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I think I've phrased the title correctly, although happy to be corrected. For some reason I'm having a huge problem wrapping my head around the use of ground in low voltage (e.g. 3.3V / 5V) circuits. I've read this answer a few times and have a better understanding, however still have this unanswered question.

As far as I understand, in the UK, if I were to plug in my DC power adapter into a socket the voltage is 230Vac, referenced, at some point, to the actual earth. Inside the adapter, a SMPS circuit using transformers, regulators and filters outputs a 5V DC voltage, for instance. What is that 5V referenced to? Is the negative line of the DC output tied to the AC earth?

If I were to take two different power adapters, hypothetically, I assume it would be possible for one of the two to have a potential of 100V on the +ve line and 95V on the -ve line? That provides a 5V potential difference, therefore working the same as the other that had a potential of 5V on the +ve line and 0V on the -ve line. What would happen if the -ve lines of these were tied together in a circuit (i.e. a common ground)?

My questions stems from the desire to use an auto switching power mux such as the TPS2113A. The grounds from the two supplies must be tied together, but what if they're at a different potential? An example circuit from Pololu is below.

CircularRecursion
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    The fact that the title makes no sense shows a misunderstanding somewhere. Voltage is simply a difference in potential, actually, any energy is. So you can't have same voltage and different potentials. The AC to 5VDC adapter would probably be isolated and have no reference to the same ground as the input AC. It is merely 5VDC in relation to some reference point in the circuit. Ground is just another word for a reference point. Think of a battery, what is its voltage in reference to? – Jarrod Christman Apr 29 '17 at 15:49
  • To add, for the last bit of your question. A circuit can have the same reference point (tied together ground) yet have the other end at a different potential. Think of a computer power supply that gives 3, 5, and 12 VDC all from the same ground. – Jarrod Christman Apr 29 '17 at 15:51
  • I think maybe my phrasing is slightly confusing @JarrodChristman. Is it not possible to get a voltage of 5V from two points that have a potential of 100V and 95V respectively? And it's also possible to get a voltage of 5V from two points that have a potential of 5V and 0V respectively? – CircularRecursion Apr 29 '17 at 15:52
  • That is true, but for all intents and purposes, that 95 or 100 VDC is 0 VDC. If the entire world is at 1,000,000 VDC, we don't care, only the difference matters. – Jarrod Christman Apr 29 '17 at 15:56
  • For your safety, those converters are required to ISOLATE the output from the input, using transformers. That is called galvanic isolation. No DC currents are allowed to flow between the isolated sections, for your safety. In reality, tiny leakages, through insulating layers of plastic or oiled/shellaced paper, will flow but those tiny leakages are limited by regulations of safety organizations. At AC frequencies, some line 50 / 60 / 100 / 120 frequency energy will couple (capacitively, via the electric fields) across, often a problem for precise measurements but not a safety problem. – analogsystemsrf Apr 29 '17 at 16:01
  • @JarrodChristman hypothetically, what happens when those two circuits with different reference voltages have the grounds tied together? The reason I ask, is I need to connect two microcontrollers to two different computers. Obviously these could be isolated, but I need a push button to connect to both of these microcontrollers (via Schottky diodes). That means the grounds of the two microcontrollers must be tied together. (Optocouplers aren't a possibility here for multiple reasons). I know it's probably safe to assume the USB 5V are both pretty close. – CircularRecursion Apr 29 '17 at 16:05
  • With this galvanic isolation, its your choice on how and where (earth?, your ear? the kitchen sink?) you tie the output reference line. If you tie the +5 output to earth, the other output wire will be -5v reference to earth. In hospitals, for patient safety with electric probes into helpless humans, extremely isolated (double or triple layers of isolation) are required. One challenge in efficient converters is switching activity, far faster than line rates; this allows tiny transformers; fast edges are used (0.1microsecond, e.g.) and those fast edges produce huge noise charge movement. – analogsystemsrf Apr 29 '17 at 16:08
  • In order for you to say you have a 5V potential difference between 100V and 95V, *you must have a 3rd reference point which you have designated 0V*. Without that 3rd point, there is no 100V or 95V - there is just 5V. – brhans Apr 29 '17 at 20:22

2 Answers2

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The concept you are missing is that a supply can be isolated. This means that (within reason) the output can be referenced in any way you like. If a DC supply is isolated, then you can reference the positive or negative to earth if you like. Or to some other voltage you have handy. It can even be referenced to a moving voltage such as the hot mains voltage (which of course makes it equally hazardous as mains voltage).

Also, if you have two isolated supplies, they can be connected together in series or parallel, as you choose. What is the potential of an isolated supply? In my opinion, it is best to think of it as undefined. You don't know what it is, you can't count on it being any particular value, it may drift with time, and if you try to measure it, you will probably end up changing it.

In general, commercial power supplies will either be referenced to an earth ground potential (supplied by you with a wire when you connect the supply) or isolated. You will not find a power supply that is referenced to some arbitrary voltage such as 90V.

In your specific case, you should be able to tie your grounds together without fear.

user57037
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Ideally, your power supply will be isolated from mains power. That means that its output voltage will be 'floating'; the positive output is at +5V relative to the negative output, but neither is referenced to mains earth. Should you connect the negative output to mains earth, you wouldn't see much of anything happen, you'd just now have a mains-referenced power supply. Depending on what it's plugged into, this may be a good thing and it may be a problem.

In truth, it doesn't make sense to talk about something's absolute potential; every potential is referenced to something. Only potential differences have any physical meaning. This is an important point in physics, and a foundational part of gauge theory.

Hearth
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