noise comeing from usb afecting thermistors

HI. This was my initial setup: mega 2560 rev3, ramps 1.4 and a reprapdiscount lcd controller
all was working ok. I decided to add a raspberry pi3 to the set up. I assembly all and it seemed to work, but while routing cables, I conected in a wrong way 1 of the endstops... So at the moment it trigered, it seems the voltage regulator of the arduino was fried. i say this because if the blackberry is not turned on, the arduino doent turn on, meaning it is being powered from the usb.
Anyway, everything else seemed to still be working. I foud information about this issue and decided not to fix the voltage regulator. But them I noticed this: I Have 2 degrees reading spikes in both thermistors (bed and hot end), tI checked wires, I even replaced thermistors with no luck.
I looked some info, and then started to make some experiments:
1: If I disconect the USB cable comeing from the raspberry pi3, and conect a usb phone charger to the arduino, arduino boots up, the lcd comes to life. Seems to work, but the lcd still shows some temp spikes, smaller than before, but there are still spikes.
2:: If I conect the usb cable to my laptop, the spikes are gone, temp readings are super stable, no spikes at all.

So, conclusion is that there is some kind of noise comening from the USB power cable??? Since the electronics in my laptop are suposed to be much more precise and higher quality than other, that is why powering the arduino from the laptop usb is much better?
How can I fix this, I mean,in the original setup, power to the arduino and ramps was comeing from the PSU, ni spike reading, super stable. Spikes appeared once the arduino started to be powered by USB. Can I filter somehow the usb power. Replaceing the voltage regulator would fix this since power would be suplied again from the PSU?


thanks in advance

See [duet3d.com]. Although I wrote that for the Duet, the information there applies to every 3D printer control board that I know of except RAMBO and Archim.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

... the USB ports mostly can't support higher currents, so load changes (e.g. switching drivers ON/OFF) can result in spikes and transients.

With the 5V-regulator on the board this is slightly better, but not optimal too -- especially, if you have ground loop differences between printer and USB-device.

Connecting/shorting the GND's of the printer and USB-device will help ... but this depends too on the internal PSU wiring -- there are some PSU's with weird internal ground vs. shielding handling ... had some PSU's with "floating" GND and +5V-levels, which had differences between mains PE (ambientGND) and the PSU GND's of more than 90 Volts!

So when connecting this "GND's", some sparking or high currents can occure -- had two occasions, where the USB-cable get pretty hot after short time

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Thanks for the answers. Also, thank you for that article. I will read it carefully. For now, I have solved the problem: I replaced the voltage regulator in the arduino board. And now the arduino is powered through the ramps as was before the VR was fried. And magic! no more spikes, temp reading are super stable again and I can even see the difference now: when PSU is on, the arduino get its power from the PSU, but when I turn it off, the arduino gets its power from the raspberry pi, and again the spikes shows up.
I will investigate deeper into this issue of ground loop
Thanks guys!