Sanguinololu has 2.5v on the 12v side

So, a bit of backstory. I got this Sanguinololu put together, along with a 12v power supply that has an LED next to the rail letting you know when it's live. The PS doesn't have a switch on it at the moment, so I leave it unplugged from the wall when I don't need it. So it was connected to the screw terminals on the motherboard, but without wall power. I plugged the USB cable in and got a good connection to the computer. I noticed that the LED on the PS's rail lit up, which I thought was weird, but I didn't think hard enough about it. When I plugged the PS into the wall to give the motherboard 12V my laptop instantly shut down.

Turns out that's because the 12v side of the Sanguinololu is crossed over to the USB's power side. So with just the USB plugged in the voltage regulator is providing the expected 5v on one side, but also putting out 2.5v on the 12v side. When just the PS is connected, the 12v side has 12v, and the 5v side has 5v, but the USB also has 3.5v.

That's not supposed to happen, right?

I can't have both the USB and the PS plugged in, because it puts power back through the USB cable into the laptop.

I double checked the installation and orientation of the voltage regulator and associated capacitors.

There are ground loop issues, but yours sound extreme...

have you seen [www.reprap.org]

Yeah, but I wasn't entirely sure what it meant.

Quote

The USB 5V VBUS is connected to the output of the 5V regulator. This is bad for the regulator and bad for the PC. Some users report the regulator getting very hot (because it is trying to power the PC), other users report the PC giving USB over current errors. Nophead and Nothinman recommend cutting the 5V track to the USB connector. The only downside is the board needs the 12V supply before it will do anything, but who cares?

An alternate fix for the 1.3a board is to cut the trace between the USB chip pin 4 and C13. This is functionally identical to building a Sanguinololu without the USB port and FT232RL and using an offboard USB-serial cable. The advantage over cutting the 5V trace at the USB connector is that the USB serial port doesn't disappear from the host PC when the printer is switched off, which can anger host software that is still running.

So, cutting that trace means you can still use the USB, but the board thinks you're using a USB connection that's somewhere other than physically on the board? If it works better why not build it that way? Do you lose a function?

Is it supposed to NOT send voltage back through the USB when the PS is hooked up? It seems like it shouldn't, cuz that seems stupid. So what is going wrong on the board to allow it?

I don't know how helpful this late reply is but it is much easier to cut the 5v wire on your usb cable and isolating the wire. I

am slowly realizing how weirdly designed this board is.