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



  • And couple of words about the Harvester LDO - it works. Also, being connected right to the spv1050 store it could be used to bootstrap the harvester, but if connected full time it will soak about 8ĀµA which is too much for a battery powered board. So still kinda hack, and its primary purpose is to supply the board via USB.

    Couple of Harvesters


  • Hero Member

    Your results are consistent with what I've seen with other harvesters.

    1. Their #1 goal is to get out of the cold boot mode and to a voltage level where regular electronics can power up, resulting in much more efficient energy harvesting. So, until that minimum voltage is reached, they typically keep all their energy and don't output any. I think that's the right approach, so I have no beef with that.
    2. I'm not a fan of built-in LDO's either, and for the same reason: in a marginal situation, you don't want to waste any more operational or quiescent current than you have to.

    You might enjoy this eevblog video, where the Dave Jones measurements show that the FX260 can handily power its LCD display in idle mode with just 2v and 2ua, and just barely so at 1v and 1.5ua:

    https://youtu.be/_VvEO_m3Owk

    It seems to keep its memory alive, for awhile at least, on less than that after the LCD display has gone blank.

    I've read that TI made a series of calculators (mostly in the 1980's) called the "anylite" series, which were designed to function without a battery in pretty much even dim indoor light. They used bigger than average solar cells. Asdie from their size, I'm not sure if there was anything special about them. Maybe. I haven't tried one myself, but I suspect so. However, It appears to me that these days though TI has gone the "hybrid" route of stuffing a battery in there, and the size of their solar cell is a lot smaller now. I'm not sure what the value of that is, other than simply extending battery life. The FX260 is one of the few that still offer the solar-only with no battery option. Its LCD display is quite crisp, so it doesn't appear to suffer from doing so.



  • @NeverDie What I meant is that in addition to internal LDOs in the SPV1050, there is also the MIC5205 3.2V which is sourced from the USB (can accept up to 16V). This LDO output can be connected to the spv1050 store capacitors via relevant solder jumper on the board bottom, and when connected and not powered it draws about 8ĀµA from the store. So the jumper is kind of requirement.

    I've enjoyed the calculators video. BTW my calculator from which I took the solar panel has similar characteristics. Regarding reuse of calculator panels, first of all they must match the application voltage. If speaking about the nRF52, it has to be 1.8V to 3.6V (or up to 5.5V), but a small panel can't supply it under load. Also, the radio can draw up to 16 mA which is possible only with larger panels. Of course, a capacitor can fix online use of a solar panel, but in order to work with lights off a secondary battery required, hence the solar charger.

    And now it runs out into a game of coulombs. Today I've tested the boards in direct sun light. They charge the 1500ĀµF capacitor in few seconds and then just idling. It looks like this power excess is more suitable for a battery rather a small capacitor. But a 2032 size supercapacitor could also make it. It looks slightly more interesting strategy to fast charge battery when you can and then work from the battery, than struggle to charge the battery in the lowest possible conditions and hence limit charging speed. Please also note, it is usually easier to build high-voltage panels rather high-current ones.

    Anyhow, there are too many influential factors so a practical test is required. I think I have ot try two competitive configurations placed in exactly the same environment:

    • 2 x KXOB25-02X8F panels (5.53V, 6.3 mA) and the buck-boost board
    • 2 x KXOB25-05X3F panels (2.07V, 19.5 mA) and boost only board

  • Hero Member

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

    But a 2032 size supercapacitor could also make it.

    Maybe, though I'mskeptical of capacitors that don't list ESR in their datasheet.

    This one is kinda interesting: https://www.digikey.com/product-detail/en/UMAL201421A012TA01/490-13963-ND/6152301/?itemSeq=317110112 I found it in the supercap category of digikey as a 50F capacitor, but it's actually a small surface mount rechargeable battery with what is described as a long cycle life (still has 90% of its capacity after 5K charge cycles). Looks as though it has been discontinued, but there are still a few left in inventory. If used as a backup of last resort in combination with a supercapacitor, maybe it would last a very long time. Looks as though its self discharge rate is pretty good at normal ambient temperatures, so that might be possible. If you averaged only one charge cycle per week, then it would effectively last 100 years and still have 90% of its capacity remaining. Sounds like it could be a really good way to avoid and/or manage the cold-boot phase of whatever energy harvester you might end up using.


  • Hero Member

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

    So far the Raybeacon was finally assembled and sent to Nordic for tuning - you may have noticed updated pics on its page.

    What sort of tuning is it that Nordic does? This is the first that I've heard of Nordic doing it.



  • @NeverDie They do review schematic and layout of their SoC and then can do antennas tuning. For the tuning two fully assembled devices (including housing) are required.


  • Hero Member

    @Mishka Without modifications and under dreadful low light conditions, all of the fancy pants commercially available ultra low voltage energy harvesters I've tried for solar seem to be much more current limited than voltage limited. And by limited, I mean the difference between working and not working. For that reason I wonder (?) if maybe for the boost-only configuration you'd be better off with a 0.5v cell of the same dimensions. I haven't made up my mind on the matter yet, but that's where my thinking is starting to trend.



