ElectroPulse
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 2:21 am

Hello, all!

My apoligies if this is something obvious...

Anyway, I am wondering if I could just buy a powered USB hub, plug it into the wall, plug the RPi into it through the RPi's mini USB port (to supply power), and plug one of the RPi's USB ports into the hub so as to use the extra ports for devices.

If this would work, I would only need one plug going to the wall, and I would be able to plug in an external HD if I ever decided to do so, without worrying about power constraints. I would also have some extra USB ports for whatever else I decide to plug in.

mgmt_idiot
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 2:54 am

I asked this question, and got some helpful answers. See link:

http://www.raspberrypi.org/for.....8;search=1

The short answer seems to be: yes.

However, note that the RasPi uses micro USB for power. Not to be confused with mini USB.

ElectroPulse
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 4:12 am

mgmt_idiot said:


I asked this question, and got some helpful answers. See link:

http://www.raspberrypi.org/for.....8;search=1

The short answer seems to be: yes.

However, note that the RasPi uses micro USB for power. Not to be confused with mini USB.


Alright, thanks for the response!

Hmm… thank you for that helpful piece of info about micro USB… I actually didn't know that was a standard connector. I've seen that connector type only once, so I concluded that it was proprietary… Well, something else I need to add to my shopping list, thanks!

After reading through the posts in the link you provided I have come to this conclusion: No one specifically said it, but it appears that this would work with type A RPi, but not with the type B. From what I was reading in those threads, it sounds like each port in USB hubs will put out a max of 500mA, so it should work with the A, as it requires only 500mA. Now, I was just thinking… perhaps there would be a way to get B to work with this… How about this:

Plug the RPi's micro USB cable into one of the hub's downstream plugs, and plug one of the RPi's USB out cables into the upstream plug on the hub (as was suggested previously). Now, to get the extra 500mA for the RPi B, plug one of the standard USB ports on the RPi into another of the hub's downstream ports. According to the posts in the link you provided, all of the ports are on something called "rails," so you could power them through the output ports. Is this a theoretically sound idea? (basically the RPi pulls in 500mA through it's power input micro USB, pulls in another 500mA through it's output standard USB, and sends the USB signal out through the other output USB).

...or am I incorrect in my assumption that USB hubs put out a max of 500mA per port? Do they put out as much as a device trys to pull (up to the max the hub has to offer, that is)?

Thanks!

Joe Schmoe
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 4:34 am

I *think* that the model B only pulls more than 500ma *if* it is also powering USB devices - i.e., the 700ma figure that we frequently see (hear quoted on these forums) is a high estimate based on having some fairly high power USB devices attached.

But, if you have a powered hub (and have your possibly power hungry USB devices plugged into the hub rather than directly into the Pi), then those devices should be pulling power from the hub rather than from the board.  So, the board itself should be OK with getting max 500ma from the hub.

Or am I all wet?
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ElectroPulse
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 4:38 am

Joe Schmoe said:


I *think* that the model B only pulls more than 500ma *if* it is also powering USB devices - i.e., the 700ma figure that we frequently see (hear quoted on these forums) is a high estimate based on having some fairly high power USB devices attached.

But, if you have a powered hub (and have your possibly power hungry USB devices plugged into the hub rather than directly into the Pi), then those devices should be pulling power from the hub rather than from the board.  So, the board itself should be OK with getting max 500ma from the hub.

Or am I all wet?



I'm not 100% sure... on Wikipedia it says that the power ratings are: 500mA for A, and 700mA for B: http://en.wikipedia.org/wiki/R.....spberry_Pi

But I'm not sure if this is the max it will pull, or just how much the device needs... I would assume the latter, just based on what I have read (I seem to remember something about somebody saying that the RPi using a power supply up to 1A, but I'm not entirely sure).

Anyone else care to chime in and confirm/deny anything in this thread?

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abishur
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 4:44 am

First a power "rail" is just a logical look at power distribution.  Anything that needs 5v of power is attached to a 5v "rail", anything that needs 3v3 is attached to the 3v3 "Rail".

As for your power dilemma, no your solution will unfortunately not work.  The USB ports on the r-pi have voltage protection on them that specifically prevents power from coming back down into them.

However, it's not really an issue.  Some hubs allow the USB ports to draw as much power as they want and don't use the 500 mA standard.  These are bad hubs as the makers haven't put proper voltage protection on them.  The one exception being hubs with specially colored "high current" ports that are specifically designed to offer a higher current to things like hard drives.  But they tend to be more expensive.

Fear not though!  For the simplest solution is to just buy a micro-USB to type A "Y" cable.  This is a cable that is micro-USB on one end and two regular USB plugs on the other.  This will allow you to plug into two usb ports on the hub to get 1A of power for the r-pi.

It should at this point, however, be noted that the 700 mA power estimation was based on the model B board going both at full tilt *and* having some USB devices attached.  The model A and model B are identical power wise except for the extra usb port and the network jack.  The network port will take a very minimal amount of power to run which means the main difference being estimated between the model A and the model B is any device plugged into the extra USB port.

