Me and a colleague (we are both EE-trainees) had a chat the other day about mixed voltages on the same PCB. She pointed out an interesting question that I was not able to solve.
Imaging having a PCB that use a ground plane for the 5VDC and the 10VAC/50Hz somewhere else on the board (not interconnected with any 5VDC-part whatsoever). That is, they are two completely separate voltage lines on the same board. The 5VDC is not derived from the 10VAC in the power supply, everything is separated.
Now, my colleague blurted out this one:
Imagine I make a mistake when soldering, so that one of the two 10VAC-cables also gets attached to the ground-plane (for 5VDC). Nothing will happen to the 10VAC-circuit since only one of the cables is misplaced, but how will this single cable affect the 5VDC-ground?
After two days I am still not able to answer that question. I cannot grip this question theoretically and I do not have any equipment to try it out practically.
I'm thinking on the one hand: 5VDC will be unaffected since there is no path to the second 10VAC-cable, so there will be no short or anything like that.
And I'm also thinking on the other hand: Will the 10VAC induce noise to the 5VDC-ground, making all 5VDC-circuits unusable?
What will happen?
EDIT: added pseudo circuit schematic