# node voltage analysis

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

1. ### R.SpinksGuest

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 HolmeGuest

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. ### Joe McElvenneyGuest

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 MonsenGuest

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