What does it mean when a circuit has two voltages?

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Discussion Overview

The discussion revolves around the interpretation of circuits that feature two different voltage sources, including negative voltages. Participants explore the implications of having voltages such as -X [V] and how these relate to circuit analysis and design. The scope includes conceptual understanding and practical examples from electronics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that voltages are relative measurements and emphasize the importance of voltage differences across components rather than absolute values.
  • There is a suggestion that having a negative voltage indicates a potential below the reference point, typically ground, which is often considered 0V.
  • One participant proposes that the interpretation of a circuit with a negative voltage can lead to calculations where the voltage difference is significant, such as +9V to -9V resulting in an 18V potential difference.
  • Another participant mentions that circuits can have non-symmetric voltage values, such as +9V to -3V, indicating flexibility in voltage configurations.
  • An example from the 1970s is provided, where HP mini computers utilized +5V and -2V for TTL logic, illustrating practical applications of non-symmetric voltages.
  • It is discussed that the choice of voltage levels can depend on circuit elements and sources, which may lead to different signs for voltages in a circuit.

Areas of Agreement / Disagreement

Participants generally agree on the relative nature of voltage measurements and the concept of ground as a reference point. However, there are differing views on the necessity of symmetry in voltage values and the implications of negative voltages, indicating that multiple competing views remain.

Contextual Notes

Some limitations include the dependence on definitions of ground and reference points, as well as unresolved aspects regarding the specific configurations of circuit elements that lead to different voltage signs.

magisbladius
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(Sorry for the bad title wording.)

I attached a picture of what I mean. Those are two points, each attached to a different voltage source. I'm used to seeing something like +9 [V] to 0 [V].

1) How am I supposed to interpret a circuit with a -X [V], where X = any positive integer, at the end?

2) Why would you use the negative in the first place?

3) Could the values for the circuit be +9 to -3, or does it have to be symmetric like +9 to -9?
 

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Voltages are only a relative measurement You can't say something is 9V unless you say 9V measured against something - you can only talk about a voltage difference across a component.
Normally if there is only a single voltage you assume the other side is at 0v and this is the potential of Earth.
You can calculate anything in this circuit by working out the voltage difference, having +9V on one side of a component and -9V on the other is exactly the same as having +18V on one side and 0 on the other, or +12V and -6V.

Think of it like a problem in mechanics, it doesn't matter what you call 0 height, everythign else works out the same.
 
If your diagram is symmetric, for simplicity, think of a ground attached to both vertical portions of the circuit at the center...say a ground connection in the middle of each.

Those voltages could have any values depending on the circuit elements and voltage or current sources. A mix of voltage and current sources, together with different circuit elements, can also cause different sign voltages...
 
Last edited:
magisbladius said:
(Sorry for the bad title wording.)

I attached a picture of what I mean. Those are two points, each attached to a different voltage source. I'm used to seeing something like +9 [V] to 0 [V].

1) How am I supposed to interpret a circuit with a -X [V], where X = any positive integer, at the end?

2) Why would you use the negative in the first place?

3) Could the values for the circuit be +9 to -3, or does it have to be symmetric like +9 to -9?

1) Interpret it as a negative voltage. So in your picture, the voltage across the device would be 9 V - (-9 V) = 18 volts. Calculate accordingly.

2) 0 Volts is usually taken as "ground". It is just a reference potential. Some voltages are below it, some above.

3) Yes, it can be +9 V to -3 V, and it need not be symmetric. It just needs to be what it is.
 
As an actual example, HP mini computers of the 1970's used +5 and -2 volts for their TTL logic. Switching rates were a bit faster due to the extra 2 volts, and short to ground (0 volts) or breaks in circuits could be detected more easily.
 
+9v and -9v is just the mentioning the electric potential difference. In that circuit the potential difference is 18V. It can be -3 or whatever you like it. It's just taking relative to the neutral or ground which is taken as 0V.
 

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