If I connect a 4, 6 or even 10 USB multiport HUB on one single USB port of my computer, how do the 4, 6, or even 10 connected devices get enough power?

AFAIK, the single port (which is the bottle neck) on the computer has to fulfill the standard of 5V and maximum of 500 mA in USB 2.0, and 900 mA in USB 3.0.

So, how can that work if I connect a charging smartphone, printer, scanner, little fan with a silly light, mouse, and so on, to the USB HUB?


I'll answer your question title: they can't, and do not.

There are some possibilities to solve this problem.

Use an external power supply. This usually is a common wall wart that powers all the downstream ports, but not the hub chip itself that is powered by the host.

Limit the maximum power, either writing it up somewhere or adding some protections like polyfuses or something like that.

Rely on the protection of the host. Modern usb ports are quite stiff so this might actually work well but you know... Building a usb ports killer device is not good.

What would happen if you overload the hub? In case one we're assuming you can't overload it, i.e. the external psu can deliver all the necessary current to each port. In case two and three protection kicks in and the downstream power is likely disconnected, saving the pc usb port.

As a side note you seem to rely pretty much on the fact that the USB standard says something (when you speak of max current). Remember one thing: there's no such thing as usb standard, there's literally thousands of manufacturers that do not care about the standard, or implement a subset of it. Having a full 500mA power port is actually quite rare in fact.

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Actually, if you connect it only though your PC's USB Port, you will destroy it.

Most of the multihub ports come with a external power supply or just with a connector for one, which delivers the current needed.

Mostly its a 9V DC Power Supply which gets converted internally to 5V.

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    \$\begingroup\$ I do not think that a decent usb hub has the thinniest chance of destroying a usb port. \$\endgroup\$ – Vladimir Cravero Aug 30 '15 at 20:12
  • \$\begingroup\$ -1 here. Virtually all USB 1.1 and USB 2.0 hubs that support external power supplies expect them to be delivering 5VDC. The more recent USB 3.0 hubs will be seen with higher voltage external power bricks, the reason being that the greater current requirements at the ports take advantage of modern switching technology to convert the higher voltage down to 5V whilst permitting smaller wires to cable up the brick. I have seen USB 3.0 hubs with 12V power and also seen the now common type of USB 3.0 "lap top dock/hub" units using bricks that deliver 19V. \$\endgroup\$ – Michael Karas Aug 31 '15 at 1:37
  • \$\begingroup\$ Personal experience there. Atleast the three hubs i had came with 9v Wall Adapters \$\endgroup\$ – Matze Strawberrymaker Aug 31 '15 at 4:56

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