Gas station without pumps

2015 October 6

Crawlspace ventilation—better low-voltage handling

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

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 September 27

Crawlspace ventilation again

Filed under: Uncategorized — gasstationwithoutpumps @ 22:53
Tags: , , ,

In Crawlspace ventilation, I talked about wiring up some little 5V fans, a voltage regulator, and a solar panel to ventilate the crawl space under my house. Today I wired up another regulator board (with a green LED this time).  The wiring is a bit neater this time (practice helps!):

I soldered the components to the prototype board first this time, then added the wires, rather than trying to do both at once. As before, the end with a one screw connector is the 12V input, and the other end has a pair of GND connections and a pair of 5V connections.

I soldered the components to the prototype board first this time, then added the wires, rather than trying to do both at once. As before, the end with a one screw connector is the 12V input, and the other end has a pair of GND connections and a pair of 5V connections.

The circuit is the same as before, with the addition of a green LED to light up when there is power.

The circuit is the same as before, with the addition of a green LED to light up when there is power.

I tested the regulator with 1, 2, 3, and 4 of the fans: http://www.digikey.com/product-detail/en/FAD1-06025BBLW12/Q620-ND/2600074  I got a constant 5.02V output independent of the load when powering the regulator from a 9.25V power supply (nominally 9V).  The 9V supply is pretty beefy (6A capability), so the tiny loads of the fans did not cause much change in the voltage at the input to the regulator (maybe 0.02v IR drop).

I was curious what a larger IR drop in the wiring to the regulator would do, so I tried putting a resistor between the 9.25V supply and the regulator:

resistance # fans V_in I_in
1 9.14V 60mA
2 9.02V 120mA
3 8.88V 190mA
4 8.76V 250mA
1 9.01V 60mA
2 8.76V 125mA
3 8.48V 195mA
4 8.19V 270mA
1 8.75V 63mA
2 8.22V 130mA
3 7.5V 220mA
4 6.7V 320mA
10Ω 1 8.61V 65mA
10Ω 2 7.85V 140mA
10Ω 3 6.94V 230mA
10Ω 4 6.4V 290mA

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).

It seems like the voltage regulator works fine as long as the input voltage is at least 7V (as claimed in the specs), but if the IR drop in the wiring to regulator is enough to drop the voltage below 6.8V, the regulator may not be able to supply enough current to start the fan motors. Each fan takes about 80mA@5V (400mW) once up to speed, so the regulator seems to have an efficiency around 74%—considerably less than what I expected from the spec sheet.  I’ll have to investigate that more closely.

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.

2015 September 26

Crawlspace ventilation

Filed under: Uncategorized — gasstationwithoutpumps @ 23:03
Tags: , , ,

I have a crawl space under my house that gets rather damp—my house is built where an aquifer comes to the surface, and in most years the water table is only a few centimeters below the surface.  (Because of the 4-year drought, the water table is currently about a 30cm below the surface.)  A few years back I had a solar panel put on one side of the house, connected to 12V fans in the crawl space to exhaust air from the wettest part of the space, but the fans stopped working shortly after the installation.

I have two conjectures about why the fans failed:

  • The 12V DC wires from the solar panel to the fans were broken somewhere, and so there is no voltage at the fans.
  • The voltage from the solar panel exceeded the voltage rating of the fans enough to burn out the fans.

I’m claustrophobic enough that I’ve never wanted to crawl around down there to try to debug the problem or replace the fans, so the installation has been non-functional for a few years.

So next week I’m having my general contractor/carpenter redo the fans so that they will be more maintainable.  The main thing is to make hinged “doors” for the vents, so that the fans can be accessed from outside the house, without having to crawl the width of the house under a very low ceiling.  Having access to both ends of the cables also makes checking them for continuity easier, and makes it possible to pull new cables if needed without crawling under the house.

 

I’m also going to change how the fans are hooked up.  Instead of using 12V fans, I got a number of little 5V fans that are very quiet (18.1 dB) but have reasonable airflow (13.1 CFM): http://www.digikey.com/product-detail/en/FAD1-06025BBLW12/Q620-ND/2600074 They are rated for 70,000 hours, but are only rated down to –10°C (good enough for Santa Cruz).

Putting 4 of the little fans blowing air through holes in the “door” should provide me with about 50 CFM (cubic feet per minute) at about 24dB.  With two such doors (8 fans) I should get 100CFM at about 27dB.  That is more air flow for the noise level than any single-fan solution that I found.

I’m going to put a 5V switching regulator on each door, so that the voltage from the solar panel can fluctuate wildly and the fans can still run at a constant voltage. The OKI-78SR regulators can accept anything from 7V to 36V and produce 5V output with about 90% efficiency. They only handle 1.5A, but 4 of the fans is only 0.6A, well below the rating. Each regulator will be delivering about 3W to the fans, so the power needed from the panel is only about 7W.  I believe that the panel was a 50W panel, so the fans should run even when the sky is overcast, though not at night, unless I add a rechargeable battery to the system.   (There was originally a lithium iron phosphate battery in the system, with a charge controller, but I repurposed it for other projects and it has since failed.)

I also don’t have to worry about the IR drop in the cabling to the vent fans, as it will not be anywhere near enough to drop the voltage below 7V when the solar panel is producing power.

Here is the circuit:

The Schottky diode is there to prevent damage from accidentally wiring the power backwards—I happened to have a couple on hand.

