Feather nRF52840 low voltage power supply

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fa2k
 
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Feather nRF52840 low voltage power supply

Post by fa2k »

I'd like to power the Feather nRF52840 Express from a 2.7V supercapacitor (Solar power project). In the specs it says it can run at 1.7V, but in the guide it's not recommended to connect other power supplies than USB and battery. I'm hoping that if the voltage is low enough, I can plug it in to the 3V pin and the 3.3V regulator and the other power circuitry won't do much, and the chip will still work. I understand that most wings won't work well (I'm planning to get the LCD display wing and wing it). Otherwise, the plan is to control a MOSFET driver chip, use ADC inputs and Bluetooth.

If I supply power directly from the supercapacitor, the voltage will vary with the charge. Is it a bad idea to have a changing supply voltage? If that's a problem, I could put a linear regulator to output 1.7 or slightly higher voltage.

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Re: Feather nRF52840 low voltage power supply

Post by adafruit_support_bill »

We have not tested either of those boards for operation below 3.3v.

The supercapacitor voltage will decrease linearly with the charge level. Assuming 1.7v is the lower limit of operation, you will only be able to utilize about 35% of the capacity of your supercap.

If you are using ADC inputs with a variable supply voltage, then you will need to use the 0.6v internal voltage reference. Otherwise the changing supply voltage will skew your analog measurements.

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Re: Feather nRF52840 low voltage power supply

Post by fa2k »

Thanks adafruit_support_bill, seems it's worth a try, without too high expectations:) And nice tip on the voltage reference.

I also appreciate the point about only having 35% of the energy. (Somwhat off-topic:) I actually want to use most of the energy in the supercap to power a load using a boost converter while briefly moving into the shadow and in the sun again. It would be good to run the Feather directly from the supercap and turn off the load when the voltage is too low, still being able to run the Feather. I just did the calculation and even with a boost converter, I only get about 60% of the energy from 2.7 to 1.7 - so maybe I need to reevaluate this whole idea.

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Re: Feather nRF52840 low voltage power supply

Post by fa2k »

Come to think of it - I always have a 5 to 7 V power available either coming from the solar cell or boosted from the supercap (even while being able to turn off the load). Much better to use this, and a linear regulator to bring it down to 5. Again thanks for making me rethink this & the Feather nRF52840 still looks like a great choice.

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Re: Feather nRF52840 low voltage power supply

Post by adafruit_support_bill »

I always have a 5 to 7 V power available either coming from the solar cell
If you have >=5v from the solar cell, you are much better off using a solar charger & 3.7v LiPo.
https://www.adafruit.com/product/4755

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Re: Feather nRF52840 low voltage power supply

Post by fa2k »

It's another good suggestion for sure :)

The long story (don't feel obligated to read it - just posting in case someone is interested): For over a year I've wanted to make a solar USB charger for camping/hiking etc. Some pre-made solutions have come on the market recently, but I still have some ideas, like a power gauge to help position the panel.

My first prototype was a simple buck converter set to 5V, with a solar panel on one end and an USB port on the other. It's did work, but had low efficiency because it drew as much current as it could - not even close to MPPT. I used it with a phone, and when walking under trees it would "ding" all the time, start and stop charging.

Second prototype was a "solar power manager" from Banggood, including battery functionality. It does charging, 5V conversion and solar MPPT (according to the spec, anyway). I took some laptop batteries, soldered on protection circuits, and connected them. Problems were: low power (<1 A USB charging), manual on/off switch, battery gauge didn't work well. I can't set the charge/discharge thresholds and the batteries wear out from the abuse. Overall too much hassle for not so much power.

The dream all along was to find some neat custom topology DC-DC converter that could shuffle power in all the necessary ways between USB, solar and battery, with high efficiency. That didn't pan out - found nothing better than the modules we know like buck, boost, etc. That's why I wanted to ditch the built-in battery and use a power bank instead - it's not much less efficient, and eliminates some complexity. I picked this project up again yesterday and had some new ideas.

The supercap is indeed not a good idea. The problem it solves, like intermittent shadow from trees or walking in front of the panel, is not a big problem. The solution is to not charge a phone directly, but either use a power bank or a built-in battery. They are probably fine to start/stop charging quite frequently.

Current candidates are :

* The Adafruit solar charger module, a big or small battery and a boost converter for the output. It's quite suitable for my 9W "colossal" solar panel and seems efficient. But I'd like to add intelligent logic to turn on/off the output, and even turn off everything when left alone.

* Back to just a buck converter, no battery or supercap, but implement quasi-MPPT like the Adafruit module. This works - it's possible to control the power draw of some chargeable devices I tested by reducing the voltage on the USB down towards 4V.

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