  • @NeverDie Yeah, perhaps at low light a weak PV battery simply can't sustain forward current for all the semiconductors to make them work properly. I think the 0.5V cells may be interesting to try too. My only concern is that the SPV1050 requires at least 0.15V to work which is roughly at 15% threshold of the log-alike illuminance-voltage chart - the 2V panels will have it at about 4% and look most promising. Please note, 2.6V buck-boost requirement is at 29% for two 4.5V panels.

    I want to give a try to all three kinds of panels, but little bit later using newer PCBs where I hope it should be easier to switch harvester into the boost mode. For now going to fix found issues and release v1.0 as buck-boost only.


  • Hero Member

    @Mishka I ordered a 0.5v sunpower solar cell to play around with. About 22% efficiency. It's different from typical solar panels in that all the wiring is on the back of it. No metal tape on the front of it, which is at least partly how it gets the higher efficiency (by using 100% of the surface that's facing the sun). It's also slightly flexible. I'm unsure, but it may possibly (?) be cut to size. All I know at present is that if you cut it in half with scissors, both halves continue to work (this was demonstrated on a youtube video).

    Also, on a different topic, I found this solar bluetooth beacon that cypress semiconductor had been selling for a while, but which is now discontinued:
    alt text

    Similar form factor to what you are attempting with the Nordic, except that in their case they crammed everything onto a single PCB, because all they needed was a Bluetooth beacon and nothing more.

    For its energy harvesting chips, Cypress went the route of requiring a minimum 2v for their energy harvesting to work, but their harvester can operate with just around 1uWatt of power, which if I'm not mistaken, means it can operate on about 500na, which is far less than the Linear Technology energy harvesting chips that I have so far tried.

    Lastly, but largely off topic, I found it interesting that Cypress's new generation of ARM mcu's (called PSOC 64), and due to be released sometime soon, will have dual processors (akin to the ESP32), but one operating on as little as 0.9v and the other on as little as 1.1v. And those will have integrated bluetooth and wi-fi as part of them to make for a single chip solution. That will surely help dim lighting scenarios for solar harvesting. Exciting times we live in.



  • @NeverDie Cutting panels should just work. I'm unsure how do you attach wires to it though. I'll be grateful if you will share your findings.

    This Cypress BLE sensor looks very sexy. It's definitely not so flexible as the Raybeacon, but it no doubt sets the tone and is ahead in both design and technology. I'm now thinking should I switch my nRF52 board from simple two-layers to high density interconnect and WLCSP - the aQFN-73 still not too easy to solder anyway, but it imposes serious space constraints.

    I like the size and shape (still considering rounded square though) - it's about what I expect from a battery operated embedded development board. Also, I found the board area is enough to build a custom DIY module using 0603 components. But routing and component placement on the main board is somewhat tight. Maybe issue a "pro" version - with all features of a development board, but integration level of a BLE module? At least it will allow place the buttons symmetrically šŸ™‚



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

    I found it interesting that Cypress's new generation of ARM mcu's (called PSOC 64)

    Yeah I remember how I played Doom on $1500 386-based personal computer, and now I can play it on $5 SoC powered by a coin cell. Awesome!


  • Hero Member

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

    I'll be grateful if you will share your findings.

    Yes, of course. I ordered it just today, so I'll update you after it arrives if I learn anything from it. I suspect the wiring issue you flagged may allow the width to be reduced but prevent the length from being shortened. Anyhow, if not this one, then probably some other solar cell exists that can be trimmed to fit after-the-fact--hopefully one with good efficiency!

    As for the cypress solar beacon, it actually does a little more than just that: it broadcasts the temperature and humidity from a couple of onboard sensors. Nonetheless, Cypress describes it this way: "The Solar BLE Sensor is ultra-low power and works with just solar energy." ... and yet, isn't that a small battery I see on it? Maybe it's rechargeable, and cypress opted for that so as to save space as compared to a supercap? I haven't looked into it, but that's my guess.

    I'd say they did a good job of squeezing it down to the size of quarter. It makes me wonder just how much smaller it could be made. Interestingly, even though the solar panel takes up a lot of board real estate, it's not much worse than a coin cell battery. So, if it could still run on a solar panel that's even smaller, then the whole thing could be shrunk even more, and then solar would be an even more tangible bonus than merely not needing to change batteries.


  • Hero Member

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

    still considering rounded square though

    I think one can argue that for a solar node a square/rectangular PCB (with or without rounded corners) is perfectly valid. I think the circular designs were primarily just trying to match the minimum footprint of a CR2032 or similar battery, since without solar that's what limits the smallest PCB size possible.



  • @NeverDie Well, round shape has property of manhole cover - it can't fall through šŸ™‚

    Another thing I'm thinking about is to get rid of CR2032 and build it on a 3rd party module and a supercapacitor. This way It can be much smaller, but number of other drawbacks may come. Smaller size usually means worse radio performance. Also, the smaller the board, the higher integration required thus severely impacting production cost. This especially may affect expansion modules which are usually a DIY thing.