The Schottky diode is there to prevent damage from accidentally wiring the power backwards—I happened to have a couple on hand.

I wired up one of the regulators on a prototype board:

My soldering is pretty sloppy on the prototyping board—I've gotten so used to doing custom PC boards for everything that running wires and joining them to components seems a bit strange. There's not much point to a custom board for only 2 instances, though, especially with such a simple circuit.

My soldering is pretty sloppy on the prototyping board—I’ve gotten so used to doing custom PC boards for everything that running wires and joining them to components seems a bit strange. There’s not much point to a custom board for only 2 instances, though, especially with such a simple circuit.

In this rather low-quality photo, the 12V input (7V–36V) comes in through the two screw terminal on the right, and the 5V output is on the the two front screw terminals on the left, with ground on the two back screw terminals on the left.  I added male headers for +5V, Gnd, Vin, so that I could later add monitoring circuitry for remote monitoring or logging the voltages, if I felt like adding that.  It might be interesting to log the voltages for a year.

I’m considering adding a resistor and an LED to the 5V output, so that there will be a visible indicator of power, but I’m not sure it will be worth the effort, since the board will usually not be visible and if I open the panel for debugging, I can use a voltmeter easily enough.

2013 July 10

Some failed designs

For the past couple of days I’ve been exploring variations on the Blinky EKG project, looking at alternative approaches.

For example, I looked at the possibility of eliminating the most expensive part (the instrumentation amp), and decided that building my own instrumentation amp out of op amps and discrete resistors was unlikely to be reliable.  I discovered (after doing calculations for 2-op-amp and 3-op-amp designs) that 1% tolerance on the resistors would produce poor common-mode rejection. In Common-mode noise in EKG, I reported measurements of the common-mode noise with a fairly short twisted-pair connection to the EKG electrodes (close to a best-case scenario).  I concluded that the common-mode noise was way too large for using unmatched resistors to be a reasonable design, and using an integrated instrumentation amp is still a good choice.

Yesterday,  I tried turning the question around? Could I eliminate the op-amp chip?  Currently, I’m using the op amp for two things: to provide a unity-gain buffer to make the reference voltage source between the power rails, and to do second-stage amplification after a high-pass filter removes the DC offset. To eliminate the op-amp chip, I need to replace both these functions.

Replacing the unity-gain buffer seems fairly easy—I could use a low-drop-out linear regulator to generate the reference voltage instead of a voltage divider and unity-gain buffer, which would be somewhat smaller and cheaper (11¢ rather than 25¢ in 100s).  I didn’t have an LDO linear regulator at home, so I tried using a TL431ILP voltage reference instead.  Unfortunately, it provides very little current, and was unable to maintain the desired voltage when hooked up to the reference voltage input of the instrumentation amp.  I believe that a part like the LM317L would work fine, though, and I may want to test that at some point.

Removing the second-stage amplifier is more problematic.  I can set the gain of the instrumentation amp up to 2000 or 2500 easily, but any DC component in the input differential signal results in saturating the amplifier (with a 6v output range, and a 1mV AC signal, we’d need the DC bias to be less than 1mV also to avoid hitting the power rails).  I tried putting high-pass filters in front of the instrumentation amp, but with the long time constants needed to avoid filtering out the EKG signal, the filters never settled to within 1mV of each other, and the instrumentation amplifier always saturated.

So I need to keep the first-stage gain small enough to avoid saturation, which means I need a second-stage amplifier.  I could use a single op amp for the second stage and a low-drop-out regulator for the reference voltage, which would produce a cleaner output signal (since my voltage-divider-plus-unity-gain-buffer reference introduces noise from the power lines).  The MCP6001 single op amp is only 18¢ in 100s (rather than 25¢ for the MCP6002 dual op-amp), but the MCP6001 is only available as a surface-mount component, which I think is inappropriate for a first soldering project.  The MCP6001 + LM317L would cost about 4¢ more than the MCP6002.

I considered redesigning the Blinky EKG to use the LM317L voltage regulator and the MCP6002, even though half the MCP6002 would be unused, but the LM317L needs a 1.5mA load to maintain regulation, and that seems like a lot for a battery operated device—more than the op amp or the instrumentation amp use (though less than the LED when it is lit).  Even using a TL431ILPR voltage reference (10¢ in 100s) and the unity-gain buffer would only need 1mA, and would save one resistor.  There are lower-current voltage references, like the LM385, but they cost a lot more (42¢ in 100s).

The non-rechargeable CR2032 batteries I’ve been using for the Blinky EKG have about a 225 mAh capacity, and cost about 19¢ in 100s (but the design needs 2, so 39¢).  I could probably get about a 100-hour life with the present Blinky EKG design—I need to measure the current and the duty cycle of the LED to get a better estimate.

The Blinky EKG weighs about 20g (not counting the wires to the electrodes), which is a bit heavy for a pendant or brooch. Most of the weight is in the batteries, but a lighter battery would give up a lot of running time.   The smaller batteries also cost a lot more, probably because Digikey only buys them in quantities around 10,000 rather than in the millions. (From other suppliers CR2032 batteries cost about 60¢ in 100s, not under 20¢).

It has been good to fool around with the Blinky EKG design, as it has gotten me to think a bit about design issues other than the first can-I-get-it-to-work one.  I rarely get my students past that point in their thinking, and I’m not sure how I would do so, as there is always so much time pressure to cover new stuff, that they get very little time to tinker with designs.

 

%d bloggers like this: