How to legally use USB as a power supply (without a chip)

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Hi - I'm working on a fairly simple PCB that will use USB for power. Why USB? I want to be able to hand this out to people and have them be able to power it easily. Without batteries.

Now - is there any way to draw half an amp off of a USB power supply without having a USB device controller on there? I don't plan on having any sort of smart IC on this board. Is it best to just connect my load to +5V and GND and pray?

Thanks!

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The only legal way to draw >100mA @ 5V out of USB is to either suck it out of multiple ports, or to use a USB controller. Some laptops and desktops will supply much more than the standard's 100mA to unconfigured devices, but I wouldn't rely on it.

- Dean :twisted:

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I cooked a USB port on my desktop with a dongle I was working on when the supply shorted. (through stupidity, I then also blew it's mate). Although the Micrel controller was supposed to limit the current safely at 250 mA/channel, it did not!
This has come up for discussion before and the consensus is that unproven USB dongles be run from a hub which is separately powered. This is really important in notebooks, as they have a limited number of USB ports and repairs will be expensive/impossible. This is even more important if potential users don't understand electricity.

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Lee -.-
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Last Edited: Wed. Dec 8, 2010 - 07:35 PM
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nleahcim wrote:
is there any way to draw half an amp off of a USB power supply without having a USB device controller on there?
Yes per the USB Battery Charging (BC) Spec:
http://www.usb.org/developers/devclass_docs
An example of a hub with BC:
http://www.goodway.com.tw/en/product3.asp?listid=1&id=4&subid=7&pid=822&typeid=

"Dare to be naïve." - Buckminster Fuller

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Can't use the BC specs with an uninitialized port, so to draw >100mA you need a controller.

If a PD is attached to a USB host or hub, then the USB 2.0 specification requires that after connecting, a PD
must draw less than:
• 2.5 mA average if the bus is suspended
• 100 mA if bus is not suspended and not configured
• 500 mA if bus is not suspended and configured for 500 mA

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Dedicated Charging Ports do not enumerate.

"Dare to be naïve." - Buckminster Fuller

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Nah, the hub does the hard work there - and - there is a separate power adapter. I don't think the OP wants to send one with every one of his fobs.