bttf
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Understanding Powered USB Hubs ...

Mon Jan 14, 2013 7:21 pm

Hello,

I've been trying to gather information on what USB hub to get for my R-Pi. It seems that most people like to get a Powered USB Hub and then use a USB-to-MicroUSB cable to power the Pi itself. I could do this, but since I already have a Micro USB power adapter that I'm using, I'd like to use that in conjunction with a USB Hub in order to have more USB slots available.

Is it possible to have a Pi powered by Micro-USB with a Powered USB Hub at the same time ?

I understand that backfeed is a serious issue with Powered USB Hubs. Are there any Powered USB Hubs out there that are confirmed to produce no backfeed? (I'm not entirely sure what causes backfeed or if there is any way to stop/reduce it?)

Thanks,
bttf

Rickard Andersson
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Re: Understanding Powered USB Hubs ...

Mon Jan 14, 2013 7:35 pm

There's some information on the subject here:

http://elinux.org/RPi_VerifiedPeriphera ... d_USB_Hubs

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abishur
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Re: Understanding Powered USB Hubs ...

Mon Jan 14, 2013 7:40 pm

The elinux link is a little outdated as the pi no longer has the limiting fuses and back feeding isn't really much of an issue anymore unless you bought the pi long enough ago such that it has the limiting fuses (if you have two little guys that say "0" on them then you do *not* have the limiting fuse and don't need to worry). I actually removed the limiting fuse on my first pi (darn there goes it's collectivity! :lol: ) and power it completely off a usb->sata adapter that backfeeds down the usb port
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bttf
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Re: Understanding Powered USB Hubs ...

Tue Jan 15, 2013 2:48 am

I had read through the article beforehand prescribed by Mr. Andersson which led me to inquire here on these forums. I find abishur's opinion of them being outdated agreeable.

I did not know that backfeeding was a virtually obsolete problem now; thanks for the tip.

I have to say that I don't really have a firm understanding of how the Micro USB interferes with the Powered USB Hub ... I asked around on the IRC channel and some folks mentioned conflicting charges clashing with each other ... Is there some documentation on the subject that would educate someone like myself on the inner workings of such electrical nonsense ?

Thanks,
bttf

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abishur
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Re: Understanding Powered USB Hubs ...

Wed Jan 16, 2013 3:47 pm

Sorry, I had held off on this in the hopes that some with a little more electrical knowledge would chime in. I know of no such documents. All I can tell you is if conflicting voltages were an issue then hubs would legally not be allowed to send power back down their uplink port, or usb ports would be legally required to have diodes blocking return voltage coming back down from its peripherals. Now if you were dealing with a bigger gap in voltages then yes, you might get some issues, but we're talking about conflicting voltages in the 5v +/- 25% range. There would need to be additional major problems with the pi for any conflicting voltages to cause problems on you, and even then it should be more in the "my pi's not being stable" department rather than the "My pi took up smoking and died" department.
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zardoz66
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Re: Understanding Powered USB Hubs ...

Wed Jan 16, 2013 5:30 pm

Most USB HUBS have an input port and x number of output ports. The input put on a USB HUB does not supply power, so there is no worry for backfeeding power to the RPI. you just dont want to output one of the x number output ports to one of the 2 output ports on the PI and just use one of the output ports on the HUB to supply power on the RPI's power port.
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abishur
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Re: Understanding Powered USB Hubs ...

Wed Jan 16, 2013 6:04 pm

zardoz66 wrote:Most USB HUBS have an input port and x number of output ports. The input put on a USB HUB does not supply power, so there is no worry for backfeeding power to the RPI. you just dont want to output one of the x number output ports to one of the 2 output ports on the PI and just use one of the output ports on the HUB to supply power on the RPI's power port.
That is the very definition of the way USB hubs are *supposed* to work :lol: unfortunately we've found that there are a large number of USB hubs out there that were lazy and blindly attached everything to the same power rails such that even the input port back feeds. This created issues when then pi had fuses on the usb ports, but now people have quite successfully powered the pi off such hubs. Of course that bypasses the circuit protection of the pi, but you can't do much when there's no standard on how the hubs handle power beyond "each port must provide up to 500mA"
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zardoz66
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Re: Understanding Powered USB Hubs ...

Wed Jan 16, 2013 8:50 pm

I can't help cheap/stupid manufacturers that do not follow specifications proper. if I found that this happen, I would complane and have them replace the damage equipment.
Zardoz
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nginx
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Re: Understanding Powered USB Hubs ...

