Orange
Posts: 44
Joined: Tue Jan 24, 2012 6:41 am

Re: USB Ports power boost

Wed May 09, 2012 9:39 am

Hi,

I don't have an RPi yet, but when I get one my plan is to use the 2 USB ports for a Wi-Fi dongle and a keyboard/mouse wireless receiver. Now I know that the Pi can't supply enough power to the USB ports for these devices as I believe they need around 250mA each. So my plan is to use maybe a 1.2A 5V supply and re-direct some of the power to the USB ports. I could do this by creating a couple of tiny USB extension leads and only pass through the data lines, then take the power from the supply instead of the RPi.

My knowledge is rather limited so I'm hoping some of the experts on here can let me know if this would work. I'm not sure how USB devices are detected. Do they have to draw power or is it purely done by detecting a closed connection on the data lines.

Thanks,

Scott

User avatar
TheCrazyInventor
Posts: 58
Joined: Sun Mar 04, 2012 11:39 am
Location: The Netherlands
Contact: Website

Re: USB Ports power boost

Wed May 09, 2012 10:22 am

Well, perhaps it's a better idea to just hook up a USB hub with an external power supply and use that to power your power hungy stuff. You can even power the pi off the same hub.

Make sure to get a hub that can supply sufficient power, though. I'd look for something above 1A.
"Anything that can possibly go wrong, does" -M

User avatar
RaTTuS
Posts: 10554
Joined: Tue Nov 29, 2011 11:12 am
Location: North West UK
Contact: Twitter YouTube

Re: USB Ports power boost

Wed May 09, 2012 10:36 am

^WHS - power the RPI from a HUB and use the HUB to hold / power all the USB stuff that way you have the best of both worlds
How To ask Questions :- http://www.catb.org/esr/faqs/smart-questions.html
WARNING - some parts of this post may be erroneous YMMV

1QC43qbL5FySu2Pi51vGqKqxy3UiJgukSX
Covfefe

Orange
Posts: 44
Joined: Tue Jan 24, 2012 6:41 am

Re: USB Ports power boost

Wed May 09, 2012 11:43 am

I was trying to avoid going down the powered USB hub root as I only need two USB ports so this just makes the solution more bulky (and more expensive). Also, many of the USB hubs I have looked at don't have always-on ports so only supply power when a host is connected so they would not be able to power the Pi due to chicken and egg syndrome!

I would prefer to go with my original solution unless someone tells me it won't work.

User avatar
AndrewS
Posts: 3625
Joined: Sun Apr 22, 2012 4:50 pm
Location: Cambridge, UK
Contact: Website

Re: USB Ports power boost

Wed May 09, 2012 12:10 pm

As long as it's a regulated 5V supply, I believe an easier way to supply more current to the USB ports would be to just connect directly to the +5V GPIO header?

http://elinux.org/RPi_Hardware#Power

Orange
Posts: 44
Joined: Tue Jan 24, 2012 6:41 am

Re: USB Ports power boost

Wed May 09, 2012 12:43 pm

Max power draw for model B from GPIO pins is 300mA so this wouldn't work.

User avatar
Burngate
Posts: 6284
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK Tralfamadore
Contact: Website

Re: USB Ports power boost

Wed May 09, 2012 3:26 pm

One possibility, if you're atall handy with a soldering iron, is to bypass the power from the micro-usb power input to the usb outputs, with a piece of wire. That way the Pi is protected by its own fuse as per normal, and the usb wifi and mouse/keyboard dongles take whatever they need from the psu.

User avatar
jbeale
Posts: 3641
Joined: Tue Nov 22, 2011 11:51 pm
Contact: Website

Re: USB Ports power boost

Wed May 09, 2012 3:29 pm

The R-Pi does not use current sensing to detect if a USB device is connected. So it will work to break into a USB cable and supply the +5V externally. When you do this, note

1) you need to break the red +5V wire, so the R-Pi 5V and the external 5V supply do not fight each other;

2) you need to connect the black ground wire but NOT break it (or, solder it back together), as the USB device, the R-Pi, and the external supply should all agree on ground.

