# Why do 0v have to be connected when using two power supplies and ULN2803A?

The other day I was playing with a ULN2803a and a stepper motor. I spent ages trying to work out why it wasn't working and why i was reading ~14V as an output on the PIC I was using. So why did I have to connect the 0V of both supplies together?
Dean

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Just because two different supplies both have a terminal labeled 'ground' doesn't necessarily mean that those nodes are at the same voltage. In fact, if you have two supplies that have isolated output, you could have almost any potential between their ground lugs, unless you strap them together. You could find a DC offset or even significant AC between the grounds of two isolated supplies. To establish a proper relative voltage levels within your system, each component needs to have a common voltage reference, and usually that's the ground node, so you connect them for that purpose.

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I had a very similar problem when using transistors to switch power components off an Ardurino's TTL output. On a hunch I connected all the grounds and it worked. I understand the concept on a practical level, but it's still kind of odd theoretically. Has anyone got a good link explaining the "relativeness" of voltage? –  Robert Atkins Nov 20 '10 at 15:45
"Ground" is just a label. Another word for "voltage" is potential difference. It's the difference in potential between two points. For convenience, we choose a point in the circuit and call it ground (0 V), and measure all other voltages relative to that point. That point is conventionally chosen in a specific way, but you could really choose any point and measure all voltages relative to it. –  endolith Nov 20 '10 at 17:24
For starters, all the power you have in your house comes from the AC plugs in your wall. AC doesn't have a 'ground' as a common reference. On top of that, many power supplies to convert AC to DC are completely isolated from the wall power, so even if there was a 'ground' reference for AC, you wouldn't have access to it from the DC (because of isolation). And lastly, I can put 'ground' anywhere I want in a circuit it's just a reference. It could at 5V or 1000V but as long as everyone IN THAT CIRCUIT agrees on what it is, it will work. –  AngryEE Nov 22 '10 at 13:39