Which one is ground?

Question

I have an AC adapter jack that has three terminals, L(eft), M(iddle), and R(ight). If I plug in the power supply and connect the plug to the jack, these are the readings I get from the multimeter.

L(red probe) M R(black probe) = +5V
L(black probe) M R(red probe) = -5V
L(red probe) M(black probe) R = 0V
L M(red probe) R(black probe) = 0V

So is the middle terminal ground? Or is ground the the right terminal? I heard that I only need to use two of the terminals when supplying power to components. Is this true?

Answer 1

From your readings, the middle terminal is isolated from the others, and I would surmise that it's either earth ground (if your AC adapter has a ground terminal on it) or simply disconnected, so it can be ignored. For DC power all you need are two terminals, one each positive and negative.

What's what most people call "ground" is determined in context of the circuit you're powering, and it's usually negative--here, the right terminal.

Answer 2

It sounds like the middle terminal isn't connected at all, your right terminal is ground and your left terminal is +5v.

This is all based off of the assumption that your multimeter has Black as ground and Red as voltage.