User avatar
Burngate
Posts: 6284
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK Tralfamadore
Contact: Website

Re: USB Ports power boost

Wed May 09, 2012 3:31 pm

The power for the usb sockets is via two fuses: the SoC cannot tell whether or not power is being drawn. (It can, however, be told if the fuse has blown)

So what if anything is connected to those sockets is purely down to the data lines.

User avatar
jbeale
Posts: 3641
Joined: Tue Nov 22, 2011 11:51 pm
Contact: Website

Re: USB Ports power boost

Wed May 09, 2012 5:21 pm

In case you are wondering what the USB cable looks like inside, below is one example. If you want a "hacked" USB cable to still work for data, especially high-speed USB (480 Mbps) you need to maintain constant impedance for the DATA+ and DATA- wires. That means you want to maintain the foil and braided shield intact, as much as possible, while cutting the +5V line and tapping into the GND line.  Do not use the braided shield to carry current. In USB cables the shield generally does not connect to the GND pin on both ends.



Embarrassing note: when I initially took that photo and put it on my webpage at http://www.bealecorner.org/bes.....index.html I had the DATA+ and DATA- lines labelled backwards. At the time I was only using +5V and GND, so I didn't realize it. Then a bunch of people copied the photo on other pages, and to this day, if you search for photos with "USB wiring" among the first hits you will still find the original, incorrect image. Sorry about that! Always double-check your randomly crowd-sourced pinouts!

User avatar
fulburncams
Posts: 70
Joined: Wed Nov 30, 2011 8:29 pm

Re: USB Ports power boost

Wed May 09, 2012 5:58 pm

Dear Orange

I can confirm that using a small breakout cable for the USB works.  My adapter (TP-Link 722N) consumed approx 140ma which I think is near the limit on the USB fuse. I tried using Belkin hub but that did not work and as I wanted a wireless/ headless solution extra cables / hubs were not wanted anyway.  So I made a cable up and that sorted the problem.  See this from May 3, http://www.raspberrypi.org/for.....mouse-wifi

Good luck

Orange
Posts: 44
Joined: Tue Jan 24, 2012 6:41 am

Re: USB Ports power boost

Wed May 09, 2012 6:36 pm

Excellent, that's exactly what I wanted to hear. Good point about keeping ground connected. Told you my electronics experience is rather limited!

I'll give this a go when I eventually get a Pi. Thanks for your help everybody.

User avatar
jbeale
Posts: 3641
Joined: Tue Nov 22, 2011 11:51 pm
Contact: Website

Re: USB Ports power boost

Wed May 09, 2012 7:25 pm

I'm starting to suspect that the R-Pi design may spawn a market for a new, specialized cable: "USB with power injection port".  It would be similar to the existing "dual USB power cable" or "USB Y plug" used for portable hard drives that draw more than 500 mA, except those have the +5V line in parallel. The new Pi Accessory cable would need a "Data" plug without +5V, and a "Power" plug without DATA+ and DATA-.

Actually, I'm usually not the first person to think of things- does this cable already exist?

There's another one too, a microUSB power cable with a built-in power switch, since the R-Pi itself has no on/off switch.

The inverse of that exists; a USB extension cable with a switch that disconnects the USB data lines leaving just power, in case you want to charge your iPod without syncing with iTunes.  http://eshop.macsales.com/item.....BLUSBCS2M/

embedded-creations
Posts: 2
Joined: Sun Jul 14, 2013 3:58 pm

Re: USB Ports power boost

Sun Jul 14, 2013 4:23 pm

Sorry to revive a dead topic, but I'm trying to solve a similar problem, and found a cable I think might work for myself and others. This cable looks like it should inject power from a separate power supply supplied by a barrel plug connector into the 5V line of USB, leaving 5V from the host disconnected. Has anyone tried this cable?

Image
https://www.google.com/shopping/product ... 2425401580

It seems ideal other than the small barrel jack diameter, not compatible with some popular supplies from Adafruit and others.

User avatar
Lob0426
Posts: 2198
Joined: Fri Aug 05, 2011 4:30 pm
Location: Susanville CA.
Contact: Website

Re: USB Ports power boost

Mon Jul 15, 2013 12:41 am

A topic Raised from the dead.

