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Suppose I have 2 circuits with their own individual power supply. Circuit 1 needs to send some sort of signal to circuit 2. Would I need to carry the current back to circuit 1 after the signal has reached circuit 2 (in the form of a neutral wire)? Couldn't I just send the voltage along one wire and then use the ground in the second circuit to complete the circuit, effectively having just one wire between the circuits?

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  • \$\begingroup\$ You're assuming both circuits are referenced to a common, clean ground. That may or may not be the case. If not, unexpected and dangerous currents can flow if you connect even a single wire between the two circuits. \$\endgroup\$ Jun 30, 2016 at 2:00
  • \$\begingroup\$ Please do not plan on using the mains wiring as a common for a low voltage signalling circuit. It is not safe, reliable or even sure to work. \$\endgroup\$
    – KalleMP
    Jun 30, 2016 at 7:09

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There are two ways to try and understand why you need a common reference voltage (what you call "neutral", many call it "ground" or "common").

The Voltage approach:

What is a voltage? It's the difference in potential between two points. The key thing there is the number two.

If you only have one wire connecting between two circuits it will do nothing at all* because there is no reference for the signal. The ground in one circuit (which your signal is referenced to) has no relationship to the ground in the second circuit. In effect you can't apply a potential difference in the second circuit from the first.

The Current approach:

For a current to flow there must be a complete circuit. Without a second wire there is no complete current path between the two circuits. In essence you have a circuit consisting of a wire (the connection between the two circuits) and an infinite resistance (the second connection which you didn't make). The current through an infinite resistance will, by Ohm's law (\$V=IR\$) be zero regardless of the voltage.

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  • \$\begingroup\$ This explanation is correct, however, if the 0V in both circuits is at ground level (like if both are running off of grounded DC power supplies), the 2nd 'wire' runs through one power supply, to the wall 0V/GND, and back out the other power supply. So, in certain cases you can get away without placing another wire in your circuit. \$\endgroup\$
    – dpdt
    Jun 30, 2016 at 0:12
  • \$\begingroup\$ I think I understand what you're about the voltage approach, but isn't there just one ground (i.e. earth)? Why does the circuit care which ground the current is going to? As both circuits are connected to the mains, don't they have the same ground? \$\endgroup\$
    – M-R
    Jun 30, 2016 at 0:12
  • \$\begingroup\$ @dpdt if both are running of grounded DC supplies, then you have a second wire. If you have a second wire then that is not the situation described. \$\endgroup\$ Jun 30, 2016 at 0:13
  • \$\begingroup\$ @MartinRand ground is a reference in a circuit. Connect an LED + resistor to a 9V battery, then the circuit has nothing to do with earth, but you would usually call the negative terminal of the battery your reference voltage (a.k.a ground) \$\endgroup\$ Jun 30, 2016 at 0:15
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    \$\begingroup\$ @MartinRand Voltage, as explained in the answer, is always between two points. It is convention to define the ground as being at 0V and measuring voltages relative to ground, however, there is no law saying that this must be so. What I intended to say is that certain DC power supplies hold 0V at ground potential (based on the mains), so you form the 2nd wire that TomCarpenter was referring to through the power supplies. If you were to power both circuits off of batteries, then there would be no reason for both 0V's to be at the same potential, and you would have to place another wire. \$\endgroup\$
    – dpdt
    Jun 30, 2016 at 0:27

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