I am planning on building a quite large USB hub, which final use is to connect, charge and sync let's say 40 iPads. The iPad electrical specification from Apple states that each unit needs to have available 5v/2.1A from the USB bus power to charge fast.

I still don't know if I'm gonna build the USB hub from scratch or I'm gonna buy a few cheap hubs and bridge them together, but the main question is: if I need the devices just syncing, I need 500mA in each port, which means 0,5*40 = 20A.

Does that mean I can accomplish that by removing the +5v wire coming from the PC port (or placing a diode) and powering the same path with a massive 5v/20A power supply? Will this work as expected? Isn't there risk of "burning" the USB devices or the computer?

  • \$\begingroup\$ It is not correct to use a external power supply in usb and in general rather than 5v wire . In general all power supply in circuit must be common in ground (as a loop) . if put this external power supply you will not have a loop . \$\endgroup\$ Jun 12, 2013 at 11:16
  • \$\begingroup\$ another thing 20A is far away from maximum current that usb port could sink or source so be aware . this resource would help you a lot usbmadesimple.co.uk/ums_2.htm \$\endgroup\$ Jun 12, 2013 at 11:20
  • \$\begingroup\$ Frankly, you would be better off going with a bunch of usb hubs that are designed for external supplies (Ie, not the 2 dollar ebay hubs), and then powering them from an external supply. That way all the isolation from the pc is done for you. \$\endgroup\$
    – Passerby
    Jun 12, 2013 at 23:59

1 Answer 1


Under normal operation the USB would only draw the current required from the 5V source regardless of the potential current that is available. In the event of a short circuit on one of the devices it could draw a lot of current through it and cause a lot of damage.

A low cost option would be to add fuses and LEDs to indicate which connections have lost power.

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  • \$\begingroup\$ or use a resettable fuse and led across the fuse so it comes on if the fuse trips \$\endgroup\$ Jun 12, 2013 at 13:29

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