Current drawn from an USB port

Is it true that 500mA can be drawn from a single USB 2.0 port?

A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB 3.0. A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0, which was raised to 6 (900 mA) in USB 3.0.

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Taken from Wikipedia.

Thanks.

EngIntoHW said:

Is it true that 500mA can be drawn from a single USB 2.0 port?

Taken from Wikipedia.

Thanks.

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This can be a yes and no thing as to a direct answer. I would suggest you **broken link removed** to better understand what current is actually available at 5 volts from a USB port with a focus on USB 2.0. You also may want to read the USB specification. Get familiar with inrush current when connecting things to a USB port.

It is very easy to damage a USB port therefore I suggest you be real careful plugging devices into a USB port. In the case of a desktop system there is plenty of 5 volt power available from inside the case where you don't risk damage to a USB port.

Ron

Thank you very much Ron.

Just to add some more useful info:

An external USB HDD requires about 500mA which can't be supplied through the front end USB connectors connected via ribbon cable to the mainboard.

To provide sufficient power I recommend using a USB-hub containing its own +5V output power supply.

Boncuk

the basic rules of USB are;
* The device must use more than 100ma during enumaration
* The device must not use more than 500ma from the USB Portm at any time
* Any device that uses more than 100ma must have its power from a powered USB hub

3lectrokid said:

the basic rules of USB are;
* The device must use less than 100ma during enumaration
* The device must not use more than 500ma from the USB Port
* Any device that uses more than 100ma must have its power from a powered USB hub

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Did I say anything deviating from those rules?

Thanks for the help guys.