Passive Sign Convention Misconception!?

Discussion in 'General Electronics Chat' started by vintageplayer, Mar 14, 2015.

  1. vintageplayer

    Thread Starter New Member

    Mar 2, 2015
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    The Passive Sign Convention from Wiki:

    "In electrical engineering, the passive sign convention (PSC) is a sign convention or arbitrary standard rule adopted universally by the electrical engineering community for defining the sign of electric power in an electric circuit. The convention defines electric power flowing out of the circuit into an electrical component as positive, and power flowing into the circuit out of a component as negative"

    All well and good. What I don't understand is what voltages and currents have to do with it. The page goes on to say:

    "To comply with the convention, the direction of the voltage and current variables used to calculate power and resistance in the component must have a certain relationship: the current variable must be defined so positive current enters the positive voltage terminal of the device."

    I understand the sign of power can be arbitrarily defined depending on what system you're interested in; however, how can currents and voltages be arbitrarily defined? In a conductor, the actual current will always "flow" in the direction of the Electric Field, from a greater voltage potential. You can't just define the directions of the voltages and currents arbitrarily, unless you also want to rewrite Maxwell's Equations.

    What am I missing here?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    It does not matter if we consider voltage drops as positive and voltage rises negative. The rule is still "the sum of the voltage drops around a closed loop must be zero". As such it does not matter what convention we apply, but we must apply it consistently for a given problem. With respect to currents, again it does not matter what convention we adopt to solve a problem as long as we are consistent. The sum of the currents into or out of a node is still zero.

    When it comes to power we don't use the sign of the voltage or current because there is no negative power. At least I never heard of negative power in a career spanning half a century. Since power can be expressed as (I^2)*R or E^2/R the use of a negative voltage or current would be eliminated by the squaring process. With the exception of tunnel diodes and similar devices there is no negative resistance.

    The magnitude of currents and voltages is never arbitrary, only the sign is arbitrary.

    If you think about it for a minute, the ONLY way for a collection of things like current or voltage to sum up to zero is for some of them to be positive and some of them to be negative. Then you can also say by a simple piece of algebraic manipulation that the sum of the things with a positive sign is equal to the sum of the things with a negative sign.

    Your concept of current flow is also wrong. Current is actually a flow of charge and charge can be positive or negative. In semiconductors, for example the electrons flow in one direction and the "holes" flow in the opposite direction. Wrapping your head around "hole" current requires a potent imagination. And no rewriting of Maxwell's equations is required.
     
    Last edited: Mar 14, 2015
  3. vintageplayer

    Thread Starter New Member

    Mar 2, 2015
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    I agree with you when you say it doesn't matter as long as we are consistent. The maths still works out either way.

    However, I disagree with you when you say that the sign of the currents and voltages are arbitrary. If you define current going the other way, aren't you redefining Franklin's initial definition of current flow?

    Also, why is there no negative power? This is arbitrarily defined depending on the system you're looking at. To say there is "no negative power" also flies in the face of the passive sign convention.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    I agree with what you said, and what I said flies in the face of the Passive Sign Convention. To be honest I've never heard of the Passive Sign Convention. I guess the professors I studied under, for my two graduate degrees, did not deem it important enough to mention. Hold on to your beliefs, I'm retired now and couldn't care less what you believe or don't believe.
     
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  5. WBahn

    Moderator

    Mar 31, 2012
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    The passive sign convention is merely a name given to the way that most electrical engineers and scientists have been defining their SYMBOLIC voltages and currents for the last century or so.

    In general, if you have a component (let's just use a resistor for now) in a circuit you don't know what direction the actual current is flowing or which side of the resistor is at a higher potential relative to the other, at least not until you do some level of analysis on the circuit. But to analyze all but the simplest circuits, you need to have variables to work with -- these variables are known as the symbolic voltages and currents.

    Each resistor has two variables associated with it -- the voltage across it and the current through it. Each of these variables can be defined with one of two polarities which, at the end of the day, are completely arbitrary. Thus we have four possible ways to assign these two variables. However, if we wish to apply Ohm's Law directly, namely that V=IR, then we really only get to choose the polarity of one variable arbitrarily because that fixes the polarity of the other one. So if we choose to define the symbolic current in a resistor as being positive when it is flowing from left-to-right through the resistor, we have no choice but to define the symbolic voltage such that it is positive one the left end relative to the right end. If we don't, then those two variables will not be related by the equation V=IR but rather by V=-IR. While this is perfectly valid, it is also asking for trouble and all but guaranteeing that mistakes are going to be made. When all is said and done, we will come up with the actual voltages and the actual currents and some of them may end up being negative. That's fine, it just means that the actual quantities are in the opposite direction compared to how the symbolic quantities are defined. By being consistent, we build in a number of checks, too. For instance, the signs of the voltage across and the current through a given resistor have to agree -- if they don't, then we KNOW that at least one of them is wrong.

    Similarly, if we want to use P=IV everywhere and P=-IV in some places, we need to define the voltages and currents as described above and, in doing so, when we are saying that if the power calculated for a given component is positive, that that component is absorbing electrical power while if it is negative then that component is supplying electrical power. A common variant of the passive sign convention is to decide which components you want to call sources and which ones you want to call loads and then defining the relative polarities as described above for the loads but defining them the other way for the sources. That will mean that the power for loads will be positive if they are acting as loads and the power for sources will be positive if they are acting as sources. The fewer minus signs we have running around the less likely we are to make mistakes.
     
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