Do 2 5V DC wall warts in the same AC outlet have a common DC ground?

Question

I'm trying to power a LED strand from one wart (5V), and my logic driver FPGA from another wart (also 5V). Both warts are plugged into the same outlet strip. I know I should probably power them from one, but I don't have the plugs on hand to rig both to the same plug.

Both warts are 2 prong (not 3 prong).

Can I send 5V digital signals from one domain to the other?

I tried testing voltage across the domains with a voltmeter but it just reads 0 (floating?) anytime I try to cross them.

• VDDA to GNDA reads 5V (first wart)
• VDDB to GNDB reads 5V (second wart)
• VDDA to GNDB reads 0V (cross domain)
• VDDB to GNDA reads 0V (cross domain)

I'm assuming my voltmeter reading tells me that I can't send signals from A to B. If that's true, would I want something like an opto-coupler to transfer a signal from A to B?

Answer 1

Your 2 wall-warts by themselves don't have a common DC ground. Each one is isolated from AC. As a result, they are isolated from each-other.

You should be able to power the circuits off two separate wall-warts and have them communicate. Connect the grounds GNDA and GNDB with a wire. It doesn't sound that you need to keep the domains galvanically isolated.

An opto-coupler would be useful if you want to keep the domains galvanically isolated.

Answer 2

Tie the DC grounds together. They are isolated from the mains, and each other. Use caution if you're going to send 5V to your FPGA. Not all FPGAs can take it and not all development boards will protect you from yourself. Consult the documentation.