Sat Mar 02, 2013 3:34 pm

abishur wrote:
zardoz66 wrote:Most USB HUBS have an input port and x number of output ports. The input put on a USB HUB does not supply power, so there is no worry for backfeeding power to the RPI. you just dont want to output one of the x number output ports to one of the 2 output ports on the PI and just use one of the output ports on the HUB to supply power on the RPI's power port.
That is the very definition of the way USB hubs are *supposed* to work :lol: unfortunately we've found that there are a large number of USB hubs out there that were lazy and blindly attached everything to the same power rails such that even the input port back feeds. This created issues when then pi had fuses on the usb ports, but now people have quite successfully powered the pi off such hubs. Of course that bypasses the circuit protection of the pi, but you can't do much when there's no standard on how the hubs handle power beyond "each port must provide up to 500mA"
You have no idea how many powered hubs don't even follow the 500mA per port standard. Many hubs simply connect the power line to all USB ports without any sort of limitations in place. What this means is that a single USB port will provide as much power as the connected device can draw and upto the limit of what the power adapter can deliver.

sanders
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Re: Understanding Powered USB Hubs ...

Sat Mar 02, 2013 7:18 pm

abishur wrote:The elinux link is a little outdated as the pi no longer has the limiting fuses and back feeding isn't really much of an issue anymore unless you bought the pi long enough ago such that it has the limiting fuses (if you have two little guys that say "0" on them then you do *not* have the limiting fuse and don't need to worry). I actually removed the limiting fuse on my first pi (darn there goes it's collectivity! :lol: ) and power it completely off a usb->sata adapter that backfeeds down the usb port
Would this mean I don't need a powered hub if I want to connect a usb hard drive ?
Remove the limiting fuses and enough current will go through the rpi usb ports to connect a usb hard drive (or a non powered usb hub) ?

Sander.

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abishur
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Re: Understanding Powered USB Hubs ...

Sat Mar 02, 2013 7:36 pm

Well, the limiting fuses at the USB ports have already been removed and replaced with (last I remember) 0-ohm resistors. So there's nothing you need to replace there.

That said, no you still can't use the pi to power a hard drive as there is still a fuse at the power input that limits the total input current to around 1A. That is unless you did one of the following

1. Bypass/remove fuse at input

2. Use 5V and GND on the GPIO strip to power the power

3. I suppose you could also solder some wires on in front of the input fuse, and attach them to a female usb adapter then use a y cable to plug your hard drive into the adapter and an open port on the pi... but that seems needlessly complicated

In all three situations you have created a situation where the pi could receive *way* too much power on a power surge causing damage to the components... but hey it's your pi have fun with it! :lol:
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sanders
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Re: Understanding Powered USB Hubs ...

Sat Mar 02, 2013 7:50 pm

Okay... Well I guess I am not emotionally ready yet to fry my rpi...

So I'll be going for a powered usb then :D

rickdallas
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Re: Understanding Powered USB Hubs ...

Sun Mar 03, 2013 1:52 am

Hi, since this my first post on this site, thought I would try to post some useful info. :mrgreen:

The USB spec states that a usb port support up to 500ma of current. This current amount may be more important than you realize. For instance, a usb hd or mp3 player might need that entire 500ma to work correctly while a mouse may have lower power requirements and only require say 100ma.

When you start looking around on the market for powered hubs, you will see pretty quick that most powered hubs don't meet this spec of 500ma per port. For instance, if you look at the spec on a 4 port powered hub, the power adapter SHOULD be rated at 2 amps (500ma X 4). However what you find much of the time is the power adapter is 2 amps or less. Even worse, many powered hubs don't advertise the power of their transformer. There is a reason for this.......... :shock:

So what does this mean to you? It means that if you have 4 usb devices all connected to your 4 port powered usb hub, there may not be enough power to make/keep the devices work correctly.

Many devices don't use the full 500ma and the resellers know this so they are gambling that no matter how many devices you plug into the hub, you won't exceed the power of the hub and with a bit of luck, they are correct. But if you start loading up one of these "underpowered" hubs with devices that do require the full 500ma, you are going to have intermittent failures, devices that suddenly stop working or erratic behavior. Ask me how I know ;)

There are of course exception to most everything but if you abide by these guidelines, you'll be a happy camper.

Moral of the story is when you buy a powered hub, do the math so you'll know what you can and can't do with the hub. Personally, I only buy hubs that meet the "math requirement" so I don't have to worry about what I plug into it.

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rew
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Re: Understanding Powered USB Hubs ...

