gregor3000
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USB Hub and back power

Thu Oct 17, 2013 7:59 am

I've been reading about compatible and tested USB hubs that work with RPi.

I can not find the explanation for the USb hub that offer "backpower". Is this powerback beneficial or detrimental to the use of Pi. Does it actually matter if USB plug has this option or not? as i notice a modpi website hut is selling a 8 plug USB hub where they specifically state no backpower.

for example:
D-Link DUB-4 High Speed USB 2.0 4-Port Hub DUB-H4 USB 2.0 4-Port 2.0 A Verified [8]
Charging port doesn't power Raspberry Pi

Does it matter that it doesn't charge the Pi is if has it's own PSU anyway?
i see the d-link 7 USB hub can power the pi.

and for the one from pi hut it says this:
The majority of USB hubs feed power back to the Raspberry Pi, our new hubs have been modified to stop this from happening.
does this mean that having no powerback is better for the hardware? oir why would someone disable this function?

itsmedoofer
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Re: USB Hub and back power

Thu Oct 17, 2013 9:01 am

Hi,

To a layman I would say for the majority of cases back power would be a benefit, if you have a weak main PSU it would top it up so to speak..... My hub back powers and I use it to actually power the Pi when I run out of plug sockets...

I did read about issues with fuses on the early Pi's whereby if you back powered it would pop a fuse out, maybe its because of this...

I would be interested in knowing a professional opinion, especially as I use back power quite a bit....

pjc123
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Re: USB Hub and back power

Thu Oct 17, 2013 11:04 am

The main issue with backpowering/backfeeding is that it bypasses the fuse protection of the pi. The fuse is located immediately after the power is fed through the micro usb connector, and backfeeding applies power at the other end of that circuit. Therefore you are either relying on the fact that the USB hub has some sort of protection, or you can add your own fuse in that feed path, or you can cut the power lead in that feed path and power the pi through the micro usb port, or you can avoid all this by buying a hub that does not backfeed; there are a couple of manufacturers that make these. The fuse popping issue occurs if you are backfeeding and applying power to the micro-usb at the same time, essentially feeding power to the circuitry from two different directions.
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itsmedoofer
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Re: USB Hub and back power

Thu Oct 17, 2013 11:33 am

Ahh I see, thanks makes sense.....

Found this in my lunch hour:-

http://elinux.org/RPi_Hardware

Under the Power section is a paragraph about back powering...

pjc123
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Re: USB Hub and back power

Thu Oct 17, 2013 11:47 am

As a side note, I use and can recommend the Plugable USB2-HUB-AG7 hub that does not backpower. I waited a year before I bought a powered hub because there was just not enough consensus as to one that worked well or didn't have a host of issues either in the forums, on elinux RPi Verified Peripherals, or reviews on various purchasing sites like Amazon. Then positive reviews started showing up for this hub on various sites, as well as demos, testing and recommendations by Plugable themselves, all specifically using it with the raspberry pi. It can be used to safely power the pi through the micro usb port, or if you prefer, you can use a separate power supply (I verified that with the manufacturer). It has a special port for data only, so power is not fed through the full size usb ports of the pi. I have yet to plug something in to the hub that doesn't work with the pi:

http://plugable.com/2012/10/25/raspberr ... le-devices

Here are the reviews on Amazon:

http://www.amazon.com/gp/community-cont ... 20pi&sort=
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gregor3000
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Re: USB Hub and back power

Tue Oct 22, 2013 7:45 am

But what can you actualyl put in USB hub if you use wireless USB mouse&keyboard and USB wi-fi? Let's say you add disks the data transfer could create issues, no? also the data will be slower to transmit.

though i have to say i was actually looking for a 4 USB hub to just expand it a bit it seems that it's better to get a 7 USB hub. but what could one do with so many additional USB plug ins?