If you are going to use such a cable with a separate power supply then you would be better off with a powered hub. You can power your devices and your RasPi from the same hub with only one PSU that way. The hub may seem like it uses up too much space but having an adapter and then an extra PSU is even more wasted space.

The other problem with using that adapter is whether it will back-power the RasPi through the USB port. It probably does not have a way to prevent that from happening. It really is not good to power a RasPi normally, and then have power also being supplied from the USB side also.
512MB version 2.0 as WordPress Server
Motorola Lapdock with Pi2B
Modded Rev 1.0 with pin headers at USB

http://rich1.dyndns.tv/
(RS)Allied ships old stock to reward its Customers for long wait!

embedded-creations
Posts: 2
Joined: Sun Jul 14, 2013 3:58 pm

Re: USB Ports power boost

Mon Jul 15, 2013 4:42 am

I believe the cable has the 5V line disconnected from the USB-A Male connector, so that should prevent back power, but I could be wrong.

I see what you're saying about the powered hub being a better solution. Even if the hub can only provide 500mA per port and the USB HDD needs more than that, a USB Y-cable could connect the HDD to two 500mA ports on the hub, without adding an extra power supply.

User avatar
Lob0426
Posts: 2198
Joined: Fri Aug 05, 2011 4:30 pm
Location: Susanville CA.
Contact: Website

Re: USB Ports power boost

Mon Jul 15, 2013 5:34 am

Overall the best way would be to power the HDD from a hub with your "Y" cable and then plug your USB A to micro B into another hub port. That is the simplest way to do it, reduce the cable mess and use only one PSU. This involves no cable cutting, wiring or soldering. Buy the shortest cables you can find and you will have an easy to use setup.

I would definitely check that that adapter does not back-power the RasPi or add an inline fuse and just back-power. That would probably work just fine with a 2A PSU. If you try that cable please report if it works with or without back-powering the RasPi. There are many people that are trying to run HDD with as little investment as possible. I am sure they would be interested.

I am assuming that you are not looking to back-power your RasPi, but here are some other ways.

I have back-powered RasPi with a USB to SATA with "Y" adapter. By plugging the Data/power cable into the RasPi, then plugging the power only cable into an iPad2 charger (back-powered). This does work but there is no over-voltage protection. I have also used an A male to A female with an Adafruit adapter to the lower USB and the HDD in the upper. A Seagate drive (single short USB cable) fired right up with that setup (back-powered). So there are a number of ways to do it, but look for the safest way when possible. Sometimes I just do not follow my own advice :?

If you go to my web site you can read about that setup and about back-powering in general if you are interested.
512MB version 2.0 as WordPress Server
Motorola Lapdock with Pi2B
Modded Rev 1.0 with pin headers at USB

http://rich1.dyndns.tv/
(RS)Allied ships old stock to reward its Customers for long wait!

User avatar
simonmcc
Posts: 181
Joined: Mon Aug 19, 2013 10:07 pm

Re: USB Ports power boost

Wed Sep 25, 2013 10:44 pm

embedded-creations wrote:Sorry to revive a dead topic, but I'm trying to solve a similar problem, and found a cable I think might work for myself and others. This cable looks like it should inject power from a separate power supply supplied by a barrel plug connector into the 5V line of USB, leaving 5V from the host disconnected. Has anyone tried this cable?

Image
https://www.google.com/shopping/product ... 2425401580

It seems ideal other than the small barrel jack diameter, not compatible with some popular supplies from Adafruit and others.
Received my cable from linitx today, exactly the same as the one in the picture. Unfortunately it does have the +5v line connected, so I stripped the cable and began to cut.

NOTE: The 5v wire is the black one (the last one I cut)
EDIT:: I ordered another one, and the colours were completely different, so it was the red one I had to cut!!!! My advice - strip a little of the insulation and buzz it out to find the correct one to cut!!!

With this simple modification I was easily able to inject 5v

Thanks for the pointer, it's a lot neater than DIY
simonmcc.blogspot.com/search/label/pi

Return to “General discussion”