# Homework Help: Wheatstone Bridge

1. Sep 25, 2007

### mjosephh

1.
I am having trouble finding the Vg for a problem involving a Wheatstone Bridge circuit. If I am labeling the problem correctly, R1=40, R2=70.0 R3=50.0 and R4=20.0.
Voltage - in = 20.0V

I am using the formula: ((R1*R3) - (R2*R4) *V-in) / (R1 + R2) * (R4+R3) to find Vg between points B and D.

R1=40 (resistance between points A and B)
R2=70 (resistance between points B and C)
R4=20 (resistance between points A and D)
R3=50 (resistance between points C and D)
+ -
V-in = 20V

2. I think my problem is that I don't know what to do with the resistance of Vg (between points B and D), which is 60.0 ohms.

3. The answer for Vg in the book is 0.94 V, and I am getting 1.56V. What am I doing wrong?

2. Sep 25, 2007

### mjosephh

I attached a graphic of the problem

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3. Sep 26, 2007

### mjosephh

Can anyone help with this? I have tried everything!

4. Sep 26, 2007

### bengaltiger14

Picture is very unclear. Could you try and repost it?

5. Sep 26, 2007

### bengaltiger14

nevermind about reposting. I see it clearly now. You are trying the find the voltage across that 60 ohm resistor?

6. Sep 26, 2007

### mjosephh

I will attach a larger image, but you can try and expand the other one to view it better.
Thanks for looking into this.

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Last edited: Sep 26, 2007
7. Sep 26, 2007

### mjosephh

Yes I am trying to find that voltage between B and D. I used the above formula (which is given when all resistances are known, but it didn't work out. I am sure that I need to distribute the 60 ohm resistance across the other resistors somehow, but I am not sure how to. I couldn't find anything online to help me. As most problems are geard to find an unknown resistance.

8. Sep 27, 2007

### Astronuc

Staff Emeritus
Well, I dislike using Wikipedia, but try this

http://en.wikipedia.org/wiki/Wheatstone_bridge

Follow the equations and rewrite according to the orientation of one's problem as necessary, then follow the steps, but realize that the bridge is unbalanced and Ig is not zero.

The input voltage is given, and one needs to determine the potential difference across Rg, which is related to the potential differences VAB and VAD.