Getting power from USB

peterz

AFAIK the USB bus may provide 100mA until the device is acknowledged by the host. From that moment on you can get 500mA provided·you have chosen the right socket.·Lets say I·am using·a regular PC USB socket.

For a device that wants only to·be powered·by·those 500mA, how can I get it without·managing USB at all ? Is it possible ?·Can I act to the host like an actual USB device without being one ?

·

Javalin

I think you can just pull 500ma anyway. Ideally of course you should negotiate - but think of all the usb-fans, usb-coffee cup heaters, etc available - I bet they don't have more than a +/- connection!


http://www.maxim-ic.com/appnotes.cfm/an_pk/3241

the bottom bit is interesting.

J

Post Edited (Javalin) : 7/3/2008 10:43:43 AM GMT

LoopyByteloose

Frankly, I personally feel it would be better stay away from having USB provide +5 volts to projects as the amount is so limited and when you add other USB devices, the load may brown out. Browning out during programing of chips may just become a big headache.

One can create a dump plug to just take 5volts from the USB, just don't use the connections other than +5 and ground. If you salvage a plug, you might notice that the wires are very thin gauge though. People are doing funny things with this 5 volts. I recently purchased a very sexy pair of audio computer speakers and they provide a USB plug to power the audio in lieu of the usual wall wart. But it this case, the audio performed better with the also provided 8 volt wall wart than the cute 5 volt option.

If you really want to pull power for projects from you computer, just put a splitter on the floppy drive/hard drive power plugs and extend it out of the case. You will avoid interfering with the USB ports that you depend on for communications and providing reliable programing data while having the added advantage of a +12volt source along with the 5volt. If you really want, you might access the 3.3volt as well, but I have doubts that one should really mess with that on a PC. It has limited power and is intended for specific uses, such as a standby mode. Browning it out may cause trouble.

At present I have ten USB ports available on my PC. 4 USB2.0 on an additional card, 3 USB1.0 on the original motherboard, and another 3 USB2.0 [noparse][[/noparse]4 minus 1] from a 4 way extention. If I were to suddenly rely on all these for main power to project boards, nothing on USB would work well.

On the other hand, I have an 8 volt gel cell with 9Ah that I can easily drop with a regulator for hobby work and I don't have to have the computer turned on for long trial runs. Why have a 300 watt computer turned on for a 0.5watt project?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
PLEASE CONSIDER the following:

Do you want a quickly operational black box solution or the knowledge included therein?······

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 7/3/2008 7:20:01 PM GMT