# 2 voltage sources neither in series or parallel

#### patrickbryanbedard

Joined Mar 4, 2022
14
Hi,

I'm taking a circuit analysis class and struggling with the circuits that have two voltage sources (which aren't series or parallel) I know the rules for series and parallel. I have already solved/worked my way backwards from the answer in the book on this problem. I would like to verify the answers by using Ohm's law. Does anyone know how I would get RT and ET?

#### crutschow

Joined Mar 14, 2008
32,903
how I would get RT and ET?
If RT and ET are the Thevenin equivalents for the voltage source and resistance, what circuit node do you want to use for the reference point?

#### patrickbryanbedard

Joined Mar 4, 2022
14
If RT and ET are the Thevenin equivalents for the voltage source and resistance, what circuit node do you want to use for the reference point?
So we haven't got that far in the class have not covered Thevenin or Superposition. Maybe I will read that chapter. Do I have to pick a point? Couldn't I just solve for the entire circuit? Etotal and Rtotal? There's 35mA flowing into the drain.

#### crutschow

Joined Mar 14, 2008
32,903
Couldn't I just solve for the entire circuit? Etotal and Rtotal?
But from what point?
The equivalent is different depending upon what node you use for reference.

#### WBahn

Joined Mar 31, 2012
29,164
Hi,

I'm taking a circuit analysis class and struggling with the circuits that have two voltage sources (which aren't series or parallel) I know the rules for series and parallel. I have already solved/worked my way backwards from the answer in the book on this problem. I would like to verify the answers by using Ohm's law. Does anyone know how I would get RT and ET?

What are RT and ET?

You need to define your terms.

If RT is a resistance, what is it the resistance of? Looking between which two nodes?

If ET is a voltage, it is the voltage between which two nodes?

If this is a problem that you have been given in your class, then the expectation is that you have covered enough material to be able to solve it using what has been covered.

What, precisely, is it that the problem is asking you to solve for?

Show your best effort at working the problem as far as you can, based on what you have covered. That will give us both some idea of what you have and haven't covered and also what your line of reasoning is. We can then tailor our hints and suggestions to help you progress.

#### WBahn

Joined Mar 31, 2012
29,164
So we haven't got that far in the class have not covered Thevenin or Superposition. Maybe I will read that chapter. Do I have to pick a point? Couldn't I just solve for the entire circuit? Etotal and Rtotal? There's 35mA flowing into the drain.
What drain?

#### patrickbryanbedard

Joined Mar 4, 2022
14
What are RT and ET?

You need to define your terms.

If RT is a resistance, what is it the resistance of? Looking between which two nodes?

If ET is a voltage, it is the voltage between which two nodes?

If this is a problem that you have been given in your class, then the expectation is that you have covered enough material to be able to solve it using what has been covered.

What, precisely, is it that the problem is asking you to solve for?

Show your best effort at working the problem as far as you can, based on what you have covered. That will give us both some idea of what you have and haven't covered and also what your line of reasoning is. We can then tailor our hints and suggestions to help you progress.
RT= total resistance of the circuit
ET= total voltage of the circuit
So far we have done OHM's law, series, parallel and combination circuits. In the most basic circuit using OHMS law you can find E total, I total, R total with a little information. So I guess I just want to know the total circuit resistance for starters.

#### crutschow

Joined Mar 14, 2008
32,903
I guess I just want to know the total circuit resistance for starters.
You are a hard learner.
Again, it depends upon which node you use for a reference point.
Is that difficult to understand?

#### WBahn

Joined Mar 31, 2012
29,164
RT= total resistance of the circuit
ET= total voltage of the circuit
So far we have done OHM's law, series, parallel and combination circuits. In the most basic circuit using OHMS law you can find E total, I total, R total with a little information. So I guess I just want to know the total circuit resistance for starters.
When you have simple series/parallel combinations of sources, you can reduce the sources to a single source. Then the "total resistance" you find is the resistance AS SEEN by the effective source. That single effective source is your reference.

