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Gabriel
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Location: France

Maximum intensity for the power supply ?

Sun Aug 19, 2012 10:42 am

Hello,
I'm making an enclosure box for my pi + a powered hub (4 usb).
I would like to use only one power supply for these two boards.
The pi needs about 1000mA .
The hub needs about 700mA (it is the maximum power supply given by the power supply of the hub).
Can I use a 1700 - 2000 mA power supply (5V of course) to do that ? (the hub and the pi would be plugged in parallel to share the intensity).
I'm asking this question because I'm a newbie in electricity and because Premier Farnell suggests a power supply that would be between 700 and 1200mA (so less than 1700mA).
I could find information on the wiki here :
http://elinux.org/Rpi_Hardware#Power
and here : http://elinux.org/RPi_Hardware_Basic_Setup#Power_Supply
But there is no mention of maximum intensity.
edit : here is the hub I bought http://cgi.ebay.fr/Hub-USB-2-4-ports-ha ... 526wt_1161
My home-made case : http://goo.gl/nprcI ; My presentation http://goo.gl/5hSar et en français : http://goo.gl/28Vif ; Please think to the FAQ and to the Wiki

itimpi
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Re: Maximum intensity for the power supply ?

Sun Aug 19, 2012 11:47 am

As long as the voltage is correct the power supply can be rated for any maximum current. The Pi will never actually draw more than it needs.

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Gabriel
Posts: 73
Joined: Wed Aug 10, 2011 5:43 pm
Location: France

Re: Maximum intensity for the power supply ?

Mon Aug 20, 2012 11:39 am

Thank you for you answer :) I will test the hub and write here the result.
My home-made case : http://goo.gl/nprcI ; My presentation http://goo.gl/5hSar et en français : http://goo.gl/28Vif ; Please think to the FAQ and to the Wiki

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mahjongg
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Re: Maximum intensity for the power supply ?

Tue Aug 21, 2012 1:56 am

The PI power needs are rated about 700mA, but that is including 200mA for the USB peripherals, if you do not feed the USB peripherals with the PI, then the actual power consumption of the PI would probably be only slightly more than 500mA at most.

Also the PI is fused with a fuse that -might- trip (in worst case situation, that is when its hot) when the current exceeds 750mA.

by the way, "maximum intensity" isn't a suitable term, (which explains why nobody is using it) I'm having trouble understanding what you meant by it.

obcd
Posts: 917
Joined: Sun Jul 29, 2012 9:06 pm

Re: Maximum intensity for the power supply ?

Tue Aug 21, 2012 10:10 am

The usb specification says that an usb port should be able to deliver 500mA to a device.
In case of a 4 port bus powered hub, this current is split into 5 pieces.
Every port is limited to 100mA and the fifth 100mA is used for the hub chip itself.
If you power the hub the hub with an external supply, it should be able again to deliver 500mA on each of it's downstream ports. So, basically, a 4 port hub should have a 2000mA supply, considering the hub chip can get it's power from the upstream usb port.
Most hub's however will have a power adapter with a lower maximum current delivery, simply because they are cheaper and smaller. They assume you will never connect 4 devices to it using the full 500mA.
Note that the Pi consumes a little more than 500mA. So, if you use the hub supply to power the Pi and the hub, you have less available current for the hub downstream ports.
In theory, as long as the current needed for the Pi and the hub stay below the current indication of the supply adapter, everything should work. Low cost supplies however have a bad habbit of not delivering the full 5V under heavy load. And the Pi is having a bad habbit of not running very reliable below 4.85V. I know saying this will upset people claiming they have a Pi working on 4.5V or even 4.4V. So, I will focus again on the word reliable. It might work at lower voltages, but there is no guarantee it will.
Another good thing to know is that altough some supplies are rated to be able to deliver 2000mA, they go on their knees at a much lower current.

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Gabriel
Posts: 73
Joined: Wed Aug 10, 2011 5:43 pm
Location: France

Re: Maximum intensity for the power supply ?

Sat Aug 25, 2012 9:37 pm

Thank you for these informations :)
My home-made case : http://goo.gl/nprcI ; My presentation http://goo.gl/5hSar et en français : http://goo.gl/28Vif ; Please think to the FAQ and to the Wiki

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