Why different power supplies

[tantemay]

Why should you use different power supplies for the controller board and the output board?

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[PlanetCNC]

Two reasons:
1 The relay coil acts like an inductor when power is removed, and it sends a spike of power back into your circuit. This causes electromagnetic interference which is normally filtered by power supply. If you share power supply with controller then this interference is not filtered and can cause problems.
2. Output board is optoisolated. If you use same power supply then isolation is pointless.

[Al_The_Man]

Any relay used in CNC equipment or any electronic equipment come to that, should have a BEMF (reverse connected) diode across the coil.
This eliminates the generated Spike of high voltage when the relay is turned off.
Also, IMO there are too many unnecessary relays used in much of the DIY CNC systems, often there is a opto isolated output and proceeding to an input which also has an opto isolator input, I have seen relays used unnecessarily many time in this scenario.
A good example is the post I made for the G540 to VFD input for FWD/REV, almost everyone is advised to use a relay, and in most cases it is redundant.
Al.

[PlanetCNC]

Output Board has flyback diodes.
However I had problems in the past and separate power supply solved them.
Basically I use separate (or isolated) power supplies in all my electronic projects and it worked good for me. Power supplies are cheap.

[tantemay]

Thanks for the answer. That makes sense to me. I read about it and used sepetaze power supplies. Now I also know why this is necessary.
I can now pass on the information if I recommend the Planet-CNC products to friends.

[Al_The_Man]

Regardless if separate supplies are used, I have always made all power common, IOW, connected the common of all supplies to the Earth GND star point.
This has cured and avoids spurious triggering of inputs etc.
Al.