πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK



  • @NeverDie My thought was that amorphous silicon (a-Si) cells have better spectral response to artificial light than crystalline cells (c-Si). However, after investigating this a little bit I've found that this doesn't seem to be true. Instead, it's shown everywhere that c-Si cells have better response to every wavelength:

    spec-response.png

    Moreover, when the light source has wide spectrum (like the sun or an incandescent bulb), c-Si panels take the advantage and produce significantly more energy from the same source, and this all explains why a-Si cells are almost two times less effective than c-Si (roughly 8% vs 20%). Please note, because of narrow spectrum a LED lamp will be obviously inefficient for a PV panel.

    But at the same time, there are reports of a-Si cells being 4x more effective in low light than crystalline. Indeed, both crystalline and poly-crystalline cells may degrade a lot:

    cell-eff.png

    The seem happens due to low parallel resistance of c-Si type cells. Shunt resistance of amorphous cells is naturally higher which results to less degradation of Vmpp and hence higher efficiency in low light conditions. Some paper show the shunt resistance rather low, when other mentions it relatively high, but at extremely low power conditions even 20 kOhm may be too much.

    In short, a-Si cells are tend to produce fairly better results in very low light environments. But they can't leverage from wide spectrum sources, yet are subject to the Staebler-Wronski effect when exposed to direct sun (which can be reversed to some extent by heating the panel). In case if the light source is bright enough (around 1000 lx and above) a c-Si pannel should be preferred.

    Finally, there are some other kind of solar cells, in particular those made from III-V semiconductors compound and promising even better low light performance.


  • Hero Member

    @Mishka Have you found a good candidate for an amorphous cell to try? I see a lot of cells/panels advertised as amorphous, but without a datasheet showing performance under low light conditions, selecting one seems a bit like throwing darts at a map.

    I've seen some flexible amorphous panels that claim to stack materials with different light sensitivities to get a better spectral response:
    alt text
    But are they any good, or is it just puffery?

    I've seen articles claiming that CIGS have efficiencies of 20% to other articles saying that CIGS are barely better than amorphous. Some also make claims that CIGS perform well under "low light," but without the detailed datasheet, there's just not much to hang one's hat on when it comes to selecting one to try....

    And then there's powerfilm, which I had linked to earlier above, which claims to be optimized for 200 lux and below. At least they were selected by TI for TI's BLE demo kit, so presumably they were a good choice, at least at the time the choice was made....

    Is amorphous better than these other choices at low light, and if so, which amorphous solution has the best efficiency under low light?

    NREL seems to be an objective independent source for testing, but for high brightness conditions (according to wikipedia, the standard test conditions for solar cells are "the AM1.5 spectrum as the reference. This air mass (AM) corresponds to a fixed position of the sun in the sky of 48Β° and a fixed power of 833 W/m2. "):
    https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200218.pdf
    https://www.nrel.gov/pv/cell-efficiency.html
    At least on paper, the multi-junction cell efficiency looks really quite amazing. There are some for sale on ebay in the $20-$35 dollar range, depending on quantity. So, if you absolutely had to have one to meet your size requirements, there they are. No datasheets though, so again, just a cat in a bag. One claims 35% efficiency. No indication at all as to low light efficiency.



  • @NeverDie Right, the good thing about thin-film solar cells that they can be relatively easily stacked up to gain better efficiency. Don't know about CIGS, but some III-V compounds like GaAs are known to be very effective in low light environment (please see the last paper in my previous post). Such, some manufacturers are making tripple-junction GaAs cells with power effectiveness up to 15 ΞΌW/cmΒ² at 200 lx - just compare it to Amorton which have it at about 6 to 8 ΞΌW/cmΒ² under the same conditions. Sounds like a huge difference, especially taking in account the Panasonic offers rather high quality cells. Unfortunately, the cost is as high as the satellites carrying these cells.


  • Hero Member

    Last night I hooked up the keychain solar cell to my simple solar circuit, and at 5 lux it could still charge a 100uF capacitor to 2.7v and blink a red led without any boosting. It looks like it's probably amorphous. So, pretty good performance considering its low cost, but perhaps not as small as what you're looking for.



  • @NeverDie Well, 5 lux is ridiculously low. It's about the same illuminance you may have at 45 cm from a candle. Are you sure your lux meter is working? πŸ™‚

    Interesting to measure Voc and Isc at that light. What's dimension of the cell?


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Are you sure your lux meter is working?

    I'm not at all sure that it's accurate, but that's what the lux meter said. It's one of these: https://www.amazon.com/Dr-Meter-LX1330B-Digital-Illuminance-Light/dp/B005A0ETXY/ref=sr_1_3?keywords=lux+meter&qid=1582903100&sr=8-3

    I've misplaced the manual, but someone posted this on amazon as to its specs:
    The specifications in the instruction manual reflect the following:
    Light-measuring level from .1Lux to 200,000Lux
    Accuracy +- (3%rdg+10dgt) <=20,000Lux/2,000FC
    +- (5%rdg+10dgt) >= 20,000Lux/2,000FC
    Repeatability +-2%
    Photo detector lead length ~150cm
    Spectral Sensitivity- curve shows mostly betweeen 500nm and 650nm

    Perhaps I should get something better, or else maybe find some way to calibrate it. What is it that you're using?

    I assume that for "Accuracy +- (3%rdg+10dgt)" it means plus or minus 3% of the reading, which is fine. Not sure what the 10dgt means though. If that means it could be plus or minus 10 lux, then I guess it's useless for measuring 5 lux.


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    What's dimension of the cell?

    37mm x 22mm

    alt text



  • @NeverDie said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    I'm not at all sure that it's accurate, but that's what the lux meter said.

    Wow, relatively to my built into the smartphone Sensortek STK3x1x ambient light sensor this one looks very serious.

    The cell has surprisingly high voltage (2.7V) at so low light. My amorphous cell has Voc = 1.8V at 50 lux (2 m from a fluorescent lamp). Maybe yours has many more cells in series. I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.

    I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.


  • Hero Member

    @NeverDie said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    If that means it could be plus or minus 10 lux, then I guess it's useless for measuring 5 lux.

    Well, maybe not completely useless. If the specs are valid, then it's surely less than 20 lux, assuming I'm giving the right interpretation to "10dgts".


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.

    I'm thinking of ordering the AM-1816CA, which AFAIK is the largest one rated for indoor and low lux. https://www.mouser.com/datasheet/2/315/panasonic_AM-1816CA-1196985.pdf
    My only reason for ordering the largest would be to see what the limit is on how dim things can get in that series and still have something that can function. Maybe ordering smaller panels would make more sense, though, as they could always be added together in parallel or series. Yeah, that would make more sense I think.

    In addition, if you let me know what models you order, I may order one of the same too just so we can have something in common to compare results.