  • Hero Member

    @Mishka If you do decide to try for a smaller design, there exists a tiny (3.2mm x 2.5mm) 11mF supercap that's rated for 10,000+ charge cycles and that might perhaps be just barely enough capacity to do some minimum amount of work:

    https://www.sii.co.jp/en/me/datasheets/micro-battery-2/cpm3225a/

    or possibly one of the variants that the same company makes.



  • Hi, i am actually building a similar PCB for Energy Harvesting with SPV1050. What about the 196 HVC ENYCAPā„¢ from Vishay like MAL219691262E3? Seems for me as a good Super Cap with acceptable dimension...


  • Hero Member

    @Sebastian-Walther said in šŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    Hi, i am actually building a similar PCB for Energy Harvesting with SPV1050. What about the 196 HVC ENYCAPā„¢ from Vishay like MAL219691262E3? Seems for me as a good Super Cap with acceptable dimension...

    Dividing this into pro's and con's, one pro would be, as noted in the datasheet, "No cell balancing necessary."

    Although you could work around the issue, one con would be that the ESR's look rather high, which could bite you especially hard if doing high power radio Tx's.


  • Hero Member

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

    Well, looks like that is the space mission panel

    I found a place that sells these more exotic cells in quantity 1: https://www.solarmade.com/store/category/solar-cells

    BTW, according to the news, the company that makes the highest efficiency solar cells (>30% efficiency) laid off its workforce in December, so who knows if those cells will ever be manufactured again.


  • Hero Member

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

    ... and yet, isn't that a small battery I see on it? Maybe it's rechargeable, and cypress opted for that so as to save space as compared to a supercap? I haven't looked into it, but that's my guess.

    I think I may have found the part: https://www.digikey.com/product-detail/en/elna-america/DCK-3R3E224U-E/604-1007-ND/970168
    The picture seems to match. So, not a battery after all, but rather a supercap (one with rather high ESR). Likely 220mF, or thereabouts.


  • Hero Member

    Powerfilm makes a thin film solar cell which they say is good for energy harvesting at 200 lux and less. According to their flyer, at 200 lux it says the expected power is 220uW for the panel included in their Nordic nRF52832 solar development kit. However, at 100 lux the expected power is not 110uW (which would have been my guess), but instead only "1,430uW"! (Even though it's a USA based company, I'm assuming they're using the European convention of a comma instead of a period for decimal notation)

    https://www.powerfilmsolar.com/media/cms/Indoor_Solar_Development_Kit_with_N_A90DF4062ABC1.pdf

    Since they're "official" datapoints, I thought it worth reporting, not for the absolute magnitudes (which presumably should be a function of panel size) but for the relative magnitudes. I'd be curious if other types of solar cells also degrade as quickly when going from 200 lux down to 100 lux or whether some manage to do a lot better in terms of energy made available for harvesting. If not, then without resorting to larger panels it sounds as though somewhere around 100 lux or higher is the practical limit for small footprint sensors.



  • Looks good, I am wondering about the high power, cannot imagine šŸ¤” because meanwhile I tested the ECS300, used in Enocean products, also designed for low light (200 Lux) with amorphous crystalline cells. But the MPP is only about 160ĀµW @ 1000(!) Lux.

    ECS300.png



  • @iiibelst
    ok, seems to be possible, because it is 10x the area of ECS300...


  • Hero Member

    @iiibelst said in šŸ’¬ The Harvester: ultimate power supply for the Raybeacon DK:

    I am wondering about the high power, cannot imagine because meanwhile I tested the ECS300, used in Enocean products, also designed for low light (200 Lux) with amorphous crystalline cells. But the MPP is only about 160ĀµW @ 1000(!) Lux.

    I suspect it's primarily a size thing. The ECS300 is lower power at least in part because it's comparatively tiny: just 35mmx12.8mm and with a short-circuit current of 6.5ua at 200 lux.
    https://media.digikey.com/PDF/Data Sheets/Enocean PDFs/ECS300_310.pdf
    If only we knew the actual size of the PowerFilm panel used in the demo, then we'd be able to directly compare the relative efficiencies of the ECS300 against the powerfilm. Even so, there's likely to be a lot of slop in these numbers, because they are each probably doing their 200 lux measurements using different light spectra (i.e. whatever shows off their product in the most favorable light, so to speak).



