I am designing a little device to use inside my PC. My circuit contains a microcontroller which needs to communicate with the host system, and I wanted to do that via a USB port. The microcontroller will be powered by 5V, but my circuit also contains components that need 12V, so I want to add a 4-pin Molex connector to get power from the PC's PSU.
The question here is: to power the microcontroller, should I use the 5V that the USB provides or the 5V that the PSU provides? Is there any difference between the two in terms of amperage or how clean/stable/reliable the voltage supplied is?
Additionally, I wanted to ask someone more experienced about the following potentially crazy idea (I am almost certain this is a bad idea, but want to ask anyway): could I maybe use both? (connect the 5V pin of the USB to the 5V pin of the DC power connector?) Would there be a noticeable potential difference that could cause currents to flow between them (afaik, both of them fluctuate slightly, not exactly perfect 5V)? Would it be safe?
I am asking this because I don't know yet exactly how much power my device will eventually need to draw, and if merging the two 5V inputs is safe and doesn't lead to bogus power consumption, it could be an easy way to increase the current available without needing to add a second connector for more power from the PSU.