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SOLVED: Separate GNDs for Different Voltages?

Posted: Wed Mar 05, 2014 5:32 pm
by CAP_Cadet
Hello,

I am using my Pi's GPIO to control some 12V lighting strips (LED). Can I just ground the GPIO connection to the 12V supply's ground? Or do I have to keep the RPi's connection grounded back to the board? I have heard that using 12V to GND 3.3V can create negative voltage, but I don't know.

Thanks!

Re: Separate GNDs for Different Voltages?

Posted: Wed Mar 05, 2014 5:42 pm
by redhawk
If your power supply / regulator shares a "common" ground with the Pi then ground is ground via the GPIO ground pins

Richard S.

Re: Separate GNDs for Different Voltages?

Posted: Wed Mar 05, 2014 5:46 pm
by FTrevorGowen
CAP_Cadet wrote:Hello,
I am using my Pi's GPIO to control some 12V lighting strips (LED). Can I just ground the GPIO connection to the 12V supply's ground? Or do I have to keep the RPi's connection grounded back to the board? I have heard that using 12V to GND 3.3V can create negative voltage, but I don't know.
Thanks!
The Pi's Gnd (0V) should be connected to the Gnd (0V) that 12V is "referenced" to (just like the Pi's 5V and 3V supply rails share a common Gnd (0V)). However, make sure that none of the GPIO pins can connect to the 12V ie. they should be isolated via a transistor or other buffer circuit.
Trev.

Re: Separate GNDs for Different Voltages?

Posted: Wed Mar 05, 2014 6:00 pm
by CAP_Cadet
Thanks for the quick replies.
The GPIO pins are using a transistor to control a 5V signal (from the Pi) to switch a 5V relay which turns on/off the 12V strip. I'll put up a schematic in a bit.

Thanks again for the help.

Re: Separate GNDs for Different Voltages?

Posted: Wed Mar 05, 2014 6:24 pm
by rpdom
I assume you mean a 3.3V signal from the Pi which drives transistors that control a 5V line to drive the relays which switch 12V.

If so, then you don't need to connect anything on the 12V side to the Pi as the relays will keep the two sides of the circuit isolated.

But please do post the schematic to confirm this.

Re: Separate GNDs for Different Voltages?

Posted: Wed Mar 05, 2014 6:28 pm
by hampi
It is better to avoid that the current from 12 V circuit flows through the RPi supply ground line. The circuit '12 V -- switch -- load -- ground' can have extra noise and transients from switching the load on/off. Usually this means that the more sensitive computer ground and the power circuit grounds are connected together only at one place (ground loops to be avoided). Analog signals are even more sensitive for grounding.

Re: Separate GNDs for Different Voltages?

Posted: Wed Mar 05, 2014 7:52 pm
by CAP_Cadet
Schematic as promised.

@rpdom you are exactly right.

edit: the relay is in open mode

Image

Re: Separate GNDs for Different Voltages?

Posted: Wed Mar 05, 2014 8:44 pm
by CAP_Cadet
Thanks for all your help everybody. I went with what was on my schematic and it is working well. :D
I will be posting my latest project's overview next week.
Thanks again!

For those who want to know the solution: Keep your RPi grounding separate from your higher-voltage line's grounding.