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node voltage analysis

Discussion in 'Electronic Basics' started by R.Spinks, Aug 8, 2005.

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  1. R.Spinks

    R.Spinks Guest

    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?
     
  2. Andrew Holme

    Andrew Holme Guest

    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.
     
  3. Hi,

    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
     
  4. Bob Monsen

    Bob Monsen Guest

    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
     
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