# Transform some sources in one

Discussion in 'Homework Help' started by juanjuan, Mar 22, 2015.

1. ### juanjuan Thread Starter New Member

Oct 24, 2013
17
0
I have this 2 circuits:

I need to substitute all those sources for just 1 source (1 source for the first circuit and another source for the second circuit. Those are 2 different items). I don't know how to do that and I also don't know how to search how to do it in google.

Any help will be very useful
Thanks

2. ### MrAl Distinguished Member

Jun 17, 2014
2,551
514
Hi,

Voltage sources add in series, and do not add in parallel, and must be the same if they are in parallel.

Current sources add in parallel, and do not add in series, and must be the same if they are in series.

So trace one branch and add the voltages, then make sure all the voltages in parallel add to the same value.
Add current sources in parallel, and make sure any in series are the same value.

3. ### juanjuan Thread Starter New Member

Oct 24, 2013
17
0
In the circuit of voltage sources, I can see that in parallel they have the same voltage (3+2=5 and 2-1=1) and I can see in the current sources that 2+1=4-1 but I don't know how to solve the exercise.

Any other help?

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
Look at the voltages at the nodes x, y, and z in this circuit (all referenced to the node connected to the ground symbol), paying attention to the signs. Relate V(x,y,z) to the nodes in your problem...

Note that positive current flows downward through R1. Look at the signs of the current that flows in the voltage sources...

5. ### juanjuan Thread Starter New Member

Oct 24, 2013
17
0
I see that, in x, the voltage is V1. In y, the voltage is V1+V2. In z, the voltage is V1+V2-V3, and that's the voltage that goes to R1, but I still don't know what about the circuits I posted

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
Now look at this: If you were the resistor, and were trying to determine what was heating you, could you tell if it was the box on the left or the box on the right? The entire premise of a Thevenin Equivalent is that the contents of the box on the left can be reduced to a much simpler box such that with respect to the effect on the resistor, the two boxes are indistinguishable from each other...

In this example, the proof of Thevenin Equivalency is that V(z) = V(w).

7. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
You are really trying to do two things for each circuit

1) Determine the conditions at the terminal -- that will give you the answer.
2) Determine that nothing between the terminals violates any rules -- that will tell you that the answer is possible.

For the picture with the voltage sources:
1) Pick ANY path from A to B and find the voltage difference. If a solution exists at all, that will be the value.
2) To verify that a solution exists, determine the voltage at every node within the circuit and verify that the voltage differences between any two nodes is consistent with any voltage source connected between those same two nodes. This is equivalent to applying KVL around every possible loop within the circuit, but the bookkeeping is a lot easier.

For the picture with the current sources:
1) Find out how much current is flowing from Node A. If a solution exists at all, that will be the value.
2) To verify that a solution exists, apply KCL at every node. The obvious one to start with is Node B.

Performing Step 1 for these circuits is trivially easy. Performing Step 2 takes a bit more effort.

Focus on the current sources first as it is the easier of the two. This is because the sources can all be incrementally combined in series/parallel to get to a single source which should match the value you get from Step 1 of the above process. For the voltage sources, some of those are in neither series nor parallel and so they can't be combined using those rules.