Lopy4 uses lots of mA when voltage is getting low

  • Re: 2.7V power supply
    I've been testing an combination of a 5v (.6w) solar cell with an 17 farad (super) capacitor.
    This works fine, the solar cell gets refilled during daytime during the deep sleep of the Lopy4.
    The lopy4 wakes once every hour to measure the temperature using a DS18x20, measures the voltage of on the cell using an voltage divider. A filled capacitor is good for 50 times waking up without refilling the capacitor (like on dark winter days).
    But when the capacitor is getting low (under 3v) the lopy start to use about 20mA without starting/running. This drains the capacitor even more. At this point it's getting difficult to recharge the supercap with sunlight because all energy is consumed by the Lopy4.
    I've tried to place a buck/charge pump in between (max1595eua50). But this too can't handle the load of the lopy on low voltages.
    Has any body seen this behavior?
    I've been looking into a thrysitor to cut of the power when voltage is under 3.5 v, but I have no experience with this. (I'm not an electronics engineer, just a hobbyist).
    Any suggestions.

  • @andrethemac As far as I recall, the voltage regulator is a LM3281. The docs pages have been reorganized, and it got only worse. Not only that the omissions and faults were not fix, information disappeared which used to be there. Also, this regulator is only specified down to 3V input.

  • @andrethemac this is probably the on-board regulator. The way the LoPy is designed, the regulator is meant to bring down the voltage from up to 5V down to the 3.3V required by the ESP32. The regulator has several different modes, and it probably switches mode when the input voltage gets below a given threshold.

    I don’t remember the regulator’s reference but there was a thread recently where it was discussed. This should point you to its datasheet so you can check whether this is consistent with your observations.

    The issue if you cut down the power completely is that wake up modes are going to be difficult to manage.

    You may want to check if the Pysense or Pytrack may fit the bill, I’m not sure they use the same regulator or whether it’s configured the same way.

    Otherwise you may want to explore regulators that behave like you want on those voltages, or energy harvesting circuits maybe.

    Another option would be to have more capacity and a bigger solar cell as well of course.

    Good luck and let us know how it goes!

  • @andrethemac The operation state of the ESP32 and the LoRa modem is undefined below 3V. Of course, you can use a transistor circuit to switch off power below the threshold. And I had expected that there are pre-built modules which you could use, but did not find one immediately,

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