USB hubs: electrical power

[eccerr0r]

Just wondering what you guys thought.

I think a lot of cheap USB hubs will "backpower" the uplink port along with the downstream ports. This is technically dangerous if the PSU is not current limited. A lot of cheap hubs aren't.

Then there are the hubs that have a diode that prevents back feeding. Well, this means when using host power, you just lost 0.3 to 0.7 volts times current draw of power (I would hope they would put a Schottky there at least). This is also not very pleasant.

Anyone seen hubs that have a MOSFET there to power downstream ports when the hub isn't used in powered mode? Perhaps all the "high quality" hubs are done this way?

I've only seen cheap crappy hubs so far, just wondering what people have been seeing. And yes it's related to the OTG issue I alluded to in another thread, I suspect somehow it's "good" to backpower the host port so it could charge the host as well as supply power to downstream devices? Or not, because OTG hosts could also be supplying power? Or by definition they should never supply power so you must use a powered hub?

So many different variations...

[reaper57]

Re: USB hubs: electrical power

I don't think that the manufacturers care a lot, as the usb standard has a lot of protection. I have shorted plenty of usb ports and the worse that has happened is windows giving a message about a usb problem. Usb ports are meant to have things plugged in and out of them all the time, so they have better short, over current and over load protection than most of the other common interfaces in a pc.
Even if something like back feeding occurs the usb port should shut down before it damages the psu. Of course it's not a good idea to be testing the protection of USBs, but there shouldn't be any serious problem regarding back feeding.
P.S. Can you put a link to the thread about the OTG problem?

[stj]

Re: USB hubs: electrical power

there is no protection on usb on motherboards other than a thermal fuse.
only laptops monitor and switch the power on them.

[reaper57]

Re: USB hubs: electrical power

there is no protection on usb on motherboards other than a thermal fuse.
only laptops monitor and switch the power on them.

Umm, I'm pretty sure I've had some of mine shut down on me a few times. Some desktop motherboards even have the option to keep the USBs power after the pc is shut down.

[stj]

Re: USB hubs: electrical power

that's different.

the port will lose power if you short it because the polyfuse is burning hot on the pcb, when it cools down the power will come back.

as for the "option" it's just a jumper that links the usb power to the main 5v or the standby 5v from the psu.
the only thing between the usb socket and psu is a polyfuse and that jumper.

[cheapie]

Re: USB hubs: electrical power

Still, polyfuses work both ways. If a dangerous amount of power gets "back-fed", it'll still '"trip" it.

[stj]

Re: USB hubs: electrical power

if it's a pc.

i was reading just earlier that this problem with backpowering from hubs has damaged some banana-pi computer boards!
it damages the power controller or a diode (D5)

[reaper57]

Re: USB hubs: electrical power

that's different.

the port will lose power if you short it because the polyfuse is burning hot on the pcb, when it cools down the power will come back.

as for the "option" it's just a jumper that links the usb power to the main 5v or the standby 5v from the psu.
the only thing between the usb socket and psu is a polyfuse and that jumper.

Thank you for the information on the polyfuse. I wasn't sure what way they are using to power the USBs down, but now I know.

[stj]

Re: USB hubs: electrical power

they look like a yellow ceramic disc capacitor or a green surface-mount resistor.
(usually green)

[reaper57]

Re: USB hubs: electrical power

they look like a yellow ceramic disc capacitor or a green surface-mount resistor.
(usually green)

I think I know what you are talking about. I've always thought that they were some sort of a resistor or a fuse, but I've never bothered to research them.

[goontron]

Re: USB hubs: electrical power

you know i had a cheap hub backpower the +5v rail on my old computer that caused it to refuse to shutdown. it was odd. i came home after my trip to Def-Con and found that it had been backfeeding for a whole week! on the other hand all my phones where charged.

[eccerr0r]

Re: USB hubs: electrical power

Yes the USB OTG problem is because it's not clear which way the power should be going. It's one thing in "device" mode that the USB OTG port is used to power the device, as well as charge the battery. However, when using the device in "host" mode, now the internal battery needs to boost the voltage to 5V to power devices attached to the tablet/phone...

Can't have it both ways at once... (or can you?) - best of both world is that there's a "virtual wire" that connects the two voltages together. Maybe this has been overcome somehow and now there's such thing as a "wire" that can safely connect two differing voltages together?

I was looking at two USB hubs I have here. One will definitely backpower if it's hooked up as a powered hub. Other seems not, I tried supplying power to it and don't see the power coming out of the uplink port...