If I have a project that uses multiple voltages (5v and 3,3v in this case) do I use a common g...

[G+_Chris Kurtz]

If I have a project that uses multiple voltages (5v and 3,3v in this case)... do I use a common ground?

 

I have a single power source (either 9.6v or 12v NIMH) and have voltage converters dropping the voltage to what I need (either standalone or via an arduino/teensy.)

[G+_Peter Hanse]

I would but depending on application you could keep common separate and floating i.e. Non grounded.

[G+_Black Merc]

If you have a single power source, you have common ground.0=0

[G+_John Sullivan]

Adding a bit to what "Black Merc" said, then yes, if you have a single power source feeding into multiple voltage converters, you would automatically have a common ground ( all current paths return to the same, single power source. You also have a common B+ (the opposite of ground) to the converters... it is only after the voltage exits the converters that you have separate B+'s ... but the return to the power source is a common ground.

And yes, as Peter said, you could design it with non-common (floating) grounds, but that would not be the normal scenario.

[G+_Chris Kurtz]

Thanks all! So if my supply is 9.6v and I'm using a converter to go down to 5v, do I need to make sure that the grounds from 5v devices are tied back to 5v ground side of the converter, or can I tie directly back to the 9.6v supply?

[G+_Jason Marsh]

If the buck converter has a ground output you can tie that to the ground from the power source.

 

If grounds are separate, troubleshooting can be an issue if you're not cognizant of it. Also, if one component is sending signal to another but they don't have bonded grounds, you could have performance issues unless it's a differential signal over two wires as in the setup of each pair in ethernet.

[G+_John Sullivan]

Think of it as two separate power circuits. All grounds on the 5v side tie together, but do not connect them to the ground on the 9v side.

 

Not that something terrible will happen, necessarily, but I've not seen a step-down transformer (which this circuit basically is) with the high-side and low-side grounds connected together. If someone here has a good reason (and real-life experience) why they would be, feel free to correct me.

[G+_Black Merc]

DC does not travel through transformers as AC, does which is why the switching turns on and off the current to emulate an AC current. Controlled very precisely like PWM, the current is filtered through capacitors to produce a fixed voltage.

 

The circuit is not really a stepdown transformer. The coil in the circuit that you see is an inductor. The ground line is rarely broken from input of the circuit to the output.

 

Basic buck circuit.

https://goo.gl/images/3q1nxs

[G+_Chris Kurtz]

John Sullivan This is really what I wanted to hear, thank you!

[G+_John Sullivan]

Black Merc,

 

Thanks but your taking my use of my analogy to stepdown transformer too literally. Perhaps I should have put it in quotes.

[G+_Black Merc]

The DC 'step down' would be the 'voltage divider' circuit. The reason that it is not used is that it uses alot of power. The buck unit is so much more efficient.