Eleventh video for electronics book

I’ve just published my eleventh video for my Applied Analog Electronics book.  This video is for part of §27.2, which is the first part of Lab 7, DC characterization of an electret microphone.

I filmed the video using OBS (Open Broadcaster Software), and this is the unedited seventh take. Earlier takes had problems with the order of presentation, technical problems with the green screen, technical problems with the interaction with the Analog Discovery 2, or just stumbles in saying what I intended.

This video is the first one in this series using a green screen, but I found that the lighting on my green screen is not uniform enough for OBS, particularly in a daylit room—I ended up having to do the recording at night so that I could use only artificial light. The chroma key in OBS is nowhere near as easy to use as virtual backgrounds in Zoom.  You can see some problems with the green screen even in the thumbnail shot.

Beginning design of a cat drinking fountain

One of our cats likes to drinking from running water (a bathroom sink on a trickle setting), so my wife challenged me to make a drinking fountain for the cat that recirculates water in a water dish.  This project will be mainly physical design (3D printing, gluing things together) with a little electronics to control the pump.

I started by buying a very cheap pump from American Science and Surplus: an ET 23 series pump that they are getting rid of for only $2.50.  Somewhat surprisingly, there is a data sheet available for this pump from the manufacturer: http://www.et-pump.com/brushless_23.html, but (not so surprisingly) the specs are different from what American Science and Surplus claims. The manufacturer says that the pump is submersible and can be run at 5V to 12V, while American Science and Surplus says it is not submersible and runs on 4V to 6V.  Because the pump electronics are fully potted, I tend to believe the manufacturer on this one.

The pump uses a brushless motor with two sets of windings (and only 2 transistors to power them) and seems to start pumping at about 4V.  To characterize the pump, I used my Analog Discovery 2 to sweep the power-supply voltage from 0V to 10V, measuring the current through a 0.5Ω resistor.  The results were interesting:

At low voltage, the current seems to be exponential with voltage, as would be expected from having a diode in the circuit—the nonideality of 5.6 is consistent with about 3 silicon diode drops. Above 4V, the motor behaves about like a 26Ω resistor, though with a lot of noise. The turn-on and turn-off behavior between 2V and 4V is interesting—the pump takes a lot of power at these voltages. All these measurements were taken with the pump running dry—it likely behaves differently when pumping water.

The “noise” in the I-vs-V curve is not random noise—it is fluctuation in the amount of current taken as the circuitry for the brushless motor switches between the two sets of coils. If we set the power supply to a constant 5V across the motor in series with the 0.5Ω resistor, we can observe the voltage and the current for the motor:

The two coils seem to take slightly different peak currents when the switch for them is turned on, but both spikes are about 2.8 times the average current. The frequency is around 643 Hz, which implies a speed of around 19300RPM.

I tried controlling the pump with one of the PWM LED controllers that I made for the desk lamps. With a 6V power supply, I need about 60% duty cycle to start the motor, but then can turn it down to about 15–20% duty cycle.  With an 8V power supply, I need about 40% to start and with a 10V supply about 30% to start. All these were crude measurements by turning a potentiometer until the motor started, but they are consistent with about a 3.3V average starting voltage and ability to keep running down to almost 1V.  If the pump stalls at low voltage, one has to bring it up to about 3.5V to turn it back on.

The motor runs even with fairly slow, low-duty cycle PWM. The current spikes at the beginning of each cycle are large.

The PWM control seems to work even with a PWM frequency as low as 270Hz, which is somewhat surprising.  There does not seem to be much in the way of voltage spiking, even with no capacitor or flyback diode added.  There is a short-lived initial current spike of about 3.5A (staying above 2A for about 4µs), which probably comes from charging capacitor C3 in the motor, which is across the power lines after the diode D1 (which seems to be there to prevent reversed power supply).  The 11µC spike is consistent with C3 being about a 1 µF capacitor (or maybe 2.2µF), which seems plausible.  I’m not sure why the current drops to 0 before the motor voltage drops more than about a volt.

I bought some cheap plastic bowls from a thrift store, and my next task is going to be to design a 3D-printed base to hold the pump and the electronics under the bowl and a clip to hold a ¼” ID vinyl tube over the bowl.  The pump is not self-priming, so I need to drill a hole in the bottom of the bowl and glue on the pump to make a gravity feed to do the priming.

LM2903 open-collector comparator characterization

In Last power-amp lecture, I posted an I-vs-V plot for the LM2903 comparator’s open-collector output, which I had made sometime in 2013, I think:

There are two regions of operation for the open-collector output of the LM2903. In the saturation region, the current goes up slowly with voltage (as about V^0.15, while in the “linear” region, it goes up as about V^1.5). The transition occurs when V_OL is about 0.25 V, so we are almost always in the saturation region.

I decided to redo the plot using the Analog Discovery~2, as I now include the open-collector curve in the textbook (in an optional section, since we no longer use open-collector comparators). I used a 12V wall-wart and both the function generator and oscilloscope functions. I used the “custom channel” and XY plot features to get the I-vs-V plot on the screen (though I saved the data and replotted with gnuplot, to superimpose different runs). I also averaged 10 sweeps to reduce noise.

R1 was 56Ω for testing high voltages and currents, and R1 was 2.2kΩ for testing low voltages and low currents.

