nile
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Re: Power supply needed whith powered hub?

Wed Mar 07, 2012 10:27 pm

If I have a powered usb hub do I still need a power supply to the R-Pi?  Also the opposite - if I have a power supply for the R-Pi do I still need a power hub?  In other words do I need both?

Thanks

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abishur
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Re: Power supply needed whith powered hub?

Wed Mar 07, 2012 10:52 pm

Well you *should* be able to get away with using a y usb cable to provide enough power to the r-pi from the usb hub (assuming that the ports offer a max of 500 mA, if you have one that does 1A then you can use a single cable).

What about the opposite way do you need a powered hub?  Well that depends on the devices you're plugging into the pi.  The r-pi has a 140 mA fuse on the USB ports so it won't be able to draw more than 140 mA.  If you're only attaching a keyboard/mouse/wifi adapter you should be just fine.  If you're attaching more or a hard drive, you won't be able to power all the devices from the pi's usb ports.
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Re: Power supply needed whith powered hub?

Wed Mar 07, 2012 10:54 pm

Side note I sent you a PM as asked in another thread.
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nile
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Re: Power supply needed whith powered hub?

Thu Mar 08, 2012 3:42 am

Abishur - I never got an email from you.  I checked my profile and the email there is correct so could you please try again or is there some way I can contact you or the other admins privately?  I'm flexible so whatever works best for you guys.  I do think we need to communicate however.

Thanks

nile

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Re: Power supply needed whith powered hub?

Thu Mar 08, 2012 5:10 am

It was a PM, at the top right hand corner of the forum, right next to where you see "you are logged in as" you should see a red box with an envelope to the left.  If you click on that box you'll be able to see your PMs.  That said, since the forum is being upgraded to a completely different software in the next week here it might be less of an issue
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Jim Manley
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Re: Power supply needed whith powered hub?

Thu Mar 08, 2012 8:48 am

Abishur said:


The r-pi has a 140 mA fuse on the USB ports so it won't be able to draw more than 140 mA.  If you're only attaching a keyboard/mouse/wifi adapter you should be just fine.  If you're attaching more or a hard drive, you won't be able to power all the devices from the pi's usb ports.


Whoa!  I can understand why there's a limit, particularly when it comes to inductive motorized devices like a hard drive, but, this is the first I'd heard that it was only 140 ma, which violates the USB 500 ma per port spec, IIRC (i.e., each port is required to provide at least 500 ma).

Is it 140 ma total for both ports, or 140 ma per port?

Is it really fused, or is it just current-limited (e.g., self-protection shutdown, etc.)?  Can we actually destroy the USB  ports if someone does something uninformed (trying to not say "stupid", if it's a kid who doesn't know any better, or a newbie adult, for that matter)?  This would be surprising, given its intended educational environment.
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Re: Power supply needed whith powered hub?

Thu Mar 08, 2012 9:06 am

nile said:


If I have a powered usb hub do I still need a power supply to the R-Pi?  Also the opposite - if I have a power supply for the R-Pi do I still need a power hub?  In other words do I need both?

Thanks


Even if the hub is powered, simply running a USB cable from one of the RP's USB sockets to the USB input of the hub will not power the RP.

Subject to the current drawn by the other USB devices plugged into the hub and the capability of the hub's power supply, it is OK to power the RP  through a second cable from the hub to the micro-USB socket on the RP.
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Re: Power supply needed whith powered hub?

Thu Mar 08, 2012 9:39 am

Jim Manley said:


Whoa!  I can understand why there's a limit, particularly when it comes to inductive motorized devices like a hard drive, but, this is the first I'd heard that it was only 140 ma, which violates the USB 500 ma per port spec, IIRC (i.e., each port is required to provide at least 500 ma).

Is it 140 ma total for both ports, or 140 ma per port?

Is it really fused, or is it just current-limited (e.g., self-protection shutdown, etc.)?  Can we actually destroy the USB  ports if someone does something uninformed (trying to not say "stupid", if it's a kid who doesn't know any better, or a newbie adult, for that matter)?  This would be surprising, given its intended educational environment.


until we find one in the wild we don't know - they are resetable fuses [circuit breakers?]

as the RPi has a 1000mA Power supply and the Model B draws 750mA there is no way it can output 1700mA - if it could then it has other applications.

it will run keyboards / mice and other low powered stuff ...

if you want to run it with more stuff then a powered hub is what your after

if that can only provide 500mA then you'll need 2 ports chained together to power teh RPi and then the USB leads from the RPi can go into the Hub then you can run other things off the Hub no worries

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Re: Power supply needed whith powered hub?

Thu Mar 08, 2012 9:40 am

Jim Manley said:


Is it really fused, or is it just current-limited


It is both, and neither.

They use a polyswitch, which is an electronic device that works like a closed switch when it's cold, but as an open switch when it's hot. When too much current goes through it, it gets hot.

It takes a while to cool down and start conducting again, but it is basically a fuse that you don't need to change. Just go and have a cup of tea and come back later.

Although the power supplied by a USB charger is not limited by the USB spec, a USB plug has a maximum current capacity of 1.5A, and the micro one probably significantly less than that. It is moderately easy to find chargers that will supply up to 1A, but rather difficult to find one that will supply more. If the USB ports supplied up to 500mA, the RPi supply would have to supply up to 1.5A at least.

100mA is the maximum current draw from an unpowered hub. So although the RPi is out of spec as a host device, it will not confuse any devices and most of them will work OK.

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