power from bigger computer

I'm seeking a second opinion about an idea. I'd like to power a
Raspberry Pi from a larger computer. I can't think of anything
that could go wrong with the setup I'm proposing.
I have a Raspberry Pi connected to a USB TV tuner. The tuner has
its own wall wart power supply. Currently, the Raspberry Pi has
its own power supply. There is an ethernet cable from the Pi to
an ethernet switch. The Pi sits on top of a larger desk-side
computer that is also connected to the ethernet switch. (The Pi
is a go-between so I don't have to reboot or mess with the big
computer when the TV tuner goes comatose.)
The big computer has a >1000W power supply that is seriously
underutilized. "sensors" on the big computer says the 5V rail is
at 5.09V. All equipment is usually powered on 24x7.
The problem:
The Raspberry Pi's wall wart power supply (the 5.25V unit from
AdaFruit) generates enough RFI that it mostly obliterates
reception of a local AM radio station at 860kHz anywhere inside
the house.
Proposed solution:
I'd like to run wires from the big computer ground and +5V (with
a 1A fuse) up to the Raspberry Pi's power input connector.
Why I think it should work:
The only things connected to the Pi are power input, ethernet,
and USB to the TV tuner. Ethernet is electrically isolated, so
there can't be ground loop issues there. The USB TV tuner has
its own wall wart power supply, so that can't cause ground loop
issues, either.
Is there something my analysis is missing that would be
Robert Riches 
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Reply to
Robert Riches
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I have 3 Pis in the house. They all radiate huge amounts of RF noise that I pick up throughout the HF and VHF bands when they are connected to an Ethernet cable.
I would suggest you check that it is not that which is giving you problems at 860KHz.
My solution is to remove the cat5 cable and run them via wifi. !
getting the wrong stick end since 1953
Reply to
nev young
You're over analysing it - just plug in 5v and go.
I regularly power Pi's via their 3 inputs (µUSB, main USB and GPIO) from a stable 5v supply without any issues. e.g. right now I have one powered off my desktop PC's USB via a phone USB lead.
I thought I'd be clever and power one from a TV's USB socket once - worked well until I turned the TV off )-:
Reply to
Gordon Henderson
"Gordon Henderson" wrote in message news:l8v5ci$lvt$ snipped-for-privacy@dont-email.me...
I've powered my PI from the front USB socket of my cable DVR. Works fine, and it never gets turned off, so, that is never a problem.
Bill Garber
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Reply to
Bill Garber
Just use a standard USB to micro USB cable, and connect it to one of the computer's USB ports. Yes, it's in violation of the USB protocol for power draw, but most USB ports don't enforce the restricted power draw, and they will have a polyfuse built-in to protect the USB cable from the high current available from the PC's PSU.
Might want to check which USB ports in the computer remain powered when the computer drops into standby mode.
Andrew Gabriel 
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Reply to
Andrew Gabriel
If you power if from any of the ports other than the micro USB power connector, bare in mind you have bypassed the built-in polyfuse protection. I power them over the GPIO connector sometimes, but I do add my own polyfuse in that case.
Reminds me of a monitor I had with a built-in USB hub, which was useful for the keyboard and mouse... ... until the screen blank kicked in and the built in USB port powered off, rendering the keyboard and mouse useless for waking the screen back up again ;-)
Andrew Gabriel 
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Reply to
Andrew Gabriel
I have one of those as well... What a stupid design!
Reply to
I've also done it via the SD card port (which wasn't what I intended, but it works).
My monitor does that - if you turn it on and the Pi isn't outputting signal (something wrong with the SD card, or HDMI isn't plugged in) it goes through a few seconds of 'please give me a signal' then turns off, taking the Pi with it. It would be rather embarassing if the screenblanker also did it...
Reply to
Theo Markettos

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