USB port power supply doesn't match RPi spec

[kevinchen]

Hi all,

I'm looking at buying the 5 V / 1000 mA USB port power supply to use with my Raspberry Pi: https://www.adafruit.com/products/501

The Raspberry Pi documentation says that the Model B draws 1200 mA at its peak. So the Adafruit power supply won't provide enough current and actually is not compatible with all models of Raspberry Pi. Am I correct in making this conclusion?

Kevin

[adafruit]

The power adapter can actually provide up to 1.5A without problems but we've never seen 1.2A - its supposed to be about 0.75 A as a USB port usually only provides 0.5A - can you point to official documentation with this info?

[kevinchen]

The Raspberry Pi website says 700 mA: http://www.raspberrypi.org/quick-start-guide

and Newark/element14's quick start guide agrees with 700 mA: http://www.farnell.com/datasheets/1524403.pdf

But the Raspberry Pis that I got from Newark/element14 came with a slip of paper that says 700 mA to 1200 mA. I will try and remember to scan it when I get home.

[adafruit_support_bill]

I see that. It says:

This product should only be connected to an external power supply rated at 5v dc, and a maximum current of 500-700mA for model A and 700-1200mA for model B.

It doesn't say that the board will pull 1200mA. In fact, it would be odd for them to specify 700mA as a lower range if the board is expected to pull more.

[davidh]

So from an engineer standpoint what how much amps and volts would a powered usb need to output to accommodate the Raspberry Pi?

Cheers
David

[adafruit_support_bill]

The USB spec is 500mA. Most computer USB ports will shut-down a port if it tries to pull much more than that. In my limited experience with the Raspberry Pi, the board itself seems to run just fine on a standard USB port. Where you might need the extra mA would be when you start powering other peripherals and circuitry from the RPi.

[davidh]

Ok, so if a separate power supply is used to power the Raspberry Pi and the the powered usb just supplied power for keyboard, wifi dongle and bluetooth dongle, then I am guessing that I should be ok?

Cheers
David

[adafruit_support_bill]

That should do it.

[kevinchen]

I see that. It says:

This product should only be connected to an external power supply rated at 5v dc, and a maximum current of 500-700mA for model A and 700-1200mA for model B.

It doesn't say that the board will pull 1200mA. In fact, it would be odd for them to specify 700mA as a lower range if the board is expected to pull more.

In case others want to reference in the future, here is the document we are referring to (Regulatory Compliance and Safety Information sheet that ships with the Raspberry Pi).

View original: http://cl.ly/image/0e1O1E3k1N3m

[adafruit]

Either way, the 1A adapter can easily supply 1200mA - you should not have any problems

[minerva]

On the RPi there is a 750 mA PPTC (polyfuse) in series with the microUSB power port. On the schematic this (F3) is labelled with a value of 1.1A but on my board it's 750 mA.

There's no need for a power supply with a current rating in excess of 750 mA, because if the RPi was actually drawing that much current it would trip the PPTC and wouldn't work anyway. There's no way that it can actually be drawing 1.2A through that port if you're using unmodified hardware. (Assuming you're powering it via the microUSB port and not injecting power via the +5V pin on the GPIO header.)

However, if you want to add extra peripheral devices onto the USB ports, and you don't want the extra complexity, cost, size and cable mess of adding a powered USB hub just to run two USB devices, you might remove and upgrade the PPTCs (especially F1 and F2, the two in series with the two USB host sockets' 5V rails, since they're only rated at 140mA each and therefore you can't use any USB device that draws more than 140mA) and if you replace them, and replace the 750mA PPTC on the 5V input with a higher-current one then it does make sense to use a power supply with a higher current capacity.

[adafruit]

we now have a 2 Amp powered hub in the shop, it works great and can power extra stuff if you need more current
http://www.adafruit.com/products/961

[davidh]

Thanks,

This will make life much easier now

Cheers
David

[davidh]

Will a 5V 2A (2000mA) power supply work with a Pi?

[adafruit_support_bill]

Will a 5V 2A (2000mA) power supply work with a Pi?

Yes, it will.