# KCL



## Platinum (Feb 1, 2007)

Inspired by the KVL thread........

Do you guys have any tips regarding KCL (nodal analysis)? I typically assign currents leaving a node as (+). Then, currents entering a node are (-).

From KVL i assume current flows from higher potential to lower potential.....i guess this works with KCL as wel?


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## singlespeed (Feb 1, 2007)

Platinum said:


> Inspired by the KVL thread........Do you guys have any tips regarding KCL (nodal analysis)? I typically assign currents leaving a node as (+). Then, currents entering a node are (-).
> 
> From KVL i assume current flows from higher potential to lower potential.....i guess this works with KCL as wel?


This is generally the accepted practice - having said that, I did it the opposite way! :appl: It just made more sense to me. But as long as you are consistent, it won't matter.


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## benbo (Feb 1, 2007)

As is said here, if you are just trying to get nodal voltages, all that matters is consistency. I get real confused with these directions. I always assume current entering a node as positive, current leaving as negative. I once had a filters class with a lot of opamps. They taught me a method that seems to work. Suppose I have a node A and a node B. First, assume there is a resistor R1 between A and B. I will always assume the current into the node (node A) from that branch is (Va-Vb)/R1. Now, assume there is a node C also attached to node A with a resistor R2 between. I will still start with Va and calculate the current into the node as (Va-Vc)/R2. On and on, for 100 nodes it is always Va minus something divided by a resistance (it also works for caps and coils impedance). There is a slight variation for a current source in the branch. If there is a current source arrow pointing in I call it I, away from the node out I call it -I. Then I just add it up, and set to zero. It always works for KCL and is usually a lot easier than some funky loop analysis. Especially with *ideal* opamp circuits (where you just assume V+ = V-). Hope I confused you enough.


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## singlespeed (Feb 4, 2007)

benbo said:


> As is said here, if you are just trying to get nodal voltages, all that matters is consistency. I get real confused with these directions. I always assume current entering a node as positive, current leaving as negative. I once had a filters class with a lot of opamps. They taught me a method that seems to work. Suppose I have a node A and a node B. First, assume there is a resistor R1 between A and B. I will always assume the current into the node (node A) from that branch is (Va-Vb)/R1. Now, assume there is a node C also attached to node A with a resistor R2 between. I will still start with Va and calculate the current into the node as (Va-Vc)/R2. On and on, for 100 nodes it is always Va minus something divided by a resistance (it also works for caps and coils impedance). There is a slight variation for a current source in the branch. If there is a current source arrow pointing in I call it I, away from the node out I call it -I. Then I just add it up, and set to zero. It always works for KCL and is usually a lot easier than some funky loop analysis. Especially with *ideal* opamp circuits (where you just assume V+ = V-). Hope I confused you enough.


This is exactly why is don't follow the negative in, positive out convention! Op amp circuits are fairly easily solved in this manner. :Locolaugh:


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## Art (Feb 4, 2007)

I use current in +, out -, but it doesn't matter, it shakes out in the math as long as you are consistent...

I think the convention originated from electrons = - charge...they are stationary in most metals, so the 'holes' move 'into' the node...and the electrons 'away', since they are the measurement of charge (1 coulomb ~ 1.66 x 10^18 electron charge), and I=C/sec, then the positive flow of charge is away from the node...


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## Platinum (Feb 4, 2007)

I see what you guys are saying. It does make better sense to assume incoming current as (+) and leaving current as (-).

I know it will work out either way as long as I am consistent. I'll try to do the rest of my practice problems this way and hopefully it will all come back to me again.

More questions to come i'm sure!!


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## grover (Feb 5, 2007)

With as much coursework in semiconductor theory as I've taken, I always have difficulty imagining current flowing from the + to the - when I'm working DC circuits! Curse those early physicists for getting that 50/50 guess wrong!


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