model a usb expansion

Brandon,

  The model A, if and when it appears, is expected to lack

the controller chip, the 2nd usb port, and the ethernet jack.

If you are using a hub just to power the wifi, then you might

be able to plug that directly into one of the recent model B's,

either the recent rev 1.0 ones with 0 ohm resistors in place of

the USB polyfuses, or the rev 2.0 ones.

With the new F1 and F2 fuses in place (or not - as be the case in rev2), does that mean that the new maximum draw for the USB port is 500mA (the rating of the tracks)?

And that would mean that you could power any usb device provided that it would not draw above 500mA? (for model A)

On model B it would be 1000mA - 700mA = 300mA, the maximum current availble through the 1.1A capacitor/provided by the power supply.

Could you confirm this?

I don't think there is any hard and fast 500mA limit on the tracks.

I think both the model A and B have a 700mA polyfuse connected

to the microusb input, so you would be limited by that, unless you

use an alternate source of power, such as the GPIO pins, which

bypass the 700mA polyfuse.

I don't think there is any hard and fast 500mA limit on the tracks.

I think both the model A and B have a 700mA polyfuse connected

to the microusb input, so you would be limited by that, unless you

use an alternate source of power, such as the GPIO pins, which

bypass the 700mA polyfuse.

I think both the model A and B have a 700mA polyfuse connected

I thought that that was a 1.1A polyfuse. ??

John Beetem wrote:

...you are limited to 1A by polyfuse F3...

                              on a differnet thread. http://www.element14.com/community/thread/19819?tstart=0

I will look into powering though GPIO pins.  This sounds like a good idea for a portable application.

Fergus Byrne wrote:

 

I think both the model A and B have a 700mA polyfuse connected

I thought that that was a 1.1A polyfuse. ??

John Beetem wrote:

...you are limited to 1A by polyfuse F3...

According to the RasPi 1.0 schematic, F3 is 1.1A.  This is one of the standard sizes offered by the Littelfuse 2016L series, which I would guess is a possible vendor for F3.  OTOH, we don't know what part is actually is populated on a given board, and I don't think we've ever seen a formal bill of materials (BOM) for RasPi with approved vendors.

Assuming F3 is indeed 1.1A and the RasPi SoC + P1 consume no more than 500 mA, that leaves you 500 mA for a wireless dongle with 100 mA margin.  However, the micro USB cable and F3 both have resistance so as you draw more current through them RasPi's 5V0 voltage may drop to the point that your HDMI and/or USB device(s) become sad.

There is a lot of confusion about F3.  I think the 700mA (or in some cases

750mA) comes from the marking on the actual fuse.  Apparently 700mA

is the "hold" current, and 1.1mA is the "trip" current, where the fuse starts

to heat up somewhere between the hold and the trip.  So you wouldn't want

to use more than the hold current.

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=2&t=5068&start=22

coder27 wrote:

 

There is a lot of confusion about F3.  I think the 700mA (or in some cases

750mA) comes from the marking on the actual fuse.  Apparently 700mA

is the "hold" current, and 1.1mA is the "trip" current, where the fuse starts

to heat up somewhere between the hold and the trip.  So you wouldn't want

to use more than the hold current.

 

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=2&t=5068&start=22

Yes, this sounds right.  It would really help things if RasPi would list a BOM.