MarkSpireWeb
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Running with Power port unplugged

Tue Jun 12, 2012 9:57 pm

I've been testing my Pi with various peripherals (mostly card readers and HDDs) via a Powered USB hub. I unplugged the power supply while it was still running, but power was provided to the board via the USB hub; the power cable was unplugged.

It even boots in this configuration, so I need to worry about the total current going into the board?

I'm using an HTC 5v 1A in the power port and a cheapo powered USB hub rated to 500mA in one of the USB sockets, but either unplugged it still powers up.

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Re: Running with Power port unplugged

Tue Jun 12, 2012 10:03 pm

It's a real bad idea to use a powered hub unpowered, as the PI cannot deliver more than about a 100mA to an USB port, and then only from a remarkably high resistance of up to 8 Ohm, (due to the choice of a 140mA polyfuse) which means that an unpowered hub will probably only receive 4,4 Volt or less, and it most probably won't work with that, and it will have no power to power anything else.

The reverse is also true, the PI cannot receive more current "backstreaming" from the hub than the same 100mA, with the same voltage drop, and the PI won't work with 4,4 Volt and only 100mA.
The official solution is to power the hub, and to power the PI from the hub with a USB to mini-USB cable, as long as the hub can deliver the 500..600mA needed by the PI.

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Re: Running with Power port unplugged

Tue Jun 12, 2012 10:20 pm

Thanks mahjongg

In that case would best practice be to unplug the hub connected to the full size USB port before pulling the power out of the MicroUSB?

Have to say i was quite surprised that I'm able to run the thing without using the power connector at all.

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Re: Running with Power port unplugged

Wed Jun 13, 2012 2:29 am

MarkSpireWeb wrote:In that case would best practice be to unplug the hub connected to the full size USB port before pulling the power out of the MicroUSB?
Or I guess if you're feeling adventurous you could snip the red (+5V) wire in the cable joining the hub to your raspi.

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Re: Running with Power port unplugged

Wed Jun 13, 2012 6:24 am

If I am reading this right he should not be able to power the RasPi from the hub at all, without a USB to micro cable. His poly fuse must have a solder bridge?. If that is true then he could easily overdraw the 1 amp polyfuse at the power side if he runs the hub without power. Running it the other way he would have no protection from the poly fuse as it would be sending power in backwards. So why does it not fry when both PSU's are plugged in? I don't know maybe just reading it wrong.

dis is Verwy, Verwy intewesting.

New way to power a RasPi, create a solder bridge at the 140ma poly fuses, then you just power it straight from the hub? Without an extra cable? Nah cannot be that easy! It just CANNOT be THAT EASY! :shock: :mrgreen: This would be a real possibility for powering an A model.

I just do not know why the Magic Blue Smoke has not released the tormented soul of this Raspberry Pi!!!!! :twisted:

Anyone have thoughts on this.
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Re: Running with Power port unplugged

Wed Jun 13, 2012 6:40 am

The polyswitch fuses are not instant, and they are only guaranteed to trigger at 280mA (IIUC). The RaspPi would be taking around 350mA, maybe less, so you might get several, maybe many, seconds before the polyswitch triggered.

Real wire fuses are this bad too, we just never notice.

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Re: Running with Power port unplugged

Wed Jun 13, 2012 6:52 am

Hi Lob,

You were reading correctly - no Micro USB cable at all, all power is supplied via the powered hub connected to the large USB. I've got a stock Pi with no mods.

Image

Been left on overnight like this, still running this morning and still able to use. I've not run anything processor intensive in this configuration though. My original thoughts were also along similar lines for the Model A.

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Re: Running with Power port unplugged

Wed Jun 13, 2012 9:29 am

But should there not be a diode upstream of each USB ports to prevent this?
The poly fuse is not mono directional is it, so that no diode is required?
If the poly fuse were operational it should have blown in a few seconds and shut the RasPi down. You are running HDMI and have everything else running off of the hub, it appears. You are not using ethernet so that is lessening the draw somewhat.
Another question is will it boot this way, or does it just continue running after it is already booted? The draw would be lower as you are not running power through the linear regulator which is a power waste. So less amperage to run it.

So does it power this way with both ports or just that lower port?
For this not to fry your Raspi, and also be able to work this way, there would have to be several serendipitous events.
1. A solder bridge, or a faulty poly fuse.
2. A failure of, or no diode upstream of that port. (or another solder bridge)
3. Two power Supply units that are so close in voltage that the voltage difference is tolerated; the RasPi PSU and the hub's PSU.

