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A RPI USB power question

Posted: Sun Sep 16, 2012 6:27 pm
by Menace
Hi
As many other people around the web, I also have issues with the low allowed power draw from the USB ports of 100mA.
The official solution is to use a powered USB hub, which does not back feed power as this can create issues...
I don't really like this solution, since most USB hubs are kinda large compared to the RPI itself.

I have found various solutions to this problem, ranging from modifying a small standard USB hub to become a powered hub, to fiddling with the fuses in front of the USB ports them selfs and also the 750mA fuse at the micro USB power input.

I am imagining a different solution, but since I am not an expert in electronic circuitry, I wanted to get some feedback from more competent people, I.E. I'm writing here :).

I am thinking about using a 5V 2A power supply, and then powering the RPI through the GPIO pins using a 750mA fuse. And here is where it gets a little dicey... I then want to somehow disconnect the two USB port from the supply voltage on the PCB and then power them directly from the power supply through two 500mA fuses...

Now my question is this, can this be achieved at all? If yes, then is there anything I should be especially aware of?

I thank you in advance :)

Re: A RPI USB power question

Posted: Sun Sep 16, 2012 9:25 pm
by W. H. Heydt
Or you could order another Pi... If you get either a Rev. 1 ECN (0003) or Rev. 2 (0004) board, there are no polyfuses on the USB ports. You're still limited by fuse F3, though.

Re: A RPI USB power question

Posted: Sun Sep 16, 2012 9:52 pm
by mahjongg
If you have the old PI without mounting holes, just bypassing the input polyfuse F3 by entering power through the P1 pinheader (because that is what you are accomplishing) wont guarantee that >100mA devices will start to work, because its not F3 that makes a difference but F1 and F2 the "USB polyfuses". In Rev2 boards these were removed because of this issue, and to make it possible to operate the PI with "backfeed power" from the hub.

The correct approach for a rev1 model to patch the board for this is to bypass the F1 and/or F2 polyfuses, by shorting them (soldering a wire over them, or replacing them with a zero-ohm SMT component, or even to use silver paint to bridge them). You only need to do this with the polyfuse that is preventing your >100mA device from getting enough volts, the other one may left untouched.
Note that F1 is used for the top USB connector, and F2 for the bottom one.

Re: A RPI USB power question

Posted: Sun Sep 16, 2012 10:05 pm
by 0117blocky
As you are planning to use flying leads to the usb 5V connections. You could use flying leads to Tp1 and Tp2 to power the RPI it would save use of the gpio port. I wouldn't have thought the test points would be on thin tracks. If you have fuses on your 5v usb tracks you could solder leads to the usb side of each fuse providing that you are using the same power supply to power up the RPI and usb's.

Re: A RPI USB power question

Posted: Sun Sep 16, 2012 11:00 pm
by Menace
Thank you all for your answers

Just to make it clear, I don't actually have my RPI yet... It should arrive from Farnell during this week, so because of this I don't really know which rev version I will get. But odds are it will be either a later 1.0 or the 2.0... (hoping for the 2.0 of course).

It would, of cause, be nice not to use the GPIO to supply the RPI, I just wonder if there is enough room for soldering leads to Tp1 and Tp2?

I order to make my thoughts more clear, I will just list my design criteria:
1. The RPI must be protected by a 750mA fuse or lower (except for the USB ports).
2. The USB ports must both be able to provide 500mA.

I could bypass the fuses F1 and F2, but bypassing F3 leaves the RPI without any protection at all, which I feel is a bad solution...

Another thought I have is to remove F1 and F2 completely from the PCB, thereby severing the power to the USBs from the rest of the circuit board. I will then solder one end of two 500mA fuses to the tracks leading to the USBs. The other ends of the two fuses I will then solder (via a lead) to the micro USB before F3.
This will then ensure that my design criteria is fulfilled... The only issue with this, as I see it (please correct me if Im wrong), is that the micro USB port must be able to handle this higher power level. I have no idea if the micro USB port is able to cope with this power level (5V * (0.75+0.5+0.5)=8.75W)?
I realize that the RPI will not actually draw 750mA at any time (not even during boot), since the USBs will no longer be powered through F3. But there will still be a much higher load on the micro USB port, than what the PCB was originally designed for (maybe around 6W, compared to the standard 3.5W).

