evilkitty
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Raspberry PI 2 amperage needs

Mon May 11, 2015 7:01 pm

I had ordered a used model B+ pi from a seller on amazon
i got a dead unit so they sent me a new one, i they sent a raspberry pi 2 model b instead
I know this model calls for 1.5 to 2A, can i get by with 0.9A with no USB accessories or monitor/GUI?
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kusti8
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Re: Raspberry PI 2 amperage needs

Mon May 11, 2015 7:16 pm

Yes, you can get by. I tested the pi with 0.7A and it worked.
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evilkitty
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 1:11 am

thanks
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MarkHaysHarris777
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 1:42 am

evilkitty wrote:I know this model calls for 1.5 to 2A, can i get by with 0.9A with no USB accessories or monitor/GUI?
That depends on what you mean by, "get by".

Will it boot up, idle, and run, mostly? Yes.

Will it continue to run reliably without brownout or reset? No.

That is the truth. The RPi 2B typically pulls more than half an amp at idle, and "often" spikes over 1.+ amps during certain system activities even with nothing plugged in. Here is the thing you need to know, if you don't by now already: the RPi has two systemic anomalies: 1) its a power hog, and 2) its usb port is an overworked OTG chip that essentially "everything" passes through... all four usb ports and the ethernet port (and it uses a relative heap of power too). You will sooner or later experience difficulties if you use a power adapter less than 1.5 amp. I recommend a power adapter (with a relative short cord) of 2200 to 2500 mA. If you are going to plug usb accessories into the RPi use a quality powered hub, and read the usb blog on this site first before buying a powered hub.

PS Don't listen to *anyone* who tells you they power their PI via the hdmi port, or they use a microwatt solar array, nor anything else like that... you will need a 2.2 amp power adapter with a quality plug and cable-- period.

you've been warned
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 1:51 am

MarkHaysHarris777 wrote:
evilkitty wrote:I know this model calls for 1.5 to 2A, can i get by with 0.9A with no USB accessories or monitor/GUI?
That depends on what you mean by, "get by".

Will it boot up, idle, and run, mostly? Yes.

Will it continue to run reliably without brownout or reset? No.

That is the truth. The RPi 2B typically pulls more than half an amp at idle, and "often" spikes over 1.+ amps during certain system activities even with nothing plugged in. Here is the thing you need to know, if you don't by now already: the RPi has two systemic anomalies: 1) its a power hog, and 2) its usb port is an overworked OTG chip that essentially "everything" passes through... all four usb ports and the ethernet port (and it uses a relative heap of power too). You will sooner or later experience difficulties if you use a power adapter less than 1.5 amp. I recommend a power adapter (with a relative short cord) of 2200 to 2500 mA. If you are going to plug usb accessories into the RPi use a quality powered hub, and read the usb blog on this site first before buying a powered hub.

PS Don't listen to *anyone* who tells you they power their PI via the hdmi port, or they use a microwatt solar array, nor anything else like that... you will need a 2.2 amp power adapter with a quality plug and cable-- period.

you've been warned
The Pi2 will run fine with 0.9A, even under load, although I haven't done any tests.
http://raspi.tv/tag/raspberry-pi-2-power-usage says 350 MA is the max under load. I plugged in my 0.7A power supply into my Pi2, and don't notice it was in for about a week! Headless and no USB, why waste your money on a good power supply if what you have is enough?
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MarkHaysHarris777
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 2:10 am

kusti8 wrote: The Pi2 will run fine with 0.9A, even under load, although I haven't done any tests.
Well, then, its time for some tests-- right?

You've already got python3 on-board... all you need is my pdeclib (you can get it from google code under PythonDecimalLibrary, or you can get it from PyPi) or you can google pdeclib and take either the first or second hit.

dscale(1010)
piagm2()

That will produce 1000+ digits of PI very quickly... now for the fun...

dscale(10000010)
piagm2()

While that is running, and it will take a while, measure the current draw on the 5v line... you might want to use a power adapter larger than .9 amp (hint) Now, you can tune the system to run slower for instance, or you can set the niceness and have it mostly backgrounding, but that is to miss the point.

Try it, let me know what you measure... and then consider what happens when you start to plug things into it...


