Are you supposed to assume current directions in/out of the nodes when doing node voltage analysis? I have done a few problems this way and if I assume the current direction 'wrong' (ie. different than the problem book I am using), the simultaneous equations do not solve correctly. How do you ensure you are assuming current direction properly when you do not know (at the start) what the voltages are at the nodes and you have multiple independent and dependent sources in the net?
node voltage analysis
Aug 09, 2005
3 Replies
It doesn't matter which way you choose, as long as you use the same direction for all your equations. Draw a circuit diagram, then draw an arrow next to any element. It doesn't matter which way the arrow goes. Then, traverse any loop, in any direction. As you collect the terms, any arrow that points in the same direction as the loop is a positive current, any arrow that points against the direction of your loop is a negative term. This should always work.
Regards,
Bob Monsen
If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877
It doesn't matter, as long as you keep track of your (minus) signs. If you draw a current in the "wrong" direction, it'll just turn out to be negative when you solve the equations.
The sum of currents entering a node equals the sum of currents leaving it. If you have all of one, or all of the other i.e. all leaving, or all entering, then the sum is zero.
A wrong guess means the answer comes out to be negative.
Suppose you have a DC voltage source V in parallel with a resistor R. By unfortunately guessing that the current flowed the wrong way through R we would have the loop equation V + IR = 0 giving I = -V/R.
It is very easy to make sign mistakes and sometimes even the book is wrong.
Cheers - Joe
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