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Feather M0 RFM69 - external 3.3v power?
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Feather M0 RFM69 - external 3.3v power?

by joemarshall on Wed Feb 10, 2021 7:51 am

Hi,

I've built a lovely system to enable radio control of heating using a feather m0. Right now the m0 is separately powered and all is fine. However my heating control relay box has an ESP32 board with a 3.3v supply going spare in it. If I could use that, then I could pop everything inside the box and it would be neater and remove a power supply brick.

Now I see in the docs that directly hitting the 3.3v pin with an external supply is not recommended, but I think I also see on the schematics for this particular board that the 3.3v supply for the radio is directly connected to the 3.3v pin. I get that this isn't recommended, but am I right that it is likely to work, assuming my 3.3v is powerful enough to run the radio?

thanks
Joe

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Re: Feather M0 RFM69 - external 3.3v power?

by joemarshall on Wed Feb 10, 2021 10:33 am

The alternative is that I have a 5v testpoint over the other side of the board, run that into a microusb plug along with a ground, and just connect the two 3.3v serial wires I need to go across between the two boards. I can see that it is safer to do that than just running the 5v straight to the USB pin, because if I reprogram the feather there's no chance of crossing the 5v wires. But is there any practical reason for doing this over the 3.3v pin direct solution?
(bearing in mind that I am really tight on space here, working around a bunch of parts which can't easily be moved)...

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Re: Feather M0 RFM69 - external 3.3v power?

by danhalbert on Fri Feb 12, 2021 9:52 am

You can supply 3.3V to the 3.3V pin, but as you read, there are a number of cautions. In your case, if you supply 5V power to the Feather, say when reprogramming it, you will be backpowering the 3.3V output on the other board, and you could damage it.

I think your idea of supply 5V via a USB plug is the safest, in the sense that it's an interlock that prevents backpowering anything, either the ESP32 board or the host computer that's plugged in via USB. Yes, you could be careful, but one mistake in powering could fry your computer USB port or the ESP32 board.

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Re: Feather M0 RFM69 - external 3.3v power?

by joemarshall on Fri Feb 12, 2021 10:32 am

Okay I'll harden up and solder to the testpoint instead of just putting the dev headers across. I see your point about programming.

thanks
Joe

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Re: Feather M0 RFM69 - external 3.3v power?

by beakes on Tue Aug 03, 2021 1:25 pm

I would also like to use the Feather M0 RFM69 board with a 3.3v external supply and no battery. For this board, which only uses 3.3v for the processor and radio, why not tie EN to ground (to disable the onboard regulator) and connect a clean/regulated 3.3v external supply to the board's 3.3v pin? That would allow you to connect USB for programming without any power conflicts since the regulator is disabled.

Shouldn't this be a safe and recommended way of externally powering this board? Am I missing something? Any issue with a disabled regulator seeing 3.3v on its output pin?

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