    At very dim levels I notice that my Fluke 87v multimeter actually draws too much current off the solar cell to get an accurate open circuit voltage measurement. So, I'll have to rig up some kind of voltage following op amp buffer as an aid to doing these measurements.


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.

    Only one way to know for sure, but I would guess that the crystalline one would drain off the current produced by the amorphous one (based partly on your theory as to why amorphous is better in low light). Worth a shot though: maybe as a compromise solution you can have the best of both worlds.

    Thinking out loud here, I have read about some research solar harvesters where they use a separate "pilot" solar cell to power the control electronics past the cold boot threshold. These days, with nanoamp current drains from control components, it would mostly need to produce adequate voltage and not much current, so a simple approach would be optimize the pilot configuration for exactly that--perhaps putting a few tiny cells in series. Perhaps any extra current could then spill over into the main accumulating capacitor. That would be yet another way to use more than one type of panel.

    The ideal solution would be if there were some way to re-configure multiple cells in series or parallel depending on the lighting conditions. It could default to series to push past the cold start and then switch to parallel (or some appropriate combination of series and parallel) for the energy harvesting. I haven't seen much on that topic, but I'd be keen to know if there are ways to do reconfiguring that consume very little power in overhead.


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    less than 25x25

    That probably limits you to a couple of AM-1456 (25mm x 10mm) or a single AM-1606 (15mm x 15mm) as your only choices.



  • @NeverDie said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Only one way to know for sure, but I would guess that the crystalline one would drain off the current produced by the amorphous one (based partly on your theory as to why amorphous is better in low light). Worth a shot though: maybe as a compromise solution you can have the best of both worlds.

    I mean connect them in series with bypass diodes so the amorphous cell can be used to bootstrap the harvester, and then crystalline cell will be workhorse during the day. Unfortunately, can't check it right now - left all my cells in the office.

    Thinking out loud here, I have read about some research solar harvesters where they use a separate "pilot" solar cell to power the control electronics past the cold boot threshold.

    That's a smart idea. Perhaps connect a dedicated tiny charge pump and an amorphous panel parallel to the buck-boost harvester storage capacitors?

    The ideal solution would be if there were some way to re-configure multiple cells in series or parallel depending on the lighting conditions. It could default to series to push past the cold start and then switch to parallel.

    A mechanical device? πŸ™‚



  • @NeverDie Going to order 1 x AM-1606, 2 x AM-1456, 1 x AM-5610, and 2 x KXOB25-05X3F.



  • @NeverDie said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    At very dim levels I notice that my Fluke 87v multimeter actually draws too much current off the solar cell to get an accurate open circuit voltage measurement. So, I'll have to rig up some kind of voltage following op amp buffer as an aid to doing these measurements.

    Heh, we seem dived below 1 Β΅A / Β΅W level here 🐟 πŸ™‚


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Heh, we seem dived below 1 Β΅A / Β΅W level here

    Looking at the datasheets for the op amps I have on hand, I'm guessing that the LTC2063 will allow an accurate measurement: https://www.analog.com/media/en/technical-documentation/data-sheets/LTC2063-2064.pdf I'll try it after my uv glue arrives.


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Perhaps connect a dedicated tiny charge pump and an amorphous panel parallel to the buck-boost harvester storage capacitors?

    Maybe, but which one? I would have suggested this one, but it's no longer available: https://media.digikey.com/pdf/Data Sheets/Seiko Instruments PDFs/S-882Z.pdf


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Wow, relatively to my built into the smartphone Sensortek STK3x1x ambient light sensor this one looks very serious.

    Looks as though you can get a fairly inexpensive digital light sensor from adafruit that will tell you the lux level: https://www.adafruit.com/product/4162
    https://www.amazon.com/Adafruit-4162-VEML7700-Lux-Sensor/dp/B07S9TD2W1/ref=sr_1_1?keywords=Adafruit+VEML7700&qid=1583129068&sr=8-1

    It doesn't have the little translucent plastic dome on it though that one typically finds on lux meters. Not sure how important that is or isn't. Seems like such domes would shade the light and skew low light level readings, so maybe you'd be better off without it.

    I may get one myself as a check on my lux meter.

    There's also this one, a little cheaper: https://www.amazon.com/Adafruit-TSL2591-Dynamic-Digital-ADA1980/dp/B00XW2OFWW/ref=sr_1_1?keywords=Adafruit+lux+sensor&qid=1583129190&sr=8-1
    I checked the adafruit library, and it prints sensor readings in lux.

    Not sure which one is more accurate.


  • Hero Member

    I built the op-amp circuit, and now the open circuit readings on a solar cell are much higher than when I was taking the readings with a regular multimeter. As long as I can keep the control logic current at just a couple hundred nanoamps or so, I think I'll probably have enough voltage under even very dim lighting that I doubt cold start will be an issue.


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    scne-ixys-data.png

    What method or methods are you using to characterize your solar cells? I'm guessing in this instance that for the different illumination levels you are recording the open circuit voltage and the short circuit voltage? So as to at least try to compare apples to apples, I want to collect data on my cells in the same way that you are.

    Another, complementary approach is described here: "The key characteristic of a solar cell is its ability to convert light into electricity. This is known as the power conversion efficiency (PCE) and is the ratio of incident light power to output electrical power. To determine the PCE, and other useful metrics, current-voltage (IV) measurements are performed. A series of voltages are applied to the solar cell while it is under illumination. The output current is measured at each voltage step, resulting in the characteristic 'IV curve' seen in many research papers. " https://www.ossila.com/pages/solar-cells-theory I suppose with this approach a series of curves could be produced, each for a different illumination level. Since doing that would be a lot of work, I'd like to somehow automate the testing process, but first I need to either know or decide what the process is that I want to automate.



  • What method or methods are you using to characterize your solar cells? I'm guessing in this instance that for the different illumination levels you are recording the open circuit voltage and the short circuit voltage?

    @NeverDie exactly. That was a quick and dirty measurement using a multimeter. The P (Β΅W) value was calculated as V * I * 0.8 (MPP assumed 80%, I must multiply to 0.8^2 instead). My intent was to describe the panels in dependency of different illuminance (which must be also denoted by E instead).

    Finding MPP on IV curve is the right method to characterize a cell. But that would require fixing illuminance at some point (and is more complicated), when I was more interested in different light conditions. Most cells are rated at 200 lux indoors, and one sun (more than 100k lux) outdoors. Perhaps 50 lux indoors (a typical light at home) and 1000 lux (cloudy day) is more practical for low-light purposes so I could trace IV curves for the cells I ordered at that illuminance levels.

    Looks as though you can get a fairly inexpensive digital light sensor from adafruit that will tell you the lux level

    It seems my phone uses Sensortek STK3310 or similar. At low light might be as accurate as those two, but is limited at higher levels indeed. Would be nice to replace it with more reliable solution, will try to find out a luxmeter in a local fablab or get one of those you've suggested, thanks!



