# Intro to Elec node analysis help

AJME
Hi Guys,

I am new to Electrical Engineering and am trying to figure out the voltages for (1) (2) (3) in the link below, assuming that a 5V dc power supply is connected to the terminals on the left. I am new to Elec Engineering and am struggling a bit. I thought to try Node Voltage analysis to get the answers. Obviously (1) =5V

However for (2), I am not sure how to figure out the equations.

http://photobucket.com/albums/a339/Eliatamby/?action=view&current=SV400003.jpg

I was thinking the KCL equation at node (2) would be:

(v1-v2)/100 = v2/3900 + (v2-v3)/220

Also, I have a general question. For the current entering node (2) can't I get uses Ohm's Law at the 100Ohm resistor? E.g. 5/100 so the current entering v2 would be 0.05 A. Then I could sub this into the (v1-v2)/100 equation to find out the value of v2? Would that work?

Thanks for your help, please let me know if this is not clear

AJME
Ok I came up with two equations. The first being the KCL at node 2 (slightly modified from above)

(v2-v1)/100 = v2/3900 + (v2-v3)/220

The second is the KCL at node 3 :

(v3-v2)/220 = v3/1000 +v3/1200

I simplified the first equation to 223v2 = -195v3 + 2145

I simplified the second equation to v3=300v2/179

When I sub equation 2 into equation 1, I get 3.9 for v2 which seems about right. When I sub that value into equation 2 for v3, I get around 6, which doesn't seem right at all.

I tested the circuit in my lab a last week and got v2=4.3 and v2=3 (approx). Are there things wrong with my equations? Am I not using KCL (or node analysis) correctly?

Thanks for any help, I am really stuck

AJME
I actually think I have got it!

I realized after going through my textbook that I am not denoting the voltage across the resistors consistantly.

I chose the equations:

(v1-v2)/100 = (v2-v3)/220 + v2/3900 (1)

(v2-v3)/220= v3/1000 + v3/1200 (2)

In retrospect this is a such a simple thing, but something I just overlooked because I never thought properly about it. But solving these two equations gave me figure which were with 0.2 V of what I measured myself.

No subject makes me feel so slow as Elec1011...

Last edited:
rockerboi
using nodal analysis, i arrived with 2 equations:

-(V2-V1)/100 - (V2-V3)/220 - V2/3900 = 0 <equation1>

-(V3-V2)/220 - V3/1000 - V3/1200 = 0 <equation2>

also:

V1 = 5 <equation3>

since the 3 equations are reducible to 2 unknowns, therefore

-(V2-5)/100 - (V2-V3)/220 - V2/3900 = 0 <equation4>

-(V3-V2)/220 - V3/1000 - V3/1200 = 0 <equation5>

then i manipulated equations 4 and 5 and arrived with equations 6 and 7

-12700V2 + 3900V3 = -42,900 <equation6>

60/11 V2 - 421/55 V3 = 0 <equation7>

solving equations 6 and 7 sumultaneously and using my casio fx-991ES calculator, i arrived with

V2 = 4.324203318 V

and

V3= 3.081380037 V

since the values i got were close enough to the measured value you obtained in your lab, i therefore conclude that my answer is certified true and correct<i guess!Ü>

we just had our exam for the nodal and mesh analysis about 2 to 3 weeks ago that's why the topic is still fresh from my memory

by the way, do you have examples about norton and thevenin's theorem?

if you do, kindly post some<like this problem u posted> because i only have some examples...thanks!