The triangle-wave generator and the nFET makes a variable load for the comparator, from slightly more than R1 up to about 1MΩ.

Even up to 11V, the LM2903 collector stays below the 20mA maximum current, but I’d want to make sure that there was some current-limiting resistor for any power-supply voltage above 12V.

The results with the Analog Discovery 2 are much cleaner than my old results, which were most likely done with an Arduino, which has a very low resolution ADC.

FET I-vs-V with Analog Discovery 2 again

 

In FET I-vs-V with Analog Discovery 2, I plotted Id vs. Vgs curves for an nFET:

The Ids-vs-Vgs curves do not superimpose as nicely as curves I’ve measured with PteroDAQ. I don’t yet understand why not.

Yesterday, I played with sweeping the power supply (Power waveform generator).  In this post, I used that capability to plot Id vs Vds curves for different gate voltages (Vgs) of a different nFET (since the AOI518 is an obsolete part).  The setup is the same as for the previous test—the function generator is connected to the gate, the power supply to the drain load resistor in series with the nFET whose source is connected to ground, and the two oscilloscope channels monitor the voltage across the load resistor and across the nFET.  The difference is that I use the power-waveform option to put a 1Hz triangle wave on the power supply, but put just a DC offset (AC amplitude 0V) on the function generator output, so that the gate voltage is constant as the drain voltage is adjusted.

The saturation regions are well plotted up to Vgs=2.7V. I averaged 10 or 20 scans for each of these curves, to reduce quantization noise for small voltages or small currents.

I got quite different results when I removed and replaced the nFET from the breadboard—the breadboard contacts seem to have a variation of about ±0.05Ω in resistance, which is much larger than the on-resistance of the nFET when fully on. I took measurements with a wire between the source and drain to estimate the wiring resistance, but wiggling the wire produced very different results.

In the next graph, I tried subtracting off the wiring resistance to get the on-resistance, but I’m really quite dubious about the measurements smaller than 0.5Ω, because of the unrepeatability of the bread board contact resistance.

The numbers here look good (close to the spec sheet), but repeating the measurements could result in ±0.1Ω, which makes the Ron measurements for fully on transistors rather useless.

By using a smaller power resistor, I could probably get saturation currents for slightly higher gate voltages, up to the current limit of the power supplies in the Analog Discovery 2, but better on-resistance measurements would require a better jig for making low-resistance contacts to the FET.

By using a much larger resistor, I could measure low currents more accurately, which would give me a better idea of the leakage currents—I don’t really believe the measurements for Vgs=2.1V, because the current appears to decrease with increasing Vds, which is probably an artifact of measuring a small difference in voltage with a large common-mode signal.

I tried using larger resistors to measure the saturation currents, but the results varied a lot depending on what size load resistor is used. I believe that the difference is due to temperature changes from self-heating. If I sweep out to larger Vds voltages (using a smaller load resistor, hence smaller IR drop across it), but about the same saturation current, I’m dissipating more power in the transistor, so making it warmer. This appears to increase the saturation current. Reducing the range of the voltage with the same load resistor drops the curve down, just as increasing the load resistor does. I suspect that proper measurement requires a jig that holds the transistor at a nearly constant temperature, as well has having very low contact resistance.

The saturation current seems to vary by about ±10% as I change load resistors. The effect is most likely thermal—note that using a smaller voltage sweep for Vgs=2.3V and Rload=51Ω resulted in almost the same curve as Rload=270Ω, because the power dissipated was about the same.

Note that the thermal explanation also works for explaining why the superposition does not work well for the Id vs Vgs plots—at lower load resistances, more power is dissipated in the transistor, and it gets warmer, shifting the current curve upward.

Electret mic DC characterization with Analog Discovery 2

I tried one of the standard labs for the course, producing an I-vs-V plot for an electret microphone, using the Analog Discovery 2 function generator and oscilloscope, rather than a bench function generator and a Teensy board with PteroDAQ.

It was fairly easy to set up a 0–5V triangle wave, running at a very low frequency (50mHz, for a 20-second period).  The maximum output from the waveform generator is 5V, so setting the amplitude higher did not get larger voltages.  The signal was applied across the microphone in series with a sense resistor, and the voltage measured across the mic and across the sense resistor.

I ended up using two different sense resistors: one for measuring the current at high voltages, and one for measuring the current at low voltages, and I had to adjust the voltage scales on the two channels of the scope for the different ranges.  The results were fairly clean:

The low-voltage behavior of the nFET in the electret mic is not quite a linear resistor, and the saturation current definitely increases with voltage.

I tried extending the voltage range by using the power supply as well as the function generator: I set the function generator to a ±5V triangle wave, and used a -5V supply for the low-voltage reference. This worked well for the higher voltages, but the differential signal for the mic had an offset of about 12mV when the common-mode was -5V, which made the low-voltage measurements very wrong.  This offset may be correctable by recalibrating the scope (I am currently using the factory default settings, because I don’t have a voltmeter at home that I trust to be better than the factory settings), but I’m not counting on it.  When I need measurements of small signals, I’ll try to make sure that the common-mode is also small.

One other minor problem with the Analog Discovery 2: the female headers on the wires seem to have looser than usual springs, so that the wires easily fall off male header pins.  Given the stiffness of the wires, this is a bit