  • Regarding capacitor selection for the SPV1050 and nRF52. The undervoltage protection for the SPV1050 is 2.2V. At the same time, nRF52 works starting from 1.8V. The maximum allowed voltage drop should be no more than:

    U_drop = 2.2V - 1.8V = 0.4V

    Taking in account, that the nRF52840 can draw up to 25.8mA at 1.8V when transmitting at 8dBm, it means that supercap ESR should be no more than this value:

    ESR = U_drop / I = 0.4V / 25.8mA ā‰ˆ 15.5 Ī©

    On the other hand, the online power profiler states that a single BLE advertisement event charge is 38.22 ĀµC at 1.8V and 25.53 ĀµC at 3.6V which roughly equals to max energy consumed:

    E = q * V
    E_3.6 = 25.53ĀµC * 3.6V = 81.11 ĀµJ
    E_1.8 = 38.22ĀµC * 1.8V = 68.8 ĀµJ

    And, for the worst case, assuming the energy as derived above and 0.4V as acceptable voltage drop, the minimum required capacity must be:

    E = 1/2 * C * (Vh^2 - Vl^2)
    C = 2 * E / (Vh^2 - Vl^2)
    C = 2 * 81.11ĀµJ / (2.2V^2 - 1.8V^2) = 102 ĀµF

    Of course, it's required that the solar cell must be able to charge the supercapacitor between BLE events.

    Also, I've noticed that in buck-boost mode the SPV1050 tends to charge the store and hence the battery to higher voltages. Such, the Harvester board has limited the U_eoc to 3.2V, but on the sun I can often observe it up to 3.5V. For this reason I'd recommend to slightly lower the U_eoc and ensure that the connected battery or the supercap can handle the voltage.



  • @NeverDie Yeah, you're very close. Cypress has mentioned it in the datasheet. The PV panel is AM-1606C and the supercap is DCK-3R3E204T614-E.

    It's also interesting to see that they have installed a second pool of ceramic capacitors to workaround high ESR of the supercap.


  • Hero Member

    I'm looking now at the ADP5091 boost chip by Analog Devices. It doesn't have a buck mode, but its boost mode is maybe just a tad more compelling than the spv1050's. It cold boots at 0.38v with 16ua of current, and after cold boot completes it can run on as little as 80mv. The pin pitch on its chip is 0.5mm as compared to 0.4mm for the SPV1050. Comparing it to the SPV1050 is perhaps splitting knits, but comparing it to every other chip on the market it does seem to require the least amount of power of any chip that I'm aware of.

    https://www.analog.com/media/en/technical-documentation/data-sheets/ADP5091-5092.pdf

    Analog Devices claims that its efficiency is 80%, so if running in buck mode is an option, I'm guessing that a buck converter would beat it. On the other hand, if there's enough light to provide the voltages for a buck converter to run on, then I'd wadge there's plenty of energy to be had regardless. At least, that's how I'm starting to look at it. I think the justification for a boost architecture is that it's preparation for the worst-case scenario, in the event that it ever occurs. I haven't yet decided whether it's like preparing for a once in 10,000 years flood or not. Perhaps it's just not practical. It's probably not a bad idea to have at least one to experiment with though.


  • Hero Member

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

    @NeverDie Cutting panels should just work. I'm unsure how do you attach wires to it though. I'll be grateful if you will share your findings.

    Closing the loop on your question, it looks as though they can be cut:
    https://www.ebay.com/itm/0-5W-0-5V-High-Efficiency-Back-Contact-DIY-1-6-Cut-Sunpower-Solar-Cell-36pcs-lot/291858971298?hash=item43f4267aa2:g:hsAAAOSwxp9W5tui
    I've read that cutting them with a laser is the recommended method. I only just came across this, and I haven't yet found a vendor selling just one solarpower solar cell lasercut into six pieces like that yet, although the above ebay auction demonstrates that you can buy them in bulk that way.


  • Hero Member

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

    somewhere around 100 lux or higher is the practical limit for small footprint sensors.

    I'm realizing now that this is hogwash because today after getting my lux meter out of mothballs I'm noticing that my solar calculator works just fine down to around 15 lux, which is when its LCD starts to fade out. It suggests that powerfilm probably just isn't one of the better performing solar cells out there.



  • @NeverDie Wow! Are you sure the lux meter is working properly? 15 lux is about as low as 25 cm from a candle fire which is awfully low.

    So, since the ADP5091 is also on the list now, I think it becomes necessary to put all the mentioned power harvesters side by side so we can compare at least basic parameters. Please take a look to the Google Spreadsheet listing some of the tiny harvesters. Please note, The Analog Devices has about ten harvesters which may be described as "tiny", but so far the sheet is covering only ADP509x series. I'm going to add the LTC series later.

    From brief analysis, it looks like the Startup Input Voltage and the Startup Input Power are placing the major constraint on PV panel. Of course, the panel should be also able to supply voltage required to cold-boot the harvester. Such, under the low-light conditions (50 lx to 100 lx in a dim room) power capabilities of both of my panels are simply not sufficient to bootstrap the SPV1050: the IXYS is too weak and works starting from 150 lx, and the SCNE is good for boost, but still require 150 lx to reach 2.6V at STORE (the data is for the charts I've posted earlier):

    scne-ixys-data.png

    Therefore any panel which can reach required voltage and provide enough power should make the harvesting IC work. Please also note, that after cold-boot almost any harvester can work on a lower power (usually 1.5 - 3 times lower than it was required to start).

    Also, in the table there is a class of super-tiny harvesters, namely the Cypress S6AE102(3)A and Ricoh R1800K. They can charge a store by harvesting source with less than 1ĀµW capability. At the same time, the EM8500 looks like the most sophisticated embedded solution with lots of features. The rest of harvesters are quite nice ICs for a modular system.


