Are there cases when one wouldn't want a common ground in a project?
My current project:
I have a common ground between the Arduino + the 20V power supply.
Are there any issues (for example when sinking current) when you wouldn't want a common ground?
You always need a common ground. Voltage is a measure of the DIFFERENCE in potential - if you want your voltages to make any sense, and not be left floating, you will need to make sure all voltages reference to the same level. To sink more current, use a bigger wire (larger cross-section).
you might not need a common ground when you use photogates to isolate a part of a circuit of another.. it's a comon use when we are controlling stepper's from computers
@xiserre: That's a good point - I'd forgotten about that. You'll want to isolate when you're working with high voltages/high currents, which you can do with optocouplers or with a transformer.
Hmm... so with my TIP 120 transistor I am controlling a 20V fan. I have a common ground (between Arduino, TIP 120, 20V/3 amp power supply). Is this going to be a problem?
No. It should be fine - in fact, in your case, common ground is necessary, unless you want to add in more components and a separate power supply. You only really need it if you're talking about, say, switching mains power.
Cool thanks (...it's all fine until the smoke comes out ;-))
Basically where people use optos is normally because they don't want a common ground and want isolation (between a low voltage MCU circuit and a high voltage circuit eg. mains control, projector bulbs etc). For MIDI you also use optos rather than a common ground.
mowcius:
Basically where people use optos is normally because they don't want a common ground and want isolation (between a low voltage MCU circuit and a high voltage circuit eg. mains control, projector bulbs etc). For MIDI you also use optos rather than a common ground.
...and not always just between low and high voltage circuits - you may want such isolation if one side of the circuit is producing a lot of back-emf noise, that, no matter what you do with caps and such, you can't filter out. Large brushed DC motors come to mind; you may not be able to filter out all of this noise, and it could leak back on a common ground, causing all kinds of havoc (or even damage) to the lower voltage microcontroller side of things. Isolation is a safe bet, then. Then there are the times where one side of your circuit is operating in a high-voltage noisy environment (think digital control of a Tesla coil, perhaps) - here again, isolation using optos can help.
I suppose one can call both of these examples a "high-voltage" side, especially the second example. However, the first example may not be immediately thought of as "high voltage", depending on what the motor's voltage and current rating are, and how much back-emf it generates (especially on start up)...
cr0sh - very true - I suppose I meant potentially high voltage circuits but you made it much clearer.