Talk:Passive sign convention

Direction of voltage arrows

At least in the German-speaking community, the direction of the voltage arrows in the drawings would be considered wrong. In the convention in the German-speaking countires, the voltage arrow is in the same direction as the current arrow (current flow) for a passive component (resistor), to have a positive sign in Ohm's law. In other words, the voltage arrow points from plus to minus for a positive value of the voltage; this is the same direction as the electric field.

https://de.wikipedia.org/wiki/Zählpfeil

I had a look at a few US textbooks, they don't use arrows for the voltage direction at all.

Does the British or US community really have a different convention, or is this a mistake?

--Anastasius zwerg (talk) 16:55, 15 May 2019 (UTC)

The convention that I have always worked with is that the voltage arrow points to the positive end of the pd. This is the convention I am seeing in my introductory textbooks. Although some use a double arrow with the +/− ends explicitly marked.

• Ryder, Electronic Fundamentals & Applications – arrow points to positive
• Morris, Industrial Electronics – arrow points to positive
• Ward, Electrical Engineering Science – uses double arrows
• Hughes, Electrical Technology – uses double arrows, except for emfs which use single arrows pointing positive
• Millman-Halkias, Integrated Electronics, – uses double arrows

SpinningSpark 09:53, 22 May 2019 (UTC)

Indeed different countries and authors seem to use different arrows. Many internet sources (mostly American) show the arrow pointing from - to +. While in many European countries it's the reverse (like in Hungary). I think I've read France also uses - to +. I've actually only discovered this recently. In former times I only saw a few examples and thought they were sloppy :D

The funny thing for me is shown in this wiki article. One of the images shows the E electric field. Which, curiously, uses an arrow that points from + to - (of course it's from physics). And then it has a voltage arrow pointing from - to +. As E and V are very closely related, it seems very strange to me to use different arrow notations for them.

Btw do notice that the numbers are the same, calculations are the same, no signs actually change. The other system still measures voltage from positive to negative, it's only the arrow drawing that's different. Just change it in your drawing and it's the same. (It's not so for the current directions as mentioned in the article).

I think it would be nice to mention this in the article. I don't yet have a definite list of countries' preferences, a bit hard to find. Hoemaco (talk) 06:58, 19 May 2021 (UTC)

@Anastasius zwerg: @Hoemaco: Thanks for bringing this up. I drew the diagrams and wrote the text that refers to the arrows. Like Spinningspark, in my American textbooks I have only seen the voltage arrow pointing from negative to positive terminal, didn't know there was any other convention. I could redraw the diagrams to have arrowheads on both ends of the line and just indicate the voltage polarity with plus and minus signs, to avoid any ambiguity. What do you think? --Chetvorno^TALK 15:42, 19 May 2021 (UTC)

Haaving arrowheads on both ends sounds very strange to me. I do not advise changing the image. (Okay well, I would like the other norm better of course but it's the English language page anyway). I only suggest writing in the text about it. Maybe someone knows more details about which countries or fields use which convention? I may write a bit about it when I gather my wits and energy. Hoemaco (talk) 16:44, 19 May 2021 (UTC)

@Hoemaco: What I meant by "arrowheads on both ends of the line" is to just use two arrows to indicate the nodes between which the voltage is measured, as is done on many drawings, i.e. [1], and indicate the polarity by plus and minus signs --Chetvorno^TALK 01:54, 20 May 2021 (UTC)

Tha's exactly what I thought you said. And I wouldn't prefer it. It's rarely used (don't remember seeing it ever - only in mechanical drawings indicating lengths). And + - signs are not very good. An arrow can show a reference direction even in AC while +- is kindof meaningless then. Oh btw, how is this "ping" supposed to work? I got no notification either in wiki or email, just saw this by chance... Hoemaco (talk) 07:49, 2 June 2021 (UTC)

The whole point of the diagram is to indicate polarity - either DC or instantaneous AC - the polarity of the voltage across the component's leads compared with the direction of current. So the + and - signs are necessary. What we were discussing is that according to Anastasius zwerg there appear to be different conventions in different countries for using an arrow to indicate voltage polarity, which I used to reinforce the + and - signs. This would make the diagram ambiguous in some countries. The solution is clearly to just use the + and - signs for polarity. That's what I was saying above. I'll change the diagrams. --Chetvorno^TALK 15:06, 2 June 2021 (UTC)