PS <edit> Sorry, forgot to answer your question... the reason you use a 2200 mA switching supply when you can 'get by' with 900 mA is that: 1) the supply is the whole deal, and its cheap, and 2) brown outs and resets corrupt SD cards; not a good plan. If you think you need 1000 mA have a supply that will deliver 2200 mA and run the supply more efficiently, and make sure your system does not brown out under some loads. YMMV of course. You do not have to agree with my wisdom; heck, do as you wish, its your RPi after all...
Last edited by MarkHaysHarris777 on Tue May 12, 2015 2:44 am, edited 1 time in total.
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kusti8
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 2:43 am

MarkHaysHarris777 wrote:
kusti8 wrote: The Pi2 will run fine with 0.9A, even under load, although I haven't done any tests.
Well, then, its time for some tests-- right?

You've already got python3 on-board... all you need is my pdeclib (you can get it from google code under PythonDecimalLibrary, or you can get it from PyPi) or you can google pdeclib and take either the first or second hit.

dscale(1010)
piagm2()

That will produce 1000+ digits of PI very quickly... now for the fun...

dscale(10000010)
piagm2()

While that is running, and it will take a while, measure the current draw on the 5v line... you might want to use a power adapter larger than .9 amp (hint) Now, you can tune the system to run slower for instance, or you can set the niceness and have it mostly backgrounding, but that is to miss the point.

Try it, let me know what you measure... and then consider what happens when you start to plug things into it...

Well, the OP isn't planning on putting anything in and the link that I posted, along with many others show that 0.9A is enough. I don't need to do any tests myself, it's already been proved.
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 2:46 am

MarkHaysHarris777 wrote: Try it, let me know what you measure... and then consider what happens when you start to plug things into it...
OP said they weren't going to plug anything into the USB.

(snap!)

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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 2:53 am

kusti8 wrote: Well, . . . the link that I posted, along with many others show that 0.9A is enough. I don't need to do any tests myself, it's already been proved.
So, just to clarify, you've 'heard' that there is no problem with a 900 mA supply, and so far you haven't had any problems yourself, so that which you've 'heard' must be true... even though you have not run any empirical tests yourself? right? ... I'm just asking.
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 2:59 am

ame wrote:
MarkHaysHarris777 wrote: Try it, let me know what you measure... and then consider what happens when you start to plug things into it...
OP said they weren't going to plug anything into the USB.

(snap!)
hi ame, there is no snap... the processor spikes above 1.0 amp without *anything* plugged into it. But of course the OP must be (speaking less than completely truthfully) since there really is no point in NOT plugging 'something' into the RPi. Something has to be plugged into it, or why? Some people don't consider a wipi adapter a usb accessory. Some people don't understand that the ethernet port on the RPi is a usb accessory. So, I'm not sure what is going on with the claim from the OP that nothing will be plugged into it... but that is neither here nor there... what amazes me is that without empirical testing so many people are willing to make rash claims because they read something on the net, or heard something on the street. What happened to the 'science' in computer science?
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ame
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 3:59 am

MarkHaysHarris777 wrote:
ame wrote:
MarkHaysHarris777 wrote: Try it, let me know what you measure... and then consider what happens when you start to plug things into it...
OP said they weren't going to plug anything into the USB.

(snap!)
hi ame, there is no snap... the processor spikes above 1.0 amp without *anything* plugged into it. But of course the OP must be (speaking less than completely truthfully) since there really is no point in NOT plugging 'something' into the RPi. Something has to be plugged into it, or why? Some people don't consider a wipi adapter a usb accessory. Some people don't understand that the ethernet port on the RPi is a usb accessory. So, I'm not sure what is going on with the claim from the OP that nothing will be plugged into it... but that is neither here nor there... what amazes me is that without empirical testing so many people are willing to make rash claims because they read something on the net, or heard something on the street. What happened to the 'science' in computer science?
This guy did it, so I don't have to.
http://raspi.tv/2015/raspberry-pi2-powe ... easurement

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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 5:32 am

ame wrote: This guy did it, so I don't have to.
http://raspi.tv/2015/raspberry-pi2-powe ... easurement
So, you're not at all interested in trying it yourself? Or, is it that you don't know how... just put a mA meter in series with the 5v line and the RPi and take your own measurements... something about that guy's tests should be troubling you me thinks... simple question: what is causing the brown-outs and resets on folks RPi units trying to use low output switching supplies?