  • Hero Member

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

    Wow! Are you sure the lux meter is working properly? 15 lux is about as low as 25 cm from a candle fire which is awfully low.

    I used a cheap consumer grade lux meter to take the measurement, but it's consistent with what Dave Jones reported for the same solar calculator. He did a youtube video on it, and he showed it worked at around 20lux at a coarse level and probably a bit less.


  • Hero Member

    @Mishka This is probably worth adding to the list: https://e-peas.com/products/energy-harvesting/photovoltaic/aem10941/

    If we add that, I think we have a pretty complete list for a first pass. The e-peas is pretty expensive though, so we could drop it for that reason.

    Since covering the worst case seems to be a relevant concern, I think it's important to identify which one can start-up and begin harvesting at the lowest lux level.

    15 lux doesn't seem all that dim to the eye. Setting aside the explanation that our eyes have great dynamic range, I still think we should be able to harvest from less than that. I mean, people are able to harvest from fairly weak radio waves, which have far less power.



  • @NeverDie Added, thanks! Quite interesting the IC implements some kind of Cuk converter. On the bad side it seems stocked nowhere, but the e-peas only.

    Looking for reasons why the Cypress BLE sensor has chosen a Panasonic cell I found the catalogue which also contains number of interesting charts. Such, the chart on page 3 explains why a calculator cell is more efficient in artificial light than the IXYS thing (BTW the IXYS datasheet has it pretty flat on the range from 400 nm to 1100nm).

    This all makes me think that there are basically two combos to choose from:

    • an amorphous cell and 3ĀµW harvester
    • a monocrystalline cell and 15ĀµW harvester with voltage adjusted to panel assembly

    But to be honest I'm quite surprised how well performs the SCNE cell I have extracted from the noname calculator.



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

    This all makes me think that there are basically two combos to choose from:

    Not quite. After closer look I've found the AM-5610 outdoor panel of suitable size - only 25x20mm. The panel is capable to produce up to 18mW.

    Other interesting panels are: AM-1606 as used on the Cypress BLE, 15x15mm, AM-1456 which is close to SolarBit by size, 25x10mm, and AM-1312 which is exactly of the same size the SCNE I have.


  • Hero Member

    @Mishka I suppose the SC-3722-9 may be too big for your project, but it's worth mentioning because it performs pretty decently under indoor lighting, and you can extract them for cheap from $1 solar keychains, which are widely available. That's all subjective though. I'm not sure how they compare by the numbers.

    https://www.aliexpress.com/item/32774648368.html?aff_request_id=d8e42a07ed7f4345b1133c9f4f249fed-1581813033932-00280-_rIgCIO&aff_platform=api&cpt=1581813033932&sk=_rIgCIO&aff_trace_key=d8e42a07ed7f4345b1133c9f4f249fed-1581813033932-00280-_rIgCIO&terminal_id=abc87087a3a64790a6abd96b2333811b


  • Hero Member

    This diode is a bit expensive and too large for your project, but for experimental purposes it seems like the cat's meow as a blocking diode when collecting currents at tiny voltages: http://www.ti.com/lit/ds/symlink/sm74611.pdf

    Just 26mv forward voltage drop at an 8a current, and just 0.3ua reverse leakage current at a voltage of 28v. Obviously, those numbers would be far less for the currents and voltages that we're dealing with. It seems to be very nearly an ideal diode, or at least the closest I've ever seen to that.



  • Summarizing the work on the SPV1050 (irrelevant to other ICs mentioned in this topic) please let me publish revision 0.9 of the Harvester board. It addresses some issues found on the previous boards, and introduces number of important changes.

    The most noticeable one is that the board now supports both boost and buck-boost DC-DC configurations of the SPV1050. After reviewing some tiny PV panels it was indetified that the maximum working voltage for a tiny panel is about 3V which means the boost mode is more suitable to do the job. Also, tiny high voltage panels (like some SolarBIT models) have very limited current capabilities and in low light conditions simply can't supply enough power in order to make the harvester chip work. It's also important to note that the cold-boot voltage for boost DC-DC is 0.55V which is only about 20% of maximum 3V voltage a panel can gain - please compare that with 2.6V and 4.4V respectively for some most advanced cells. Finally, if in the boost mode the SPV1050 supports TEG modules.

    Therefore the BOM was adjusted to the boost configuration with the following thresholds:

    Symbol Parameter Value
    V_uvp Battery under voltage protection threshold 2.4V
    V_eoc Battery end of charge voltage 3.1V
    V_oc Source open circuit maximum voltage 4.7V
    V_mp Maximum power point voltage 78% * V_oc

    Please note, in the boost mode the SPV1050 will effectively set V_eoc = V_in for all V_in values greater than 3.1V which may cause damage to the battery or the nRF52 SoC. To prevent the negative impact please carefully consider the source.

