Odd power situation - can power Pi without a powersupply, just a powered usb hub...

[rmwebs]

I was just aout to plug my pi back in to test a few things and basically plugged a powered USB hub into one of the two USB sockets.

For some reason this powered up the pi. Bearing in mind I had nothing plugged into the microusb power supply.

I've seen this happen with my Sheevaplug and IIRC it was deemed ok, but that there would likely be not enough power.

Is this the same case for the Pi, or is there something wrong here?

Is this a 'ok' or safe way to power the pi, as if it is it saves having another cable!

Edit:

Did a few tests, it does seem that its not enough power to run the pi. I was however able to have the Pi powered from the hub using a USB -> MicroUSB, meaing the hub is basically powering the pi, and its USB addons.

[plugwash]

Some hubs violate the USB spec and backfeed power onto the upstream port. Looks like yours is one of them.

If you want to power the Pi this way without the polyswitch on the USB port tripping out you will need to bypass said polyswitch or replace it with a higher rated one.

[zardoz99]

I was under the impression that the maximum current from a powered USB hub was 500mA? Isn't the required minimum for a Model B 700mA?

[rmm200]

People keep saying the Pi needs 700 ma, but I have never seen an actual measurement.

I am still expecting something in the 100 ma range...

I wish someone would measure and post, because most of the cell phone chargers seem to be in the 500 ma range.

[rmwebs]

If its any help or measure, I tried pulling the main power cable out while the USB hub was connected via standard USB - it was ok for about 30 seconds then blackscreened. When I plugged the microusb back in it rebooted.

So it does seem that there is not enough power coming through (or its being blocked by the limiter).

[plugwash]

If a hub is cheap enough that it's backfeeding power to it's upstream port I highly doubt it has any circuitry to limit the power supplied to any one downstream port either.

[cjdelphi]

forget the hub, my cheap PSU does the same thing, if i ended up placing a diode to prevent my Arduino board powering all the USB devices on the computer (which it did lol)...

Simply not enough current, use a regular power supply and if you know what you're doing take a current reading find out how much current it actually uses, then you'll be able to work out the cheapest power supply you'll need...  i want a R PI i'm geting more curious by the minute...

[trikidiki]

plugwash said:


Some hubs violate the USB spec and backfeed power onto the upstream port. Looks like yours is one of them.

..........

Slightly O/T but my wife had a problem where she couldn't turn her car off. Eventually traced it to her mobile phone which was connected via a USB lead to the cigarette lighter socket. The phone was providing enough power back up the line to keep the engine management system alive and hence the car running.

[johnbeetem]

plugwash said:


If a hub is cheap enough that it's backfeeding power to it's upstream port I highly doubt it has any circuitry to limit the power supplied to any one downstream port either.


I have a very cheap hub and here's how it's wired:  All downstream ports share the same 5V wire, which is also shared with an (optional) external 5V power jack.  Since the downstream ports share the 5V wire with no current limiting, there is no problem using it for 700 mA.  The upstream port has a 1A Schottky diode so the upstream host only provides power if it's at least 0.2V greater than the external 5V power input.

My hub's upstream diode prevents it from powering RasPi through a RasPi downstream port.  OTOH, I should be fine powering RasPi using a hub downstream port connected to RasPi's micro USB power input.

[mahjongg]

Hm, I heard the R-PI had a small polyfuse for the USB, reportedly just 100mA, so in principle this should not work, at least not for long.

Perhaps the reports were wrong when they said that the primary input fuse was 100mA and the secondary (USB power) fuse was 100mA.

Re: ah, I see, it did not work for long!

Maybe the current in "non active mode" is just on the threshold of tripping the 100mA fuse, and it needs about 30 seconds to heat up and trip.

[Jim Manley]

Others have posted that the Pi with nothing else plugged in and just idling it draws ~330 ma (~300 ma was originally reported by Eben, IIRC), with a keyboard it draws ~385 ma, a keyboard, mouse, and network cable it draws ~450 ma (three LEDs lit), and when various ARM CPU-intensive software is run at up to 99% load in that latter fully-connected setup, it drew up to ~485 ma.  It hadn't been tested with the GPU running full-bore, yet, but, the remaining ~215 ma to get to 700 ma maximum is otherwise available for other USB devices.

[GizmoB73]

It is the polyfuse that is the limiting factor here.

I posted about this on the Farnell forum and PeteL confirmed this.

The Polyfuse although rated at 100mA ish allows more through, but when the Pi gets to the parts where it starts switching on all the USB stuff, then the power requirement exceeds the fuses capacity and the Pi shuts down.  The Pi then starts again and you end up in a loop.  Well at least that is what I found.

[rmwebs]

That makes sense I guess.

So the option here is that we need a better USB Hub, something that meets the standards better and hopefully doesn't backfeed back into the pi.

