Solar Energy Harvesting for wireless motes
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Here's my current thinking on what to do next:
https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v000.pdfIt's a roll-my-own LDO, which has two advantages: 1. I can pick whatever maximum voltage I want for the solar input (I'm thinking a max of 40v would cover everything), and 2. It gives me access to the charge finished signal (similar to a power_good signal), which will set things up for the next stage, which is dumping surplus charge into a much larger capacitor. i.e. that next stage after this will give priority to quickly charging a small capacitor (say 100uF) to immediately power-up the MCU, and then only afterward to charge the really big supercap.
What do you guys think?
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I forgot to mention: another advantage is that it can start charging the capacitor at lower solar voltages than what a pre-made LDO (at least the ones that can withstand 40v) can. AFAIK, the pre-made 40v LDO's don't pass current until the voltages are 2v+, or thereabouts. In theory, this one could start charging at around 0.4v to 0.8v (depending on how many diodes I end up needing to guarantee a full-shutoff at the PMOS).
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I have doubts the earlier version would have charged the capacitor when the harvested solar was of weak voltage. However, this new version should do that: https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v001.pdf
It somewhat depends on the behavior of the voltage detector during the power-up phase, so I won't know for sure without building it and then testing it. Also, it does not address how to cleanly disconnect the charger so that the MCU can take open circuit voltage measurements of the solar cell. -
OK, hopefully this new version has addressed all the issues, including the clean disconnect for the voltage measurement: https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v002.pdf
I guess if no one has any comments, then like the little red hen I'll be going this alone.
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Personally, I'd just use the AEM10941, ah ah.
It has MPPT module, starts charging the storage medium when your solar cell reached 50mV... It's hard to beat that :P
That being said, it's made to work with small solar cells, so if you plan to use "big" panels (via V > 5V or I > 110 mA), then building your own circuit will be better.
I can't say much about your design though, I haven't enough knowledge on that matter ;)
Come have fun with me on IRC: #mysensors on Libera.chat :)
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Personally, I'd just use the AEM10941, ah ah.
It has MPPT module, starts charging the storage medium when your solar cell reached 50mV... It's hard to beat that :P
That being said, it's made to work with small solar cells, so if you plan to use "big" panels (via V > 5V or I > 110 mA), then building your own circuit will be better.
I can't say much about your design though, I haven't enough knowledge on that matter ;)
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Here's a simplified version: https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v004.pdf
How it works:
This particular NCP301 voltage detector goes high when the voltage reaches 2.7v (most voltage detectors only go high when they fall below a target voltage). Thus, that should trigger the NFET to open, which should provide a positive bias to the base of the PNP transistor, which I'm hoping will be enough to completely turn-off the current flow through the PNP transistor. If I'm lucky, fewer than 3 diodes will be needed. The resistor values may need tweeking. For the NCP301, the typical quiescent current is 500na, which is lower than any of the hysteresis chips I checked.Summarizing:
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This should allow charging of the storage capacitor at any voltages above the diode and PNP voltage drops, which should be low compared to any pre-made LDO (at least all the ones I'm aware of).
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It will allow input voltages up to 40v from the solar cells, which is higher than what the pre-made LDO's allow (again, at least all the ones that I'm aware of). Thus in a very dimly lit environment, a string of solar cells could be put in series to counteract the dimness and yet still charge the capacitor.
As always, comments are welcome.
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OK, I think this one's the winner: https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v005.pdf
It uses only 4 parts and is compatible with any input voltage: https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v005.pdf Thus you can stack as many or as few solar cells in series as you want to, and the circuit should work the same regardless.
How it works: current from the solar cell/panel flows through the diode to charge a capacitor (either surface mounted to the PCB or attached to the PCB using the provided through-holes). When the voltage reaches 2.7v, the voltage detector goes high, burning off 50ma of current through the 56 ohm resistor until the voltage drops below its hysteresis point. As long as the solar cell/panel's current does not exceed 50ma, this design should work. If you need to handle an input current of greater than 50ma, then simply modify the circuit to instead connect the voltage detector output to an appropriately sized mosfet for that current, and then use that mosfet to dissipate the surplus current through a suitable resistor to ground.
In my case I'm be choosing a diode with a maximum of 100na reverse current leakage, but you can choose whatever diode you want to fit your particular trade-offs.
I presume that by choosing a different voltage detector you could just as easily charge a battery instead of a supercap, if that's what you wanted to do.
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I just now sent the files to fabrication. If it tests out as expected, then I'll post the gerber files.
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Just for fun I added an LED that will flash each time the capacitor discharges a little to stay within its maximum 2.7v. Although brief, it indicates that solar harvesting is working and that the capacitor is fully charged.
BTW, I found a voltage detector that consumes just 150na, so I'll probably switch to using that because it will be important for the nextgen version which prioritizes the charging of a bootstrap cap before dumping solar charge into a much larger supercap.
