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Similar to Can I power a Pi through a 5V pin?, but I want to acquire the 5V power source from another Pi. The first Pi gets its power via micro USB, and then I want to use pins 2 and 6 from that to power the second Pi via its 2 and 6 pins.

The trouble is every time I complete the connection, all power is lost on the first Pi and it shuts off (and the second one never lights up AFAIK).

I have confirmed that I can power the second Pi via pins 2 and 6 with 3 AA batteries. And of course pins 2 and 6 is supposed to deliver 5V power to another device. So why then can't I chain them together in this way?

Strictly speaking, this are Pi Zero W's.

Andrew Arnott
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The Pi Zero has a basic power configuration i.e. NONE!

All the 5V pins are interconnected; there is absolutely no protection.

You could power one from another, but this seems pointless, and just involves running more current through the board traces.

Just run BOTH from the same 5V supply; it doesn't matter whether you use the dedicated USB port or the 5V pins.

In fact you can do this for ANY model Pi; the Foundation has recommendations on how to do it, but as the Zero has no protection, adding any seems superfluous. See Raspberry Pi Power Limitations

There is absolutely no way this could cause a shutdown (unless the power supply shuts down), although doing any connection to a running system, particularly if using significant current, is inadvisable.

You may have confirmed "I can power the second Pi via pins 2 and 6 with 3 AA batteries" - BUT don't expect it to run for long (or reliably). These are totally unsuitable for even the modest current required by the Pi Zero.

Milliways
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  • My power supply is an AC to microUSB adapter. I can't readily connect it to two Pi's. My idea was to use my adapter to power the first, and then use pins 2 and 6 with standard wires between the two. Your idea that the problem was because the first Pi was already running and thus had a sudden voltage change when the second was connected is interesting. The 3 AA batteries test wasn't because I was considering using batteries. I just wanted to confirm that power delivery via those pins was valid. – Andrew Arnott Jun 27 '21 at 23:18
  • Yes you could do this. If you consulted the schematic https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/ it would be clear and you are unlikely to overload the traces with a light load. – Milliways Jun 28 '21 at 00:19
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Your question is unclear wrt how the batteries are connected. However, battery connections aside, what you're doing is probably not a good idea. If you'd like to edit your question to include a schematic, perhaps we can answer more definitively.

However, your comment changes your question substantially:

One reason I want to chain them is to connect their GND pins, so that one Pi can decide to "short out" the RUN pin on the other simply by pulling a GPIO pin down to 0V.

You can tie ground pins from different RPi together without incurring the issues you'll face in tying the 5V input of one RPi to the 5V bus of another.

Seamus
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  • The AA batteries were connected to each other in series, delivering 4.5V to one Pi via pins 2 and 6. That worked, but that was just a test. It isn't what I intended to stick with. – Andrew Arnott Jun 27 '21 at 23:20
  • @AndrewArnott: Good - because it won't work well :) Did you understand my comment re the ground connections vs the supply connections? – Seamus Jun 27 '21 at 23:45
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That will work provided

  1. the power supplied to the first Pi is sufficient for its needs and the needs of the second Pi.
  2. that any polyfuse fitted to the first Pi will let sufficient current flow for both Pis.

I don't think the Pi Zeros have polyfuses so any failure is down to case 1.

joan
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  • Thanks. I am using a 2.5A capable power supply as shipped with https://smile.amazon.com/gp/product/B0748MPQT4/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1. I expected that would be plenty. I am also powering two relays (which draw very little current). Could it be an insufficient power supply? – Andrew Arnott Jun 26 '21 at 21:08
  • it can't be. Either it doesn't supply 2.5A or you are using more. – joan Jun 26 '21 at 21:42
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Some USB power supplies are not great in maintaining 5v with peaky current requirements, even when they are quoted at 2.5 Amps. And poor power cabling will cause additional voltage drops along the way. With my Raspberry Pi battery power supply I did an experiment where I used one unit driving a Pi4 + LCD, USB Hub with Keyboard, Mouse and Webcam to also power a second battery board with a Pi3 and LCD. The Pi4 board was drawing around 2.5 Amps, the Pi3 another 2 Amps (as it was also charging that battery) when running a full graphics test on both. Note that power regulation will be critical to maintain a steady voltage. I made a video of it at https://youtu.be/DS3TtVOnquk .

The RedReactor
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use a usb hub for multiple raspberry pi power supply.

user135142
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