  • @NeverDie said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    I built the op-amp circuit, and now the open circuit readings on a solar cell are much higher than when I was taking the readings with a regular multimeter. As long as I can keep the control logic current at just a couple hundred nanoamps or so, I think I'll probably have enough voltage under even very dim lighting that I doubt cold start will be an issue.

    There is the Ricoh R1800K which consumes just 144nA and can start from a 0.72 Β΅W source. It requires at least 2V to operate, but schematic is very miniature - only three more components needed.


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    R1800K

    Interesting chip. On the one hand, it seems aimed at small solar cells because of the low quiescent current and because it can't handle more than 1 or 2ma tops. On the other hand, it doesn't have MPPT but instead wants you to pick a single MPP voltage (out of the choices that RICOH provides) that it should operate at. Not sure how good being tied down like that would be in actual practice. Maybe it would be fine in an office environment where you could perhaps assume steady, uniform lighting...?


  • Hero Member

    I switched to a deadbug build using an MCP6S22 opamp for a buffer because I was getting too much conductance/noise on a pCB with the other op amp. Everything has to be soldered together, because otherwise current gets lost through the connectors when dealing with such miniscule currents.

    Having done that, for the keychain solar cell I measured open circuit voltage at 2.66v at 1 lux (according to my lux meter that I mentioned above, so take that measurement for whatever it's worth) and a short-circuit current of 88na, according to a uCurrent Gold (but with the voltage measuring opamp circuit still soldered into place).

    This has me wondering now how much of a voltage (non-boosted) it could eventually generate onto a capacitor, so I suppose that's the next thing to try. I'll try it first with my simple solar charger: https://www.openhardware.io/view/620/Supercap-solar-charger
    since that's easy, but for a more accurate measurement I may need to construct a deadbug equivalent using just a diode and capacitor. That would be a lower bound for the dead simple approach which then perhaps some harvester could improve upon, though I'm not sure any of the commercial energy harvesters are spec'd at that low of a power.

    This also explains why measuring the voltage of the solar cell with just a volt meter (with no op amp circuit to help it) is hopeless at such low light levels: 2.66v divided by 10MOhm is 266 nanoamps, where 10MOhm is the typical digital volt meter input resistance. i.e. the 266 nanoamps drain through the volt meter would be approximately 3x the amount of current that the solar cell can generate, thereby causing a large error in the voltage measured by the DMM.

    Edit2: I connected a 100uF ceramic capacitor in parallel with the solar cell (I didn't bother with adding a diode), and it charged up to 2.778v. Somehow that's slightly higher than the previously measured open circuit voltage of 2.66v. Not sure how that is, but perhaps the orientation of the solar cell was a little more favorable when this measurement was taken. In any case, I think whatever the open circuit voltage is, you can probably charge up to that amount with any size low leakage capacitor that you want to use. πŸ˜€ So, from this point of view, choosing a solar cell which generates high open circuit voltages in very dim light is perhaps more important than any other decision if what you want is something that can get past startup even if the available power is only minuscule.

    The only thing needed is a simple control circuit which, if possible, consumes little or no energy until it reaches the desired voltage range, whereupon a more sophisticated control circuit can take over. Something like a schmitt trigger might work, but it would need to draw extremely little current, which not all schmitt triggers do, especially as they approach the threshold voltage. Any ideas?

    Perhaps something like: https://hackaday.com/2018/07/19/energy-harvesting-design-doesnt-need-sleep/
    or perhaps a solar engine control circuit might work: http://beambuilder.blogspot.com/p/solar-engines.html
    or...???
    Since they all do more or less the same thing (charging a capacitor to a threshhold voltage and then "turning on"), the challenge would be to find (or invent) a circuit which achieves that result but while consuming the absolute least amount of power that current technology allows. A lot of the published designs use older technology, and so I suspect better possibilities exist if leveraging newer, more capable components.



  • @NeverDie Wow, I'm really surprised with so high voltage of the panel. Thank you for sharing the measurements!

    My understanding is that OC voltage is defined by amount of free electrons in the depletion zone, and hence by the width of the zone. When in the light, more electrons will fill the zone, but there seem to be some saturation threshold limiting the max voltage. It would be interesting to somehow measure the electric field in the full absence of light. Also, capability to emit new electrons in the depletion zone defines the max current from the cell. It looks like crystalline cells can do it more effectively than amorphous, but the latter have wider depletion zone in the dark.

    I don't know how to use so ultra-low current sources. The harvester should be able to work from 100 nanoamps or below. This limits design to a linear charger only (at least at frontend) - anything more complex (like a boost or buck circuit) would require higher quiescent current which will collapse the cell.

    A MOSFET may draw as low as few nanoamps so virtually it could be possible. The PV cell needs to be isolated from the load to prevent voltage drop on the FET which may cause it defunct. Perhaps an isolated capacitor will be required to sustain the FET state while input capacitor releases its charge. The FETs may require resistors to shift voltage level, but again, they need to be hundreds of megohm. This will also impact switching speed. Perhaps some sort of hiccup switching circuit may make it. Also, I see some similarities with how dynamic RAM implemented.

    For a usual solution, there are some ideal diode like the MAX40203 with 300 nA quiescent current. I suspect that the charge pump of the SM74611 may draw microamps when in ON state - it's unclear from the datasheet.

    All in all, it looks like a puzzle πŸ™‚


  • Hero Member

    If you're able to run in some kind of duty cycled mode, where the control circuitry is only active for brief periods of time, then perhaps the quiescent currents get averaged down to a more manageable level. As a first step, I think I'll just blithely assume the control circuitry can access at least some conventional voltage levels (either saved up from earlier energy harvesting or else gathered in a crude way like in my example above). If I can make good progress doing that, then I can always revisit that assumption at a future date.



  • Found a nice paper on charge pumps design: https://www.mdpi.com/2079-9292/8/5/480.


  • Hero Member

    TPL5100, which draws just 30na, looks promising for duty cycling the control circuitry:
    http://www.ti.com/lit/ds/symlink/tpl5100.pdf
    It has both a PGOOD pin as well as a mosfet driver pin. The edge case would need confirming that it can slowly power up from zero volts to its minimum 1.8v operating voltage with only just over 30na source current without itself drawing more than 30na. Since it has a PGOOD pin, I'd wager that it's unlikely to emit false positive signals while still charging at below 1.8v, because if it did the PGOOD pin would be worthless. πŸ€”


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Found a nice paper on charge pumps design: https://www.mdpi.com/2079-9292/8/5/480.

    @Mishka I read a similar paper (http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=D650012CC6F5208E02BF41AE55DF0E95?doi=10.1.1.128.4085&rep=rep1&type=pdf) which says that the best charge pumps use static charge transfer switches. That said, I'd be happy if I could build any kind of ultra low energy harvesting charge pump using discrete components as long as the component count is low.


