My Arduino megas FTDI broke… we are assembling a sumpod alu and every thing seamed fine. It did run well by it self, but when we connected the USB port to the arduino, the laptop screen and the external screen got black for a few sec, and then when the image came back on the screens the external lcd screen was unhappy (locked like a crt on video, visible waves ). when we disconnected the usb and turned off the power to the steppermotors and reconnected the Arduino we get “unknown device”. The Atmega seems to be fine, we can still reach it through rx0, tx0. We guess that the chineese power supply is to blame. The output from the power supply do not have the same ground as the USB and therefor sends spikes through the usb cable. What should we do to get a common ground?
The USB shielding should do a good job unifying the ground rails. Maybe check that there is in fact shielding all the way through your USB cable with a continuity tester.
We only connected ground to ground and not to both ground and com as described on this page.
http://reprap.org/wiki/RepRapPro_Mendel_power_supply
Hmm. We should probably have looked this up before connecting it, but it seemed obvious at the time…
So you’re saying that you basically haven’t grounded the power supply? That wouldn’t be good.
Wiki:
“Important: Check the resistance between the Ground pin on the mains plug and the metal case of the supply. This should be 0 ohms.”
The power supply was grounded (gnd), but there where no connection between com and ground. so:
com != ground.
ramps ground = com
usb ground = ground
there was occasional spikes between com and ground on ~12v that went through the USB cable.
usb = 5v
5v << 12v
This resulted in a permanent sad FTDI chip and luckily only temporary damage to my laptop and display.
And our power supply seems to have stopped working now. the fuses are ok, but it wont power on. This is going well.
Com should always be at ground level where possible. If you’re using a cheap unshielded cable and a non-grounded com from a Chinese Psu, that calls for trouble - you might have injected a deadly Esd spike into the Arduino’s power supply (and into your now malfunctioning PC) .
Do ground your Psu and possibly replace the Arduino. It might work now, but could malfunction at any time (given that the Ftdi is already fully malfunctioning ).
I bought one of these cheap chinese power supply’s. The fan was very noisy. Is that how it is supposed to be?
our does not have a fan, so its quiet.
Personally i’m using ATX supplies. You can get a good quality 80+ - certified unit for about 35€ (BeQuiet L7 350W), which will in all circumstances work with the RepRap and most like not end up destroying things. The only “hack” sometimes required is loading the 5V line, but my PSUs all work fine without it.
I just wanted one because it is a bit easier to mount on/in the printer. Use ATX without balanced 5V on My Prusa 2. Ordereded a fanless 20A Chinese of eBay for my next printer 
30A