From a quick price-check on ebay,
- Passive (bus-powered) USB Hubs – ~ $2-$5
- Powered (externally) USB Hubs – ~ $15-$20
My use-case is to expand the no. of USB ports on a ARM-based SBC, and to embed the hub in the same enclosure, so I was wondering, if it is possible to just add a regulated (7805 based) wall-wart power adapter, to supply extra current to the devices.
- Is there some per-port and up-stream protection circuit required ?
- Does it make a difference if the host-port is USB1.0 / USB1.1 / USB2.0 (from the point of view of converting the hub to a powered one) ?
- Are there some gotcha's with this approach, which I might be missing ?
BTW, if someone is aware of an existing "embeddable" USB hub circuit schematic, which isn't more expensive (BOM-wise) than the ebay consumer devices, would appreciate a pointer.
Best Answer
The (externally) powered USB hub I have just connects the power supply to the appropriate pins of the USB connectors. Power from the PC is connected trough a diode, so the hub can be powered by the PC or the power supply.
USB specification says that devices should negotiate for the power with the hub or PC, but in practice, just connecting the 5V to the required pins works just as well, especially if the power supply is powerful enough for all devices. Some USB devices (battery chargers, lamps, fans etc) do not negotiate for the power, just connect the power pins to whatever is needed.