Believe me mine will not do this. It will power the hub and the power light will stay on the hub PSU until I unplug the Raspi PSU. Since I only have the one I really cannot test this to the possible destruction. If I only had a second RasPi to throw upon the Altar. It would be interesting to see what dom or Gert would have to say about this.

MarkSpireWeb: I would be very careful with that RasPi. It does not have the normal protections that other RasPii have. If you switch out one of those PSU's you may just see the Magic Blue Smoke. I am surprised you already have not seen it. But you have a unique little berry there among the many raspberries that have been produced. It shows unintended consiquence and possibilty. :idea:
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Re: Running with Power port unplugged

Wed Jun 13, 2012 9:44 am

http://www.raspberrypi.org/phpBB3/searc ... er+usb+hub also throws up a few relevant posts, if people want to investigate this further. Not all USB HUBs are actually 'capable' of sending power "back upstream" - dunno if it counts as a violation of the USB specs or not? *shrug*

Edit: Heh, just spotted that this is in the "Power Users" forum :lol:

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Re: Running with Power port unplugged

Wed Jun 13, 2012 9:58 am

If I saw 350mA from my Pi, and my keyboard, mouse and Ethernet use add up to 100mA, which they might, then the power draw for a Pi without keyboard, mouse and network atttached might be 250mA. That is just inside the current at which the polyswitch is guaranteed to trigger. So it is not necessarily a fault, just rather fortunate. I would still find it rather surprising though.

What happens if you use the other USB slot?

@Lob0426, there is no diode. A diode would drop too much voltage.

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Re: Running with Power port unplugged

Wed Jun 13, 2012 10:10 am

AndrewS: Good point. I will have to test my hub to see if it can. It may have safeguards to stop this from happening. But the RasPi should still be protected against this and his isn't. An "A" male to "A" male USB cable into one of the ports, rather than the normal hub input, should get around any safeguards. I will have to test the hub tomorrow.
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Re: Running with Power port unplugged

Wed Jun 13, 2012 4:42 pm

I believe @rurwin about no diode, but I have to disagree with him about the poly fuse working within spec. Under moderate use it should have blown the fuse. He is right that it would not blow immediately. @AndrewS has a point about the hub. It really should not be sending voltage into the RasPi through the link port. If MarkSpireWeb can afford it, he should replace that hub with another that works right. I believe that hub will eventually do harm to his RasPi under dual power.

So the RasPi may be working properly except for the poly fuse. This still opens up a possible avenue for a some research. I still have not tested my hub but I bet it will not send power through the link. But a hub is able to send power through the other ports. Only 500ma if they are working within spec. So far they are testing the production RasPi at 2watts which equates to 400ma if my math is not slipping up. This would be lower if you are not wasting power through the linear regulator that is so inefficient. The LAN9512 (onboard hub) traces for the PCB go directly back to incoming power. This side of the 1amp main poly fuse. The 140ma poly fuse is the only thing between if I understand @rurwin. So it is very possible that that RasPi is normal, other than a possibly defective or solder bridged fuse.

I know I am restating a lot of previous post. Sorry helps me think.

The potential: power a RasPi through its USB port, OTG style, just no data to SoC due to the LAN9512. "A" models do not have the LAN9512.

So how to do it for other RasPii. Bridge one poly fuse (voiding your warranty). Possibly modify your hub. Put a block into the onboard power connector, just so you will not forget and plug that in. I do not believe any of us will be as lucky as @MarkSpireWeb and not fry our RasPi under dual power. Use at least a 2.5amp PSU and you have a RasPi that only needs one PSU and wastes power with only one regulator. This is just begging to be the future for an "A" model. Need a powered hub with a LAN port and at least three USB. And I have seen those. The question is whether you will be able to get the Ethernet working under Debian.

RasPi revision 2: No onboard power from production. Comes with a hub and PSU. You lose a regulator, a fuse or two, power connector and the LAN9512 from the PCB. PCB could be a bit smaller minus those parts. Mini or micro USB connection to hub to reduce size of PCB further. Could you balance the cost of the hub and PSU, into the goals of the foundation, after losing some parts and smaller PCB? You would need less to get a working system from that point.