I gues, they imagined their brownouts and resets... and I suppose Element14 is recommending and selling 2200 mA power units for the RPi because they just want to over-charge non suspecting clients? Or, could there be a reason that Element14 recommends 2200 mA power adapters?
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 6:12 am

MarkHaysHarris777 wrote:
ame wrote: This guy did it, so I don't have to.
http://raspi.tv/2015/raspberry-pi2-powe ... easurement
So, you're not at all interested in trying it yourself? Or, is it that you don't know how... just put a mA meter in series with the 5v line and the RPi and take your own measurements... something about that guy's tests should be troubling you me thinks... simple question: what is causing the brown-outs and resets on folks RPi units trying to use low output switching supplies?

I gues, they imagined their brownouts and resets... and I suppose Element14 is recommending and selling 2200 mA power units for the RPi because they just want to over-charge non suspecting clients? Or, could there be a reason that Element14 recommends 2200 mA power adapters?
I am quite capable of measuring my Pi's power consumption if I cared to. It is running happily on a 1000mA PSU.

Element 14 recommends 2.2A power adapters because they state the Pi's power consumption is 1.8A.
http://www.element14.com/community/docs ... tions-faqs

OP wanted to know if they could get away with 900mA, I say yes. YMMV.

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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 6:19 am

ame wrote: Element 14 recommends 2.2A power adapters because they state the Pi's power consumption is 1.8A.
http://www.element14.com/community/docs ... tions-faqs
So, the world renowned electronics distributor Element14 'claims' that the RPi 2B power consumption is 1.8A... hmmm I wonder, did they pull that number out of the air, or did they, um measure it? ... the world renowned eletronics testing company Element14 just , um, guessed at that number, or do you think they ran some tests?? 'ey Barny ? :roll:

... I rest my case.
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 6:30 am

MarkHaysHarris777 wrote:
ame wrote: Element 14 recommends 2.2A power adapters because they state the Pi's power consumption is 1.8A.
http://www.element14.com/community/docs ... tions-faqs
So, the world renowned electronics distributor Element14 'claims' that the RPi 2B power consumption is 1.8A... hmmm I wonder, did they pull that number out of the air, or did they, um measure it? ... the world renowned eletronics testing company Element14 just , um, guessed at that number, or do you think they ran some tests?? 'ey Barny ?

... I rest my case.
They probably calculated that the USB ports can deliver 1.2A in total and added 600mA for the base load of the SoC.

Or, to put it another way, a Pi with nothing plugged into USB needs less than 900mA of supply current. Which was the OP's question.

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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 6:57 am

ame wrote: They probably calculated that the USB ports can deliver 1.2A in total and added 600mA for the base load of the SoC.
I love that word "probably". You have no idea.

And, maybe you will go back and 'read' what I initially answered (yes, it will work; no, it will not be reliable). The main point really ame is that your answer is a mute point in practical regards, because its silly to have an RPi that has nothing plugged into it! If the switching supply is inadequate (and you inevitably start plugging things into it) at some point you will experience a brown-out or reset (lots of folks have had this experience). I would like to spare folks that specific experience and tell them the truth up front. Element14 is doing the same thing, just telling people the truth so they won't believe a fallacy and get bit...
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thsBavR10
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Re: Raspberry PI 2 amperage needs

Tue May 12, 2015 1:55 pm

The foundation states:
https://www.raspberrypi.org/documentati ... /README.md
"The device is powered by a 5V micro USB supply. Exactly how much current (mA) the Raspberry Pi requires is dependent on what you connect to it. We have found that purchasing a 1.2A (1200mA) power supply from a reputable retailer will provide you with ample power to run your Raspberry Pi.
..."
Another source: http://elinux.org/RPi_Hardware#Specifications -> Power ratings
and http://elinux.org/RPi_Hardware#Power

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Re: Raspberry PI 2 amperage needs

Wed May 13, 2015 12:10 am

on the note of the quality of my 0.9A source
it is the USB 3.0 port of my custom desktop
Seasonic m12ii-520 PSU and a AsRock Z97 Killer motherboard
my USB power is pulled from the +5v standby rail rated for 2.5A, i assume the motherboard caps it to 0.9A as that is the specked amperage for USB 3
i measured the voalge while i had it on, i was reading 4.98v using a decent quality (26AWG) USB cable (the crappy ones only get 4.6v to 4.7v, i tested a dollar store cable)
i do plan to make my own cable with 20AWG wire if my USB A plugs ever come in

