wowfood
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stupid question on power

Tue May 29, 2012 9:10 am

Okay I get that the cable needs to be microUSB which can provide 700mA (i'm assuming that's the same as 0.7 amps) and 0.5v

One thing that confused me a tad though was, why should you not plug it in via USB hub / computer? I mean, can't you get powered hubs which only provide well, pretty much what the rasp needs per port? Is there some other reason you shouldn't do it?

Just curious really.

bredman
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Re: stupid question on power

Tue May 29, 2012 9:14 am

A lot of USB hosts will provide only 500mA, and this is not enough for the RPi. If you are lucky, your USB host will supply more than 500mA and you can use it OK.

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rurwin
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Re: stupid question on power

Tue May 29, 2012 9:40 am

(It's 5V, not 0.5V)

Hubs and computers are not required to supply enough current for the Raspberry Pi. USB ports other than dedicated charging ports are only required to supply 500mA, and that after software handshaking which the RaspPi cannot do.

Many hubs and Laptops do work, because their designers did not bother to limit the current, but the only way to find out is to try it.

Jaz_knos
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Re: stupid question on power

Tue May 29, 2012 10:52 pm

Having a good run with Belkin hubs so far- both 4 and 7 port are running the RPi with no problem!

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mahjongg
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Re: stupid question on power

Tue May 29, 2012 11:13 pm

Sometimes using an "Y-cable" to power the Pi from the combined power of two USB ports helps.

mikebiggs
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Re: stupid question on power

Thu May 31, 2012 3:29 am

If your laptop has a dedicated USB "charging" port, that will provide a full 1 amp (1000mA), as the yellow charging port on my Thinkpad does. More than enough for the RPi :) ...or just use a double cable as suggested above.

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jbeale
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Re: stupid question on power

Thu May 31, 2012 4:10 am

I don't have a Pi yet. Out of curiosity I measured the actual current drawn by a 320 GB USB-powered HDD (Seagate FreeAgent Go, p/n 9ZA2AG-500). It averaged around 650 mA when running with peaks around 850 mA. I was surprised the current was that high, because the drive works fine on the three desktop PCs I've tried and also a laptop USB port, without using a dual cable. The USB ports only need to provide 500 mA per standard, but apparently they often do somewhat more. Based on that data, my forthcoming R-Pi ought to work powered from a USB port as well. Needless to say, your mileage may vary depending on your particular system.

willhy
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Re: stupid question on power

Fri Jun 08, 2012 12:23 am

I've just received my Raspberry Pi after months upon months of waiting, and I'm afraid to power it on.

The documentation I received in my package from element14 states "This product shall only be connected to an external power supply rated at 5v dc, and a maximum current of 500ma."

However, these posts below, and several others sources I've read seem to imply that the current can and should be over 500ma.

I've searched the forums and web and I'm at a loss. Does anyone know if there is a definitive source that explains what the current should be? I can't tell if the Element14 documentation is entirely misleading or if there's some other form of confusion here.
mikebiggs wrote:If your laptop has a dedicated USB "charging" port, that will provide a full 1 amp (1000mA), as the yellow charging port on my Thinkpad does. More than enough for the RPi :) ...or just use a double cable as suggested above.
bredman wrote:A lot of USB hosts will provide only 500mA, and this is not enough for the RPi. If you are lucky, your USB host will supply more than 500mA and you can use it OK.

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rurwin
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Re: stupid question on power

Fri Jun 08, 2012 11:44 am

willhy wrote:I've just received my Raspberry Pi after months upon months of waiting, and I'm afraid to power it on.

The documentation I received in my package from element14 states "This product shall only be connected to an external power supply rated at 5v dc, and a maximum current of 500ma."
That is a very badly worded instruction. It should read "This product shall only be connected to an external power supply rated at 5V DC, and a maximum current of at least 500mA."

It is a matter of physics that any current rating over the given number will be perfectly OK. The RaspPi will take what it needs and no more. If there is not enough current available then the voltage will drop and it may become unstable or fail to boot. You will not break it whoever you choose to believe. There is a fuse on the board that will prevent it taking excessive currents.

The foundation's advice is at least 700mA. There have been problems with inadiquate power supplies which have led some to advise using supplies capable of supplying at least 1000mA. However in actual use the board seems to take only about 400mA, (although I didn't try watching any movies,) so I can believe that 500mA may be sufficient.

willhy
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Re: stupid question on power

Fri Jun 08, 2012 5:03 pm

Thank you for clarifying. I suspected as much.

I asked Element 14 directly on twitter and they said that "It should read: "greater than 700mA recommended""

I'm not sure if the misprint is only in my manual or if others might see this as well, but either way it's been brought to their attention.

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abishur
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Re: stupid question on power

Fri Jun 08, 2012 5:52 pm

willhy wrote:Thank you for clarifying. I suspected as much.

I asked Element 14 directly on twitter and they said that "It should read: "greater than 700mA recommended""

I'm not sure if the misprint is only in my manual or if others might see this as well, but either way it's been brought to their attention.
I *REALLY* wish someone would take the 5 minutes to fix that! :roll:
Dear forum: Play nice ;-)

grrrrr
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Re: stupid question on power

Fri Jun 08, 2012 6:01 pm

USB ports that appear identical physically may easily not be identical when it comes to providing power. Some *will* provide the power that the Pi needs - some won't provide enough unless the Pi "asks for it" - which the Pi cannot do as its power socket isn't configured to do so. Hence the "sure to work" solution is to use a power brick and not a USB port to power the Pi. That doesn't mean that a Pi won't work when powered by one (or more using a Y cable) USB port(s) - it just means that it may not work when powered that way. Some motherboards now provide "charging" power on one or more of the motherboard USB ports, but this may require a BIOS update to enable, if the motherboard is a year or two old.


