Same ground for multiple voltages?

Question

This is a follow-up for my christmas-light project, first referenced in this discussion: http://www.element14.com/community/message/88712#88712/l/re-beginner-question-12v-wall-wart-to-power-both-a-msp430-microcontroller-and-an-led-strip

This seems like an even more noobish question, but when I run lights on 12 volts and a microcontroller on 3.3 volts through a regulator, can they still use the same ground? My gut feeling (and high school basic knowledge) tells me that there should be some kind of division, to keep current going from 12v towards 3.3v. (the left side towards the right side).

Would it be enough simply to insert a diode at the blue line like so?:

Or is it more complex than that?

I am not sure I am still in the right group, but the structure of Element14 still baffles me

shabaz · Accepted Answer

Hi Niels,

At a first level of analysis, you can run as many circuits as you want from the 12V supply, all in parallel, with no problem, so the diode is not needed. Your first circuit will be fine.

At a next level there could be considerations due to interaction and then there are needs for separate wiring of supplies all the way back from the battery/supply. For this circuit you should be fine.

By the way, across the IC on the right, something like a 100nF ceramic capacitor should also be close to it's supply rails, unless that is a symbol for a complete microcontroller board, in which case it will already contain the capacitor.

pettitda · Answer

Yes, they should use the same ground. There are circumstances where you might want to isolate grounds, but they don't apply here.

mlease · Answer

Niels, to expand on the other comments. If your 12V circuit generated much noise or had a highly variable current flow you would want to do a "star" approach so that the 12V circuits and the other circuits each connect to ground right at the power supply. That would keep the current flows separate.

For a circuit board layout,it's common to use zero ohm resistors to create a different ground nets to help physically separate the grounds on the board. In your case, you could have GND at the power supply, GND1 for the 12V circuit and GND2 for the 3.3V circuit. Once the layout is done the resistors can be removed and the nets all tied together again for a single common ground. Depending on the layout package, that can be easy or a real pain so it pays to play with that first to decide if it's worth the trouble. You can also leave in the zero ohm resistors for a few more pennies.