Passive Sign Convention

Discussion in 'General Electronics Chat' started by ihaveaquestion, Dec 21, 2009.

  1. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    314
    0
    Hello,

    I was taught (and I thought it was widely accepted) that we always use the passive sign convention and look at current flowing from the positive voltage terminal to the negative terminal.

    http://www.allaboutcircuits.com/videos/36.html

    In this video at 22:00 I see that he goes against this.

    Could someone explain why? I've seen this done in ideal Op-Amp circuits as well for example here I think (11.9):

    http://img683.imageshack.us/img683/1842/imggi.jpg

    Both + and - are @ Vs volts... but current at the - terminal doesn't split up into two paths (1 going down to ground, 2 going to the output terminal), but rather current 'flows from ground through that node' (not to the - terminal).... so it's a little confusing.

    Thanks
     
  2. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    ihaveaquestion,

    Sign convention has been discussed extensively. See http://forum.allaboutcircuits.com/showthread.php?t=21280&highlight=conventional

    Current exists, but does not flow. Charge flows alternately back and forth between both output branches of the OP amp output to the ground. The voltage is the same across the OP amp inputs, but the impedance of the between the OP amp inputs is so high that for practicable purposes, current it does not exist.
     
  3. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    314
    0
    Could someone please explain the reasoning behind the way he explains the way the charge flows in the video at that time (22:00)?
     
  4. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    314
    0
    Anyone know the reasoning behind the way he explains it?
     
  5. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    ihaveaquestion,

    Yes, he is using the negative electron flow to define current. Using the direction of charge carriers to define current direction can really get your mind tied up in a knot. That is because there are two polarities of charge to confuse you. If you do electrochemistry where positive ions exist along with negative ions or electrons, or p-type material where positive holes are the charge carriers and interact with electrons from n-type material, a lot of confusion can result. So the conventional current direction was developed to keep things straight. For mathematical computational purposes, current is assumed to exist in a direction from the positive terminal of the source to its negative terminal. Current existing in that direction is designated a positive quantity. Notice that we don't care what the polarity of the moving charges are. Then when we get a number from the current calculations, if it is necessary and important, we can say that a positive carrier moving in one direction is the same mathematically as a negative carrier moving the the opposite direction. If we are determining the direction of charges in a wire, where negative electrons are the predominate carriers, then by the above reasoning, we know that electrons are moving opposite to what we calculated.

    Conventional current is the industry standard. All diodes are marked with a arrow designating the conduction direction of conventional current. Ammeters are marked with a + and - to show that if you hook up a battery to that polarity, the meter will show a deflection in a positive conventional current direction. So you were correct to notice the difference in the video.

    Ratch
     
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