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I'm looking at putting together a small stack (maybe 4 - 6) of Pis to host my currently externally hosted services, plus a router and a media server. Software setup is not a problem and basic hardware stuff I'm ok with, but I have no idea about power - volts and amps are a mystery to me.

Would something like this actually power 7 Pis? What do I need to look for in a powered usb hub to determine if it will be suitable?

TIA

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Power needs:

The Raspberry Pi runs at 5 Volts and a current of 1 Ampere.

The voltage is fixed. For the current: You can run the RPi at lower currents (especially the model A), but to make sure it runs stable, provide at least 1 Ampere. This is true, if you don't connect anything to the USB ports of the RPi. Every device you directly connect to the USB port will draw additional

Power calculations:

If you connect more than one RPi to one power supply (like a USB hub with only one power adapter), the voltage does not split up, so it would provide 5 V for every RPi. But the current would split up. You can simply sum all current needs of all the devices, you connect to one power supply. If you connect three RPis to a power supply, it needs to provide 3 Amperes. Every additional device, you connect to the RPis USB port will also sum up.

The 7-Port hub you linked, comes with a power adapter providing 15 Watts. Watts are calculated Amperes * Volts. The power adapter outputs 5 Volts, so you can calculate the available current by dividing the Power (Watts) by Volts. If you divide 15 Watts by 5 Volts, you get 3 Amperes (15 W / 5 V = 3 A). The linked hub would be sufficient to power tree RPis at maximum if you don't connect anything else to the RPi's USB ports.

Current Limit per Port:

The next thing to look for, when buying an USB hub, is how it limits the current on each port. The linked hub can provide 3 Amperes, so it seems like it is no problem to power three RPis connected to three of it's ports. But some hubs are limiting current on each port sometimes even to 0.5 Amperes. For the linked hub, this seems not to be the case. The part about iPads means, the iPad will limit the current, not the hub. If the hub is not intelligent and says "I have a lot of current for you", the iPad will draw only 0.5 Amperes (500 mA).

Power for 6 Raspberry Pis:

To power 6 Raspberry Pis, you would need a power supply that is capable to provide at least 6 Amperes. That is a lot of current. But for example, this USB charger claims to be able to deliver 8 A but it only has five ports. But as it claims to limit each port to 2.4 A, it would be possible to connect two RPis to one port. This would be possible for example with a passive USB hub.

Note on additional USB devices:

Again: If you want to connect additional devices like USB drives, keyboards etc to the USB ports of your RPis, you need their current needs to the whole sum. But as the USB ports of the A and B model are not protected that good, I would recommend to connect devices only via active USB hubs to the RPi, especially if you want to connect USB hard-drives. If you want to connect USB flash drives, they won't add much current needs. Maybe 100 mA maximum. But some of them are drawing dangerous spikes of current for a really short time if you plug them in. I had some flash drives crashing my RPi model B every time I plug them in. With an active (powered) USB hub, that does not happen. Or plug the flash drive in before powering up the pi and never unplug it again.

The Model B+ claims to have better protected USB ports and should be able to provide at least 1 Ampere to connected devices (if the RPi is provided with at least 2 Amperes). This should be enough to power a modern 2.5 inch form factor hard-drive. I did not look at the schematics of model B+, nor tried that, so I cannot verify that information.

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  • +1 I think you are correct about the 500 mA issue in this case being specific to Apple proprietary devices ("the iPad will limit the current, not the hub") -- as it turns out there is actually a link down the right side to an article with photographs of a pi attached to the hub. So it could presumably power three of them. – goldilocks Nov 3 '14 at 13:53
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Each Pi will require 1A (1000mA) of power to run. They will run at slightly less than 1A power, but you risk brown-outs (loss of power leading to a reboot). So, you need a hub that will deliver 1A for each USB port. Which means that you will need a power supply for the hub that will provide at least 7A (7000mA) to the hub. It's not clear from that product page how many amps the power supply provided will generate so I used this calculator (http://www.supercircuits.com/resources/tools/volts-watts-amps-converter) to work it out. It says that each port will be provided with 2.8A, which sounds okay.

I recommend asking the manufacturer of the hub how many amps each port will get before buying it. Anything 1A and above per port and yes, that hub would do.

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  • Any compliant hub will only deliver 100mA without a data connection, which the Pi μUSB does not supply. You cannot power a B+ through USB ports. – Milliways Nov 1 '14 at 9:42
  • @Milliways Many (perhaps, most at least WRT inexpensive consumer ones) are not compliant in this sense, nor do they limit themselves to 500 mA per, so they can be used to power a pi. I and lots of other people do this. Here is a list of powered hubs that are known to work (or not work) for this purpose. Note that being able to power a pi does not mean they will be able to power one from every port at the same time since the total amperage is very unlikely to be that high. – goldilocks Nov 1 '14 at 11:46
  • However, that hub does look to be compliant; it says iphones will only "trickly charge" because they will be limited to 500 mA. That's not enough. Further, @recantha says above "it's not clear how many amps..." but if you read down the page, you'll find "Output is 5V 3A" and "the available 3000 mA can be divided among the 7 ports" -- meaning if you use all 7 of them you'll actually get less than 500 mA. – goldilocks Nov 1 '14 at 11:52
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    What did you enter in that calculator to get a result with 2.8 A per port? The hub linked in the question has a power supply capable of providing 15 W in total. When you enter that along with 5 V, the result is 3 A. But that is the current available in total. Like goldilocks says. – Pascal Rosin Nov 3 '14 at 12:58
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As others have said, no, that one won't cut it. Qv. my comments on recantha's answer explaining why.

There are, however, USB charging stations that can do this kind of thing -- this one, for example, could run up to 5 pis.

Output: 10 watts (5 volts DC @ 2.1 amps) for each charging bay.

The "for each" there is important; most powered hubs report the total output (e.g., the one from the question has a total output of 3 amps). Note that a charging station is a distinctly different thing from a USB hub.

So if you can find a charging station with 7 ports (w/ at least 1 A each), you're set. However, you could likely buy 7 power supplies and a 7 plug power bar/surge protector for less. Then if one fails you don't have to replace the entire thing ;)

I doubt very very much you will find a powered hub that will stack up. The USB standard limits power to 500 mA; while hubs are often not regulated to limit this properly, their total output won't exceed that (500 mA per port) and is likely less.

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Probably not. A hub is not the ideal device to power a Pi (depends on model). You should power from μUSB port. You need much more detail for a definitive answer.

What you call "volts and amps" is just basic high school physics, hardly rocket science.

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    No need to put the guy down. Volts and amps may not be rocket science, but some people just don't "get it". Best to stick to facts when answering questions. – recantha Nov 1 '14 at 7:13

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