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allfox
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I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 8:46 am

I'm setting up an external USB to SATA hard disk container. It has a USB wire whose shape like "Y", one end is a single USB, other end has two USBs, which I think is for draining power from two USB ports.

My hard disk says it require 700mA current to work, so I used two Pis to power up the container. Single USB end connected to container, the double USB end connected to two Pis, each USB for one Pi.

The interesting part is, I find that if I unplug one Pi's Micro USB power, that Pi won't stop. It seems it is draining power via the "Y" cable from other Pi.

However, it can't boot via "Y" cable's power. Booting require Micro USB power.

I just feel it's interesting.

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MarkHaysHarris777
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 9:02 am

Its way worse than just interesting...

... it can seriously damage your RPi. The phenomenon is called back-feeding. You should be using a powered usb hub (a quality one) which does not permit back-feeding.

You should be able to power down your RPi and remove the micro usb connector from the power jack... and your powered hub should 'still' power the devices attached to it, yet, your RPi should be 'off'... no LEDs, nothing.

The problem with back-feeding is that if there is a power spike (common depending on where you live) your RPi is NOT protected from the spike through its normal internal voltage regulators-- and it could get zapped; this happens way more than it should.

(just words to the wise)
marcus
:ugeek:

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allfox
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 9:08 am

Ah, so it's unprotected and dangerous, got it and removed.

Thanks for quick reply!

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PeterO
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 10:02 am

MarkHaysHarris777 wrote: The problem with back-feeding is that if there is a power spike (common depending on where you live) your RPi is NOT protected from the spike through its normal internal voltage regulators-- and it could get zapped; this happens way more than it should.
There is no regulation of the 5V supply on the PI. It relies on the external supply being well regulated. All other lower voltages ARE regulated on the PI and back powering will make no difference to them.

However, I believe USB back powering will not work on latest models (B+ and Pi2) due to presence of current limiting circuits on the USB ports.

PeterO
Discoverer of the PI2 XENON DEATH FLASH!
Interests: C,Python,PIC,Electronics,Ham Radio (G0DZB),1960s British Computers.
"The primary requirement (as we've always seen in your examples) is that the code is readable. " Dougie Lawson

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MarkHaysHarris777
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 10:09 am

PeterO wrote: There is no regulation of the 5V supply on the PI. It relies on the external supply being well regulated. All other lower voltages ARE regulated on the PI and back powering will make no difference to them.
Not so. If you back-feed via usb the 3v3 is not safe, and you can zap it... you've been warned.

This is from the FAQ on this site:

2. CAN I POWER THE RASPBERRY PI FROM A USB HUB?

It depends on the hub. Some hubs comply with the USB 2.0 Standard and only provide 500mA per port, which may not be enough to power your Raspberry Pi. Other hubs view the USB standards more like guidelines, and will provide as much power as you want out each port. Please also be aware that some hubs have been known to “backfeed” the Raspberry Pi. This means that the hubs will power the Raspberry Pi through its USB cable input cable, without the need for a separate micro-USB power cable, and bypass the voltage protection. If you are using a hub that “backfeeds” to the Raspberry Pi and the hub experiences a power surge, your Raspberry Pi could potentially be damaged. The additional information column provides details as to whether the Raspberry Pi can be powered directly off the hub and if the hub “backfeeds” power to the Raspberry Pi.

Again, you've been warned.
marcus
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PeterO
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 10:43 am

MarkHaysHarris777 wrote:
PeterO wrote: There is no regulation of the 5V supply on the PI. It relies on the external supply being well regulated. All other lower voltages ARE regulated on the PI and back powering will make no difference to them.
Not so. If you back-feed via usb the 3v3 is not safe, and you can zap it... you've been warned.
The 3v3 IS regulated on the PI. The 3v3 regulator doesn't know if the 5V source is comming via USB or micro USB or GPIO. The 5V source makes no difference to the 3v3 suppy !


PeterO
Discoverer of the PI2 XENON DEATH FLASH!
Interests: C,Python,PIC,Electronics,Ham Radio (G0DZB),1960s British Computers.
"The primary requirement (as we've always seen in your examples) is that the code is readable. " Dougie Lawson

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allfox
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 10:45 am

PeterO wrote:However, I believe USB back powering will not work on latest models (B+ and Pi2) due to presence of current limiting circuits on the USB ports.
I'm using two Pi 2 with max_usb_current=1.

Connecting them with a USB wire directly, without hub.

It won't boot via USB power, but one could last after removing Micro USB power.

Didn't work more on it.

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rpdom
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 10:49 am

allfox wrote:I'm using two Pi 2 with max_usb_current=1.

Connecting them with a USB wire directly, without hub.

It won't boot via USB power, but one could last after removing Micro USB power.
Yes, it will work like that. When the Pi2 or B+ starts the USB power is off, so there is no way to back power at that point.
ut once it has started booting the USB power circuit is activated and will allow the 5V to flow in both directions so it can keep powering the Pi even if the main power is unplugged.

The A, B and A+ don't have that power control circuit and will power up immediately if power is connected to the USB ports.

ame
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 11:06 am

MarkHaysHarris777 wrote:Its way worse than just interesting...