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RaTTuS
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Re: USB Hub and back power

Tue Oct 22, 2013 8:27 am

most HUBs are built round 4 port chips
a seven port hub will be 2x 4 port with one 4port chained to the next
a 10 port will be 3x4port

your main problem will be power of the usb hub

if you have a 7 port hub
you can power the RPi from one
mouse / keyboard / wireless / HD [may need 2 ports] usb stick etc..
but if you have a 1Amp PSU you will be underpowered
think that each port should be able to provide 500mA , 7 ports and you should really have a 3.5Amp PSU - [YMMV and some things will not pull 500mA but others may pull more...]
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gregor3000
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Re: USB Hub and back power

Mon Oct 28, 2013 8:22 am

But these hubs have their own power. for example this one from mod my pi: https://www.modmypi.com/raspberry-pi-ac ... pi-hub-eu=

why would power be the issue here? i think the issue would be more likely speed over which the data can be transfered since all these ports will use 1 USB plug.

donburch
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Re: USB Hub and back power

Sat Nov 02, 2013 1:22 pm

Like gregor I have read several threads about USB hub and back power ... and pieced together an understanding (which may not be particularly correct). I feel that there is still a need for a definitive and well explained (for us noobs) guide.

Can you confirm one point for me ... If the RPi's micro-USB socket is connected to power only and we want the power to come from the USB hub (so only one power socket is used); and we don't want backpower coming from the hub to our USB port ... we will need to use 2 of the hub's sockets and 2 USB cables (one for power and one for data) ???

I have noticed that some USB hubs have an extra USB (sometimes a mini-USB or micro-USB) socket ... which i guess is an uplink USB port for connecting to your PC ? If so, I assume it would not be powered ? And so we get:

Power
to hub --> H -- uplink port <-----------------<-- USB port -- R
U -- USB port -- -- USB port -- P
B -- USB port -->----------------->------ power -- i
-- USB port --

If the hub doesn't have a separate uplink port then we end up with only 2 extra usable USB ports from a 4-port hub.

My apologies for my question being overly wordy - I am trying to be precise yet avoid drawing a couple of diagrams.

thanks in advance

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Lob0426
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Re: USB Hub and back power

Sat Nov 02, 2013 6:37 pm

All hubs have an uplink port!

The uplink port will usually be a mini B or a standard B (D shaped) female port.

The micro USB power port does not carry data. I have seen at least one person who modded their RasPi so it would carry data. They connected data wires to one of the USB ports. This left them with F3 to protect the RasPi.

And no you cannot assume that the uplink port does not supply "back-power". That is how back-powering was discovered. Cheap hubs tend to be the ones that do not follow the the USB standard, that the uplink should "not" supply power.

That said. If you want to power from the hub you will use the uplink port for data and one other port for power, leaving three useable ports for your USB devices on a four port hub.

If your hub allows you to back-power then you only use the uplink port and have four useable ports for USB devices. Only one cable needed.

Back-powering bypasses the polyfuse at F3. So if your PSU goes bad it has the potential to ruin your RasPi. Powering by the GPIO has exactly the same potential.

I modded my hubs to back-power making sure to take power from the outgoing side of the USB hubs polyfuse. That means I am protected at least by that polyfuse in case the PSU burns.

I have had no issues at all with using back-power to my RasPii. My web server has been running for over a year this way. But people are still nervous about using back-power, although they seem less nervous about powering by GPIO, even though both bypass F3. Cannot figure out why they think GPIO is any safer!
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MattPhelps29
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Re: USB Hub and back power

Sat Nov 02, 2013 7:13 pm

Personally I think I'm going to go with the PiHub. http://shop.pimoroni.com/products/pihub It's designed for the Pi and I contacted the manufacturer about backpowering, which it does not.

samscorp
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Re: USB Hub and back power

Sun Jul 13, 2014 9:22 am

Sorru to bring this up again. I have an M4-ATX car PSU, rated at 250W and max. 15A at the 5v line.

If I solder a micro usb lead to one of the 5v lines of the mentioned psu, will I still need a powered usb hub?

I do need a hub for wifi, bluetooth and a touchscreen.