Of course we need to see the "A" model into production first. And enough available stock so we can have more than one. Some people already have two from the get. I wish I had followed their lead.
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Re: Running with Power port unplugged

Wed Jun 13, 2012 8:54 pm

Lob0426 wrote:RasPi revision 2: No onboard power from production. Comes with a hub and PSU.
Are you suggesting that the Raspi2 only gets reverse-powered via this 'special' hub? I think that's never gonna happen - the whole point of the U is for Universal i.e. you can swap out one hub for any other ;)
And I think expecting reverse-power from a USB device port is against the USB specs, so you wouldn't be able to advertise the board as having a "USB port".

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Re: Running with Power port unplugged

Wed Jun 13, 2012 11:25 pm

Actually if you design it to power that way, it is not reverse powered. What I really am suggesting is that the RasPi II incorporate a fully powered USB hub and share this power with the SoC. Overall "cost of ownership" would be reduced. Right now we are buying two PSU's and the hub. The Alpha board was closer to this, as it had fully powered USB ports, so you could have used a less expensive unpowered hub. These limitations have been placed by the choice of switching to a "cell phone" charger. There are a small number of people that are finding that choice to be a pain. The quality of these chargers covers a wide range. This power option was suggested in these very forums. So I am suggesting something else. Price wise it could be possible to meet a low price by reducing components and supplying the most needed components together as a package.

RasPi Average decent PSU $15
Powerred Hub of average quality. $22
additional cost plus RasPi $72
I think that price could be beat and with good components. The A model will be even more powered hub dependent, or keyboard only unless you invest in a wireless combo keyboard/mouse for even more additional cost. I had a lot of stuff lying around. Do you think most kids are going to have this stuff lying around? For example I never had a need for a USB hub before so I had to buy one. I had no less than 4 SD cards and at least that many USB sticks lying around. I had three USB HDD's lying about also. I have tons of stuff around, but I do not think the average house has computer gear lying around. They have a keyboard and mouse and it is on the family computer. They gave away the old computer with their other keyboard and mouse. but the likelyhood of a keyboard and mouse lying around is a lot higher than a powered USB hub. Of course the current power scheme is predicated on there being likely there is a cell phone charger lying around. So why are so many people asking where to buy a PSU? :)
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Re: Running with Power port unplugged

Thu Jun 14, 2012 6:09 am

If you have a good powered hub then you do not need a separate power adapter as you can power the Pi via the hub itself (at the cost of losing a port on the Hub to powering the Pi).

This is what I am now doing and it works well. I also use the hub to power any other devices that need USB power such as USB 2.5" Hd. It also gives a single item I can power on/off when I want to restart the Pi and HD via a power-cycle.

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Re: Running with Power port unplugged

Thu Jun 14, 2012 10:30 am

Or if you don't mind chopping up cables, you could do something like: https://sites.google.com/site/bifferboa ... om-one-psu

(and before you ask ;) the Bifferboard doesn't have polyfuses - the 5V power input is connected directly to the 5V USB output)

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Re: Running with Power port unplugged

Thu Jun 14, 2012 10:46 am

itimpi wrote:If you have a good powered hub then you do not need a separate power adapter as you can power the Pi via the hub itself (at the cost of losing a port on the Hub to powering the Pi).
As I understand it you can only do this if you have a poor quality powered hub that breaks the rules and exceeds the 500mA limit without negotiation. With a "good" powered hub you would need a "Y" power cable and take 2 ports for power. These are fairly expensive to buy, and a bit tricky for most people to try and make.

I got an electric shock last night from touching the end of a usb cable on my Pluscom 7 port hub powered with the adapter that came with it. It was enough to make me jump and say a rude word.

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Re: Running with Power port unplugged

Thu Jun 14, 2012 12:09 pm

Um, am I missing something here? How can you get a shock from 5V?
And some folks need to stop being fanboys and see the forest behind the trees.

(One of the best lines I've seen on this board lately)

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Re: Running with Power port unplugged

Thu Jun 14, 2012 12:14 pm

Probably some kind of AC signal. The same way you can get a 'tingle' from a TV COAX signal.

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Re: Running with Power port unplugged

Thu Jun 14, 2012 1:41 pm

Joe Schmoe wrote:Um, am I missing something here? How can you get a shock from 5V?
Most DC supplies are not connected to a common ground so each floats with respect to the other and their 0V potentials can differ; You can measure the voltage between the two and get that tingle if you put yourself between them.

