[arm-allstar] Adventures with RPi3 5V power

[David McGough]

Hi Everyone,

Since the question of what power supply to use with the RPi3 is a regular
question asked on this list, I thought I'd post some results I've
obtained, including real photos of live operation.

After a quantity of experimentation, I've adopted the cheap high-frequency
5 volt, 3 amp buck-mode DC-DC switcher as the ONLY power supply (PS) I
use.  In addition, for all my "production" systems (including repeaters,
remote base radios, etc.), I SOLDER the power leads to the RPi3 board--see
the photos below as for why I adopted this method.

Here is an example of this PS: https://www.ebay.com/itm/173040030132
....the "Micro USB(Model 1)" version. I set my bench power supply to 10 
volts, for easy power calculations.

For ALL these tests, I connected the RPi3 boards to the network via WiFi.

Here is the RPi3 board with a fob and Asterisk/app_rpt running. Note the 
input power is about 10V * 0.17A = 1.7 watts. The measured USB voltage is 
4.96V. See: https://hamvoip.org/photos/RPi3-usb-unloaded.jpg

Here is the same RPi3 board which additionally has the CPU "maxed" 
out doing an intensive parallel compile of Asterisk on an NFS mounted 
filesystem. The input power is 10V * 0.42A = 4.2 watts. BUT, also note 
that the USB voltage has DROPPED 0.1 volts! See: 
https://hamvoip.org/photos/RPi3-usb-loaded.jpg

Now, we move to an example with a RPi3 which has a soldered-on power 
supply--one of the same style 5V 3A switchers as above, the only 
difference being leads for the output rather than a USB plug.

Again we have an RPi3 board with a fob and Asterisk/app_rpt running.
Note the input power is about 10V * 0.16A = 1.6 watts. The measured USB
voltage is 5.04V. See: https://hamvoip.org/photos/RPi3-soldered-unloaded.jpg

And, the same soldered RPi3 board which additionally has the CPU "maxed"
out doing an intensive parallel compile of Asterisk on an NFS mounted
filesystem. The input power is 10V * 0.42A = 4.2 watts. But, notice that 
the USB voltage is still 5.04 volts! Wow! No voltage drop under load! See:
https://hamvoip.org/photos/RPi3-soldered-loaded.jpg

So, my reasoning for soldering is simple: the micro USB plug may not be 
long-term reliable in an unattended, non-thermally-controlled repeater 
cabinet.  Here is a photo showing how the leads are soldered to the RPi3 
board: https://hamvoip.org/photos/RPi3-soldered-power.jpg

Here is one of dozens of ebay listings for leaded PS units. This is the 
exact PS seen in the photos: https://www.ebay.com/itm/261335563607

Using these PS units, soldered connections and WiFi, I've found RPi3 
boards to be at least as reliable as embedded PC computers.


Merry Christmas and Happy Holidays!


73, David KB4FXC