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I'm trying to understand what would happen when connecting two grounds from diffident power supplies together. A more in depth explanation of what I'm trying to accomplish is here: Connect Arduino to existing circuit with seperate power supply

One power supply is 12V and I don't know if it's floating or not in relation to earth ground. The second power supply will be 5V floating. If there's a voltage potential between the two grounds and I connect them together will there be current flow between the two grounds or will the floating 5V supply "adjust" itself to the 12V supply's ground potential?

From my understating I thought that when connecting two spots that have a different voltage potential you will get current flow between these two spots. However I'm not sure if the whole floating voltage situation changes this.

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    \$\begingroup\$ Replace 'floating' with 'connected by a very high resistance' and see how far that helps your understanding. Draw a circuit, where the insulators are represented the same a you would represent a lossy capacitor. \$\endgroup\$ – Sean Houlihane Oct 19 '16 at 18:23
  • \$\begingroup\$ I suggest you may want to read about electro-static discharge. \$\endgroup\$ – user117772 Oct 19 '16 at 18:36
  • \$\begingroup\$ Do you want to understand what already happened, or just theoretically? If you don't know if the 12V supply ground is floating, why don't you try to measure this with a simple DMM? Connect your 5V supply ground with 12V signal ground via, say, a 10k resistor, and see what kind of leakage do you have. \$\endgroup\$ – Ale..chenski Oct 19 '16 at 18:52
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    \$\begingroup\$ Hint: a usual AC-DC 5V wall adapter usually has a leakage of up to 100uA to hot AC wire. So, when probing an ungrounded PS relative to Earth ground with a high-impedance probe (1M-10M), do not be surprised to see an AC waveform with 50-60V amplitude (on a 115V AC mains). \$\endgroup\$ – Ale..chenski Oct 19 '16 at 19:00
  • \$\begingroup\$ I'm hoping to understand from a theoretical perspective not so much what already happened. I'm hoping to eventually connect this to an existing and expensive device so I want to make sure I'm understand things correctly first. My biggest concern is that current will flow between the two grounds and fry the existing electronics. \$\endgroup\$ – Harry Muscle Oct 19 '16 at 20:23
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Just think of connecting several batteries together. Each battery is floating if not connected. You may connect the minus point of several batteries to the same ground, you may have different strings of batteries connected in series. If you know for sure that the 5 V supply is floating, you may connect both minus points together regardless if the 12 V supply is grounded or not. You may use a multimeter to measure the resistance between the ground connection of the AC plug and the minus output of the 12 V supply. Please be sure to measure at the ground connection only, not at the hot wire! If the other power supply is really floating, you should measure a high resistance there.

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