I have to realize a relays PCB board controlled by a MCU (arduino micro in the first stage) and a ULN2003, as for example in the attached figure I found in Internet with some other components.
What I would like to understand is if I need a separate power supply line to power the ULN2003+Relays. I ask this question because in almost all the industrial application I have seen, they use 3.3/5V to power the MCU and a dedicated 12V line to power the relay control even if they use separate discrete transistor in place of the ULN2003.
In terms of required current, also with 5V type relays I should stay safe, because the ULN2003 can supply up to 500mA (single output) and a generic 5V relay datasheet indicates a nominal current required to switch to about 80-100 mA. A 12V relay requires 30-40 mA to switch.
So, why in this industrial applications they use 12V to control the relays?
I would like to know if they need to isolate the MCU power supply to avoid some spikes/noise generated from the coil on/off that could reset the MCU (although they use the reverse diode to avoid this). Or, they need to reduce the current to drive the relay in order to match with their discrete transistor driver (a 12V relay need less current than a 5V type)?
If you suggest to separate the power supply, could I use two separate (with two regulator) 5V line instead of a 5V + 12V? And in this case, I could connect togheter the two grounds of the two power lines in the PCB?