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here is an unusual question. I have NOT found ANYTHING answering my question which is why I'm here.

My question is this: I am about to make a server cabinet for 4 Raspberry PI's it USED TO BE the 2 modB... That is until I learned about the new Pi 3 on the 29th of February. One PI is for hosting a Web server, one for an email server, and the other two for either support, or various other projects as to not slow down the Web and email servers. Everything in the cabinet is designed to be minimalist. The switch I plan on using is a 4 way micro switch for ethernet connection to the network.

Here's what I am wondering. I want to Daisy chain at least 2 of them together using one power source... So, the 1st Pi would be plugged into the primary power source of 5.1V 2.5A... BUT the second PI, I want to use the 5V and GND GPIO posts on the first PI to micro USB power input on the second PI. Is this possible? How well would it work? How would I go about making the secondary power source meet 2.5A (if I need to) as to not fry the second PI?

ANY info yall can give me about whether or not this is possible and how to make it happen would be great!

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This doesn't feel like a good idea to me.

Assuming that both Pi's are drawing their maximum current of 2.5A, wouldn't the first Pi in the sequence need to draw 5A from the wall (+ some amount due to inefficiencies). There are some resettable fuses on the Pi that I imagine have a lower current rating than 5A, and may trip to protect the Pi. This, of course, depends on how the circuit is designed with regard to providing power to the 5v GPIO post.

You might be better off trying to find a powered USB hub that outputs a steady 2.5-3A on each port, and using that to power your Pi's.

  • re:RPi2 - The resettable fuses are called "polyfuses", and are in series with the micro-USB connector's power terminal(s). Despite being soldered to the PCB (for added mechanical strength), the rest of the connector's terminals aren't used. The polyfuses protection only happens when the Pi tries to draw too much current via micro-USB. The resetting of the polyfuses happens automatically (within limits), but "may take up to a day". The +5V pins, power supply lines to the USB ports, and the Pi itself, are all tied together AFTER polyfuse protection, so daisy chaining isn't as safe as "Y" cables. – user39664 Jul 18 '16 at 13:03
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The amps aren't going to fry the second Pi (maybe you're thinking volts?). Anyway, 5V at the input is appropriate, so that's no worry.

The trouble might be the opposite, that your primary power supply won't provide enough amps for both Pis. Benchmarks seem to indicate a 360mA max draw for the Pi 2 and 750mA for the Pi 3. That's between 720mA–1.5A for both, but if you have any USB devices attached, that could go up quickly. If so, you might want a beefier power supply (maybe 5A for both).

As for power delivery, probably easiest to split at the power plug instead of trying to daisy chain, if that will fit in your enclosure. A Y cable should work fine for that.

  • Well, at this current moment in time, the server cabinet is in the design phase. So I have LOTS of room to make any kind of adjustments needed. So pretend I screwed up and bought a 5A power supply, you don't think it will hurt the 1st PI2 OR EVEN the PI3... I would hate to fry something and have spent the money I did and simply broke it. As yall can tell, I am quite new to the Raspberry Pi, and I have no idea what I'm doing as far as electronics electrical systems. – Pi_Guy Mar 1 '16 at 21:36
  • No. Too-high voltage would fry something. As long as your power supply puts out 5V, you're good. More amps just means you have a larger reservoir of power to draw from, when you need it. – bobtiki Mar 2 '16 at 6:00
  • Aaaahhh... I see. Well, I've been doing some research, and I have found a 4 plug USB household outlet (wired into the wall usually). I think I am just going to wire it up in a circuit to a regular household plug (for things like the switch and router) using 12-2 romex wire and then wiring the romex to a male plug to plug into the wall...this would make my cabinet portable in case I move to a new house. – Pi_Guy Mar 3 '16 at 5:44
  • 12-2 romex wire is WAY more than is necessary. Save it for a hot water heater or stove! ;+D An ordinary 18-2 extension cord will easily handle the AC supply, be much cheaper than romex, but most of all will be nice and flexible compared to working with a rigid, stubborn, unruly length of heavy-duty romex! – user39664 Jul 18 '16 at 13:29
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If you have a clean 2A power supply, you could use max_usb_current=1 in config.txt to enable 1.2A power output from the USB ports. Then, you can power your second Raspberry Pi from the first using a standard USB A to micro USB cable. This is assuming both Raspberry Pi devices do not have any extra USB peripherals connected, and can get by with ~800mA.

I'd still recommend using a multi-port power supply, instead of daisy chaining, so as to not cause a power bottleneck.

  • Is there any other way to get 2.5A out of the USB ports? For the web server and the email server, I want to have available the MAX possible power for surges of activity. – Pi_Guy Mar 1 '16 at 21:32

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