How best to find the "best" small solar panel of a particular size?
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You mean the BQ25504? I dropped that quite a while ago. It was rather expensive in the first place, but it was also miserably hard to solder.
One of the simplest circuits is just LDO plus diode plus one 2.7v supercap.
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That should be plenty. I have a node that wakes up every 100ms and listens for 1ms each time to see if another node is trying to contact it. It also updates the gateway with its cap voltage level every 5 minutes. So, on average it's probably busier than your weather station. It can do all that for 24 hours on less than 5F charged to 2.7v. So, with 100F, you should have plenty of margin.
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@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.
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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?
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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.
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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.
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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
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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.
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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.
