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I wanted to connect two power supplies, one is AC (240V , 6A) and the other is DC (3,3V, 300mA). But i read in some referrence that connecting AC and DC power supplies could be dangerous if the two signals have are from different grounds. So I was thinking about using a floating ground to avoid a ground loop.
Is the use of the floating ground can secure the power supplies from damage? Or the power supplies need to be isolated ?
If not, what is the best way to connect the two power supplies safely?

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    \$\begingroup\$ what exactly do you mean by "connect"? have the 240V/AC and 3.3V/DC parts of your pcb share a common ground? \$\endgroup\$ – PlasmaHH Apr 21 '15 at 16:08
  • \$\begingroup\$ Depends on how you want to connect things. What is the AC Supply? If it is transformer isolated from the line, you might be okay, but then I'd probably ground one leg. If you are directly connecting the AC line to your 3.3V source, I'd say no to that under any circumstances. I'm old enough to remember hot chassis electronics from several decades ago... never fun for a user. \$\endgroup\$ – R Drast Apr 21 '15 at 17:11
  • \$\begingroup\$ @PlasmaHH The AC is an external power supply to the PCB, is isolated with an solid state relay (like an optocoupler). The solid state relay is in the same PCB as the DC power supply \$\endgroup\$ – R Djorane Apr 22 '15 at 7:26
  • \$\begingroup\$ An isolation transformer may serve your purpose, maybe two. \$\endgroup\$ – Optionparty Apr 22 '15 at 21:34
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A circuit with a floating ground requires that circuit to be isolated - they are one and the same thing. Try looking at a 3V3 isolated power module from Traco, Murata or XL for example.

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