Potential Difference: Can 2 Negatives Equal a Positive?

[Ranger24]

Hi, I'm new to Physics Forum and I can figure out what it seems should be an easy problem. I have two DC power supplies both of which are putting out 24vdc. In my mind I should be able to test for potential difference between the (-) of one and the (+) of the other and read how many volts they are away from each other. If I do that test I get zero difference in voltage. How is this possible? How can both negatives be the same voltage as the positive on the other power supply?

 

[davenn]

In my mind I should be able to test for potential difference between the (-) of one and the (+) of the other and read how many volts they are away from each other. If I do that test I get zero difference in voltage. How is this possible? How can both negatives be the same voltage as the positive on the other power supply?

if the power supplies were not connected together, how would you expect to get a combined reading between them ?

you can ONLY get a reading between the + and - of each individual supply

 

[Ranger24]

if the power supplies were not connected together, how would you expect to get a combined reading between them ?

you can ONLY get a reading between the + and - of each individual supply

Thank you for your response! I guess that right there is what confuses me. Even if they are not connected why wouldn't there be any difference in voltage between them. In my mind that means that the (-) of one and the (+) of another would have to be the same value which seems impossible and wouldn't make sense when you tested the other two pos/neg. Is there any potential difference between those points even if the circuits are not tied together?

 

[davenn]

Thank you for your response! I guess that right there is what confuses me. Even if they are not connected why wouldn't there be any difference in voltage between them. In my mind that means that the (-) of one and the (+) of another would have to be the same value which seems impossible and wouldn't make sense when you tested the other two pos/neg. Is there any potential difference between those points even if the circuits are not tied together?

because you need a complete circuit. The positive or negative terminals on one supply are completely unrelated to the ones on the other supply.

( now just a note ... there is one circumstance where there may be a voltage read between the 2 supplies
IF and this isn't the way it's normally done on a good quality supply, the negatives of each supply are connected to the metal chassis of the supply and the chassis which should ALWAYS be connected to the mains power Earth wire. THEN there may be oddball voltages read between various configurations of testing between the + and - of the separate supplies.
EVERY GOOD PSU should have the + and - terminals floating above ground)

Dave

 

[davenn]

Even if they are not connected why wouldn't there be any difference in voltage between them. In my mind that means that the (-) of one and the (+) of another would have to be the same value which seems impossible and wouldn't make sense when you tested the other two pos/neg.

OK consider this drawing I have done ...

can you now see that if there isn't a connection between the centre + and - terminals, there is no complete circuit
so the voltmeter cannot read a value other than 0 ?

cheers
Dave

 

[Ranger24]

OK consider this drawing I have done ...

View attachment 93056

can you now see that if there isn't a connection between the centre + and - terminals, there is no complete circuit
so the voltmeter cannot read a value other than 0 ?

cheers
Dave

Yes, I'm anot idiot lol thank you for taking your time out to clear it up. That makes perfect sense!

 

[davenn]

Yes, I'm anot idiot lol

dunno where that came from ??
wasn't inferring that you were

 

[Ranger24]

dunno where that came from ??
wasn't inferring that you were

Sorry that is not how I intended that when I typed it. I meant yes I understand and I'm sarcastically an idiot because it seemed so obvious once I looked at your drawing. Sincerely thank you for taking the time to explain it to me.

 

[rootone]

'A picture can be worth a thousand words'.
This old saying is as valid today as it ever was.

 

[davenn]

Sorry that is not how I intended that when I typed it. I meant yes I understand and I'm sarcastically an idiot because it seemed so obvious once I looked at your drawing. Sincerely thank you for taking the time to explain it to me.

no probs mate ...
look forward to seeing you around PF regularly