  • Hero Member

    I tried it with a TPL5110 just now, but it gets caught in a boot loop: voltage rises to 1.440v and then suddenly drops to about 1.400v. I think this is because when the TPL5110 starts up for the first time, it draws ~200ua current to read the resistor settings, which it then stores and uses for the time delay.
    So, if there exists a similarly low current timer that can be set without a heavy drain step like that just described, then I'd go for that instead.

    Meanwhile, this is the lowest current (88na) voltage detector that I know of: https://www.torex-usa.com/products/voltage-supervisors/low-power/xc6136/
    That would limit me to light sources something greater than 1 lux (as measured by my lux meter) if I am to harvest anything using the brute force simple approach for a cold start, but once I get enough of a power reserve I could maybe harvest lesser amounts by duty cycling something like an LTC3108.



  • @NeverDie Maybe try to bootstrap it with external voltage source applied parallel to the cell?


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    @NeverDie Maybe try to bootstrap it with external voltage source applied parallel to the cell?

    I'm not sure what that would look like. Do you have anything concrete in mind?



  • @NeverDie So when TPL5110 has passed the boot phase does it work after that?


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    @NeverDie So when TPL5110 has passed the boot phase does it work after that?

    Ah, good question. Then it seems to work just fine. I had it waking once every 10 seconds and weakly flashing an amber LED, all on just 88na of collected solar current. Essentially, the capacitor voltage would drop to just below the forward voltage of the LED during the flash (effectively self terminating the flash duration) and then it would charge back up from there.



  • @NeverDie Oh, nice! It's interesting that the full circle including oscillator consumes so low power. It seems really possible to build a discrete harvesting circuit which can collect enough charge to execute a single duty cycle of an MCU.

    Such, assuming (88-35) nA/s = 53 nC charge it will require less than 5 minutes and 22 Β΅F capacitor in order to shot a single BLE event from an nRF52 MCU. And that's at so ridiculous low light. Quite awesome, I think.

    The only issue is that the timer can't optimize it for faster charge, but a voltage driven latch could.


  • Hero Member

    @Mishka Just FYI, in my experiment I drove the LED directly from the DRV pin (I didn't use a MOSFET), and I didn't bother with setting the DONE pin, since I wasn't using a MCU. That gave a maximum possible flash duration of 50ms once every 10 seconds, but like I said, it self-terminated before the full 50ms was up because the capacitor voltage dropped below the forward voltage of the LED. Using a mosfet and an MCU, as intended, would give a little more control, since the the MCU could issue a DONE signal.

    So, yeah, it really is impressive what can be done with so little light, and it could actually go with even less light and a longer charge time, provided the startup hurdle can be gotten past.

    Unfortunately, the XC6136 doesn't yet seem to be widely available at the the all the different possible voltages that can be detected. Digikey doesn't have any, and mouser has only just 3 different types. Perhaps that will improve in the future.

    So, perhaps this is a case where powering the TPL5110 from a primary cell would be an acceptable "cheat". At just 35na, that primary cell should last a very long time.


  • Hero Member

    I received a 0.02% accurate 500,000 count DMM that should make measuring changes by small voltage and current amounts a bit easier:

    http://youtu.be/M3xDX1Jq55M

    If you're in the market for such a thing, now is a good time to buy, as prices are lower than I have ever seen before and a number of the models previously available from Extech, Brymen, GreenLee, AmProbe and other labels have been discontinued (permanently, it would seem). The models still in production cost 2-3x as much, as did the discontinued models up until fairly recently.


    Interestingly, in the dead of night the keychain solar cell can nonetheless pull down 1.3v from a window facing a streetlight that's across the street, as measured with the op amp buffer using a DMM. That amount of light is so low that it registers as 0.2 lux on my lux meter. On the other hand, it also measures 0.2 lux even with the lens cover on, so I think it's below my lux meter's ability to measure it, as the 0.2 lux appears to be just an offset that should be calibrated to zero.

    An alternative to the opamp buffer would be to have the solar cell charge a 0.1uF capacitor, which then gets quickly read by an arduino ADC. I haven't wired that up yet, but I expect the results would be about the same.

    Or, you could charge a larger capacitor for a longer period of time and perhaps try to snag it with a peak voltage reading when you first connect to it with your DMM. I haven't tried this. I expect it would work, at least to some degree, if you used a big enough capacitor and charged it for long enough, so it might be worthwhile if you have lots of patience.


    Interestingly, the typical input resistance for an oscilliscope is only 1 MegaOhm. For a typical DMM, it's 10 MegaOhm, and for an atmega328p ADC, it's 100 MegaOhm. Thus, if measuring 5 volts, the Arduino ADC would experience a 50 nanoOhm drain. That's too high for measuring weakly sourced solar voltages under very dim lighting. 10 gigaohm would be preferable, but then it will take some time to charge up an input capacitor for the ADC to read.

    It would be better to leave the input impedance as is but use software to disconnect the input pin when it's not being used. That's certainly possible with an nRF5x, but I'm not aware of that being possible on an Arduino Uno. Is it?

    I could connect/disconnect it with a mosfet or a transistor, but then we're back to supplementing the arduino uno with more hardware again, and the voltage drop across such hardware needs to be adjusted for, since the whole point is to get an accurate voltage measurement.



  • @NeverDie said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Interestingly, in the dead of night the keychain solar cell can nonetheless pull down 1.3v from a window facing a streetlight that's across the street, as measured with the op amp buffer using a DMM. That amount of light is so low that it registers as 0.2 lux on my lux meter.

    It would be interesting to measure voltage when the panel is dead black. Should be possible by wrapping it into paper and then aluminum foil. In theory it should be perfect zero, but connecting wires and the cell itself may work as antenna and hence the opamp may show some bias.

    An alternative to the opamp buffer would be to have the solar cell charge a 0.1uF capacitor, which then gets quickly read by an arduino ADC. I haven't wired that up yet, but I expect the results would be about the same.

    Yeah, the charge capacitor is part of some ADC implementations. But instead of use of comparators it might be possible to measure charge / discharge time and derive current and voltage from that. Also, knowing the charge current it will be easy to derive time to full charge and select proper capacitor.

    It would be better to leave the input impedance as is but use software to disconnect the input pin when it's not being used. That's certainly possible with an nRF5x, but I'm not aware of that being possible on an Arduino Uno. Is it?

    From my understanding, input impedance of most of MCU ADC pins (when disabled) are defined by MOSFETs and hence is subject to implementation and input voltage. But with the charge capacitor large enough it should be not an issue, at least as long as the capacitor wasn't connected to the ADC port during the charge (otherwise the impedance must be gigohms in order to be negligible small). Perhaps a mechanical switch could better solve it for the task. And then MCU can be used to measure time to discharge and do the math.

