No one in my class can figure this out!

Discussion in 'Electronics Homework Help' started by SirExcedrin, Feb 3, 2011.

1. SirExcedrin

2
0
Feb 3, 2011
I just wanted to find out the total resistance in this simple circuit. MultiSim 11.0 is telling me that the resistance is 937.83 ohms as shown below. But how did they get that value??? Or how is this circuit properly redrawn to make dealing with R5 more obvious?

2. Laplace

1,252
184
Apr 4, 2010
I don't know of any way to simplify an assymetric bridge but the equivalent resistance can be calculated using generic circuit analysis. Once you have the equivalent resistance of the 5-resistor bridge, it's easy enough to get the resistance of the whole circuit. I wrote the node equations for either side of R5, then another equation stating that the sum of the currents flowing through R1 & R2 is equal to the sum of the currents flowing through R4 & R8. Then a little algebra using Mathcad gives the resistance of the assymetric bridge.

Req:bridge = (R1R2R4 + R1R2R5 + R1R2R8 + R2R4R5 + R1R4R8 + R1R5R8 + R2R4R8 + R4R5R8)/(R1R4 + R1R5 + R2R4 + R2R5 + R1R8 + R4R5 + R2R8 + R5R8)

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3. SirExcedrin

2
0
Feb 3, 2011
Incredible

You answer was absolutely awesome! Thanks so much. Now I see why I could not solve this by just redrawing the schematic in a simplified equivalent form - which I tried for several hours. I'm looking ahead in my textbook now but is your answer what is known as nodal or mesh analysis?

Loops and node analysis is in two more chapters, but this is so cool that I'm going to get into it this weekend. Thanks so much for your answer!