... it can seriously damage your RPi. The phenomenon is called back-feeding.
It will not seriously damage your Pi. I use it to power my Pi media centre and external hard drive. It's an original Pi B. I don't think back-feeding works with the B+ or 2B.

Ongelma
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 11:08 am

Maybe a little off topic but this reminded me to post this here... Here goes:

When i received my pi2 last month i purchased a new and shiny hdmi cable with it. Now that i connect the pi with hdmi to tv and the power it using micro usb.... the pi starts and boot goes to end it gives me the login question. I can then remove the micro usb power and the pi stays on with only HDMI cable connected.... Works like a charm and been running like this for 2 weeks now. :)

Dunno what really is happening, maybe a bad connection in the pi itself but im happy it works like this.

So if you guys have pi2, could you try this. Is it just my pi or all the pi2's ?!

BMS Doug
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 11:14 am

Ongelma wrote:Maybe a little off topic but this reminded me to post this here... Here goes:

When i received my pi2 last month i purchased a new and shiny hdmi cable with it. Now that i connect the pi with hdmi to tv and the power it using micro usb.... the pi starts and boot goes to end it gives me the login question. I can then remove the micro usb power and the pi stays on with only HDMI cable connected.... Works like a charm and been running like this for 2 weeks now. :)

Dunno what really is happening, maybe a bad connection in the pi itself but im happy it works like this.

So if you guys have pi2, could you try this. Is it just my pi or all the pi2's ?!
All Pi2's and B+'s...

just like Ame said for USB, the same applies for HDMI.

The backpowering prevention only works until the Pi has been powered up, then it doesn't work anymore, this does have the disadvantage of bypassing the fuse.
Doug.
Building Management Systems Engineer.

ame
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 11:17 am

Ongelma wrote:Maybe a little off topic but this reminded me to post this here... Here goes:

When i received my pi2 last month i purchased a new and shiny hdmi cable with it. Now that i connect the pi with hdmi to tv and the power it using micro usb.... the pi starts and boot goes to end it gives me the login question. I can then remove the micro usb power and the pi stays on with only HDMI cable connected.... Works like a charm and been running like this for 2 weeks now. :)

Dunno what really is happening, maybe a bad connection in the pi itself but im happy it works like this.

So if you guys have pi2, could you try this. Is it just my pi or all the pi2's ?!
Actually, you should have started a new thread, but since it's here I will say I've seen the same thing. My Pi 2 was connected to a pico-projector, and when I shut it down and unplugged the USB power connector the red LED stayed on. Presumably the Pi 2 was drawing power from the pico-projector via HDMI.

I haven't looked into it at all, but your message has prompted me to think about it again.

I'm not sure it's a Good Thing. However if it's a legitimate design feature then I suppose I can choose to use it or not.

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allfox
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 11:22 am

Ongelma wrote:Maybe a little off topic but this reminded me to post this here... Here goes:

When i received my pi2 last month i purchased a new and shiny hdmi cable with it. Now that i connect the pi with hdmi to tv and the power it using micro usb.... the pi starts and boot goes to end it gives me the login question. I can then remove the micro usb power and the pi stays on with only HDMI cable connected.... Works like a charm and been running like this for 2 weeks now. :)

Dunno what really is happening, maybe a bad connection in the pi itself but im happy it works like this.

So if you guys have pi2, could you try this. Is it just my pi or all the pi2's ?!
It sounds great.

Why we ever need a USB power control circuit anyway? I think if Pi is able to power up via USB/HDMI/GPIO/Micro USB etc. is great. The more the better.

The HDMI way just like a Chromecast, sounds really cool.

BMS Doug
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 11:25 am

allfox wrote: It sounds great.

Why we ever need a USB power control circuit anyway? I think if Pi is able to power up via USB/HDMI/GPIO/Micro USB etc. is great. The more the better.

The HDMI way just like a Chromecast, sounds really cool.
it won't power up by either USB or HDMI but once it has been powered up (by Micro USB or GPIO) it will continue to run while 5v is present on USB or HDMI.
Doug.
Building Management Systems Engineer.

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Lob0426
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Re: I`m watching a Pi powering via USB, not Micro USB.

Thu May 07, 2015 4:03 pm

Back-powering on RasPii has been working since the early boards (B and A) it ended with the B+. The A+ back-powers just fine. I have tested it on an A+, no problem.

Back-powering is no more, or just as dangerous as powering by GPIO. It just depends on the quality of the power.

What should not be done is to have power at the power connector and back-powering from the USB at the same time. The difference between the two is what can fry your RasPi.

I back-powered my web server, powered it only by the USB port, for about a year and it still operates today. I used a modified 4 port USB hub, that I intentionally made to back-power. This gave me 5 powered USB ports. It was hardwired for much of the time. Then I added pins and a connector to it later. It was finally exchanged for a 512 B model. But again it still works to this day. Just have to plug it in.

The advantage was that I only had two wires into the server, power to the hub and an Ethernet cable. I used to plug my iPhone into for charging. For a time it ran WiFi with a USB HDD with no issues. Something that was rarely done on model B's. Of course it was a hub!

Never power a Raspberry Pi with both micro USB and power coming into the USB ports (power coming from a USB device) together. One or the other works just fine.
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