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Hi need to use a Raspberry PI in a project. I use a 5V power supply for it (by now a 2A phone charger), and it will controll a "power board" with some MOSFET, etc running at 12-24V. The power board is connected to an external 12-24V switching power supply.

Can/should I connect the two power supply GND togheter?

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    \$\begingroup\$ Can/should I connect the two power supply GND togheter? Yes, generally you must unless there is a good reason not to do so. But then you would know that. In general circuits need to share the same ground in order to work. Also if you don't connect the grounds together, there is the risk of damaging one or both boards. \$\endgroup\$ – Bimpelrekkie Feb 28 '17 at 8:50
  • \$\begingroup\$ Yes, you should. pls share the setup of your wiring as a sketch on a sheet is also ok. \$\endgroup\$ – User323693 Feb 28 '17 at 8:53
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    \$\begingroup\$ It might be a good idea to consider "formally" accepting answers to questions you have raised that you have considered useful. It may not apply to all questions you have raised - just those where you have had a satisfactory result. Call it "the fee" for getting good information! \$\endgroup\$ – Andy aka Feb 28 '17 at 9:29
  • \$\begingroup\$ Multiple non-connected 'GND' nodes in any system is either a documentation error or a wiring fault. If the 'GND' is a safety feature, it's worse. \$\endgroup\$ – Whit3rd Feb 28 '17 at 10:19
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You have to connect them, otherwise your circuit will most probably not work. If you do not connect both grounds, the voltage between these two grounds is somewhat random (determined by surrounding air, environment,...) and your signals might not be what you intended.

The best method is to connect the two grounds at exactly one point. This is called the star point. Make sure the ground connection from your power board has low resistance (thick wire) to keep the voltage drop over it small.

If you want to dive deeper into grounding, you can read this article, but for first experiments this is not necessary.

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  • \$\begingroup\$ What do you mean with "The best method is to connect the two grounds at exactly one point."? Is it ok connecting both GND to the same GND plane on the PCB? \$\endgroup\$ – Noisemaker Feb 28 '17 at 9:24
  • \$\begingroup\$ You take ONE wire and connect it from Ground to Ground, you do NOT take two wires and connect them to different points on the same ground. \$\endgroup\$ – Christian Feb 28 '17 at 9:26
  • \$\begingroup\$ I don't understand what does "ONE wire" means in a PCB layout. I usually use a ground plane to connect all GND pins of PCB components. Do you mean I need to create two separate ground planes (one for 5V and the other one for 12V) and connect them by a single trace? \$\endgroup\$ – Noisemaker Feb 28 '17 at 9:59
  • \$\begingroup\$ Using a ground plane is good. If you really want to switch 30 A (as said in the other comment), then a split ground plane would be a good idea. Have a look at figure 3 in the article I linked. Also magnetic fields might be a problem if you want to stack the boards \$\endgroup\$ – JLo Feb 28 '17 at 10:30
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A voltage only have a meaning with a reference which is, most of the time, the ground (GND). In your exemple, if both GND are not connected, the 12V / 24V source voltage can ether be +100V or -200V ( or whatever random level ) seen from the +5V voltage.

However, if both ground are connected, voltages sources with have the same reference, and so be coherent to each other. Please mind you have to connect grounds with little resistance.

So yes, most of the time you have to connect GND together.

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If you connect ground of two voltage systems like 5V and 24V then any fault in 5V will burn out other 24V lines. We in industry keep voltage groups isolated (this also safeguard minor overcurrents flowing from high voltage elements to low voltage elements). Ultimately, all power supplies will be having shared line-neutral at input side so no need to connect two voltage systems at output side

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  • \$\begingroup\$ This is an DIY project, so the two power supply will be (1) a Raspberry PI dedicated power supply, (2) a 12V 30A switching power supply. I need to create a PCB of the entire board, with connection from GPIO of Raspberry to power mosfet which will drive 12V loads. So the "neutral" you're talking about I think is not possible in this way, right? \$\endgroup\$ – Noisemaker Feb 28 '17 at 9:57
  • \$\begingroup\$ Source inputs of Raspberry PI dedicated power supply and 12V 30A switching power supply from same wall socket. As load is 30 amps and load spikes may damage controller side, I would always avoid connecting load circuit to controlling circuit \$\endgroup\$ – SACn Feb 28 '17 at 10:02
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    \$\begingroup\$ I'm not sure what industry you are in SACHIN, but it is common practice in power electronics to bond all low voltage supply commons together, not isolate them. \$\endgroup\$ – R Drast Feb 28 '17 at 10:11
  • \$\begingroup\$ Relying on shared line-neutral is dangerous. If one of your power supplies has isolated output (e.g. because it uses a transformer), there is no defined potential between the power rails. For example, Lab power supplies have isolated outputs (for safety), so you need to take care of the grounding on the output side. \$\endgroup\$ – JLo Feb 28 '17 at 10:12
  • \$\begingroup\$ I agree with you, but load side chaining is risky, and input side chaining is always done. At overhead cranes (steel plant) we've seen that load side surge cross controls and destroyed PLC cards so isolation of load side voltage groups is always followed. I know its not conventional for small requirements - in your case load is 30amps very high than micro-controller port capacity. If i mistakenly perform a quick short or spike on 30amps side, am sure micro-controller port will blow away. \$\endgroup\$ – SACn Feb 28 '17 at 10:19

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