Squirrelking
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Re: stupid question on power

Sat Jun 09, 2012 2:35 am

I've been wondering this myself as I have a power supply that will provide the required 5V but at a higher amperage.

I know that it will only draw what it needs so I reckon I should be safe enough connecting it up (with a bit of butchering to make up a connection) but with an inline fuse to be on the safe side (my RS instructions say a max supply of 1.2A).

By this logic would it be safe to say that it's perfectly acceptable to connect the Pi to any power supply with a 5V rail (ATX, XBOX, whatever) as long as a fuse is fitted and polarity is correct?

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nick.mccloud
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Re: stupid question on power

Sat Jun 09, 2012 8:53 am

Squirrelking wrote: By this logic would it be safe to say that it's perfectly acceptable to connect the Pi to any power supply with a 5V rail (ATX, XBOX, whatever) as long as a fuse is fitted and polarity is correct?
Almost, it would be OK to connect it to the 5v rail of a power supply as long as the polarity is correct, fuse optional but a good idea.

Your version leaves wiggle room for someone to connect it to the 12v rail of a power supply that also has a 5v rail. Then, after the smoke settled, they'd come and tell us all about it!

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rurwin
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Re: stupid question on power

Sat Jun 09, 2012 10:30 am

Squirrelking wrote:By this logic would it be safe to say that it's perfectly acceptable to connect the Pi to any power supply with a 5V rail (ATX, XBOX, whatever) as long as a fuse is fitted and polarity is correct?
A massive power supply such as those you list can have other requirements, such as a minimum current. If you don't draw the minimum current then the 5V is not regulated. That would be a bad thing. It's perfectly possible to use supplies of this size with the RaspPi, but you should know what you are doing. I'm using one myself with a maximum current of 9A. (That one you really have to know what you're doing; there's 400V DC on that circuit board.)

Squirrelking
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Re: stupid question on power

Sat Jun 09, 2012 7:18 pm

Well, yeah but then if they connect it to the 12V rail then a part of me feels they shouldn't really be playing with things they don't understand. Harsh but fair, measure twice cut once and all that.

My reasoning for an ATX supply (or equivilent) is that it allows for greater flexibility in terms of connecting hard drives, optical media and such. I'll take a look at the minimum current issue, is that likely to be displayed on the rating plate of the device or will I have to go hunting for data sheets?

bredman
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Re: stupid question on power

Sat Jun 09, 2012 7:34 pm

Squirrelking wrote:I'll take a look at the minimum current issue, is that likely to be displayed on the rating plate of the device or will I have to go hunting for data sheets?
You will find out if you search the internet for "ATX power supply as bench supply".
In general, you will need to put a dummy load on the 5v and 12v rails, otherwise the ATX supply will refuse to turn on. You will probably need to burn at least 30 watts. Not very practical to power a Raspberry Pi that only needs a few watts.

Squirrelking
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Re: stupid question on power

Sat Jun 09, 2012 10:23 pm

No I guess not, Loading the 12V is less of an issue if I was running hard drives but as you say it's not particularly practical and I still need to load the 5V line. Guess the XBOX idea is off for now.

Other plan is to use a Gamecube power supply which is much smaller and possibly a better bet. The power supply itself comes from a transformer on the cable which converts AC to 12V 3.5A DC before plugging into the case. There is then an inline hard switch which goes down to a regulator where the lines are split. From looking at the pinout there is a 5V line which powers the mainboard and would be suitable for tapping off into the Pi. From looking at various "portable" forums (basically hacking the cube into a gameboy type device) the sole 12V line for the optical drive is the primary power consumer and nobody seems to have had any bother running the inverter at lower power on the 5V lines.

Thanks for the help, I do know a little but certainly not enough electronics by the look of it (I prefer to hit things for a living, mechanics are far easier to predict) :lol:

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rew
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Re: stupid question on power

Sun Jun 10, 2012 6:43 am

bredman wrote: You will find out if you search the internet for "ATX power supply as bench supply".
In general, you will need to put a dummy load on the 5v and 12v rails, otherwise the ATX supply will refuse to turn on. You will probably need to burn at least 30 watts. Not very practical to power a Raspberry Pi that only needs a few watts.
I have the impression that this is a myth, with everybody repeating what they've heard. Some powersupplies might require this, but I'd think most will not.

To prove my point, I have a powersupply lying around here. It's not inside a computer because there was something wrong with it.

So I connected up the mains connector, put my multimeter black lead in the black pin of a disk-connector. and I found a neat 5.02V on the "5V standby" pin. In the document I found as the first hit on google I found on page 27 that the 5VSB should provide at least 2.5A of current. This should be enough to power three raspberry pies! No fan noise! great! Guaranteed to be 5V even when unloaded.

Next I turned on the powersupply. Now I know why it was not inside a computer: the fan is bad. Now I found a "great for a 'pi" 5.06V on the 5V lines. Regulated and all. No problems.

Now I don't claim that ALL powersupplies will work unloaded. I just claim that SOME will. At least the one I have here works. But if you want to run off an ATX powersupply I suggest you look into using the "5VSB" line, as not having the fan running sounds (pun intended :-) ) like a good idea.
Check out our raspberry pi addons: https://www.bitwizard.nl/shop/

Squirrelking
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Re: stupid question on power

Thu Jun 14, 2012 4:43 am

So...

Any more thoughts people?

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