But when you have more complex circuits, there are many choices for what the reference is and you need to specify what reference you want because the results are, in general, different for each.

Again, what were you tasked with finding for that problem?

#### patrickbryanbedard

Joined Mar 4, 2022
14
You are a hard learner.
Again, it depends upon which node you use for a reference point.
Is that difficult to understand?
Yes I do understand that voltage is a potential between two points so let's just forget about the Etotal for a minute. What is the total circuit resistance (RT)?

for instance: in this circuit RT(total resistance)=R1//R2//R4+R3= 19.99666722 ohms

#### patrickbryanbedard

Joined Mar 4, 2022
14

#### patrickbryanbedard

Joined Mar 4, 2022
14
When you have simple series/parallel combinations of sources, you can reduce the sources to a single source. Then the "total resistance" you find is the resistance AS SEEN by the effective source. That single effective source is your reference.

But when you have more complex circuits, there are many choices for what the reference is and you need to specify what reference you want because the results are, in general, different for each.

Again, what were you tasked with finding for that problem?

#### patrickbryanbedard

Joined Mar 4, 2022
14
This is the exact problem although as I mentioned in my original post I have already solved the HW (problem below). I'm trying to verify that my answers are correct by solving in a different fashion.

#### WBahn

Joined Mar 31, 2012
29,164
Yes I do understand that voltage is a potential between two points so let's just forget about the Etotal for a minute. What is the total circuit resistance (RT)?

for instance: in this circuit RT(total resistance)=R1//R2//R4+R3= 19.99666722 ohms
That is the total resistance SEEN by the source.

With simple circuits like this, asking for the total resistance IMPLIES that you are asking for the total resistance as seen by the source.

But you could also ask for the resistance as seen by R2. Or R4.

Once you have a circuit that is complex enough that it is not obvious which component is being used as the reference, you must specify that.

#### WBahn

Joined Mar 31, 2012
29,164
ground...
There's NO current flowing into that ground connection (because there's no way for it to flow back out!).

#### WBahn

Joined Mar 31, 2012
29,164
This is the exact problem although as I mentioned in my original post I have already solved the HW (problem below). I'm trying to verify that my answers are correct by solving in a different fashion.
You said that you solved it by working backwards from the answers in the book. That's not solving the problem.

How would you have done so?

If you would have done it using loop voltages, the solve it using branch currents. If you would have done it using branch currents, then solve it using loop voltages.

The problem as given is not properly defined. It asks you to find two currents, but doesn't define what the directions of those currents are. So be sure to define those for your solution.

#### patrickbryanbedard

Joined Mar 4, 2022
14
That is the total resistance SEEN by the source.

With simple circuits like this, asking for the total resistance IMPLIES that you are asking for the total resistance as seen by the source.

But you could also ask for the resistance as seen by R2. Or R4.

Once you have a circuit that is complex enough that it is not obvious which component is being used as the reference, you must specify that.
You said that you solved it by working backwards from the answers in the book. That's not solving the problem.

How would you have done so?

If you would have done it using loop voltages, the solve it using branch currents. If you would have done it using branch currents, then solve it using loop voltages.

The problem as given is not properly defined. It asks you to find two currents, but doesn't define what the directions of those currents are. So be sure to define those for your solution.

So how can I verify this or solve in another way?

#### crutschow

Joined Mar 14, 2008
32,903
What is the total circuit resistance (RT)?

for instance: in this circuit RT(total resistance)=R1//R2//R4+R3= 19.99666722 ohms
Once again, that's from a particular node (in this case the one connected to V1).
So what's the node you select for your homework circuit?
If it's from a source, then there are two sources nodes, take you pick.
.
Is that clear?

#### RoofSheep

Joined Mar 7, 2023
36
You know Vb = 60V. You can now calculate the current through the 8k resistor. You can also calculate the current through the series combination of the 10k and 5k resistors. Use this current to calculate Vc. Now calculate Vab and then I1. Rather use basic principles than forcing the circuit to look like somethings that you are familiar with.