Voltage control relays, Should I use separate power supply circuits?

Question

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?

Answer 1

So, why in this industrial applications they use 12V to control the relays?

While 5V relays are available, they are less commonly used probably because 12V is already available for most applications (especially industrial) and relays with a lower voltage have higher current requirements. A 3.3V relay would have an even higher current requirement if the relay is in the same family, some relay families don't go as low as 3.3V in voltage.

The problem with relays is the high inductive load. The coil can sink a lot of power when it is turned on. This causes power dips on the power rails of the power supply, so a voltage source that is low impedance and can source current well is recommended.

In the projects I have done I usually use 12V or 24V relays depending on what my main supply voltage is. If an 5V or 3.3V LDO is used it will regulate any dips in power to digital electronics because the relays might drop the rail momentarily but not below the LDO voltage. Also, Sometimes even using bypass caps with a relay does not solve the problem.

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 together the two grounds of the two power lines in the PCB?

I recommend putting the digital/analog on a separate rail than relays. Realize that relays downstream from an LDO creates large thermal losses and loss of power efficiency because relays use large amounts of current. Relays don't need a high amount of precision with an LDO. It would also be better to consider a DC to DC converter for voltage regulation for relays as they are more efficient.

The grounds can be connected and do not need to be separate. However, realize that currents return to the source, which could affect low level measurements (under mV's), if the ground currents from the relays pins run through an analog section it could create problems. Read up on some of the split plane questions like this one Single ground plane vs split planes?