    Also, I must note that to charge the capacitor with tens of nanoamps, the harvester control circuit must consume something in picoamps πŸ™‚ And this makes me think that, first, there must be a reasonable bottom limit, and, second, a combined RF / solar harvester may be an interesting option to go, especially taking in account they can be connected to the same input.


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    It would be interesting to measure voltage when the panel is dead black. Should be possible by wrapping it into paper and then aluminum foil. In theory it should be perfect zero, but connecting wires and the cell itself may work as antenna and hence the opamp may show some bias.

    OK, I'll try it and let you know.

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Also, I must note that to charge the capacitor with tens of nanoamps, the harvester control circuit must consume something in picoamps

    You're right, there just aren't going to be any control circuits that run on mere pico-amps on a continuous basis, and that sets the limit on how low you can go. It's for that very reason that I'm hoping to find some kind of ultra low current, very low frequency, low voltage self starting circuit that effectively draws almost no current until it starts up. It wouldn't have to start at a precise voltage. Just in a general ballplark. Maybe something like this, except lower voltage than 3v?alt text
    http://www.discovercircuits.com/DJ-Circuits/3na-osc.htm
    Seems like it should be possible, given progress in the components since that circuit was drawn, which is now quite a while ago.

    If so, maybe it could even be used to drive a boost converter, similar to:
    alt text
    and with a high enough voltage, perhaps a voltage multiplier as well:
    alt text
    http://dangerousprototypes.com/blog/2013/07/20/avalanche-pulse-generator-and-some-scope-porn/

    Basically, the circuit needs to remain inert until enough charge builds up and a trigger gets tripped. And, it needs not to bootlooop even though it ramps up using just very little current. A tall order, I know. Not sure if the right kind of circuit exists, but that's what I'm in the hunt for.

    If not a multivibrator, then maybe a ring oscillator. Or, if not that, then a blocking oscillator. And if not that, ...., who knows? There are lots of research papers published where people have been able to do it, but unfortunately a lot of them are IEEE published, and so I don't have access to the details of how it has been done. For sure, a lot of it is instantiated into a CMOS chip, which is beyond my reach anyway, but some of them do seem to use discrete components.

    If you have any suggetions, I'm all ears.


  • Hero Member

    @NeverDie said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    @Mishka said in The Harvester: ultimate power supply for the Raybeacon DK:

    It would be interesting to measure voltage when the panel is dead black. Should be possible by wrapping it into paper and then aluminum foil. In theory it should be perfect zero, but connecting wires and the cell itself may work as antenna and hence the opamp may show some bias.

    OK, I'll try it and let you know.

    @Mishka 8mv was as low as I could take it, but I suspect even then there may have been some slight amount of light getting at it. The room was very dark, but I could make out shapes with night vision, and the backlight on my Fluke 87v was leaking light all over the place, even though I tried to shield it. To really do it properly I'd probably have to set up a wireless link so that I could be in another room to read the voltage. Either that or set up a logger and check it after-the-fact. So, summarizing, I apologize I didn't do a more thorough job, but for being conducted in the middle of a pandemic I did the best that time allows, and besides, 8mv is pretty close to zero, so I hope that answers your question well enough. πŸ™‚ As a cross-check to rule out the possibility of it being an artifact, I'll sometime soon take the measurement in a brief snapshot using an arduino, without the aid of an op-amp, and see how that compares. I'll probably use a load switch to disconnect the arduino so that the solar cell has a chance to charge up a capacitor between readings, and I anticipate that given enough time it will eventually charge up to the open circuit voltage.

    Should I start a separate thread for this, or continue it here? It seems that your project is completed, and although this is all relevant, maybe it would be better to split it off? @Mishka Since you're the OP, what's your preference? Continue as is, or fork your thread and continue in a separate thread? I'm enjoying the collaboration, and hope you feel the same. I'm fine with either choice.


  • Hero Member

    In case anyone is curious, this is the dead-bug setup I used to do the op-amp assisted measurements:
    20200318_221533.jpg

    The op-amp calls for a bypass capacitor to be soldered within one millimeter of the input signal, so I soldered a small surface mount ceramic cap directly to that pin and then ran a wire to it from the GND pin. Not sure how well you can see it, but here's a photo of that:
    20200318_221635.jpg

    The LDO had similar capacitor requirements, and I was able to solder those directly between its pins:
    20200318_224136.jpg

    Maybe because of that, despite all the long wires, noise didn't seem to be a problem. The reason for the deadbug design and the DIP op-amp was to avoid any leakage currents that might happen if it were all mounted properly to a protoboard, as I've read accounts from others who have tried doing that but who ran into leakage problems.

    So, while I admit it looks awfully scruffy, it doesn't matter, because it's purpose built just to help get accurate open circuit voltage measurements (and short-circuit current measurements with a uCurrent Gold, not shown here).


  • Hero Member

    And here is Version 2, which uses an Arduino UNO to and a load switch to measure an E-PEAS solar module that's charging a 100uF capacitor using a solar keychain solar cell. This time I did use a prototype board.
    20200319_014102.jpg
    It's too soon to evaluate, but it seems as though it's off to a good start....

    Edit: nope. I'll have to migrate the Arduino design to dead-bug, because the voltage is only very slowly crawling up. Maybe that's just how it is with this the E-PEAS, or else maybe mounting leakage (the higher the voltage the higher the leakage) is what's causing the drag.

    Edit2: confirmed. These measurements were taken at 1 minute intervals, under varying cloud conditions, and it's clearly not able to hold on to the charge the E-PEAS has accumulated on the 100uF ceramic cap:

    734, raw = 652, volts = 3.267
    735, raw = 689, volts = 3.453
    736, raw = 371, volts = 1.859
    737, raw = 399, volts = 1.999
    738, raw = 445, volts = 2.230
    739, raw = 556, volts = 2.786
    740, raw = 616, volts = 3.087
    741, raw = 671, volts = 3.362
    742, raw = 371, volts = 1.859
    743, raw = 444, volts = 2.225
    744, raw = 510, volts = 2.556
    
    

  • Hero Member

    I punted on the Arduino and hooked up the E-Peas AEM10941 module to the deadbug op-amp for measurement, as the dead-bug op-amp appears to work flawlessly. The results were: the AEM10941 does build up to a voltage of about 4.135v max when under fairly bright lighting, and then all that disappears and it reverts to starting to charge up again from a voltage of about 1.8v, and it cycles over and over like that. Kinda weird, but that unexpected result may explain the above 1 minute interval readings: probably (?) it wasn't current leakage but instead this kind of cycling that explains the measurements.

    Edit: Unfortunately, the AEM10941 breakout board can't seem to rise above 0.352 volts when tested with the same solar cell and same dead-bug op amp assembly under the same 1 flux light source, so I'm afraid I have to label it a FAIL for use in boosting, just by itself, from that particular low light scenario. Given its datasheet, I'm rather disappointed. I did clean the board and my soldered connections pretty thoroughly with IPA, but perhaps there are other leakages inherent in that board, or perhaps the chip itself has limitations that maybe aren't apparent from its datasheet.

