Potential Difference: Can 2 Negatives Equal a Positive?

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Discussion Overview

The discussion revolves around the concept of potential difference in electrical circuits, specifically examining the scenario of measuring voltage between two DC power supplies. Participants explore the conditions under which voltage readings can be obtained and the implications of circuit connections.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why there is no voltage difference when measuring between the negative terminal of one power supply and the positive terminal of another, despite both supplies outputting 24V.
  • Another participant asserts that a voltage reading can only be obtained between the positive and negative terminals of each individual supply, emphasizing the need for a complete circuit.
  • A later reply suggests that if the power supplies are not connected, the terminals are unrelated, leading to no measurable voltage difference.
  • One participant introduces a condition where there may be a voltage reading if the negative terminals are connected to a common chassis that is grounded, although this is noted as not typical for good quality power supplies.
  • Participants discuss the implications of circuit connections and the necessity of a complete circuit for voltage measurements, with one participant expressing confusion about the relationship between the terminals.
  • Visual aids, such as drawings, are mentioned as helpful in clarifying the concepts being discussed.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for measuring voltage between the two power supplies. There is no consensus on the implications of the voltage readings or the relationship between the terminals when the supplies are not connected.

Contextual Notes

Participants reference the necessity of a complete circuit for voltage measurements and the potential for odd readings under specific conditions related to grounding, but these points remain unresolved in terms of broader applicability.

Ranger24
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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?
 
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Ranger24 said:
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
 
davenn said:
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?
 
Ranger24 said:
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
 
Ranger24 said:
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 ...

power connection.JPG


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
 
davenn said:
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!
 
Ranger24 said:
Yes, I'm anot idiot lol

dunno where that came from ??
wasn't inferring that you were
 
davenn said:
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.
 
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'A picture can be worth a thousand words'.
This old saying is as valid today as it ever was.
 
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  • #10
Ranger24 said:
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 :smile:
 

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