powerpower supplyusb hub
I bought a cheap Hub for powering my 5 arduino(s). It has an AC wall power adapter. Can i determine if it actually powers my USB devices and what amount of current per USB port can deliver?
The USB hub has one LED, that lights when i power it with the AC adapter, and until now everything works ok.
In the control Panel, in the USB ports i can't find any useful information. I can see the Hub as port 4 hub 3, but says power required "unknown".
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.
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.
Best Answer
The simplest way to know if your hub is providing power to the ports or not is to just plug something into it while it's not connected to the computer. If it gets power then the only place it can possibly be getting it from is the hub's power supply.
How much current it can deliver depends very much on how they have designed the hub. Many cheap hubs just have a direct connection between the power supply and the power pins of the ports. The only limit then is what the power supply can provide (less what the hub itself needs to operate).
Better ones include power control and over-current sensing, etc. In these cases, if they are to adhere to the USB 2.0 specs, then 500mA is the minimum current they should be able to provide whilst self-powered, or 100mA when bus powered. Any more than that and, at the hub's discretion, the power may be shut off.
Of course, the hub is free to provide considerably more current, in which case it can be classed as a CDP - Charging Downstream Port - which can be used to charge high capacity battery systems like mobile phones, etc, where they request (or expect) much higher currents than the USB 2.0 spec allows for a standard port.
So the only real way of knowing just what your hub can do is to pull it apart and see what kind of power circuitry is connected to the power pins on the ports, and look at the datasheets for any chips involved.