IMO they really should have used a USB 3 port and/or added 2 pins like the GPIO near the plug as a second/additional power source input
Last edited by evilkitty on Wed May 13, 2015 12:32 am, edited 1 time in total.
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Re: Raspberry PI 2 amperage needs

Wed May 13, 2015 12:20 am

because its silly to have an RPi that has nothing plugged into it!
And what about all those people who run their pis headless without anything plugged into it (including one of mine)?
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Re: Raspberry PI 2 amperage needs

Wed May 13, 2015 12:35 am

kusti8 wrote:
because its silly to have an RPi that has nothing plugged into it!
And what about all those people who run their pis headless without anything plugged into it (including one of mine)?
How many is that, I wonder?

So, let me try to understand this... you have an RPi powered up with 'nothing' plugged into it (no gpio, no eth0, no wipi, no usb anthing, 'nothing') ... why? ... I know, you're using it for a dummy load on your switching power supply...

I have several headless PI(s) running but they all have at 'least' one thing plugged into them (one of them has an eth0 and the other two have a wipi adapter)... there are cheaper ways to power a glowing LED in a translucent case!

Or, is this just an academic argument ... yes, its possible to run an RPi with minimal current drain... ?

No, I've got it... you're one of those who use their RPi as an mp3 music kiosk...? ... no no no, I get it, its 'dead' and you use it to prop open your bathroom door?
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Re: Raspberry PI 2 amperage needs

Wed May 13, 2015 4:23 am

Mark,

ethernet doesn't require any additional current from the 600/1200 mA additional current provided for USB.

You seem to have no idea what 'running headless' means.

And yes, a headless Pi 2 will run happily with round about 1A. It's a simple calculation. If it can supply up to 1,2 A to the USB ports, all the rest shouldn't use more than about 800-1000 mA.

Repeating the same wrong stuff dozens of times doesn't make it more true.
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Re: Raspberry PI 2 amperage needs

Wed May 13, 2015 5:53 am

gkreidl wrote:You seem to have no idea what 'running headless' means.
That's possible... correct me if I'm wrong... a headless RPi is one which has no connected display, keyboard, or mouse in the classical workstation sense; the display (the head) has been removed. All of my servers run this way as do three of my RPi(s). I access them via ssh &| vnc from another console quite a distance away.

gkreidl wrote: Repeating the same wrong stuff dozens of times doesn't make it more true.
I have said 'nothing' wrong. I have qualified all of my statements, and I stand by them. I will continue to make the same power recommendations (as it pertains to the RPi) until something changes. I am certain that Element14 will do the same (I will also continue to qualify my statements and to the best of my ability be as 'correct' and accurate as possible; you of course are free to disagree with me or my technical stance).
Last edited by MarkHaysHarris777 on Wed May 13, 2015 6:24 am, edited 1 time in total.
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Re: Raspberry PI 2 amperage needs

Wed May 13, 2015 5:59 am

gkreidl wrote:And yes, a headless Pi 2 will run happily with round about 1A. It's a simple calculation. If it can supply up to 1,2 A to the USB ports, all the rest shouldn't use more than about 800-1000 mA.
If you were to 'read' the rest of this thread then you would know that you and I are closer in opinion here (on this issue) then some of the other posters... who claim that the RPi can be run with less than .9 A (some have said less than .7 A. Well, I did not disagree with them either... it will... BUT NOT RELIABLY. And, that's what I have said from the beginning of this stupid thread. Element14 recommends something greater than a 1.8A supply (as do I) and they sell a 2200 mA supply to meet that need (I have purchased several of them, and will continue to). I will also continue to recommend at least a 2200 mA switching supply for the RPi 2B because that is what tests show is required for reliable operation. I'm right.
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Re: Raspberry PI 2 amperage needs

Wed May 13, 2015 6:52 am

MarkHaysHarris777 wrote:I'm right.
Statistically speaking I suppose it might happen sometimes.

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Re: Raspberry PI 2 amperage needs

Wed May 13, 2015 7:18 am

ame wrote:
MarkHaysHarris777 wrote:I'm right.
Statistically speaking I suppose it might happen sometimes.
ame we're gonna be friends yet...

... my humanities professor used to like to tell his freshman smart-arses, "Even a stopped clock is right twice a day!"

Happy Wednesday!
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