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Re: Running with Power port unplugged

Thu Jun 14, 2012 5:07 pm

My previous post was about a new version of RasPi. You will find that I have commented in other threads about powering from a hub port. I have a Motorola Lapdock enroute. Which will have essentially the same power scheme.

There are hubs out there that have charging ports for cell phones that have 1amp ports. Also a "Y" connector will work. This thread started with a RasPi that was running without it being plugged in itself. It was being back powered by the hub. I still think it is worth an investigation of the circumstances that allowed it to work. My hub does not port power back through the link port. It is obvious that OP's hub does. So will other RasPii "run backwards"? I think the poly fuse would prevent it. I also believe one of his USB poly fuses has a solder bridge that is allowing it to work. @rurwin believes it could run as long as it does not exceed 280ma. I wish Gert or Dom would weigh in on this thread!
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Re: Running with Power port unplugged

Mon Jun 18, 2012 8:03 am

While trying to find a decent powered hub online tonight I ran across [a thread on Dealextreme.com (http://club.dx.com/forums/Forums.dx/threadid.433192) that explained about the missing diode on a particular 7 port powered USB hub. When I got home I figured I'd dig out a hub I'd bought online months ago and sure enough it looked identical to the one on the dealextreme thread except it was rebranded as "TomTop". My guess is that this is a very popular hub and gets rebranded a lot, I also think a lot of people find it with the intention of getting it for the Raspberry Pi, in fact the last guy to comment in the thread I linked to even says he wants to use it for his RasPi, don't know if he's blind or not because after reading the issues I'd steer clear.

I took mine apart and couldn't actually see any pads labelled D1, there were zero ohms installed but they all appeared to just be used to jump tracks since it wasn't a multi layer board. Seeing the LED (and being half asleep) I assumed they were using the LED as a block so I plugged it into my RasPi while the microUSB power port on the Raspi was unplugged and I did get a power light on the RasPi.

So in short, be careful when using cheap powered USB hubs, especially ones that look like the product in the dealextreme forum I linked to. I remembered seeing this thread here on the raspberry pi forum so I thought I'd post about what I'd read over at dx.

Also be aware of pretty much all wall adapters from China claiming to be 2 or 2.5 amps, under load I'd be surprised if you'd get more than 500mA out of them.

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Re: Running with Power port unplugged

Mon Jun 18, 2012 12:33 pm

Lob0426 wrote: This thread started with a RasPi that was running without it being plugged in itself. It was being back powered by the hub. I still think it is worth an investigation of the circumstances that allowed it to work. My hub does not port power back through the link port. It is obvious that OP's hub does. So will other RasPii "run backwards"? I think the poly fuse would prevent it. I also believe one of his USB poly fuses has a solder bridge that is allowing it to work. @rurwin believes it could run as long as it does not exceed 280ma. I wish Gert or Dom would weigh in on this thread!
I would like to add that I bought the Logik hub that's listed on the eLinux peripherals page this weekend. The hub I was using prior to that caused problems with the Pi (from a USB perspective - problems with mouse, keyboard etc.) Anyway, my old hub (Gensys chipset) *does not* power the Pi when connected to either Pi USB port. However, the Logic hub *does* power the Pi through USB, on either port that I connect it to. Clearly it's not just the Pi that's possibly acting strangely, it is also influenced by the hub that's being used. I can't really tell from this thread, but I think my general question would be, is it safe to run the Pi connected to a USB hub that can actually power the Pi via the USB ports (with main Pi power disconnected)?

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Re: Running with Power port unplugged

Mon Jun 18, 2012 4:05 pm

bob_binz wrote:Clearly it's not just the Pi that's possibly acting strangely, it is also influenced by the hub that's being used. I can't really tell from this thread, but I think my general question would be, is it safe to run the Pi connected to a USB hub that can actually power the Pi via the USB ports (with main Pi power disconnected)?
No, because the Pi can potentially draw more current than the polyfuses on the Pi's USB outputs allow before tripping.

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Re: Running with Power port unplugged

Mon Jun 18, 2012 4:14 pm

I've got a no-brand USB hub that "powers" the RasPi -- it doesn't seem to provide enough juice for it to actually run, but the power LED on the RasPi stays lit when I unplug the micro-USB powersupply. After spotting it did this I've erred on the side of caution and not used it again!

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