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It finally dawned on me that a very solid minimalist circuit can be accomplished using just two diodes: https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v008.pdf
The trick to making it work is selecting a diode D2 that has a forward voltage drop of 2.7v. For instance, CMF05(TE12L,Q,M) is such a diode, and on Digikey it costs a mere 40 cents: https://www.digikey.com/product-detail/en/CMF05(TE12L%2CQ%2CM)/CMF05(TE12LQM)CT-ND/2310627
The result is a circuit that's not only inexpensive but can withstand any voltage that might be applied to it and, in realistic terms, any charge current that it's likely to encounter as well. And by picking diode D1 to have a low forward current (and for that, any common diode will do), it will charge quickly as well. So, better, faster, cheaper. Usually you only get to pick two of those. :-)
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It finally dawned on me that a very solid minimalist circuit can be accomplished using just two diodes: https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v008.pdf
The trick to making it work is selecting a diode D2 that has a forward voltage drop of 2.7v. For instance, CMF05(TE12L,Q,M) is such a diode, and on Digikey it costs a mere 40 cents: https://www.digikey.com/product-detail/en/CMF05(TE12L%2CQ%2CM)/CMF05(TE12LQM)CT-ND/2310627
The result is a circuit that's not only inexpensive but can withstand any voltage that might be applied to it and, in realistic terms, any charge current that it's likely to encounter as well. And by picking diode D1 to have a low forward current (and for that, any common diode will do), it will charge quickly as well. So, better, faster, cheaper. Usually you only get to pick two of those. :-)
@neverdie what will prevent your supercap from discharging through D2 ?
When current flowing through the diode gets low, the forward voltage gets lower too so your supercap will be drained. -
@neverdie what will prevent your supercap from discharging through D2 ?
When current flowing through the diode gets low, the forward voltage gets lower too so your supercap will be drained.@nca78 Thanks for pointing that out. I don't have in my possession the diode with the 2.7v forward voltage drop, so I ran some tests on a red LED insteaad. According to my multimeter, the red LED has a forward voltage drop of 1.8v. I hooked it into a uCurrent Gold to measure current and then decreased the voltage below 1.8v to see how the LED current reacted. You are right. Voltage had dropped all the way to 1.4v before I could no longer see any detectable current on the micro amp scale. Then, switching to the nano-amp scale, it wasn't until I had reduced the voltage to one volt that I could no longer discern any current on the nano amp scale. I had thought the current would cut-off much sooner than that, but I was wrong. Thank you once again.
Back to the drawing board!
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@neverdie what will prevent your supercap from discharging through D2 ?
When current flowing through the diode gets low, the forward voltage gets lower too so your supercap will be drained. -
@nca78
What if a 2.7v zener diode, reverse biased, were used instead? Would it have essentially the same problem?Scratch that. Probably not, except in limited cases, and even those might require hand selected zeners.OK, for an extra 40 cents, this new circuit should work perfectly without more than 150na of current drain: https://www.openhardware.io/dl/5b8ff8a3d376570b051a91ed/design/schematic_solarLDO_v011.pdf
I suspect 150na is less than the self discharge rate for any supercap.
For larger solar panels, expunging surplus capacitor charge through an nfet would be the prudent way to go, but this circuit should work for the tiny solar panels that I'm currently focused on, which for sure are producing less than a milliamp of current. The circuit retains the ability to withstand any input voltage, provided that the input current is guaranteed to be less than 10ma, which is the absolute maximum provided by the datasheet.
It's a good time to be alive. Not long ago these ultra low current drain parts didn't even exist--at least not at $0.82 for single unit quantities. :)
That said, there are a number of different ways to attack this problem. Maybe an even cheaper way exists that can withstand any input voltage (say, up to 50 or 60v DC worst case)? That's really the only complicating factor. If one assumes less than 12v input voltage, or even less than 20v input voltage, then I can see at least some other possibilities that would work just as well, if not better.
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I forgot to mention: another advantage is that it can start charging the capacitor at lower solar voltages than what a pre-made LDO (at least the ones that can withstand 40v) can. AFAIK, the pre-made 40v LDO's don't pass current until the voltages are 2v+, or thereabouts. In theory, this one could start charging at around 0.4v to 0.8v (depending on how many diodes I end up needing to guarantee a full-shutoff at the PMOS).
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I forgot to mention: another advantage is that it can start charging the capacitor at lower solar voltages than what a pre-made LDO (at least the ones that can withstand 40v) can. AFAIK, the pre-made 40v LDO's don't pass current until the voltages are 2v+, or thereabouts. In theory, this one could start charging at around 0.4v to 0.8v (depending on how many diodes I end up needing to guarantee a full-shutoff at the PMOS).
