuits course — gasstationwithoutpumps @ 21:29
Tags: , , ,

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
Tags: , ,

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.

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