Power Blue Pill and Maple Mini from external 3.3 source?

Post here first, or if you can't find a relevant section!
Post Reply
visinet
Posts: 18
Joined: Mon Dec 05, 2016 6:30 pm

Power Blue Pill and Maple Mini from external 3.3 source?

Post by visinet » Fri Oct 13, 2017 5:05 pm

Have a question on powering these, or any board for that matter, that has a build in 3.3V LDO along with connections to post-LDO 3.3 volt for use of external 3.3 power source directly. What issues, if any, are there when connecting an external 3.3V source post-LDO due to the fact that this results in back-feeding the LDO since it has no input voltage applied to it's input. Is this generally bad practice that could lead to failures after long term use? In commercial products I usually see a jumper to select the power source which basically removes the LDO from the circuit if it not being used due to some external power source.

Thanks a bunch any any insight.

Cheers

User avatar
ahull
Posts: 1630
Joined: Mon Apr 27, 2015 11:04 pm
Location: Sunny Scotland
Contact:

Re: Power Blue Pill and Maple Mini from external 3.3 source?

Post by ahull » Fri Oct 13, 2017 10:03 pm

It probably is bad practice, but I have done this in the past. As to damaging the LDO, that would depend on which LDO is fitted, and whether the manufacturer allows for this mode of operation. Check the datasheet.
- Andy Hull -

visinet
Posts: 18
Joined: Mon Dec 05, 2016 6:30 pm

Re: Power Blue Pill and Maple Mini from external 3.3 source?

Post by visinet » Tue Oct 17, 2017 9:22 pm

Thanks a bunch, that is what I thought. What specifically would I look for in the data sheets? I have looked in the past and did not see anything that specifically addressed this issue. I even contacted one manufacture and they really didn't have a straight forward answer. I would think that in today's world with boards allowing power from different sources and having battery backup this would one of the most asked questions. Thanks again

User avatar
ahull
Posts: 1630
Joined: Mon Apr 27, 2015 11:04 pm
Location: Sunny Scotland
Contact:

Re: Power Blue Pill and Maple Mini from external 3.3 source?

Post by ahull » Tue Oct 17, 2017 10:28 pm

If the datasheet mentions reverse protection, or shows some sort of schematic of the output stage, you might get some clues. I suspect, given how the output stages tend to work, it would care if you feed it 3.3V, but I'm no expert, so I'll throw that one open to anybody else that wants to chime in.

Typically if you are feeding two PSUs in to one device, you put a diode in line with each to stop them feeding each other, but often there is effectively a diode in line with the output, so this *may* be unnecessary... try it and see.. what is the worst thing that can happen.. you pay the $2 entry fee to the magic smoke club. :twisted:
- Andy Hull -

stevestrong
Posts: 1748
Joined: Mon Oct 19, 2015 12:06 am
Location: Munich, Germany

Re: Power Blue Pill and Maple Mini from external 3.3 source?

Post by stevestrong » Wed Oct 18, 2017 7:36 am

I have a blue pill working 24/7 with external 3.3V supply since 6 months, with the on-board 5V->3.3V regulator.
And it still works...

User avatar
zoomx
Posts: 526
Joined: Mon Apr 27, 2015 2:28 pm
Location: Mt.Etna, Italy

Re: Power Blue Pill and Maple Mini from external 3.3 source?

Post by zoomx » Thu Oct 19, 2017 10:03 am

Cheap Arduino Pro Mini clones works if powered with external supply but when in sleep mode the LDO uses some mA so if you want to use sleep mode to save power you have to desolder LDO or cut a trace.

Post Reply