On Sun, 3 Feb 2019 17:30:20 -0000 (UTC)
Markus Robert Kessler  wrote:

>Am Sun, 03 Feb 2019 16:53:55 +0000 schrieb David Higton:
>
>> In message 
>>           Markus Robert Kessler  wrote:
>>
>>>Hi,
>>>
>>>I just bought a Raspberry 3B+ and now I am setting it up.
>>>
>>>Nice little box and it's really fast in comparison to my other RPis like
>>>Zero/W, A, B... It's fun to work with it.
>>>
>>>But, besides this, one thing makes me really wonder:
>>>
>>>Upper right on the screen there appears a yellow flash all the time and
>>>one of the boot messages reads something like "undervoltage detected".
>>>
>>>The new box is working fine, though, but I connected a USB voltage
>>>measuring device and saw that the power input on micro USB connector of
>>>RPi3 never goes under 5.01 Volts, but, the USB power on those 4 USB port
>>>(I only checked 2 of them with identical results) is only 4,55V (!).
>>>
>>>Total input current is at 0.6 A in average, so I doubt that the 5V/2A
>>>power supply is too weak.
>>>
>>>Am I doing something wrong?
>>
>> Are you actually measuring the voltage at the RPi end of the cable,
>> or are you measring at the other end and assuming that the same voltage
>> arrives at the RPi?
>>
>> Many people have had trouble caused by USB cables that have too much
>> resistance, and thus a significant voltage drop between the ends of the
>> cable.  The shorted and fatter the cable, the better!
>>
>> Dave
>
>
>Hi Dave, hi Alister,
>
>I am using such a device
>
>    https://www.amazon.com/PowerJive-Voltage-Multimeter-chargers-capacity/
>dp/B013FANC9W
>
>between power supply out and raspberry power in.
>
>So, I see exactly what RPi gets. With the same voltmeter I then checked
>the (2 out of 4) USB ports on the RPi.
>
>The funny thing is, meanwhile I found a power supply which was part of a
>"Raspberry 2 starter kit" from Vilros. This one delivers 5V and 2A also,
>but in reality, there are 5.37 Volts without load, measured with the
>device above.
>Surprisingly RPi did not complain about "over voltage".
>
>Best regards,
>
>Markus
>
>

This will never give an accurate result as it is in line with the very thing it
is measuring.

The only way to do this reliably is with a proper battery operated test meter.
These days you can get them for about a tenner - and once you've got one you'll
wonder how you ever managed without it :P

--
W J G

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