 Done --Chetvorno^TALK 19:24, 2 June 2021 (UTC)

Don't know why you didn't get the ping, I'll look at it. --Chetvorno^TALK 15:06, 2 June 2021 (UTC)

@Chetvorno and Hoemaco: Sorry I'm late to this discussion, I've been on an extended wikibreak. The reason the ping didn't work was that Chetvorno added it as an afterthought after the initial post. The notification system requires a new valid signature to be posted with every ping. If you forget to ping someone you have to make a new post with a new signature to go with it. SpinningSpark 13:06, 26 July 2021 (UTC)

Thanks, good to know. --Chetvorno^TALK 16:12, 26 July 2021 (UTC)

at any given instant it obeys the PSC

For AC circuits, it says: at any given instant it obeys the PSC. Obviously this isn't true for inductive and capacitive loads. For induction motors it will be true more than half the time, but not all the time. Gah4 (talk) 23:18, 21 August 2020 (UTC)

The passive sign convention has to do with defining reference directions for current and voltage. The statement "at any given instant it obeys the PSC" is misleading. The assigned reference directions either obey the PSC or don't. They cannot possibly obey some of the time and disobey at other times. The convention does not depend at all on the actual algebraic sign of the voltage or current variables. Doug iowa (talk) 03:23, 12 September 2020 (UTC)

I wrote that. Yeah, I agree, that was erroneous. Thanks for correcting it.--Chetvorno^TALK 15:48, 19 May 2021 (UTC)

Labels vs. variables

In this article the labels that define a circuit variable such as i or v should be distinguished from the variable itself, which could be positive or negative in sign.

The section presently labeled "The convention," has the phrase, "The passive sign convention states that in components in which the conventional current variable i is defined as entering the device through the terminal which is positive as defined by the voltage variable v, the power. . ." This is correct, but most readers not familiar with the convention will read it as, "The passive sign convention states that that when the current enters the device through the positive terminal, the power. . ." or some other such incorrect reduction of the true meaning of the given sentence.

I propose that the article should state, "The passive sign convention states that in labeling the voltage and current for a circuit element the current arrow must be directed into the positive labeled terminal of the circuit element. Then the power. . ."

The whole article needs to be re-written get rid of ambiguous phrases such as, "Defining the current variable as entering the positive terminal means that. . ." In this phrase the terminal that is actually more positive is irrelevant to the discussion at hand. The phrase should be, "Defining the current reference arrow as pointing into the positive labeled terminal means that. . ."

Background: I teach electrical engineering courses. This matter of the passive sign convention is a stumbling block for students. This wiki article has nothing in it that is per se wrong, yet it does not debunk wrong ideas students may have because it does not clearly distinguish between variables and labels. Many students do not understand that the labels define the variables and that a positive labeled node can be the more negative node in fact if the voltage variable is negative in sign, etc.

I don't have time to do a "bold rewrite" now. Let me know what you think before I even give it a try. --Doug iowa (talk) 03:25, 12 September 2020 (UTC)

Thanks for the great feedback! It really helps to know what problems students have with the article. I appreciate you catching the error "Defining the current variable as entering the positive terminal means that..." which I wrote. It should be "Defining the reference direction of the current variable as entering positive reference terminal..."

Besides one or two errors like the above, the article actually does a pretty good job of distinguishing between the reference direction of the variables and the current or voltage, which is what I think you mean by "label" and "variable". What you call the "arrow" is in textbooks called the "reference direction" of the variable v or i. The article explains:

"The current flow in a wire has two possible directions, so when defining a current variable i the direction which represents positive current flow must be indicated, usually by an arrow on the circuit diagram. This is called the "reference direction for current i." If the actual current is in the opposite direction, the variable i will have a negative value."

And a similar definition of the reference terminal for a voltage.

"To understand the passive sign convention, it is important to distinguish the reference directions of the variables, v and i, which can be assigned at will, from the direction of the actual voltage and current, which is determined by the circuit."

My feeling is, replacing the term 'reference direction' with the ambiguous term 'arrow' would be more confusing, particularly since the reference direction of a voltage difference v between two nodes is often not indicated by an arrow but by plus and minus signs. But I think the wording could be improved, and paranthetical use of 'arrow', as in "...the reference direction (arrow showing the direction of positive current) of i is..." would be helpful. --Chetvorno^TALK 20:20, 12 September 2020 (UTC)