    Which raises an interesting question: are some types of PCB's less prone than others when it comes to leakage currents? It might make a difference in what kind I order from a PCB fabhouse if some types are better than others. Anyone know?

    I didn't test whether the AEM10941 might have some other beneficial use if that barrier is somehow passed, or assisted in being passed, but for now I'm moving on to test some other chips and see how they perform under these conditions to see whether they perform any better and without needing help.


    Edit: BQ25504 has no problem charging under the 1 lux light, under the same conditions, even from a starting voltage of zero volts.

    Edit2: Well, not quite. Despite a promising start, the BQ25504 peaked at 0.812v and couldn't seem to pull itself above that. It was able to charge up further than the AEM10941, but it hit a ceiling nonetheless. So this time I gave it some bright light to bring its supercap voltage up to 0.814v, and now the voltage appears to be climbing again, al beit slowly. Assuming it is able to continue on its own from this point forward, then my theory is that internally there is a schmitt trigger which is causing this effect: as the voltage slowly rises, the current drain increases up to a peak and then declines again as the voltage continues to rise. At least that is how TI describes typical schmitt trigger behavior in Figure 1 of TI's application report entitled "Understanding Schmitt Triggers": http://www.ti.com/lit/an/scea046/scea046.pdf
    alt text
    Vertical axis is current and horizontal access is voltage.
    So, if that peak current drain is greater than the current generated by the solar cell at the 1 lux light level, it simply can't push past it to the other side.

    Both chips seem rather pathetic if, as seems to be the case, a solar cell alone and without assistance can charge up a capacitor to its open circuit voltage of somewhere around 2.7v but they can't. To be fair, there could be other factors in play, though, like the PCB material type, the dialectric the manufacturer used to impregnate the FR4 to make the PCB, how much humidity had gotten into the FR4 (resulting in extra leakages), component choices as well as layout. That was my earlier hunch, but that hunch may get upended and replaced with the schmitt trigger theory if the BQ25504 continues its upward charge. We shall soon see.

    Meanwhile, somewhere I have laying around a BQ25570 on a chinese breakout board, and it might (?) possibly avoid this problem that the other two seem to share....

    Edit3: Nope. The BQ25504 simply got stuck again, this time at 0.851v. According to the datasheet, it's still in cold-start mode until the voltage on VSTOR reaches at least 1.6v (or possibly higher), and that matches what I recall about the BQ25504 from earlier experimentation with it: it performs pretty terribly while in cold-start mode.

    Punt!

    Unfortunately, BQ25570 also remains in cold start mode up to the same 1.6v+ as the BQ25504, so I'm losing optimism that it might be any better... And like the BQ25504, the BQ25570 also requires typically 15uw to get its mojo on, and at 88na of current that just isn't going to happen.
    So, there's no rush to test the BQ25570. If the datasheet is right, it's almost certainly another FAIL. 😞 Judging from the datasheet, the BQ25570 is largely just the BQ25504 mashed together with a buck converter. And the BQ25505 looks about the same as the BQ25504, except with a little bit less quiescent current.

    What a disappointment! I would have thought that TI had the in-house talent to do a lot better than this.


  • Hero Member

    Well, doing a little back of the envelope calculations: if a silver zinc 8mah SR416 primary battery, which is just 4.8mm in diameter and 1.6mm thick, were used to continuously drive a 35na TLV5110 timer, then assuming all 8mah could be extracted and ignoring self discharge, it would last for 26 years. Then consider that a properly designed energy harvesting circuit could relieve the battery from needing to run whenever there is adequate harvested energy available, and the expected lifespan of the system would probably be even longer.

    The only purpose of the timer would be to pulse accumulated energy into an off-the-shelf energy harvester, probably none of which can handle extreme low energy accumulation. One maybe can't know what the optimal duty cycle should be, but one could make educated guesses, and perhaps a more refined circuit could even self adapt to some degree.

    Anyhow, I'd rather not go that route, but as an exploratory tool, it would be fun to make these harvesters work in "scotopic" darkness and yet still accomplish something useful with whatever energy they can somehow squeeze out of it, all while remaining tiny. 😎



  • Hi @NeverDie, thanks a lot for doing the experiments!

    Basically, the circuit needs to remain inert until enough charge builds up and a trigger gets tripped. And, it needs not to bootlooop even though it ramps up using just very little current. A tall order, I know. Not sure if the right kind of circuit exists, but that's what I'm in the hunt for.

    Yeah, this is would be very interesting to achieve indeed. Looks like the water bucket from aquapark. Unfortunately, have no practical ideas at the moment. Maybe a FET + BJT combo where the FET generates spike of the current which activates the BJT which then drains the input capacitor? The idea here is to utilize the inrush current from the FET before it will be stabilized. A comparator may have higher quiescent current, or may not.

    8mv was as low as I could take it, but I suspect even then there may have been some slight amount of light getting at it. The room was very dark...

    8 mv is fair enough. So it seems all about the structure of the amorphous cell. Interesting!

    Should I start a separate thread for this, or continue it here? It seems that your project is completed, and although this is all relevant, maybe it would be better to split it off? @Mishka Since you're the OP, what's your preference? Continue as is, or fork your thread and continue in a separate thread? I'm enjoying the collaboration, and hope you feel the same. I'm fine with either choice.

    Although the discussion went beyond the original project, the topic is very interesting. While most of existing harvesters are aimed at low-voltage sources, it seems that we're trying to address the unique property of a-Si cells to have high-voltage bias in the extremely low light. This is not only enjoying, but might also have (and I hope will do) some practical extension. Of course, if there is a better place for the discussion - it's completely okay to move it, I'll be glad to follow-up there.

    Regarding the project, it wasn't finished yet. I'm currently waiting for newest PCBs - they're still based on SPV1050, fully configurable, the components selection is for the boost. Appropriate solar panels are also on the way.

    BTW, I had a chance to try the SPV1050 (buck) and nRF52833 with a single one SolarBit I have, no battery attached. In the direct sunlight it works without any issue, even with 1 mA red LED blinking 50% of time. This is definitely not the best setup, so the mentioned PCBs and panels should make it more useful and especially for a cloudy day. Also, for the version 2.0 I'm considering to replace the harvester IC with the AEM1094. I also have somewhat different idea about form-factor, but that's for another topic.

    And here is Version 2, which uses an Arduino UNO

    Perhaps the right thing would be to charge the capacitor first, and only after that connect it to the Arduino. The Arduino has to read the ADC often so it should be possible to determine highest voltage before it decays.