Maybe something needs to be mentioned about this on the official docs, that the Pi doesn't play well with cheap hubs. It's not really a major issue, but I can see people doing what the rest of us did, went to somewhere like eBuyer, Play and Amazon and picked up the cheapest powered hub they could find.

Ideally we could do with a list of known-compatible hubs.

[GizmoB73]

rmwebs said:


That makes sense I guess.

So the option here is that we need a better USB Hub, something that meets the standards better and hopefully doesn't backfeed back into the pi.

Maybe something needs to be mentioned about this on the official docs, that the Pi doesn't play well with cheap hubs. It's not really a major issue, but I can see people doing what the rest of us did, went to somewhere like eBuyer, Play and Amazon and picked up the cheapest powered hub they could find.

Ideally we could do with a list of known-compatible hubs.



Well I guess that depend what you are doing with it!

Mine is a cheap powered hub, just like you describe.  It works well for me because when I want to remove power from the Pi, I don't need to touch the flimsy micro USB connector, I just halt the system and pull the 2.5mm DC jack that powers the Hub.

[rmwebs]

GizmoB73 said:


rmwebs said:


That makes sense I guess.

So the option here is that we need a better USB Hub, something that meets the standards better and hopefully doesn't backfeed back into the pi.

Maybe something needs to be mentioned about this on the official docs, that the Pi doesn't play well with cheap hubs. It's not really a major issue, but I can see people doing what the rest of us did, went to somewhere like eBuyer, Play and Amazon and picked up the cheapest powered hub they could find.

Ideally we could do with a list of known-compatible hubs.


Well I guess that depend what you are doing with it!

Mine is a cheap powered hub, just like you describe.  It works well for me because when I want to remove power from the Pi, I don't need to touch the flimsy micro USB connector, I just halt the system and pull the 2.5mm DC jack that powers the Hub.


Out of interest, what hub are you using? My one wasnt able to power the Pi and power the USB devices at the same time, it was one or the other

[GizmoB73]

rmwebs said:

Out of interest, what hub are you using? My one wasnt able to power the Pi and power the USB devices at the same time, it was one or the other

I bought a cheapo 7-port hub with a 3A PSU from ebay a while back.  The Hub was fine but as many have experienced here, the PSU was pants.  As it happened, I had a decent 3Amp switched mode PSU laying around so I just stuck a 2.5mm DC jack on it.

This one looks the same:

ebay: Item number: 180733957512

[cpldave]

Slightly O/T, but can anyone who has a Pi confirm a make/model (and possibly UK buy link) for a powered USB hub that will allow you to plug a USB->Micro-USB cable into the Pi's power socket and power it from that, as well as power peripherals plugged into the Pi's USB port?

As I understand it (I'm not electronically inclined) but most "quality" hubs have a component to prevent such circular-connectivity like this. I'm wanting to build a case with a powered USB hub inside, so all I have to plug-in is the hub at the back of the case and it'll be right to go... at least that's what I'm hoping for hehehe

[plugwash]

The problem is you want a hub that isn't too clever (as that may make it impossible to run the Pi off it) but you also want a decent PSU. Only way to find out if a hub meets those critera is to buy it and test it.

another option is to buy a cheap hub and then buy a decent PSU seperately. Then run power from the decent PSU to both the Pi and the hub. Yes this will involve cutting and solding some wires but that shouldn't be too big a deal.

[zippity]

I"m getting somewhat worried reading this – could a usb hub that backfeeds power into the Raspi damage the board?

[abishur]

zippity said:


I"m getting somewhat worried reading this – could a usb hub that backfeeds power into the Raspi damage the board?



There are fuses on the R-pi which should prevent power from flowing back into the R-Pi and causing damage.

[rmwebs]

Abishur said:


zippity said:


I"m getting somewhat worried reading this – could a usb hub that backfeeds power into the Raspi damage the board?


There are fuses on the R-pi which should prevent power from flowing back into the R-Pi and causing damage.


Just to confirm, is there also a fuse on the microUSB power-in too? I only ask as I'd like to test out a few hubs I've got but if the MicroUSB isnt protected I dont want to risk frying the board.

[plugwash]

There are polyswitches on both the power input and the power output. Further there is a system designed to protect against overvoltage. I'm not going to say it's impossible for a bad PSU to fry a Pi but I think it's pretty unlikely unless the PSU does something like put mains on it's DC output.

[GizmoB73]

As I have found out, the most likely event is that there will be an under-voltage on the board.

Although my cheap ebay hub is hard wired to the power outputs, it looks like the soldering was poor, and the tracks are not really sufficient to be able to supply 700mA at 5v. I guess the latter is a little unfair as it was probably never designed to supply more than 500mA. Out with the soldering iron now.