USB Hub power

This is a pretty basic question but I would like to know how a USB hub's power supply work's, that is, if you have a powered hub does each individual device still need to be plugged into a power socket.
Marina2006Asked:
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Gary CaseRetiredCommented:
Marina2006 -- the USB standard defines a limit of 500ma (1/2 Amp) at 5v for each port.  If you plug a non-powered hub into a normal USB port, then the total current draw of ALL devices on that hub cannot exceed this limit (and 2.5w is not much power to share).   A powered hub provides this power for all of the devices plugged into the hub -- so you don't have this restriction.

As for whether each device has to be plugged into a power socket -- that's simple:  If they normally require a power socket, then Yes, they still do.   If the use of external power is optional (e.g. some small USB hard drives), then you probably don't need to use the external power if the hub is powered.

As irwinpks noted, some PC's -- notably notebooks -- have USB ports that can be unreliable even at the defined power capability for USB ports;  in those cases a powered hub can often improve the overall reliability of the USB subsystem quite a bit.

My standard advice for using a USB hub:  ALWAYS use a powered hub.   Non-powered hubs are just inviting trouble with the PC's power capabilities (whether a desktop or a laptop), and are very likely to eventually result in failure of the USB controller on the PC.
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Irwin SantosComputer Integration SpecialistCommented:
" if you have a powered hub does each individual device still need to be plugged into a power socket."
depends on the device..

A powered USB hub relieves the hosting system from expending it's own power. such as a notebook...some notebooks can't handle the extra draw of power. in this case the USB powered hub would take over the burden.

If your USB device requires external power.. don't rely on the USB hub to provide that, use the devices power adapter, UNLESS the USB device is powered by the USB connection.
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Marina2006Author Commented:
I'm sorry I don't understand what you mean by "UNLESS the USB device is powered by the USB connection".
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Irwin SantosComputer Integration SpecialistCommented:
If I have a USB camera, it has no other connections other than the USB cable". in this case, the USB connection is powering the camera

if I have a USB printer, then more than likely the USB connection will NOT power this, as most alll printers have an AC connection.
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Gary CaseRetiredCommented:
... a device that's "... powered by the USB connection ..." simply means any device that uses the power available on a USB port (that 2.5 watts I discussed above -- 500ma at 5v).
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Zuhir ElgmatiApplications and Systems AdministratorCommented:
this link clear how the usb ports works :   http://computer.howstuffworks.com/usb.htm
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Gary CaseRetiredCommented:
... just to confuse the issue a bit;  there are also hubs you can get called USB Plus Power hubs; and many devices are beginning to support the USB Plus Power standard (it uses a slightly different connector).   With these devices, substantially more power can be provided -- this was designed to do exactly what you inferred in your question:  eliminate the need for external power "bricks".   These are used a good bit in the retail industry -- receipt printers, scanners, etc. are all powered by their USB-plus connections.

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Marina2006Author Commented:
Does this mean devices which support USB Plus power standard can only be used with USB Plus Power hubs or are adapters available for the connectors and are they reverse compatabile such as with USB 1 and USB 2, that is, you can use the USB 1 device  in the USB 2 port you just won't get the USB 2 speed.
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PCBONEZCommented:
USB1 and USB2 only refers to the speed. Nothing to do with the power.
USB1 is the slower.
USB1 and USB2 are generally front/back compatible but will always operate at the slowest speed.
Exceptions to the front/back (if they even exist) are rare. I haven't seen or heard of one.
However, some things won't work -well- with USB1 because it's just too slow.

USB plus is some kind of advertising thing.
Never was an official standard and so it really doesn't mean anything.
Anyone could call their device "plus" and it means whatever they want it to mean.

There are 4 wires in USB.
2 are for the signal and two are for power. (It's 5 volts DC.)

A non-powered hub takes all the power from the ports on the computer.
A powered hub has an AC adapter that plugs into the wall to provide the 5 volts.
The powered hub takes the load off the computer's USB port.
Also, a device that has it's own AC adapter (ot that runs on AC) is not using the USB port for power.

You can configure any way you want as long as you don't attach devices that use more than 500 miliamps (1/2 amp) to the computer. - That's the total of all devices to one USB port.
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I'm sure I stomped all over things that were already said.
Asker was still confused so I tried other wording.
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Marina2006Author Commented:
I seem to have confused you in the process I was merely using USB 1 and USB 2 as an analogy.
What I was really asking was can you use a device which does not support USB Plus Power standard in USB Plus Power hub.
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PCBONEZCommented:
What do you mean by USB "Plus". . USB Plus is not a standard.
It's some advertizing hype thing and it has no meaning so far as technical specs go.

You can plug any USB device into a powered hub.
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Marina2006Author Commented:
I was simply responding to the garycase comment which differs significantly to what you are saying . My original question really has been answered my last question was out of interest I don't think we need to pursue this any further.
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PCBONEZCommented:
You don't get me:
Here are two companies calling their product "USB Plus" yet the two products are completely different because the term does not apply to an Industry Standard.
http://www.digi.com/products/usb/usbplusseriesspecs.jsp
http://www.usbpluspower.org/
I'm sure there are others....
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Irwin SantosComputer Integration SpecialistCommented:
cool. thank you!
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Marina2006Author Commented:
No you don't get me.

You are obviously very knowledgeable that is not in question I really just wanted to know whether I still needed to plug the devices into a power socket if the USB hub was powered.

My other comments related to garycase's comments I really can't argue the point the argument is his.

I don't know whether you didn't read my question and all the responses in their entirety or whether I didn't express myself well but we are getting lost in far more detail than is required.

That said you have none the less provided a lot of interesting information for which I thank you.
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Gary CaseRetiredCommented:
Good morning Marina -- looks like my comment on the Plus Power devices generated  abit of discussion :-)     The answer to your question is that (a)  devices that support Plus Power can also be used with a normal cable -- they simply need to use their own Power "brick" in that case;  and (b)  "normal" devices can still be used with Plus Power hubs -- they just don't have a connector for the "plus" power part.

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