Beginner Circuits Questions!

Discussion in 'Homework Help' started by David Lin, Oct 11, 2015.

  1. David Lin

    Thread Starter New Member

    Oct 11, 2015
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    0
    Hi, I'm pretty new to circuits and just trying to get a rigid foundation in basic concepts. Right now, I'm stuck on simple direction conventions of currents. I ran a simulation of the following circuit using LTspice. I'm very confused as to why the current through R1 is opposite in direction to the current through R2. Also, which direction is positive current moving? Counterclockwise or clockwise? And how do you know? Any help is greatly appreciated.

    upload_2015-10-11_19-26-54.png
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,788
    4,808
    The convention used by almost all circuit simulators is that (conventional) current flowing INTO a pin is positive. This convention was adopted because, in general, the simulator has no way of knowing whether a particular pin is an input pin or an output pin (or even a bidirectional pin).

    For a two pin device, such as a resistor or a voltage source, the convention is that the current through the device is the same as the current into pin #1.

    For a voltage source, pin #1 is the positive terminal.

    Unfortunately, most of the component libraries do not mark components such as resistors and such with any kind of a pin #1 indicator. This is why I go into the component libraries on my machine and add a slight perpendicular tick mark on pin #1 so that I can tell where it is on my schematics.

    In your circuit, current is flowing clockwise. Thus the current flowing INTO the positive terminal of the voltage supply is negative. It also appears that pin #1 for each resistor is on the right hand side of the resistor, which makes sense given that you probably instantiated both resistors the same way.
     
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  3. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,653
    768
    For the resistors, if you reverse them, the direction of the current will also be reversed.

    WBahn, I can see, posted a good reply while I was typing on this little tablet.
     
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  4. David Lin

    Thread Starter New Member

    Oct 11, 2015
    4
    0
    Oh I see.. Thank you all! I'll probably try adding that tick mark as well.
     
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