USB voltage and amps

Discussion in 'Computing and Networks' started by Cackles, Aug 2, 2010.

  1. Cackles

    Thread Starter New Member

    Feb 7, 2008
    I always thought that USB runs at 5v with a max of 500mA. But someone has said to me that USB ports have been running at 1A for a while now. Someone else said they work for a company and the devices 'sense' how many A a USB port can kick out.

    I have a couple of issues with these replies Ive had. The first being that if modern devices running at 1A were plugged into an old motherboard with a 500mA USB port they would fry , but Im also told the motherboards have detection for this, which confuses me. Because then when you short the -/+ of a USB port the mobo wouldnt fry.

    Im just curious because Ive made up a few cables and the devices would benifit from the extra.
  2. zxsa


    Jun 11, 2010
    The specification is indeed 5V and 500mA. If you are designing a USB device which is bus-powered (ie it draws current from the computer), then stick to 5V and 500mA.

    Yes, some computers would be able to supply more current than 500mA. This is because you can have more than one USB device connected to the PC at one time (think using an external USB hub without its own external power supply).

    Yes, a decent motherboard (USB controller) design will detect the amount of current that a device is trying to draw from the USB bus and, if it is above the capability of the motherboard, the USB controller will shut off that device.

    No, this does not mean you can short circuit the terminals of the USB port! Over current faults and short circuits are two different kinds of faults. For a short circuit fault the controller must be able to react MUCH faster than for an over current fault.

    500mA is actually quite a lot of current. If your USB device requires this much current, you probably already need to consider using an external power supply for it.
  3. CoolBeer

    Active Member

    Mar 29, 2010
    This is cut 'n pasted from Wikipedia:

    Code ( (Unknown Language)):
    2. A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB
    3. 3.0. A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0,
    4. which was raised to 6 (900 mA) in USB 3.0.
    5. There are two types of devices: low-power and high-power. Low-power devices
    6. draw at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0,
    7. and 4 V in USB 3.0.
    8. High-power devices draw the maximum number of unit loads supported by the
    9. standard. All devices default as low-power but the device's software may
    10. request high-power as long as the power is available on the providing bus.
    So basically 100mA is the limit until you get permission from the controller.