- I welcome your
questions and comments. -Charles Kim.
Email to me at firstname.lastname@example.org
The Laws of Circuit
- you can learn
and practice by just reading
Copyright. Charles Kim 2006
to know for Node Voltage Method
- On Node voltage method.
First thing you remember is a method is not a law. A
method can be derived from a law, though. The nodal
analysis or node voltage method comes from the KCL. So I
would love to claim that node voltage method is a KCL in
disguise. Then why do we have to disguise, instead of
using the law? The main reason is that, if you apply KCL
directly to a circuit problem, you may have to apply KVL
along with KCL to solve the circuit. In other words, KCL
and KCL in their original forms would end up with too
many a equation to solve. This does not mean the laws
cannot solve the problem. They need to change their shape
to minimize the number of equations (or variables). So
the 'disguise' fits in here. Second, you have to
have a clear understanding on the term 'node voltage'.
Again all voltages are defined by two terminals and the
potential difference between two (i.e., 'voltage across' a
and b is Vab). This
does not change at any condition. Then, a node voltage is
a voltage between a node and the reference node. The
reference node is a node in the circuit but selected as a
reference (i.e., ground or 0[V]). That means a node
voltage is also a voltage between two terminals (a
and ref, or b and ref, etc): Va-ref and Vb-ref. However, since the reference is
0V level, we can symbolize them as: Va0 and Vb0. Again, since the other
terminal is always reference with 0V, we just ignore the
ref terminal in our symbol of node voltage: Va and Vb. But don't be fooled by the
notation of the node voltages. They are voltages between
two points: the other terminal is always the reference.
Third, we apply KCL at a node and the currents are
to be expressed by applying Ohm's Law: 'voltage across'
over resistance . KCL: The sum of currents-in is the same
as the sum of currents-out at a node. Fourth,
once you have an equation at a node, make sure every term
must be current, whether a current directly from a
current source or a current expressed by voltage across
over a resistance.