USB as electric power supply

[gnasen]

Hi folks,

I have an external device which needs 12V at 4.1W (~350mA). Instead of using a battery I think about using the USB3 as power supply. This delivers 900mA on USB3 with around 5V which results in 4.5W.
Therefore I would put in the USB cable and use a transformer to get the needed voltage.

However I read that you have to tell the USB that you want to use the hight state (900mA) instead of the low state (100mA). Does anyone know how to achieve this (in Linux)? What happens if I dont tell to switch to high state?
I think the easiest thing would be to grab a power cable for an HDD, cut it off and use this. Would this work?

Im looking forward for your ideas

[IdeasVacuum]

If using a laptop, the device might make your battery drain too quickly. Most USB drives use a separate power supply, USB is for comms only. However, you have not told us much - what is the device, how much time does it need to be running?

[Tenaja]

Hi folks,

I have an external device which needs 12V at 4.1W (~350mA). Instead of using a battery I think about using the USB3 as power supply. This delivers 900mA on USB3 with around 5V which results in 4.5W.
Therefore I would put in the USB cable and use a transformer to get the needed voltage.

However I read that you have to tell the USB that you want to use the hight state (900mA) instead of the low state (100mA). Does anyone know how to achieve this (in Linux)? What happens if I dont tell to switch to high state?
I think the easiest thing would be to grab a power cable for an HDD, cut it off and use this. Would this work?

Im looking forward for your ideas

For short term this would work fine. Just remember, that stepup circuits are often only 80-90% efficient--but what is even more important is the quiescent current (the power draw with no load).

[gnasen]

For short term this would work fine. Just remember, that stepup circuits are often only 80-90% efficient--but what is even more important is the quiescent current (the power draw with no load).

The efficiency loss for a stepup circle is very interesting, thanks for that hint. The device is a camera (very expensive, no experiments!) and the running time is not that important. It should work for an hour.
Hence I still have no clue about this low/high state of the USB. Does it activate automatically on load? How to register at the USB root? Questions over questions.

[Little John]

I think your external device has to communicate with the USB host controller.
Some additional information is here (in German).
For details see http://www.usb.org/developers/docs/

[gnasen]

I think your external device has to communicate with the USB host controller.
Some additional information is here (in German).
For details see http://www.usb.org/developers/docs/

That sounds like a non-trivial task. I would need to build some kind of controller
Is there maybe an USB device which just works as a power supply? So instead of feeding an USB bus with an external power supply, just a device which reverses this? I would be surprised if something like that is not available.

[Little John]

Maybe something like this?
(There are lots of other external accumulators.)

[gnasen]

Maybe something like this?
(There are lots of other external accumulators.)

The idea was to avoid an additional external accumulator (this is how it is done at the moment).

[Little John]

The idea was to avoid an additional external accumulator

Oops, you already wrote that in your initial post. Sorry.

Maybe something like this or this?

[gnasen]

The idea was to avoid an additional external accumulator

Oops, you already wrote that in your initial post. Sorry.

Maybe something like this or this?

Yes yes! promising! I will have a closer look at this, thank you very much.