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I'm considering making a Pi cluster (Let's say 8 Pi 3s), and instead of buying an 8-port USB charger and 8 USB cables of exactly the right length, could I use a powerful enough (8A?) 5V wall wart and then hookup wires to plug it directly into the 5V header on the GPIO on each of them. Would this be safe electrically and not damaging to the Pis, and if not would there be a method to make it so? Or is it best to do it the 'normal' way?

Thanks

  • See Raspberry Pi Power Limitations for Foundation recommendations on external power – Milliways Oct 5 '16 at 22:48
  • @Milliways in raspberrypi.stackexchange.com/a/51616/17669 this answer, it mentions the new power IC on the Pi 3/2/B+. Do you know if powering through the 5V pin would still take advantage of this or would it bypass it like the fuse and polarity diode? – Luke Moll Oct 6 '16 at 6:25
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    Powering through the expansion connector bypasses the polyfuse, the ideal diode, and reduces the surge protection, but otherwise is identical. The section "Can the Pi be powered through the expansion header" provides a summary of the Foundation recommendations. The Pi3 and B+ schematics are available. – Milliways Oct 6 '16 at 6:37
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Yes, you can power all your Pis from one power supply with sufficient capacity.

You lose the polarity and fuse protection of powering via the microUSB socket.

You can mitigate any risks by adding your own in-line fuse per Pi and taking care you plug 5V into a 5V pin and ground into a ground pin.

  • Are there diode ICs and breadboard-friendly fuses if I didn't want to use SMT components? – Luke Moll Oct 5 '16 at 14:08
  • I presume so, just a question of searching the catalogues of the suppliers in your neighbourhood. – joan Oct 5 '16 at 14:22
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I have a couple of remarks which won't fit in the comment:

First, while 1A per Pi should be sufficient (assuming no USB devices plugged in and WiFi/BT not used), you may get quite close to the limit at full load. You may want to disable HDMI and Bluetooth to increase the margin:

 # disable HDMI, to be run at every reboot
 /opt/vc/bin/tvservice -o

 # disable BT, to be run once
 echo "dtoverlay=pi3-disable-bt" >> /boot/config.txt
 systemctl disable hciuart

Second, don't use thin wires to distribute power. A wire carrying 8A should be at least AWG21, or 0.41mm² copper. You will lose about 0.7V per meter with wires of this diameter, so thicker wires are recommended for lengths exceeding 20cm or so.

  • For the wires from the individual fuses to the GPIO headers, should they be treated as carrying 8A or 1A? And what would be the best way to connect the thicker wire to the Pi header? – Luke Moll Oct 5 '16 at 16:42
  • Individual wires will carry 1A of course, but you'll need 16 of them (power and ground). And even 0.4-0.5mm² wires are not that thick, you should be able to fit compatible connectors on them with no problem. – Dmitry Grigoryev Oct 5 '16 at 16:48
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    Excellent point on the problem with using thin wires to distribute power. Typically, when you connect to the 5V GPIO power pin, you're using very thin wire (#28 to #34) with a female dupont connector. This thin wire has high resistance which will result in a voltage drop and may get overheated with the current level needed to power the Pi. – PhilM Oct 6 '16 at 2:32

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