    Unfortunately, the AEM10941 breakout board can't seem to rise above 0.352 volts when tested with the same solar cell and same dead-bug op amp assembly under the same 1 flux light source, so I'm afraid I have to label it a FAIL for use in boosting, just by itself, from that particular low light scenario.
    ...
    Despite a promising start, the BQ25504 peaked at 0.812v and couldn't seem to pull itself above that.

    From my (perhaps not too careful) review I did earlier in this thread the AEM10941 requires 3 Β΅W input, and the BQ25504 requires 15 Β΅W. Either of those are far beyond the 3V*80nA condition.

    Unfortunately, by most of manufacturers a nanoamp source seem usually considered as zero current.

    if a silver zinc 8mah SR416 primary battery, which is just 4.8mm in diameter and 1.6mm thick, were used to continuously drive a 35na TLV5110 timer, then assuming all 8mah could be extracted and ignoring self discharge, it would last for 26 years.

    Hmm... taking in account those 80 nA collected in 10 seconds will be wasted in one millisecond, and then the next 9.999+10 seconds it will wait for another portion, it sounds like bargaining 35 nA for 40 nA. Well, fair enough! 🀠


  • Hero Member

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Hmm... taking in account those 80 nA collected in 10 seconds will be wasted in one millisecond, and then the next 9.999+10 seconds it will wait for another portion, it sounds like bargaining 35 nA for 40 nA. Well, fair enough!

    TI makes a range of different TLP5xxx chips, and Adafruit makes convenient breakout boards for at least two of the different models. I even used one in an earlier leak detection project: https://www.openhardware.io/view/534/Extremely-Simple-Arduino-Pro-Mini-LoRa-Water-Leak-Detector

    What I haven't yet tested (and haven't read nor heard) is whether ia TPL5xxx can self excite and start-up normally if powered from the very slowly rising voltage created by a tiny solar cell in weak lighting.


  • Hero Member

    I stumbled across this: http://www.prc68.com/I/JouleThief.shtml
    which is a fascinating goldmine of information about blocking oscillators and their use in just about every cheap solar circuit you've ever seen or heard of, including some that maybe you haven't. Be that as it may, for tiny panels in ultra low light (1 lux and below), I'm pretty sure they'll need to be spoon fed, just like these commercial chips we've been examining.

    As for a proper DIY trigger circuit, about 5 years ago David Pilling made some very interesting posts regarding the use of PUTs (programmable unijunction transistors): https://www.davidpilling.com/wiki/index.php/PUT
    and on his wiki he built some solar harvesters around that. What I really like and appreciate about his work is that he published ltspice models of his circuits, so it's very easy to download them and run the simulations. Earlier today I emailed David Pilling to see if he'd like to join the discussion here. A lot of technological progress in ultra low power has happened over the last 5 years, and so I think maybe he would be interested and perhaps he'd want to upgrade his circuitry to take advantage of the much lower-voltage/lower-energy components commonly available now that simply didn't exist back then.

    @Mishka said in πŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Hmm... taking in account those 80 nA collected in 10 seconds will be wasted in one millisecond, and then the next 9.999+10 seconds it will wait for another portion, it sounds like bargaining 35 nA for 40 nA. Well, fair enough!

    What I forgot to mention in the post directly above was that the tlp5xxx chips can be resistor programmed for much longer cycles than 10 seonds. e.g. the TLP5110 can be set anywhere from 100ms delay all the way up to a 2 hour delay. So, that's a very pliable range for collecting tiny amounts of solar on a cap, which can then be fed into a harvester as a unified kick. The 10 seconds you're referring to just an arbitrary number that I had picked and which just happened to work in the earlier circuit. The time delay could be set much less or much greater than 10 seconds. It's whatever you choose.

    I think I'll try a tpl5xxxx timed collector and then pipe the accumulated current into the LTC3508 circuit through an ultra low leakage load switch. Since the LTC3801 needs only 20mv, it should be easy to collect at least that voltage level, even in very dark conditions, using the solar keychain solar cell (amorphous silicon): https://www.openhardware.io/view/732/Extreme-Energy-Harvester So, while the hunt is on for something better and more elegant as a trigger than the crudity of just how much time has passed, this is a brute force approach whose virtue is that it's pretty much guaranteed to work provided that leakage currents are tightly controlled to extremely low levels Fortunately, because of ohm's law, ultra low voltage is likely to make ultra low leakage easier to achieve during the accumulation phase. 😁 What will be interesting is: 1. how big a cap is needed and 2. how long the cycle time needs to be, because boosting extremely low voltages still needs to meet the minimal power requirements (i.e. a lot of current). Unfortunately, nowhere that I can find in the LTC3108 datasheet does it specify the minimum input power to operate. Just the 20mv minimum voltage. Therefore, I'm guessing the minimum power is probably rather high, since companies often hide their bad news by simply not reporting it in the datasheet. Anyhow, I'll just have to derive the minimum power as best I can through experimentation. πŸ™„

    The main downside to the TPL5xxx is that it reads the resistor values exactly once during the startup phase, and then never again. Although that has the benefit of limiting forever after the amount of current the TPL5xxx needs to operate, it also means that you can't easily change the periodicity anytime after the TPL5xxx starts up: even if you change the resistors after it gets going, it never reads them again. Thinking ahead, it might (?) be possible to hack around that limitation by changing the resistor values and then power cycling the TPL5xxx so that it reads the new values and incorporates them. The tradeoff for that result though is the extra circuitry needed to accomplish that. It would be much easier if (?) one of the TPL5xxx variants had a reset pin, so perhaps I'll look soon into whether or not any of them have that feature....


  • Hero Member

    I put David Johnson's 3na oscillator circuit:
    http://www.discovercircuits.com/DJ-Circuits/3na-osc.htm
    into LTSpice and ran the simulation, and it looks promising:
    simulation1.png

    It also runs just fine at 2v. Fairly easy to get a shorter or longer cycle by tweaking the resistor values and/or capacitor values.

    The voltage swing is even better than I was expecting: it drops all the way down to around 30 or 40mv. πŸ˜‹

    Of course, it would be nice if it could run at even lower voltages than 2v. Seems like that should be possible. Anyone have suggestions for which transistors to try for that?

    The simulation shows that there's a very nice current pulse of about 4.2ma through Q2 during the very brief discharge phase, so I'm guessing that could drive a buffer transistor to turn on hard, which in turn could, in theory, drive a meaningful load without disturbing the underlying timer circuit. 😎 If that's the architecture, though, there may need to be a separate, isolated capacitor to drive the load that charges up in parallel with the capacitors in this oscillator circuit--unless perhaps there's some way to recycle/reuse the current that gets dumped and otherwise wasted during each discharge.

    Or, quite possibly, it could be used to drive a flyback type circuit, in which case I possibly wouldn't need a commercial boost chip at all and could instead do all the boosting with a homemade DIY circuit made out of discrete components. 😍 That's the promise of what this type of low-level control could grant.

