# Superposition Question Help

#### Ronny2412

Joined Sep 2, 2014
17
Hello!

So, this is a question I received for my Homework & I decided to tackle it using Superposition Theorem.

1)
Firstly they have asked to find out Voltage at B with respect to GND.
In this case, 1st I ignore V2
Considering V1, I calculate the Voltage @ B wrt GND which turns out to be I*(R1+R2)
To Calculate I: I take V1/(R1+R2+R3+R4)

I then repeat the above step ignoring V1.

Finally Vb wrt GND= V1st step-V2nd step.

PLEASE LET ME KNOW IF THIS IS CORRECT

2)
Secondly they ask me to find out Voltage between A&B
I ignore V2.
Considering V1:
I used Voltage divider rule to get Vab= V1*(R2/R).

I then ignore V1 and repeat the step.

Finally Vab=V1st step-V2nd step.

PLEASE LET ME KNOW IF THIS IS CORRECT.

Thank you

#### Jony130

Joined Feb 17, 2009
5,186
Considering V1, I calculate the Voltage @ B wrt GND which turns out to be I*(R1+R2)
I * (R1 + R2) is equal to a voltage drop across R1 + R2, but this is not Vb voltage.
In this step V1step = I * (R? + R?)

To Calculate I: I take V1/(R1+R2+R3+R4)
OK
Finally Vb wrt GND= V1st step-V2nd step.
Why you put a "minus" sign ?

Secondly they ask me to find out Voltage between A&B
I ignore V2.
Considering V1:
I used Voltage divider rule to get Vab= V1*(R2/R).

I then ignore V1 and repeat the step.

Finally Vab=V1st step-V2nd step.
Again why "minus" sign. And are your sure that you need all this steps ?
In part A you already find the current, so Vab = I * R2

#### Ronny2412

Joined Sep 2, 2014
17
Dear Jony,

Thank you so much for your time.
Can you please explain to me where I am going wrong when I calculate Vb wrt GND?

#### Jony130

Joined Feb 17, 2009
5,186
In step Vb1 you replace V2 with a short circuit. So Vb = I*(R3 +R4) or Vb = V1 - I*(R1+R2).

#### anhnha

Joined Apr 19, 2012
881
It is OK to use superposition method here but honestly I don't like that approach much!
You just need to find the current I = (V2 - V1)/(R1 + R2 + R3 + R4) and then everything is simple to calculate.

#### Ronny2412

Joined Sep 2, 2014
17
I don't think you understand how superposition aproach works.

Let me ask a couple of questions.

Ok. So we are doing superposition in a circuit that has two independent voltage sources. We start with V1. You said to ignore V2. What do you mean by ingnore? Do you mean to replace it with a short circuit? Do you mean to replace it with an open circuit?
I meant we short circuit V2. .

#### Ronny2412

Joined Sep 2, 2014
17
Yes. I got that. I calculated Vb wrt ground across R3 & R4. I used Voltage Divider Rule to arrive to the formula:

Vb= V1* (R3+R4)/(R1+R2+R3+R4).

Is this correct?

Also, we did this by shorting V2. Shouldn't we then Short V1 & do the whole process again with V2?

#### MikeML

Joined Oct 2, 2009
5,444

#### Ronny2412

Joined Sep 2, 2014
17
yeap! I calculated Vb using V2 over R1+R2 only!
and Finally I do Vb2-Vb1. .That is correct right?

#### Ronny2412

Joined Sep 2, 2014
17
Oh yes. I get it. Since both voltages are in the same direction, we add Vb2 & Vb1. OK. Thanks all of you #### Jony130

Joined Feb 17, 2009
5,186
and Finally I do Vb2-Vb1. .That is correct right?
No, this is incorrect.
Vb1 + Vb2

#### Ronny2412

Joined Sep 2, 2014
17
Thanks all of you . .I have understood the concept perfectly.

#### shteii01

Joined Feb 19, 2010
4,644
Thanks all of you . .I have understood the concept perfectly.
I appologize. I was mixing Node-Voltage and Superpositon. My mistake. I finished doing Node-Voltage and I got Vb=7.8 volts like Mike showed.