Any comments or advice will be greatly appreciated!!!

Re: A RPI USB power question

Posted: Sun Sep 16, 2012 11:39 pm
by 0117blocky
I would wait until you receive your RPI before thinking about any modifications, of course I do not know what you plan to connect to the usbs. The test points on my RPI which has usb fuses, look quite easy to solder wires too. The Micro usb I think is rated to at least 1amp maybe as high as 1.5amp.

Re: A RPI USB power question

Posted: Mon Sep 17, 2012 12:04 am
by Menace
Thanks for your quick reply 0117blocky

I plan to connect an external HD permanently to one of the USB ports, which needs 500mA to function.
The other port will be used for a USB DVB-T tuner, or a wifi dongle when Im on the move. Both of these devices need 500mA to work.

You are right of cause, that I should wait until I receive my RPI, before buying any components to modify it, as I don't know which rev version I will get. But as I need 500mA from both USBs, the F3 fuse will become an issue no matter which version I will receive... For this reason I just wanted to bounce some ideas of the knowledgeable RPI community :).

If the micro USB port can indeed handle 1.5A as you say, this is very good news!!! Because then I see no problem in extracting power from here (before the F3 fuse), through a couple of 500mA fuses to power the isolated USB ports...

But if anyone else have any comments, ideas or warnings... It will again be much appreciated.

Re: A RPI USB power question

Posted: Mon Sep 17, 2012 12:29 am
by Menace
I found this:

http://pinouts.ru/Slots/USB_pinout.shtml

Here they state that a USB plug is able to provide 1.8A when it works as a dedicated charger:
A simple USB charger should short the 2 data lines together. The device will then not attempt to transmit or receive data, but can draw up to 1.8A, if the supply can provide it.
They don't, however, state if this is true for all USB typer (A, mini and micro)...

Re: A RPI USB power question

Posted: Mon Sep 17, 2012 2:07 am
by 0117blocky
A revision 2 board has the 5V supply directly connected to the usb ports, looking at the photo of the revision 2 RPI. So you could uprate the 750mA fuse.
Personally I would use a power usb hub, but you have stated you don't want to do that.

Re: A RPI USB power question

Posted: Mon Sep 17, 2012 9:00 am
by Burngate
You want to power the Pi either through the micro-usb (and therefore the fuse F3), or through a separate fuse to the GPIO pins or TP1/2; and you want to supply up to two high-current devices through the usb ports, preferrably via separate fuses.

On Rev 1 boards, this should be do-able. Remove F1 and F2, and supply power directly to the pins of the usb socket, via a fuse of your choice. You can separately supply power to the Pi via its micro-usb socket, or to any other place of your choice.

Rev 2 boards are more problematic, as there aren't any fuses to remove, and the usb socket pins are tracked directly to the main 5v rail.

Re: A RPI USB power question

Posted: Mon Sep 17, 2012 9:18 pm
by Menace
Hi Burngate

So this means that it would be much easier for me to accomplish my goal, if the board I receive is a rev 1.0. Guess its good that I already ordered a while ago, which will increase my chance of getting the 1.0...

My plan, as I am imagining it now, is to power everything from the micro USB port, using a 5V 2A power supply that I found:

https://www.elextra.dk/main.aspx?page=a ... tno=H33069
(The page is in Danish, sorry).

If I receive a rev 1.0 RPI, I then plan to remove F1 and F2, while leaving F3 in place.
I will then solder two 500mA fuses between the USB ports and the micro USB port, before the F3 fuse.
This way the chips are protected by the F3 fuse and the USB ports are protected by the new 500mA fuses.
The only issue that I can see with this, is that I don't know if the micro USB port is be able to handle this much current? (Maybe 1.5 - 1.8A)

Thanks