Maximum Amp and voltage for PSU ?

7 posts
by alexcamp » Wed Oct 10, 2012 8:58 pm
I'm working on a custom case with an integrated external powered USB hub.
I'd like to use a single power supply to power both the Pi and the USB hub.

I can read anywhere what is the minimum requirements. Fine. I want to know what is the MAX Voltage/amp it will support.

For example, my powered USB hub comes with a 2.5A/5V power supply. I am looking at getting a power supply that will allow enough juice to both the pi and the usb hub.

Anyone good at this wants to dig in ? Any help will be greatly appreciated.

Posts: 7
Joined: Wed Oct 10, 2012 12:32 am
by alexcamp » Wed Oct 10, 2012 9:05 pm
ok, with some googling I found out the equation is quite simple, simply add the AMP requirements of both devices to get the required PSU.

So to power
USB hub 2.5A / 5V
RPi 1A / 5V

I would need a 3.5A / 5V PSU. Is this right?

Can I go for a 4 or 5A psu or will it cause problems?
Posts: 7
Joined: Wed Oct 10, 2012 12:32 am
by Wheel_nut » Wed Oct 10, 2012 9:09 pm
Like everything, it depends .......

....on what you are going to plug into the Hub and Pi USB Ports. If you are going to attach Hard Drives and other power hungry devices then you may push the limits but otherwise, your 2.5A Power supply should be enough to power the whole rig.

Could you post a linky to the source of your Hub and PSU ....

In reply to your second post ...

NO, an un-loaded Hub only uses a few milliAmps. You need to add up the current drawn by all devices plugged into ALL ports.
Posts: 136
Joined: Tue Sep 18, 2012 7:51 pm
by alexcamp » Wed Oct 10, 2012 9:22 pm
A USB Hard drive is among the things I had in mind, but those basically come with their own PSU.

Aside from that, USB keys, USB wifi adapter, wireless keyboard/mouse etc.
I also did not consider powering the Pi from the Hub itself at first, but it makes sense after all.

I just want to build it so I don't run out of milliamps.
Posts: 7
Joined: Wed Oct 10, 2012 12:32 am
by W. H. Heydt » Thu Oct 11, 2012 6:17 am
You don't need to worry about having a power supply capable of providing more current than everything wants. Devices will only draw the current they need. You *do* need to worry about overvoltage, though. The power supply should provide (under load!) between 4.75v and 5.25v at the board connector. Ideally, it should be a smooth, solid 5v. (Internal resistance in the power supply will cause the output voltage under load to drop, unless it's a well designed--and built--power supply operating with its current limits.)
Posts: 6711
Joined: Fri Mar 09, 2012 7:36 pm
Location: Vallejo, CA (US)
by penguintutor » Thu Oct 11, 2012 9:35 am
Supply voltage needs to be 5v (+/- 0.25v). You do not normally need to worry about a maximum current from a power supply as the equipment will only draw what is required, unless there is a fault. Assuming the Raspberry Pi is supplied power through it's micro-USB port then it is protected by a 1.1A fuse.

The Pi itself normally runs at less than 500mA, it's the external devices that push it above that and make the requirement for 700mA -> 1A. If the USB devices are being put into the hub then they should be included in the power to the hub and do not need to be counted twice.

If you are supplying power direct to the GPIO then you should consider including protection and if you are back-feeding from a USB hub (revision 2 boards only) the official guidance is to make sure that the HUB does not supply more than 2.5A under fault conditions.

As hubs do not provide that kind of information if your hub does back-feed the best way is to stick with a power supply to the HUB that is limited to 2.5A. Note that back-feeding is not an official USB feature so technically any HUB that back-feeds is actually breaking the standards.

Unless you are connecting up multiple USB-powered hard disk drives I can't see you needing more than the 2.5A power supply that came with your HUB.
User avatar
Posts: 356
Joined: Tue May 08, 2012 9:11 am
Location: UK
by alexcamp » Thu Oct 11, 2012 5:44 pm
Thank you all, for your detailed answers. It is much appreciated.

Posts: 7
Joined: Wed Oct 10, 2012 12:32 am