How does this affect you?  You're already using a USB hub.  Just plug devices into that instead of the r-pi and a single 500 mA USB port should be more than enough to power the r-pi!  No need for special ports or cables!
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ElectroPulse
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 5:06 am

Ah, tyvm Abishur!

Well, that solution makes sense. I"ve never encountered one of those Y USB cables (that I recall at least), but that sounds like it would fit the bill perfectly! So, order hub, Y USB cable, and a standard USB to mini USB cable to connect to the hub"s upstream. I really like the idea of this setup because: 1. Don"t need to figure out what phone charger would work with the RPi, 2. I won"t have to worry about power consumption of devices I plug in, 3. I only have one power cable going to the wall (because RPi would be pulling power from hub), and 4. I would have several extra USB ports.

Thanks again for the info!

P.S. What exactly is the peak power usage of each model with no peripherals attached?

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abishur
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 5:18 am

Both models are identical on that issue the max power the r-pi can draw is 1A.  Once it hits 1A a self resetting fuse latches until you unplug the power.  If you like looking at things mathematically, then the formula for figuring the max amount of power available for any attached peripheral is

1 - x = y

Where x is the amount of power current being used by the r-pi (which varies based on demand) and y is the amount of power left over for the attached peripherals (and of course 1 is 1A).
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ElectroPulse
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 5:32 am

Well, I understand that part... But what is the maximum known value for X? (Basically, in the future if I ever run a RPi without a hub, I would like to know what the maximum power is that I can pull through it to power devices, without infringing on the amount of electricity required to keep the RPi running. I"m mainly wondering for HDs and optical drives).

Jock in a Frock
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 10:18 am

ElectroPulse said:


Hmm… thank you for that helpful piece of info about micro USB… I actually didn't know that was a standard connector. I've seen that connector type only once, so I concluded that it was proprietary… Well, something else I need to add to my shopping list, thanks!


Just FYI, the EU has now mandated that the micro-USB connector become the standard for all new mobile phones. The idea behind that is that the amount of redundant mobile wall-warts has grown to ridiculous proportions and is environmentally harmful.

In future, when you buy a mobile phone, you won't get a charger. If you don't already have a charger with a micro-USB connector, you buy one separately.

hyena
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 1:59 pm

is it possible to feed the pi more than 1A @5V = approx 5w (if i remember my o level physics)

reason is : a 2.5" HD eats around 3W at max pelt .. so its getting likely if you add up the pi B's needs at full pelt there may be a power problem ? which usually means hard drive read write failures

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abishur
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 3:05 pm

I would assume that the max value for x would be around 300 mA since this is the max *estimated* for the model A.  This should leave you with 700 mA for the USB peripherals.

@hyena Yes.  There are always works arounds for any given problem

1) Desolder the self resetting fuse.  While you could technically just wire a jumper across the pads and deliver an unlimited amount of current this is a phenomenally bad idea and I would strongly suggest soldering a high Amperage self resetting fuse in its place.  Of course you'd need to be able to solder (and surface mount solder at that!) and it would void any sort of warranty that might come with the board.

2) Use the source/sink pins.  There are two pins in the GPIO strip that instead of being connected to the processor are connected to the 5V power rail.  You could provide power through these pins as they bypass *all* voltage protection.  If you did this I would strongly recommend making an add on board with your own voltage protection on it or risk burning your pi (I'm also guessing using this method would void your warranty

3) The recommend, non-warranty voiding way is the one the op is using.  Use a powered USB hub since a 2.5 inch hard drive is going to need two usb ports anyways.  Ergo you wouldn't be able to have anything else attached to the pi, ergo you'd need a hub anyways if you wanted to use a keyboard or mouse with your drive.
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 5:47 pm

The Y type USB cables are frequently used with 2.5" hard drives as it is not abnormal for a standard USB port to provide insufficient power. Therefore I would have thought you could have one arm of the Y (the data part) plugged into the RPi and the other arm plugged into a powered USB hub. However as I do not (yet!) have a RPi that is just supposition.

Another point from experience is that one needs to be careful with powered USB hubs as to the total power they can supply. Many hubs do not provide the full 500mA to each USB port.

WereCatf
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 6:01 pm

I am seeing 700mA mentioned as the max draw, but what is typical power draw of type B board with, say, HDMI, USB mouse and USB keyboard connected?

ElectroPulse
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 7:23 pm

Thank you for the responses!

Alright, just wanting to confirm, this is the type of connector I would need to get, correct? http://www.newegg.com/Product/.....6812200832

@Abishur: Just curious, if option 1 works, why did they use a 1A polyfuse, vs. a 1.5A (like the limit on a lot of cellphones is, I heard), or even 2A? Is keeping it that low just a safety precaution? Also, I could potentially see someone wanting to us a HD without a hub... it would work if they weren't using a GUI (or if they were using some form of remote desktop to control it)

But yea, with a base of 300mA, then I'm guessing that HDs would work? (or how much do they really draw?) I am also not sure of how much power is required to run HDMI, mouse and keyboard, so I am also looking for the answer to WereCatf's question as well

Joe Schmoe
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Re: RPi Power by USB Hub?