    Edit: I posted the LTSpice circuit simulation file for David Johnson's 3 nano-ampere circuit here:
    https://github.com/rabbithat/3nanoAmpOscillator

    Edit2: I anticipate a potential problem though: the 3na oscillator has very high input impedance. PV cells are modeled as having a shunt resistance, and unless that shunt resistance is exceptionally high, then most of the generated solar cell current won't be entering into the oscillator but will instead be lost as wasted current through the shunt resistor. I'm hoping that doesn't preclude the oscillator from working, but it might if that reduced current translates into reduced voltage at the inputs to the oscillator. The best case scenario would be that the oscillator simply has a much longer cycle time with the PV cell as compared to a battery. In any case, shunt resistance doesn't seem especially easy to measure, so one strategy would be to just build the circuit and see how it works with the target PV cell rather than fuss too much over constructing an accurate equivalent circuit to plug into the simulator.

    Edit3: Good news. Using the method published in IEEE to calculate PV shunt resistance (https://ieeexplore.ieee.org/document/1483817), I calculate the shunt resistance on the keychain solar cell to be 30,681,818 ohms. So, more than likely the oscillator will work when hooked up to it. This also finally explains why these solar cells perform so well at even ultra low lighting conditions.

    This thread seems to have petered out, so I guess that's the end of it. It was nice while it lasted. πŸ™‚



  • Dear @NeverDie, you've done tremendous amount of work! The topic is extremely interesting, but I admit I can't keep up the pace, especially when discussion dived so deep and requires fair amount of research and simulation. Let's just keep it floating and open for everyone (I really hope David might kick it up). I personally try to follow up your recent posts a bit later, sorry 😞

    P.S. Asked a colleague about it and he's like: "Nano... what?!" πŸ™‚


  • Hero Member

    @Mishka I recently had some convivial email exchanges with David Pilling after I reached out to him. He seemed interested in this thread, or maybe he was just being polite. Regarding his previous efforts, he mentioned that he was eventually able to run his PUT oscillator at 200 nanoamps.


  • Hero Member

    @Mishka I've got good news, and I've got bad news. The bad news is that according to the LTSpice circuit simulator, the Dave Johnson circuit, as given, is nowhere near 3na of power consumption. It's much higher than that. Here's what it shows as the current passing through the R10 resistor in the figure below:
    Johnson_current.png

    The good news is that by increasing the resistance and capacitance, I've confirmed it's possible to run the oscillator at 1 lux on the keychain solar cell:
    P1020037 (2).JPG
    If measured at the output pin of transistor Q2, it produces a 2 volt pulse every couple of seconds:
    2volt_1LuxOscillation.png
    I'm pretty confident it will run at even lower lux, al beit producing a lower voltage, but I'm not yet setup to test at less than 1 lux yet.
    Here is an approximation of the modified circuit and its current consumption:
    solar_works_v000.png
    As you can see, both the average and the instantaneous current consumption are less than for TI's TLV5110 chip.

    And yes, I've confirmed through testing that it can self-start at 1 lux even if it had been pitch black prior! In that case it starts a pulse train at lower magnitude but higher frequency and gradually works it's way up to the 2 volt magnitude at the 0.5 Hz frequency, which at 1 lux is where it settles.

    πŸ˜„ πŸ˜„ πŸ˜„ πŸ˜„ πŸ˜„



  • @NeverDie Oh, nice! You may eventually turn it out into a PWM/PFM charger.


  • Hero Member

    @Mishka The circuit is more stable and consistent than the 3v simulation would suggest. I'm now totally sold on the value of simulation, but it's a bit problematic when a solar cell/panel is involved because for an accurate simulation you need to find an accurate "equivalent circuit" to use in place of the cell/panel, and for accuracy that means a 5 element circuit: two diode, shunt resistor, series resistor, and a current source. However, figuring out the correct values for those parts requires a lot of measurements to get the desired accuracy and is a project in itself.

    That said, I'm optimistic that there are some less mainstream transistors that will allow the circuit to run at lower voltage.


  • Hero Member

    Here's a courtesy heads-up.

    I just now stumbled across a circuit:
    alt text
    published here:
    https://www.edn.com/solar-powered-motor-runs-on-10-na/
    that allegedly can operate on as little as 10na while collecting energy, which it then uses to power a small pager motor once a threshold voltage is reached.

    It also has the virtue of utilizing inexpensive jelly bean parts and not relying on gigaohm resistors, which in the Dave Johnson circuit turned out to be so large that I lack the means to verify their specs through measurement after they are delivered.

    This other guy instantiated the circuit as a PCB, and he made the gerber for it available as a free download: https://hackaday.io/project/159691-electron-bucket-extreme-power-management-module

    If it turns out to be true that the circuit can both collect the current and trigger at a threshhold voltage all with just 10na of overhead, then on its face it sounds better than the David Johnson circuit turned out to be and possibly also better than many/most/(all?) of the commercial chips that we've reviewed on this thread if paired with an appropriate amorphous solar panel.

    Edit:
    But wait! There's more. There appears to exist an equivalent single chip voltage detector that also consumes a mere 10na of current: https://www.akm.com/content/dam/documents/products/power-management/power-ic-for-energy-harvesting/ap4405aen/ap4405aen-en-datasheet.pdf
    It's itty bitty, so it's probably a great fit for your uber-compact design.

    "But I want more!" I can hear you say. "I want a total step-up solution! And I want one that doesn't use a transformer!" Well, of course you do. Who wouldn't? Apparently, a 0.2v transformerless step-up solution does exist as well. I'm just not sure where. They developed it for a customer who wanted to harvest energy from... bacteria. Actually, the official term is "microbial fuel cell." The chip is the AP4470, and thankfully it can also be powered by solar, without bacteria.
    https://solutions.akm.com/us/en/applications/energy-harvesting/
    But can we buy it? Or is it just another inaccessible research project? I don't yet know. Can you read Japanese? The trail of bread crumbs written in English seems to run cold after the above link, but there's more about it that's written in Japanese. Argh.



  • @NeverDie This. Is. Stunning!!!

    I must admit that I were stuck with a CMOS driven circuit, but there are BJT circuits with amazing level of practicality. The decision to employ a LED is simply brilliant. I don't know shall we put it into a SPICE, perhaps to facilitate selection of real components, but taking in account the Hackaday project it should simply fall into place. Going to examine the project. It's definitely worth implementing it, thank you very much for finding the project!

    The AP4470 looks very interesting too. With reported 7Β΅A current consumption when boosting starting from 0.2V, and fixed high to low output voltage from 2.6V to 3.55V, it looks like a strong competitor to the AEM10941. I'd still stick to the latter though, not only because of availability (including documentation), but the e-peas product also has very appealing buck-boost configuration.

    I'm also thinking about even more modular design of the boards (details will follow later), so having two harvesting circuits targeting different scenarios is the right way to go.

    Thanks again for your interest!



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