[chbot] stm32f103 blue pill power advise?

[Mark Atherton]

Hi Peter, Andrew,

I agree with Andrew, a Schottky diode in series with each rail should 
work well.

However, if you don't want to modify the board then you need to put a 
silicon diode in series with the external 5V supply. This is a bit rough 
from an engineering point of view, but it may be OK.

If either USB or external 5V are applied with this situation then the 
input to the regulator will be 5V (less any cable drops) with the USB 
connected, and 5V less silicon diode volt drop (0.7V ?) = 4.3V if 
external 5V is applied. So far so good.

If both USB *and* external 5V are applied, the silicon diode should be 
reverse biased, and the unit should be powered off of USB power.

I say 'should' because if you have a poor quality USB cable (with thin 
power wires), then a different justification is required; you may well 
not get 5V from the PC at the processor board end. Under these 
circumstances, the higher of the rails will actually feed power to the 
CPU, and in the case of a cheap USB cable, there may be a little 
back-powering of USB power to the PC.

However, as long as [External Supply - 0.7V] is below [PC 5V rail] any 
back powering may only result in current being shared between the two 
supplies.

To even consider my argument above as being valid, you need to 
accurately measure USB supply at the processor end without ext 5V 
applied, you then need to remove USB power, apply 5V via a silicon diode 
and repeat the measurement.

What are you using for an external 5V supply ?

Please, can someone review my argument above, I am not sure if i have 
had quite enough coffee yet :)

Regards, Mark

PS The CPU needs 3.3V, and the LDO regulator should be OK down to about 
3.8V input rail.

PPS Jaycar sell diodes, see part number ZR1100

PPPS I have seen 1m cheap USB cables with 1 ohm resistance is the power 
wires. Turned out that read and black wires were 90 microns(!) diameter.


On 23/12/2018 10:10 PM, Andrew Errington wrote:
> Hi Peter,
>
> It looks like everything on the board runs at 3.3V from an on-board 
> regulator therefore your 5V supply only needs to be nominal. How about 
> running your power inputs into a pair of Schottky diodes? The voltage 
> drop is small, and you could build a tiny circuit board with a micro 
> USB socket and whatever you need for your other 5V supply as input, 
> and a micro USB plug as output.
>
> Best wishes, Andrew
>
> > On 23/12/2018, at 1:58 AM, Peter Ellens <ellensp at gmail.com 
> <mailto:ellensp at gmail.com>> wrote:
>
>     > Is this on topic?  I don't know....  not sure what on topic
>     encompasses any more!
>     > (maybe Mark could clarify on that)
>     > I'm playing with a project that has a stm32f103 blue pill at its
>     core. Many of you will know this board and I'm hoping that some of
>     you will have more practical experience with this board than I.
>     > What I am wanting is to be able to power the board from either
>     USB or a external 5v source, preferably being able to remove one
>     or the other supply and it keeps going, without modifications to
>     the blue pill board or the USB cable.
>     > The issue is the 5v line from the USB is directly connected to
>     the 5v pin of the module, so you can have one or the other supply,
>     but not both. This problem is well known, but I have not seen a
>     solution.
>     > Any thoughts or ideas?
>     > Thanks
>     > Peter E
>

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