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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

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1 Answer

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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.

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"Insulated" or "isolated"? – The Photon May 28 '12 at 17:46
Hmmm, I understand now, thanks dude! – Calvin May 28 '12 at 17:54
@ThePhoton - sorry, I'm not a native English speaker. In Dutch we use the same word for both, so this is a bit confusing. Both insulated and isolated then? Feel free to edit my answer if you think it's necessary. – stevenvh May 28 '12 at 17:55
@stevenvh, there is insulation that provides the isolation but it would be standard to say it is isolated as mains is still powering it but they do not have a reference. – Kortuk♦ May 28 '12 at 18:01
@Kortuk - Thanks a million. Edited. – stevenvh May 28 '12 at 18:05

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