doveman
Posts: 174
Joined: Wed Dec 07, 2011 11:52 am

UBEC to hub as well as PI?

Fri Dec 07, 2012 12:17 am

I've got a UBEC I'm going to take to the GPIO pins to power the PI.

Do you think it would be a good idea to feed both the RPI and the USB hub from the UBEC output (rated [email protected]) to give clean power to both or could that cause problems and would it be best to just feed the RPI from the UBEC and feed the hub direct from the PSU?

Dilligaf
Posts: 283
Joined: Wed May 23, 2012 6:48 pm

Re: UBEC to hub as well as PI?

Fri Dec 07, 2012 1:38 am

It should be fine depending on what's plugged in to the hub and what's powering the ubec. Most ubecs are rated at 3A continuous, 5A peak, in order to supply the 5A peak it would require 25 watts (5v x 5A = 25w) if supplying 12 volts in to the ubec and taking efficiency into account you would need around 2.5A (12V x 2.5A = 30W minus 15% =25.5watts) To supply just the 3A continuous it would require 12V 1.5A input. A 3A ubec should be capable of powering 5 full power devices after powering the PI so a PI and a 4 port hub should be fine.

doveman
Posts: 174
Joined: Wed Dec 07, 2011 11:52 am

Re: UBEC to hub as well as PI?

Fri Dec 07, 2012 2:16 am

Hi Dilligaf

I actually posted about this on the Raspbmc forum here as well but feel free to ignore that and continue here if you prefer.

My UBEC is indeed a 3A one and I'll be using a 5v/4A PSU which came with the hub.

Can I can use GPIO pin 2 OR 4 for +5v and 6 for GND? The UBEC has the two lines going into adjacent holes on a header, so if I can use pins 4 and 6 I can just plug it in without modifying.

The only thing I'm plugging into the Pi is a 16GB USB stick (and SD card) with all the other devices plugged into a powered hub (7-port but three are on the inside of the case and it's likely only one of those will be usable due to the case layout) , which will be connected to the other port via the data lines only. Would a 2A fuse be good between the UBEC and the GPIO pins or should I use something lower?

Would this be a suitable fuseholder? I could get something like this or this instead but they look like it would be fiddly to replace the fuse.

For the fuse, would these quickblow ones (available in various ratings) be suitable or would this (only available in 1A) be better?

I'll probably have a dual DVB-T tuner, RF remote receiver, WiFi dongle, Wireless keyboard/mouse receiver and occasionally another USB stick plugged into the hub and possibly in the future a USB HDD, so assuming the Pi takes 1A max (I know it's probably less but I'm being generous), that would leave 2A from the UBEC or 3A direct from the PSU. So with all those devices I'm not sure whether the possible benefits of feeding the Hub from the UBEC outweigh the reduced ampage.

Dilligaf
Posts: 283
Joined: Wed May 23, 2012 6:48 pm

Re: UBEC to hub as well as PI?

Fri Dec 07, 2012 3:04 am

2A fuse is fine, powering from pin 4 to 6 is fine. If you're using a 5V 4A power supply then the ubec is pointless as you've already got 5 volts. The ubec is a step down transformer if you start out with 5V there's nothing to step down. With a 7 port hub try just plugging one of the pi usb outputs to the hub input and see it it works, if it does then you're done, if not then also plug one of the hub outputs to the PI micro usb input and see how that works

doveman
Posts: 174
Joined: Wed Dec 07, 2011 11:52 am

Re: UBEC to hub as well as PI?

Fri Dec 07, 2012 4:10 am

Thanks. I was led to believe the UBEC would smooth out the quality of the power supply to avoid any issues from that. I've bought the thing now anyway, so I might as well use it and it's got a nice header on it, just right for the GPIO ;)

I have been powering the Pi from the hub into one of the USB output ports but had some issues which may be power-related. I could take another lead from the hub to the micro usb as well but because of my case layout the hub and the Pi's micro usb are very close so the cable would only need to be about an inch long. I'm already cutting the USB A end off the lead to the hub (mini USB B) so that I can shorten the lead and solder the wires to a USB A plug I've bought, as there wasn't enough room for the existing plug (it was mostly the molding that made it too long but the actual plug and metal surround were a lot bigger than the ones I've bought now as well and the whole assembly disintegrated in my attempts to shorten it!)

doveman
Posts: 174
Joined: Wed Dec 07, 2011 11:52 am

Re: UBEC to hub as well as PI?

Tue Dec 11, 2012 2:06 am

Hmm, seems I was misled about the UBEC as I've just tested it and it's only putting out 4.54v, which is obviously no good for the Pi :roll:

Return to “HATs and other add-ons”