    If the only source you have is a high voltage solar panel, it's possible to adjust the R1-R3 resistors ladder (please refer to the SPV1050 datasheet) and switch the Harvester board to buck-boost mode as follows:

    Harvester DCDC config

    Hint: If the solar panel is really big (like 2W / 12V or so) and you don't need MPPT, it's possible to close the USB Charge jumper and connect the panel to VBUS and GND solder pads in order to employ the 3.2V USB LDO.

    The MPPT fixed voltage ratio is set to 78% with resistors R2=2.2M and R3=8.06M. For a TEG module with MPPT ratio at about 50% just replace both R2 and R3 with 5.2M resistors.

    Other notable changes included into this release are:

    • Fixed some silk layer errors
    • The MIC5205-3.2 LDO got missing input filtering capacitor
    • The current limiting resistor between the SPV1050 and the tantalum capacitor has been removed
    • Connection to the ground plane in some isolated areas was improved

    And last, but not least, I'd like to thank the MySensors forum and in particular @NeverDie for his tremendous contributions. It's pure fun to discuss tiny boards with tiny harvesters working from tiny power sources šŸ¤™


  • Hero Member

    @Mishka Looks like a winner. šŸ‘ Once you put it together I'll be interested to hear what the lowest light levels are that you're able to run it at and which solar panels/cells you end up liking the best.

    I think there's a good chance it will outperform the eval kits from enOcean, Cypress Semiconductor, Cymbet, and others that rely on a high cold start voltage. In order for them to win they would need to harvest at a lower power than what your chip can manage but somehow also at the higher voltages and with enough power that their chips require, and I'm not sure whether or not those two conditions can be generated simultaneously by real world solar panels.



  • @NeverDie Well, the SPV1050 has a nice set of features I need and offers impressive flexibility in a small package. However, when speaking about efficiency the AEM10941 seem outperforms it in every single bit.

    The current design reached some level of stability so I think the AEM10941 is what I should try next.



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

    The current limiting resistor between the SPV1050 and the tantalum capacitor has been removed

    Why removing the resistor? Why not placing it after the (optional) Tantal to protect a downstreamed Batt or Cap?



  • @iiibelst There is one. The R7=549Ī© is limiting current between the tantalum capacitor and the extension socket. Its purpose is to keep it under 2mA for ML2032. You can bypass it with relevant solder jumper on the board bottom.

    The dropped 50Ī© resistor was previously located between SPV1050 and the tantalum capacitor.


  • Hero Member

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

    The current design reached some level of stability so I think the AEM10941 is what I should try next.

    Yes, at a 3 microwatt minimum, that chip may be very tough to beat. It has the same 380mv cold start voltage as the ADP5091, but it requires only half the energy. With these tiny solar panels that extra margin might really make a difference under dim indoor lighting conditions.

    I guess it's no accident that the AEM10941 is the newest chip. Perhaps it's the constant improvements in cmos technology that it leverages. In which case.... we can probably look forward to even better chips in the future! For sure solar cells and panels continue to improve their efficiency. The markets are finally big enough to support the required R&D for continual improvement. And the mcu's and radios are constantly improving their efficiency so less power is required. It's great to be in the nexus riding a few waves like this, where we can get the benefit of multiplying the improvements together.



  • @NeverDie That's all true.

    Please also note the AEM10941 can regulate up to 5V of input when ADP5091 upper limit is 3.3V.

    Again, when speaking about BLE, a beacon (as a low-power application example) has to advertise at least once every 1000ms to be generally usable. For nRF52840 it approximates to about 50ĀµW of power consumption. By adding up some microwatts of the harvester itself it may be safe to expect a panel should be able to produce 60-70ĀµW of energy in average. In turn, this means that those 3ĀµW or 15ĀµW are rather an edge scenario, and there is must be a timeframe when the system can collect all the required electricity. Such, when running from a daylight it should be expected that in February the harvesting will be efficient for at most 8 hours a day. The system must be able to offer minimum (24h/8h)*70ĀµW = 210ĀµW during the light period of time. For the reference, a couple of my IXYS panels can provide only about 150ĀµW when located in 1m from window on the north side.

    From the experiments, to me it currently looks more not about possible minimums, but rather about higher efficiency at nominal values. But I admit the difference between 3ĀµW and 50ĀµW doesn't look big either. The sleeping current of the nRF52 is already less than 3ĀµW, so perhaps some time the source and the load can converge.


  • Hero Member

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

    Again, when speaking about BLE, a beacon (as a low-power application example) has to advertise at least once every 1000ms to be generally usable.

    I read in a couple different places that the maximum bluetooth advertising interval is 10.24 seconds (i.e. 10x your assumption) so if the rest of your math is right that should provide ample headroom for being bluetooth compliant.



  • @NeverDie Yeah, there are also scenarios when a beacon advertises only when it was charged enough - it may wait for several hours before send a packet. But that's rather uncommon application. A typical beacon usually advertises every 100 ms to 1000 ms - this way it can be located quick enough.


