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<div class="moz-cite-prefix">Hi Peter, Andrew,<br>
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<div class="moz-cite-prefix"><br>
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<div class="moz-cite-prefix">I agree with Andrew, a Schottky diode
in series with each rail should work well.<br>
</div>
<div class="moz-cite-prefix"><br>
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<div class="moz-cite-prefix">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.<br>
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<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>What are you using for an external 5V supply ?</p>
<p>Please, can someone review my argument above, I am not sure if i
have had quite enough coffee yet :)</p>
<p>Regards, Mark</p>
<p>PS The CPU needs 3.3V, and the LDO regulator should be OK down to
about 3.8V input rail.<br>
</p>
<p>PPS Jaycar sell diodes, see part number ZR1100</p>
<p>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.</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 23/12/2018 10:10 PM, Andrew
Errington wrote:<br>
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<blockquote type="cite"
cite="mid:CAGZkoWR-SmL+oVy-NWRSGa0UzVPWPf6126OexSZhqWKa_Q+RNA@mail.gmail.com">
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<div>Hi Peter,
<div dir="auto"><br>
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<div dir="auto">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.</div>
<div dir="auto"><br>
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<div dir="auto">Best wishes, Andrew</div>
<br>
<div class="gmail_quote">> On 23/12/2018, at 1:58 AM, Peter
Ellens <<a href="mailto:ellensp@gmail.com"
target="_blank" rel="noreferrer" moz-do-not-send="true">ellensp@gmail.com</a>>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">> Is
this on topic? I don't know.... not sure what on topic
encompasses any more!<br>
> (maybe Mark could clarify on that)<br>
> 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.<br>
> 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.<br>
> 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.<br>
> Any thoughts or ideas?<br>
> Thanks<br>
> Peter E</blockquote>
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