Sun Jan 29, 2012 7:43 pm

I've heard that the base draw for the B is, indeed, the same as for the A - 300mA.

But (and this is a big but) that doesn't include the onboard ethernet, which is (IMHO) the main reason to prefer the B in the first place.  According to what I read somewhere on this forum, the ethernet adapter can draw as much as 230mA, so that would put you over the magic 500 threshold number.

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digital_addict
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 12:32 am

The information Abishur gave is irrefutable.



Here are some numbers to flesh out other concerns.



Assuming a type “A” running flat out. 300ma + 500ma for one usb peripheral,so the largest psu you would require would need to supply 800ma. The current for the hdmi is irrelevant as this is supplied by the gpu and is therefore included in the 300ma.



The type “B” is a bit more complex. 700ma + 300ma available for usb consumption. This is not quite as bad as it looks, a mouse and a keyboard require a fraction of this. AFAIK the ethernet connection is provided from the usb section and is included in the 700ma. As any R-Pi is not likely to be running flat out permanently all of these numbers are subject to change.



As an afterthought, a usb wifi dongle will draw approx 350ma on transmit and 250ma on receive, with approx 35mw of RF.





To Abishur

If you feel there are errors in this post, please correct.

ElectroPulse
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 12:45 am

Thanks for the extra information!

So, is there really any disadvantage to experimenting around with the power drawn by the RPi? Like, if I plug something in (such as an external hard drive) that draws more power than it can handle, what could be some problems with that? Anything irrepairable? (I would assume not... unless important data was lost of course)

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Chromatix
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 1:05 am

If the polyfuse trips, you will only have to remove the load to reset it.  It's a pretty good system actually.
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ElectroPulse
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 1:15 am

Chromatix said:


If the polyfuse trips, you will only have to remove the load to reset it.  It's a pretty good system actually.


Ah, good point... I had just been imagining that if I tried to draw too much it wouldn't have enough electricity going through it, and it would just shut off or something.

Also, a question I asked previously (but was last post on previous page, so probably got missed):

Just curious, why did they use a 1A polyfuse, vs. a 1.5A (like the limit on a lot of cellphones is, I've heard), or even 2A? Is keeping it that low just a safety precaution?

digital_addict
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 1:36 am

ElectroPulse said:



So, is there really any disadvantage to experimenting around with the power drawn by the RPi?


You are always able to experiment.

Consider this, how quickly does your car accelerate?

Now add 6 average size passengers, what's the difference?



As far as the fuse issue is concerned, it's probably a recommendation from the chip manufacturer.

ElectroPulse
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 2:29 am

Well yes, of course I am always able to experiment... I can conduct an experiment to determine how much damage a stick of dynamite will do to my house too, but I won"t because I know I will damage something (What I meant by that post was that I was basically asking if it could potentially cause any damage to experiment with it... And it turns out the polyfuse is a safeguard against damaging the unit yourself, too )

axiomprime
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 3:34 am

Anyone know of a credit card sized powered USB hub? I don't suppose it'll be long before someone makes one built into a Pi case.

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abishur
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 4:58 am

We were discussing this in another thread and someone pointed out that I was forgetting that the max draw of the ethernet port under 100% load will be 231 mA.  Now the model A has a little room allocated for a keyboard plugged into which means even with the additional power the model B (without a heavy load on the USB ports) *should* squeak in under 500 mA, but my original statement of the model B power draw should be around 300 mA was incorrect (unless you're not using the ethernet port ).  Sorry about that.

As for why they went with the 1A fuse instead of a 1.5... I don't know sorry Maybe Gert or James or Liz will see then and drop some official goodness on us but my only speculation is that their focus is on getting a device who main accessory is a keyboard (and possibly mouse) into the hands of needy children.  As such they aren't considering a heavy draw on the USB port in which case a 1A limit makes sense.  If the board is drawing a full amp then something is probably amiss and they need to clamp that shut!
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Vindicator
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Re: RPi Power by USB Hub?

Mon Jan 30, 2012 5:11 am

I have been researching the powered hub issue and one problem is that a 4 port powered hub cost more than a 7 port or even in most cases a 10 port powered hub, I believe this is because 4 port hubs are desired more often than a seven or ten port hubs.

There are many 4 port hubs that can be powered that are close to credit card size but they cost comparably more than larger hubs and usually do not include the power adapter.

A seven port hub with power adapter that is 3 to 3.5 amps at 5 volts cost less than a 4 port hubs power adapter that only has 2 amps (mind that I am only saying the power adapter for a 4 port cost more than a 7 port hub with adapter)

The dimensions of USB hubs in general are not that well documented, even from the manufacturer sights quite often.

4 port hub

http://www.mcmelectronics.com/.....t/831-2054

data on hub

http://www.tripplite.com/en/pr.....delID=2524

size

3.5"L x 1.5"W x .5"H

can be found for around $20 us
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