  • Hero Member

    Closing the loop: today I finally received the sunpower solar cell, so I was able to take a closer look at it. Basically, the traces on the back are interdigitated. So, it looks as though it could be cut along the horizontal axis (if, say, the connection pads are on the left and right) almost as narrow as whatever you might want to. However, it would be ruined if you were to cut along the vertical axis: one pad would remain fine, but all the traces to the other pad would be severed. Maybe in theory they could be re-attached to a new pad with a lot of careful soldering, but that doesn't seem very practical, as the pitch between traces is quite narrow. On the other hand, if one were to use a custom flex film pcb with connection traces that aligned to the severed traces, it might be possible, but still a nontrivial amount of work.

    Looked at from the point of view where large surface area is OK: one of the nice things about these cells is that they are reasonably inexpensive considering their 5"x5" width and height, and yet they are quite thin and still easy to connect. However, I suppose they maybe should be coated with something to protect them. A 2K automotive epoxy spray would probably be ideal, but perhaps even a hard automotive acrylic lacquer would be sufficient, as either should be both non-yellowing and moisture proof. Unfortunately, not much seems to be written about what types of coatings work best. Obviously, the commonly used chinese epoxy solar cell coating would be a poor choice, as that stuff degrades under uv and yellows/browns and clouds up quite rapidly.



  • @NeverDie Interesting. I think the cell may be carefully cut with laser and then properly remetallized. Perhaps can be done with a typical tin based solder paste with some proper flux (I don't know, originally some kind of silver paste is used). Fixing the cell itself into epoxy should be easy.

    The nice thing about the process is that it should be virtually possible to create cells of arbitrary shape. However, in order to get usable voltage it might require to build a panel.


  • Hero Member

    @Mishka On second thought, if you were cutting it to a small size then there wouldn't be many traces remaining to be reconnected, so from that point of view it might actually be practical.

    For me it's academic because I don't own a laser cutter, and I have no idea what kind of power would be required to cleanly cut one of these cells even if I were to buy one for that purpose. I'd be interested to know though. Even 20 watt lasers are pretty cheap these days. Hooking a laser up to a CNC, which I do have, to execute the cut would be fairly easy.



  • @NeverDie My nearest laser service costs about 3-4 dollars for one running meter, can cut 2 mm steel, so never thought about that. On the other hand, with enough number of passes it virtually should be possible even on a DIY CD-ROM laser engraver, especially if mounted on a CNC which is usually more precise than lasers.

    I'm expecting that at least three crystals will be required to gain 1.5-2V. For a circle, it sounds reasonable to cut three or four sectors of equal shape. Maybe a 3D printed pallet can be used with top layer protected by epoxy. But I think for the best result additional metallization will be required anyway.


  • Hero Member

    @Mishka Since you could convert one large cell into lots of tiny cells, it might actually be cost effective.

    I once looked into directly etching the copper on pcb's using a laser mounted cnc, but apparently that requires a much higher power and more expensive laser than what's commonly used by hobbyists. AFAIK, simply running a low power laser over the same isolation traces over and over with just a low power laser won't get you anywhere. I'm guessing that's because of both the copper's reflectivity as well as pretty excellent heat dissipation to the surrounding copper. It may be that a solar cell wouldn't be as difficult, but I couldn't say. For sure your cutting service's laser should be able to handle it though. Please do post how it goes if you decide to pursue it.

    For POC you could cut the cell using just a box cutter or something like that. What happens is that the cell shatters near the cut mark, but enough is left over that the cell still works. So, it's not really the proper way to do it, but it could be done, at least for larger cells. It's hard to know a priori how far the shattering/cracking might travel in a small cell. Maybe not much useable area would be left. Or maybe there would be. I guess that would require experimentation to find out. I only know what I saw in this youtube video:
    Cutting Supower Maxeon Solar Cells? - Mikes Inventions ā€“ 06:37
    ā€” Mikes Inventions

    His was just a rough and ready test to see what would happen. Perhaps cutting it on a circular saw with a suitable tile cutting blade, tightly sandwiched between lots of rigid support might cause less shattering/cracking. Or perhaps borrow techniques used for cutting thin glass. Perhaps multiple passes with a diamond drag bit on a CNC could do it with minimal shattering/cracking: https://www.amazon.com/Diamond-Spring-Loaded-Engraving-Degree/dp/B07F9L62C3/ref=sr_1_6?keywords=diamond+drag+bit&qid=1582661720&sr=8-6
    I think that might stand a decent chance of working. However, aside from a POC, it's easy to see why a laser would avoid these problems altogether, and without producing dust.


  • Hero Member

    Art Resin tested a large number of different epoxies, and it seems that all of them yellowed to some degree over time, but some a lot more than others:
    Non-Yellowing Epoxy Resin Third Party Testing from ATLAS Labs ā€“ 01:46
    ā€” ArtResin

    Of course, since it was a test designed to make Art Resin look good, perhaps they omitted epoxies that really do never yellow. I just don't know which ones those would be. Eight weeks, which was the limit of their study, doesn't seem like a particularly long time.



  • @NeverDie Wow, nice collection! How do you think, may it be reasonable to glue cut cells to a quartz or glass base? Would it compromise effectiveness?

