I have two different DC power supplies (6 and 12 volts, let's call them A and B). They connect to the wall and provide a negative and a positive terminal each.
I tried to measure the voltage between the positive terminal of A and the negative terminal of B and measured 0V.
I don't understand why the potential difference between the + and – terminals of different power supplies is 0V. One could say that they don't complete a circuit, but they should, they both come from the mains of my house.
Can someone please explain me what I am thinking wrongly?
Thanks in advance,
Calvin
Best Answer
They come from the mains, but they're isolated from it by a transformer. You're correct about the complete circuit, because that the condition to have a potential difference (just say voltage difference, that's also OK).
If you would take a transformer and measure the resistance between a contact on the primary and a contact on the secondary you'll see that the resistance is very high, almost infinite. There's no closed circuit. If you would measure the voltage between the pins of power supply A you would have a closed circuit: from one pin through the multimeter's resistance back to the other pin.
A closed circuit is the condition.
edit
However! Note that this is for DC. AC signals can pass though transformers and capacitors, which also would block DC.