How best to find the "best" small solar panel of a particular size?
-
@gohan said in How best to find the "best" small solar panel of a particular size?:
What LDO did you use?
MAX8887EZK27
However, if you're going to be ordering new parts, I suggest you wait a bit. I'll be posting a better circuit after I receive and test the pcb.
-
The best deal I've seen is 10F supercaps on Digikey for $2 each, which would be good enough for most applications.
The good news is that the pricing on supercaps is favorably non-linear. By that I mean you can, for example, purchase a good 400F supercap from Digikey for around $12, not $80.
Which buck-boost converters did you buy?
-
I was checking my orders and I forgot to order it... :confused:
Anyway, this was what I was looking at: it was the only one I found with a low input voltage. That actually is the 5V version, but there was another one that had 3.3v output that I can't find it right now -
I was checking my orders and I forgot to order it... :confused:
Anyway, this was what I was looking at: it was the only one I found with a low input voltage. That actually is the 5V version, but there was another one that had 3.3v output that I can't find it right now@gohan said in How best to find the "best" small solar panel of a particular size?:
I was checking my orders and I forgot to order it... :confused:
Anyway, this was what I was looking at: it was the only one I found with a low input voltage. That actually is the 5V version, but there was another one that had 3.3v output that I can't find it right nowWell, the chart in the description foretells of a problem you're likely to encounter: to get 50ma of 5v output, you need to provide it with 260ma current at 1.2v. I don't know your setup, but I'm guessing your solar panel won't be delivering that. Will it scale to the available current, or just fail entirely? Who knows, because there is no datasheet. Of course, at 3.3v, it won't require as much current, but still.... you might want to look into that.
-
of course, but I am counting for the supercap to supply the power and the solar panel to charge it during the day. With 2 100F supercaps voltage should never reach that low level within one night. I found the 3.3V version. The values in the table are just for reference, so you need to scale it down to the power required for an arduino pro mini and few sensors.
I could also very well be that I may only need a voltage regulator to 3.3V because the voltage from the 2 caps would never drop below that. -
of course, but I am counting for the supercap to supply the power and the solar panel to charge it during the day. With 2 100F supercaps voltage should never reach that low level within one night. I found the 3.3V version. The values in the table are just for reference, so you need to scale it down to the power required for an arduino pro mini and few sensors.
I could also very well be that I may only need a voltage regulator to 3.3V because the voltage from the 2 caps would never drop below that. -
of course, but I am counting for the supercap to supply the power and the solar panel to charge it during the day. With 2 100F supercaps voltage should never reach that low level within one night. I found the 3.3V version. The values in the table are just for reference, so you need to scale it down to the power required for an arduino pro mini and few sensors.
I could also very well be that I may only need a voltage regulator to 3.3V because the voltage from the 2 caps would never drop below that.@gohan said in How best to find the "best" small solar panel of a particular size?:
of course, but I am counting for the supercap to supply the power and the solar panel to charge it during the day. With 2 100F supercaps voltage should never reach that low level within one night. I found the 3.3V version. The values in the table are just for reference, so you need to scale it down to the power required for an arduino pro mini and few sensors.
I could also very well be that I may only need a voltage regulator to 3.3V because the voltage from the 2 caps would never drop below that.Well, given your approach, you may want to try this: https://www.openhardware.io/view/279/Adjustable-Boost-Converter#tabs-instructions
Seems better than anything I see on AliExpress.
Or this, which has a very low start-up voltage of just 250mv:
https://www.openhardware.io/view/281/Solar-Energy-Harvester -
Also, do you have any particular reason you want to boost to 3.3v? If not, then if using an RFM69W, it would be more efficient to boost to a lower voltage, like maybe 2v.
-
Here's a link to a basic LDO 2.7v supercap solar charger:
https://www.openhardware.io/view/382/Basic-27v-Supercap-Solar-Charger -
@gohan
I found a module from the same seller, but it can convert from 0.7v:
https://www.aliexpress.com/store/product/0-7-5V-to-3V-3-3V-5V-DC-DC-Boost-Converter-voltage-Step-up-Module/2348129_32800430445.html?spm=2114.12010615.0.0.SYUdHL -
Also, Pololu sells a pre-made one that can take an input voltage of 0.5v and has an adjustable output voltage: https://www.pololu.com/product/2560
-
The one from aliexpress is only step up, the other it's interesting but I don't need the variable output voltage since pretty much everything runs at 3.3v.
-
They're cheap enough that I decided to just try a few and compare empirically. A conservative yet simple test: if it can make a blue LED glow, then it's good enough to power your mote. It turns out that even very small solar panels can make a blue LED glow indoors with nothing but indirect sunlight, including this one: https://www.digikey.com/product-detail/en/ixys/KXOB22-01X8F/KXOB22-01X8F-ND/4840081
Out of the panels I've tried, everything rated at 5.5v or thereabouts has worked, including on overcast days. That's important to me, because I don't want to assume direct sunlight. -
@NeverDie Wow, that digikey solar cell is tiny at 7mm x 22mm! I'd love to see one of these worked into the plant monitor, as really that only needs to transmit a few times during the day at most and would be fantastic without need of batteries.
@tomtastic said in How best to find the "best" small solar panel of a particular size?:
@NeverDie Wow, that digikey solar cell is tiny at 7mm x 22mm! I'd love to see one of these worked into the plant monitor, as really that only needs to transmit a few times during the day at most and would be fantastic without need of batteries.
Yes. And since plants need light....
-
@gohan I think you may run into a problem with your step-up design. From a cold-start with dim light, very likely the step-up will drain current from your buffer capacitor faster than it's being added, and it will just spin its wheels and produce no usable output. Please do let me know if that's NOT what happens or if you have a design which avoids that happening.
-
I thought about that too, that's why I choose to use big capacitors, so that voltage would never go that low.
In that case I think there should be some components that are able to wait a minimum 1volt or such before allowing current to pass through -
@NeverDie did you get anywhere with this? I'm building a low-power solar node and would like to know as well :)
@mfalkvidd said in How best to find the "best" small solar panel of a particular size?:
@NeverDie did you get anywhere with this? I'm building a low-power solar node and would like to know as well :)
The truth is worse than I imagined. For instance, I purchased four of these panels:
https://www.aliexpress.com/item/6V-0-6W-Solar-Power-Panel-Poly-DIY-Small-Cell-Charger-For-Light-Battery-Phone-Toy/32573510541.html?spm=2114.13010608.0.0.QVU7RI
and measured the open circuit voltage of each under the same deliberately dim ambient room light. Each had a different open circuit voltage: 3.2v, 2.7v, 2.5v, and 2.2v. That's quite a spread in performance! I'm not sure what accounts for the difference, but you can't really guess just by looking at them which would be better or worse. So, really, there's no way to predict from the advertising alone what you're going to get.
