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Long version (full story, so skip and go direct to questions below if you want to):

I have small arduino mini pro board powered by 230mAh Lithium coin battery used as wireless sensor (with NRF24L01 and different sensors) I use a 1.8V or 2.4V LDO regulator and modified bootloader (lower frequency = lower consumption). Periodically it take measurement, send it and go to sleep (Board modified to go down to nA consumption). This work well multiple months, I'm happy.

But I found a new greener challenge: Power it without battery only by ambiance luminosity. I want to use a 12V (1.5W max) solar panel (I hope to get at least 2.8V with artificial light, I will verify this by testing) combined with a supercap (10 Farad for example)

I found some examples with IC's to control voltage and limit current: Bicycle example But I wonder if I could make it simpler directly with only a LDO regulator.

For a 10F supercap and an 2.5V (180mA max) LDO regulator I get: Charging time = (2.5/0.180)*10 = about 2 minutes 32 seconds

Questions:

  • Will my first 2.5V LDO regulator survive without current limitation, or do I need a mosfet like this : Instructable. I could add a resistor too, but that's energy wasted no?
  • Do I need more schottky diode to prevent some current leakage?
  • Can I use a 2.7V LDO regulator instead 2.5V one or It's better to stay a little away from supercap voltage limit even by loosing some power?
  • (I will see if power consumption is better with second 1.8V LDO regulator and a lower working frequency or if it's totally useless against using only the first one)

Scheme

(Sorry for my poor English)

Edit 17/09/2018:

By searching other regulators with current protection I wrote this small tab: LDO regulator comparison

MCP1702 is a better choice cause of thermal protection? But expensive for a SOT23 package : 0€42/piece. LP2985AIM5X-2.5 seems to be at 0€18/piece. I don't find other with Vin >= 12V on aliexpress for 2.5V output (most are 3.3 or 5V).

Edit 17/09/2018-2:

I found this example but for now I doesn't understand it all. After @HarrySvensson comment suggesting to use MPPT to get more efficiency, I wonder if using some MPPT board for charging Lipo Like this one and putting the LDO regulator after it could be a good idea or absolutely not? (I looked the CN3791 datasheet I don't find any way to adjust output voltage)

Edit 18/09/2018:

And what about This one? Voltage and intensity are apparently adjustable. But I'm not sure this is a real MPPT board, it seems more like a buck converter.

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    \$\begingroup\$ You should look up MPPT. My first guesstimate of your efficiency from converting solar power to useful electrical power is 1%. Reason: You are not treating the solar panel for what it is. \$\endgroup\$ – Harry Svensson Sep 16 '18 at 17:26
  • \$\begingroup\$ @HarrySvensson thank for your comment. I will read more about MPPT, it is interesting, but is it not to complicated/overkill for my small application? The most current is when luminosity rise to charge the super cap, after it is only a uA/nA consumption from arduino and the supercap leakage current compensation. \$\endgroup\$ – ROUGEXIII Sep 17 '18 at 10:41
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    \$\begingroup\$ My point is that if you would design it properly then you could downsize the solar panel by a factor of 20-50 and get the same results. - Sure, your design will probably work, you can also use your hand as a hammer when hammering nails. It's just a bad design. \$\endgroup\$ – Harry Svensson Sep 17 '18 at 10:41
  • \$\begingroup\$ @HarrySvensson Oh ok. For keeping this as simple as possible, do you think it is possible to find some "all in one" MPTT chip/board like this simple LDO regulators? ( My electronics skills are quite limited :/ ) \$\endgroup\$ – ROUGEXIII Sep 17 '18 at 10:48
  • \$\begingroup\$ I have no idea, I got 0 experience with it. \$\endgroup\$ – Harry Svensson Sep 17 '18 at 10:49
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The HT7325 has 500mW of maximum power consumption. If your solar panel is really able to achieve 1.5W output most of it will be lost as heat in the HT7325 and it will be too weak.

I think there are LDOs with reliable overcurrent and overheating protection which could be safely operated in this rare condition.

I think you could leave out the second 1.8V LDO if you don’t need a specific, stable voltage (for example for interfacing with other chips (powered from a different source) or ADC/DAC). The ATmega328 has an operating voltage from 1.8 to 5.5V.

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  • \$\begingroup\$ Thanks for your answer @Michael So the LDO regulator couldn't limit itself? Maybe could I add a 15 ohms 1/2W resistor between first regulator and junction to limit the current : imgur.com/a/t1IGUva I know the second LDO is optional and I will make some tests to see if I got more obscurity working time with it at lower frequency or not. \$\endgroup\$ – ROUGEXIII Sep 16 '18 at 17:10
  • \$\begingroup\$ The LDO has to drop the voltage difference of 12V-2.5V=9.5V. The resistor would limit the current to 166mA. So with this output limit resistor you could still get 9.5V*0.166A=1.6W at the LDO. Of course this assumes that the solar panel provides its maximum power at 12V which is probably not the case. I’m not sure how to take the power curve of a solar panel into account here. \$\endgroup\$ – Michael Sep 16 '18 at 17:30
  • \$\begingroup\$ Mhhh, would the solar panel voltage stop to rise as soon as it’s above 2.5V and the LDO is doing its thing? As long as it doesn’t provide more than 180mA of current (that’s the output current limit of the HT7325) you’d be fine. You could measure how much current the solar panels produces when you short it with 3 Si diodes in series (for a 2.1V drop). \$\endgroup\$ – Michael Sep 16 '18 at 17:37

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