    IMHO the right way to cut them is either laser or high speed CNC. Also, CNC cut crystals may require extra polishing.

    Just asked a couple of local vendors for a single cell, waiting for their reply. BTW those cells are usually of 18-19% energy efficiency, so the only way to beat Amorton or IXYS is to cover larger areas.


  • Hero Member

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

    Also, CNC cut crystals may require extra polishing.

    I think polishing would probably damage them. These cells are different than generic monocrystaline cells. Allegedly, at a microscopic level, they are built using tiny pyramids to increase their surface area. I can believe it, because when taken out of the package they look a bit like velvet. For that reason they apparently scratch extremely easily. The two that I received were in their raw form and totally unprotected, so I am right now in the middle of applying layers of an acrylic lacquer to them as a guard against scratching the active surfaces.

    A water clear urethane coating might have been a better choice, as it's probably harder, but acrylic lacquer is all that I had on hand. I hope to handle differences in co-efficients of thermal expansion by coating both the front and the back equally. Otherwise, it will probably warp.

    I soldered the dog-bones to them. I used rosin core solder, because that's all I have on hand, but next time I think I would use pure solder without the rosin, because I'm not sure whether the resin will interfere with a protective coating. I'll have a better idea about that when I finish coating this batch. Because of the cell's fragile nature and tendency toward scratching, I don't have the guts to clean off the resin with IPA without a protective layer in place. Perhaps I should, though, after the coating on the front finishes curing, and before coating the back of it.


  • Hero Member

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

    BTW those cells are usually of 18-19% energy efficiency, so the only way to beat Amorton or IXYS is to cover larger areas.

    The C-60, gen3 solar cells I received supposedly have a higher efficiency than that: https://us.sunpower.com/sites/default/files/media-library/spec-sheets/sp-sunpower-maxeon-solar-cells-gen3.pdf

    That's the main reason why I ordered them.


  • Hero Member

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

    How do you think, may it be reasonable to glue cut cells to a quartz or glass base?

    Yes, totally reasonable. It would protect them from breaking.



  • @NeverDie No no, by polishing I mean only the edge after cutting. I'd prefer to have it nice and clean just to avoid possible impact of cell layers which might cause shortenings. It's also very interesting to know that the cell has 3D surface - cool.

    How easy it was to solder anything to the cell? Have you tried to solder anything to crystal raw surface? My concerns is that after the cell will be cut it will lose interconnection of conductors so it would be nice to restore the metallization layer.


  • Hero Member

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

    How easy it was to solder anything to the cell? Have you tried to solder anything to crystal raw surface?

    I soldered on the dog bones (a kind of bus connector) to the edges and gave each cell a brief test before applying a protective coating. They each work. That's about all I know. What's a bit confusing is that the solder pads look as if they they are made out of solder mask, but clearly they must be some kind of white conductive material that doesn't look like metal. I'm not exactly sure what's going on with that. I haven't yet found a "how-to" guide for this type of cell that explains anything in any detail. Its construction is completely different from any other kind of solar cell I've tried.

    I don't know what country you're in, but in the US the ebay sellers fullbattery and theHeartOfTheSun sell them at reasonable prices. Do an ebay search for C60.



  • @NeverDie The pads are usually made of silver. If thin enough it may look like the crystal. But the crystal itself may also be light enough - they produced with painting added for better light absorption.


  • Hero Member

    Not really surprising: they do much better with sunlight than with LED or fluorescent light.


  • Hero Member

    At 28lux of really lousy LED lighting, a C60 cell produces 0.66ma short circuit current and 96mv open circuit voltage. So, maybe not so terrible after all.



  • @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 48Ā° 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 Ī¼W/cmĀ² at 200 lx - just compare it to Amorton which have it at about 6 to 8 Ī¼W/cmĀ² 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? šŸ™‚

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


  • Hero Member

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

    What's dimension of the cell?

    37mm x 22mm

    alt text



  • @NeverDie said in šŸ’¬ 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 šŸ’¬ 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 šŸ’¬ 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 šŸ’¬ 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 šŸ’¬ 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 šŸ’¬ 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? šŸ™‚



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



  • @NeverDie said in šŸ’¬ 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 ĀµA / ĀµW level here šŸŸ šŸ™‚


  • Hero Member

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

    Heh, we seem dived below 1 ĀµA / ĀµW 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 šŸ’¬ 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 šŸ’¬ 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 šŸ’¬ 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 (ĀµW) 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 šŸ’¬ 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 ĀµW source. It requires at least 2V to operate, but schematic is very miniature - only three more components needed.


  • Hero Member

    @Mishka said in šŸ’¬ 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. šŸ˜€ 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 šŸ™‚


  • 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. šŸ¤”


  • Hero Member

    @Mishka said in šŸ’¬ 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 šŸ’¬ 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 šŸ’¬ 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 ĀµF 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 šŸ’¬ 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 šŸ™‚ 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 šŸ’¬ 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 šŸ’¬ 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 šŸ’¬ 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. šŸ™‚ 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).



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