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I am looking to get a new power supply for my Raspberry Pi 2 Model B (I am currently using a 1A supply). Since my setup has multiple elements which require USB power I am looking at USB power strips.

Most of these strips have 1A and/or 2.4A ports. I have seen several times on this site people recommend 2A chargers, however the FAQ recommends 1.8A (so far I haven't seen a single one of those).

Is 1A too little? Would a 2.4A port work? I have also seen many which have a "Smart USB" feature which claims to be able to figure out and supply what the device needs. I am particularly interested in these, would one of the "Smart" hubs work?

Note: I have seen this post, but it is asking about 2 chargers in particular. Its somewhat contradictory answers have also just served to confuse me further.

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It's likely that 1A will be too little. As you may know, the USB 2.0 standard limits output to 0.5A. The way USB is supposed to work is that each device asks the host for the amount of power it needs and the host is responsible for either granting or denying those requests based on the available power.

However, there are many devices which simply don't actually follow the standard. Paradoxically, this means that well-made and well-engineered USB hubs that actually do follow the standard will not be able to supply enough current. Devices that don't follow the standard may or may not. There's no way to tell because ... they don't follow any standard.

With that said, many dumb "wall wart" style chargers omit the active host hardware and solely supply the +5V for charging or powering phones or other devices. They don't actually do the negotiation phase and simply put out power. The way voltage regulators in such devices are typically designed, they will put out a steady and reliable 5V until they get near their rated design capacity, usually specified in amperes (A) or milliamperes (mA). Cheaply designed and built devices will start to let the voltage sag before they get to their design capacity. Better designed devices will hold their voltage all the way to their rated capacity.

So with all of that, you're better of powering your RPi 2 with a "wall wart" style power supply that is rated for 2A (2000 mA) or more, and then only plugging the USB devices you need into the Pi. If you need a powered USB hub (as I did with my Pi Zero), I'd recommend powering it and the hub separately.

A note of caution

Normally, in a working USB connection, the power flows from the host to a peripheral device. So, for instance, your USB keyboard draws its power over the USB cable from the Raspberry Pi's USB connector (assuming they're connected directly together, as they are with my Pi2.) However, the current isn't supposed to flow the other way. That is, USB was not designed to have your keyboard power the computer it's attached to. In fact, the USB standard is pretty clear that this should be actively prevented by including a diode in the case that the external device is powered by anything other than the USB connection itself. In other words, a properly designed USB hub will not allow current to flow from itself toward an attached computer -- only the other way around is allowed. Electrically, the device that accomplishes this one-way flow is called a diode and it only allows current to flow one way.

Unfortunately, as mentioned here some USB hubs are not equipped with a diode to prevent reverse current flow. (They probably figured they could save $0.05 and so it was worthwhile.) However, that can mean that if there's a power failure for the Pi but not for the hub, the hub will "backfeed" the Pi. This isn't necessarily fatal, but the Pi is not designed to take its power from the USB port and so backfeeding it that way bypasses the voltage regulation (and voltage protection) built into the Pi. The result is that as long as the voltage is 5V, everything will be fine, but if the voltage goes too far above that, the Pi has no defense and damage may occur.

  • What do you mean by "powering it and the hub separately"? You mean I shouldn't power my peripherals from the same power strip (such as this one) as the pi? – PGmath Dec 21 '15 at 1:33
  • No, it's perfectly fine to power them both from the same outlet (120V in your case). I meant that you shouldn't try to power both the hub and the pi from the same "wall wart" power supply. – Edward Dec 21 '15 at 1:39
  • Oh, right. So you think the one I linked would work? – PGmath Dec 21 '15 at 1:54
  • Yes, I suspect it would if the device actually meets the stated specifications. – Edward Dec 21 '15 at 1:57
  • The keyboard actually has two USB plugs, one for power and one wireless dongle plugged into the Pi. Its power is one of the several other things I would like to plug into the USB power hub, so the Pi won't actually be powering the keyboard. – PGmath Dec 21 '15 at 1:59
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Electronic devices only take as many amps as they need, so as long as you supply it with at least 1A (assuming you have little to no usb devices plugged in) it should be fine. The more devices you have plugged in, the more amps you need to supply it. At a certain point, it becomes easier just to use a powered usb hub, but that's just a side note. All in all, just be sure to supply 5v and at least 1A, if you notice the PWR led not staying on, you may need to give it more amps and make sure your supply gives 5v.

  • I am looking for a power hub, so I won't have anything plugged into the Pi for just power. So there shouldn't be too much demanding power from the Pi. But I have heard that giving it too much current could damage it, is that not true? – PGmath Dec 21 '15 at 0:18
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    @PGmath It shouldn't. I power one of my Pi's with a 5v 3A power supply. It's really the volts that you need to watch closely. – Patrick Cook Dec 21 '15 at 0:36
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The Pi2 has a 2 amp polyfuse so if you are powering via the microUSB socket there is no point having a higher amperage power supply.

If all you are powering is the Pi2 (i.e. no power to attached peripherals) then you would probably get away with a 0.7 amp power supply or less.

  • Would a 2.4A smart USB be able to deal with this? They are supposed to just supply what the device needs, but they're mostly meant for phones and tablets so I don't know if it would work with the Pi. – PGmath Dec 21 '15 at 14:02
  • What is a 2.4A smart USB? If you mean can you provide power from an item rated at a higher than needed amperage then the answer is yes. The Pi will draw the power it needs, no more. I provide power to the Pi via the expansion header using a 4 amp power supply. – joan Dec 21 '15 at 14:06
  • They supposedly figure out what the device needs and supply that, up to max 2.4A. – PGmath Dec 21 '15 at 14:07
  • I'm not sure. The Pi microUSB socket is not actually a true USB socket. It only has the power lines connected. The data lines would be used in a proper USB socket to negotiate the power supply requirements between the USB device (Pi) and the power supply. This negotiation won't happen so who knows what the power supply will decide. It might supply the minimum, maximum, or some randow figure in between as the amp setting. – joan Dec 21 '15 at 14:11

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