# Connecting Two Grounds Together

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.

• 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. – Sean Houlihane Oct 19 '16 at 18:23
• I suggest you may want to read about electro-static discharge. – user117772 Oct 19 '16 at 18:36
• 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. – Ale..chenski Oct 19 '16 at 18:52
• 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). – Ale..chenski Oct 19 '16 at 19:00
• 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. – Harry Muscle Oct 19 '16 at 20:23