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I'm making a small project that drives a few DC motors with H-bridges.

+12V DC will come from one DC power supply (regulated to 10V for motors,) and then +5V will come from another (different) DC power supply for some logic controls.

Since these are two separate power supplies, should the GNDs be connected or isolated from one another? If so, how? Should I just connect the GND of both DC power supplies to the GND plane on the PCB, or should I bring in the +5V and not connect the +5V GND at ALL, and just reference the GND from the +12V on all the ICs?

Schematic

schematic 2

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    \$\begingroup\$ Your schematic doesn't show anything being powered with 5V, so based on this schematic, you could leave the 5V supply fully disconnected and it would work. You need to show something relevant to the question, like the H bridge IC with 10V supply in and 5V control signals from MCU being powered with the 5V. \$\endgroup\$
    – Justme
    Dec 4, 2023 at 20:55
  • \$\begingroup\$ @Justme I added the other page! Didn't think it was relevant, my apologies. \$\endgroup\$
    – Rydberg
    Dec 4, 2023 at 21:04
  • \$\begingroup\$ Did you mean to use 7404 inverters? 06 is open-collector and I don't see a pull-up. \$\endgroup\$ Dec 5, 2023 at 0:53
  • \$\begingroup\$ @TimWilliams I just added some pull-ups a little while after posting. Good catch! Thank you \$\endgroup\$
    – Rydberg
    Dec 5, 2023 at 1:33

2 Answers 2

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You would usually connect all the supplies on a single board together back to one GND. Some would suggest you seperate digital and analog grounds in sensitive systems but I don't think this applies here (and I wouldn't recommend it personally anyway.)

I presume at the point of the H-bridges, the logic part of your circuit that's powered from 5V, will need to controll the flow of the 12V for your motors. At this point, if your two supplies are isolated (GNDs unconnected) this part of the circuit might not work at all, or as expected. In this application, you should connect the two GNDs with a strong connection (ideally a GND pour) and ideally have the two connectors mounted reasonably close together.

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  • \$\begingroup\$ Thank you for this excellent response and explanation. \$\endgroup\$
    – Rydberg
    Dec 4, 2023 at 21:06
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    \$\begingroup\$ I don't recall seeing anyone recommending to "isolate" analog and digital grounds. Maybe "isolate" is not the correct word, or maybe the interpretation is wrong. They can be kept separate maybe, and organized carefully for managing power, digital and analog currents, but generally, they can't be isolated. \$\endgroup\$
    – Justme
    Dec 4, 2023 at 21:12
  • \$\begingroup\$ @Justme very valid, isolate has a key electronic meaning and I've used it wrongly. I'll edit \$\endgroup\$
    – LordTeddy
    Dec 4, 2023 at 22:03
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Yes the H bridge IC references signal inputs and motor drive to same ground pin, so you must connect the 5V and 12V grounds tohether.

If you connect only the 5V of the 5V supply but not the 0V of the 5V supply, it would not be able to power anything, as it would be missing a return wire.

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