Mon Mar 04, 2013 8:00 am

abishur wrote:Well, the limiting fuses at the USB ports have already been removed and replaced with (last I remember) 0-ohm resistors. So there's nothing you need to replace there.
No, the last few batches of "rev 1" boards had the 0-ohm resistors. Rev 2 boards simply don't have the footprint.

The reason some people are afraid of the "backfeed" of the usb HUB is that they think that if you connect two powersupplies together they will fight each other to death. "The voltages will not be exactly the same, so one might supply say 5.05V and the other 4.95V, so they will fight eachother". This is not true. All powersupplies "sag" under load. So the supply with the highest unloaded voltage will provide most of the initial current. So what? And the powersupply who delivers only 4.95 does not have the option to absorb current. It will provide the nano-amps to keep the voltage at 4.95 when the voltage sags below 4.951, and it will simply stop doing that when the voltage goes above that.

You have to be a bit careful though: When the two powersupplies have widely separate voltages, the one with the highest output voltage may be delivering more than his fair share of the current. So suppose you have 1.5A of load, and two 1A powersupplies, then the "unfairness" may require one of the supplies to deliver more than 1A. Devices differ in what they do when the overload. Some simply smoke a little and break down. Not good. Others have a "protection mode". So they switch themselves off, and wait for better times. Also not good. But nowadays many devices go into a "constant current mode". So, in that case, the worst that would happen with the two 1A supplies needing to deliver 1.5A, one will be delivering 1.1A (slightly more than' its specification, because when they claim 1A they don't want the constant current mode to kick in accidentally at 0.99A) and the other might end up delivering only the remaining 0.4A. This is the most likely scenario.
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Cloudcentric
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Re: Understanding Powered USB Hubs ...

Mon Mar 04, 2013 8:58 am

The issue I had was that the USB mains Powered Hub was back feeding to the USB Port and when I plugged in the USB Power Lead it would not fire up. So had to plug the USB Hub in afterwards.

Reagrds 500ma, actually the maximum draw is limited to 400ma I believe.

As said before I am now powering my Pi from a Belkin Hub, and works fine..
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abishur
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Re: Understanding Powered USB Hubs ...

Mon Mar 04, 2013 2:40 pm

rew wrote:
abishur wrote:Well, the limiting fuses at the USB ports have already been removed and replaced with (last I remember) 0-ohm resistors. So there's nothing you need to replace there.
No, the last few batches of "rev 1" boards had the 0-ohm resistors. Rev 2 boards simply don't have the footprint.
Thanks!
Cloudcentric wrote:Reagrds 500ma, actually the maximum draw is limited to 400ma I believe.
From USB.org on how much power has to be given to a USB header:
500mA per USB port. See section 7.2.3 for the details of device behavior during suspend and resume
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Ray_GTI-R
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Re: Understanding Powered USB Hubs ...

Thu Mar 07, 2013 2:23 am

See also http://www.raspberrypi.org/phpBB3/viewt ... 68#p303568 esp 4th post.

HTH, Ray

FWIW I've linked before ... cheap PSUs=tiny amps on-load inc hilarious vids. For example I ordered a 7-port powered hub. It came with a 1 amp PSU. There is no way that PSU could ever cope with 7 devices asking even 400 mA let alone 500mA ... some USB devices require more than 1A stand-alone (2 x power leads).
Worse ... a cheap powered-hub with high-draw devices attached can drag down the voltage of the RPi even with an otherwise-perfectly good RPi power supply. Read ... devices WILL suck amps from everywhere & anywhere. And volts will suffer if the PSU output is unregulated.
Best get a big-amp, regulated-voltage PSU for 4+ socket USB hubs and take it from there ... :?: Hope the PSU doesn't blow up BTW but that's a different story :roll:

Bashster
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Re: Understanding Powered USB Hubs ...

Thu Mar 07, 2013 8:56 pm

I had precisely this problem the other day with a cheap Chinese psu. The packaging clearly stated the psu was good for 2A out at 5V, but the device inside stated only 1000mA at 5V! It was clear the thing was struggling under very modest loads, so I took the opportunity to perform a quick experiment. Under no load conditions, the psu did indeed provide a whisker over 5V. However, when 'asked' to supply 1A by the application of 5X 1ohm 10W resistors in series across it, the output dropped like a stone to 1.96v!!
I'm guessing the average customer hasn't the stuff at hand to carry out a basic test like this so the manufacturers/dealers really take advantage. It's bordering on the criminal I reckon. :shock:

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