I still have the psu from my full (desktop) carpc mod but it keeps crashing now. I know it's overkill but i'm not willing to buy a new psu when I allready have an expensive DC-DC psu.

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Burngate
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Re: USB Hub and back power

Sun Jul 13, 2014 9:53 am

The power supplied to the uUSB connector of the Pi is limited by the Pi's poly-fuse, and almost certainly won't be enough to power wifi, bluetooth and touchscreen, so you will have to power those separately.
A powered hub is the way to do this.

If you choose a hub which back-powers the Pi, the poly-fuse will be bypassed, so your Pi will be unprotected.

A 250W power supply (of any sort) may or may not be happy supplying only a few watts - the Pi itself only uses about 5W - so you may or may not be in luck
If it's happy, and the 5v line stays properly regulated, then yes, a uUSB cable attached there can power the Pi. Another similar one could power your hub.

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startrek.steve
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Re: USB Hub and back power

Tue Jun 30, 2015 2:17 pm

Im confused as to what back-powering actually is - I have a four port powered hub (The one recommended by the foundation) I have a cable plugged into one socket of the hub, and the other (micro) end going into the Pi2's power input socket, is this back powering? to my knowledge the power protection is on the Pi's power input connection, so Im not circumventing this am I? I've run the PI2 like this with no problem for around four months.
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rpdom
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Re: USB Hub and back power

Tue Jun 30, 2015 3:34 pm

startrek.steve wrote:Im confused as to what back-powering actually is - I have a four port powered hub (The one recommended by the foundation) I have a cable plugged into one socket of the hub, and the other (micro) end going into the Pi2's power input socket, is this back powering? to my knowledge the power protection is on the Pi's power input connection, so Im not circumventing this am I? I've run the PI2 like this with no problem for around four months.
No, you aren't back-powering. You're powering the Pi via its Micro-USB power socket, as it is supposed to be.

Back-powering is when the USB hub feeds power backwards from its USB "input" socket back to the Pi through one of the main USB "output" sockets.

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startrek.steve
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Re: USB Hub and back power

Wed Jul 01, 2015 7:24 am

Ah right, cheers bud, thats what I thought I was doing.
Steve
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shkaff
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Re: USB Hub and back power

Tue Sep 22, 2015 9:07 am

Hello

What is the way to check if my hub powers back?
If I have a hub, should I simply test the presence of voltage on the input USB of the hub?

I like the idea of using only one powerful power supply unit which powers only the hub having all devices including Pi powered from it. But the warning about bypassing the fuse is making me feel uncomfortable

Many thanks

Moe
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Re: USB Hub and back power

Tue Sep 22, 2015 8:40 pm

This issue is a lot more complicated than it needs to be.

'Back powering' is just sales-speak for connecting the Pi's 5V rail to the same 5V source over two different routes. The Pi doesn't care which wire its current comes down, but it only has a fuse at one end of the rail. The problem is that in the event of a fault the Pi will draw its current from the unfused end, making the fuse completely useless.

There is no fancy electronics involved here - a USB socket that doesn't 'back-power' is just one with the power wires disconnected. You can check this by checking for 5V across pins 1 and 4, the two outer pins on a standard type A plug or socket.

If in doubt, cut the 5V wire (the red one) in the USB cable or make up your own cables
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shkaff
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Re: USB Hub and back power

Wed Sep 23, 2015 7:34 am

I painted a schema what we are talking about I guess
Image
So I put a cross on the power line between the raspberry unit and the hub.
And it should work properly, right?

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RaTTuS
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Re: USB Hub and back power

Wed Sep 23, 2015 8:19 am

shkaff wrote:I painted a schema what we are talking about I guess
Image
So I put a cross on the power line between the raspberry unit and the hub.
And it should work properly, right?
YES
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shkaff
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Re: USB Hub and back power

Thu Sep 24, 2015 6:31 am

Aha! I see the approach...

ImageImageImage

Okay, now I know the way to go!

Moe
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Re: USB Hub and back power

Thu Sep 24, 2015 7:56 pm

Job done! :)
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