"David Higton"  wrote in message
news:9741bf8157.DaveMeUK@my.inbox.com...
> In message 
>          bob prohaska  wrote:
>
>>Making it easy to verify the Pi's supply voltage will come in
>>handy when you report problems to others. One of the first questions
>>I get when seeking advice with a problem is "What's the supply
>>voltage?".
>
> Particularly so in this case, as the thing is teling him clearly
> that it is receiving undervoltage!
>
> And, yet again, I remind the OP (and everyone else) that what matters
> isn't what's sent, but what arrives.  The only place that makes any
> sense to measure the voltage is within the RPi.

And also, it's important to measure the voltage *under load*. Not only
because of contact/cable resistance, but also because of supply regulation.
A well-regulated supply should maintain the supply voltage between suitable
limits for a wide variation of current being drawn. I'd expect that a 2.5A
supply (as stated on the "plate") should be able to maintain the supply
within the normal tolerance that USB specifies for all currents up to that
value.

Obviously there will be greater voltage drop for greater current (due to
cable resistance), even with a perfectly regulated supply, but I think some
cheaper supplies can't keep it up (!) even at the PSU end of the cable as
more current is drawn.

I'm always amazed at how small switched-mode power supplies are these days:
the ones that Amazon supply for the Kindle fit the mains transformer, the
diodes and the PSU control circuitry into the size of a normal-pin plug.

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