# Thevenin Worksheet Problem #40

Discussion in 'Homework Help' started by Red Bow Tie, Jul 8, 2012.

1. ### Red Bow Tie Thread Starter New Member

May 3, 2012
12
1
Am I correct in re-drawing the unbalanced bridge as 2 voltage dividers in parallel connected by the 2.7K resistor? Removing RL leaves me a circuit with 3 loops. Calculating branch currents and applying them to find Vth does indeed give me 7.6922 Volts.
But, how do I calculate Rth for this circuit? If I short out the 9V supply, it (I think) leaves me with the 1.5K and 3.3K in parallel. Is this correct? The 2.7K and 1K being in series produces a 3.7K that will be in parallel with the equivalent of the 1.5K and 3.3K? Now matter what I do I cannot come up with the textbook answer of Rth=766.84 ohms. I come up with roughly 806 ohms.
I'm obviously reading the resistors series/parallel combinations incorrectly. Can someone help me to correctly deduce Rth?

2. ### mlog Member

Feb 11, 2012
276
36
Would you happen to have a schematic diagram or a figure that you could upload for us? Thanks.

3. ### Red Bow Tie Thread Starter New Member

May 3, 2012
12
1
This is taken directly from the worksheet of Thevenin's Theorem on this site

Observe the following circuit:

Note that it is not reducible to a single resistance and power source. In other words, it is not a series-parallel combination circuit. And, while it is a bridge circuit, you are not able to simply analyze the resistor ratios because it is obviously not in a state of balance!
If you were asked to calculate voltage or current for any component in this circuit, it would be a difficult task . . . unless you know either Thévenin's or Norton's theorems, that is! Apply either one of these theorems to the determination of voltage across the 2.2 kΩ resistor.
Hint: consider the 2.2 kΩ resistor as the load in a Thévenin or Norton equivalent circuit.

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
1,115
This imagine shoudl help

File size:
10.7 KB
Views:
139
5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Yes the answer seems to be for an entirely different problem.
The values I have are
VTh=3.375V
RTh=1.719k

Ok I missed it was for the 2.2k case.

Feb 17, 2009
3,990
1,115
7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Yes we are. I was in autopilot. I took it to be the standard bridge problem with the voltage across the 2.7k being of interest. You have the matter well in hand.

8. ### Red Bow Tie Thread Starter New Member

May 3, 2012
12
1
Thanks so much for the help. I had the exact same re-written circuit as your top image but somehow could not see how to re-write it into your bottom image. Thanks especially for including how you got to the bottom image re-writing the circuit yet again.