# Need help with basic circuits.

Thread Starter

#### Marasha

Joined Feb 6, 2018
10
Greetings,

I really need help with some assignments. I am horrible with circuit analysis, so please keep it as simple as possible.

If you have the time, please explain to me, preferably step-by-step how you would solve this using KVL.

We need to find the current running through the equivalent R1+L1 resistor using KVL. Can I apply KVL when there are inductors in the circuit, or do I have to turn them into an equivalent (complex) resistor? Thank you!

Edit: Deleted 2nd circuit.

Last edited:

#### MrAl

Joined Jun 17, 2014
8,251
Greetings,

I really need help with some assignments. I am horrible with circuit analysis, so please keep it as simple as possible.

If you have the time, please explain to me, preferably step-by-step how you would solve these.

Circuit 1:
View attachment 240684
We need to find the current runinduning through the equivalent R1+L1 resistor using KVL. If it even has current, how does the Voltage transfer from the source to R1+L1? Can I apply KVL when there are inductors in the circuit, or do I have to turn them into an equivalent resistor?

Circuit 2: Again, we need to use KVL. I started with Superposition but got stuck. Can I use KVL with a capacitor and an inductor in play or do I need to combine them in equivalent resistors?
Hello,

If you are 'horrible' at circuit analysis that is probably because you never learned a general analysis method. If you were to learn something like that you would suddenly be very good at it, as long as you have some math to back it up. As a prerequisite algebra, simultaneous equations, and complex numbers and complex algebra.
If you dont have much algebra such as simultaneous equations behind you then you are going to have a difficult time so i would suggest learning that first. After that, get proficient at DC circuits with DC sources and resistors. After that start getting into purely AC circuits (like you have here). In all these cases you can use Nodal Analysis because it is very general.
So here is a list:
1. Algebra including simultaneous equations.
2. Complex numbers and algebra.
3. DC circuits.
4. AC circuits.

For an example, for the first circuit you show, i would first convert the inductors into their impedances, then proceed almost as if it was a DC circuit but using the complex quantities.

For the second circuit, do the same thing, and if the two sources are in phase then just proceed as you would with a DC circuit but if the two sources are not of the same phase then convert each to their complex form first, then proceed almost as you would in a DC circuit except using complex numbers and/or variables.

So you see DC circuit analysis is important, and a general method like Nodal Analysis allows you to approach each and every circuit in the same way basically.

Maybe you should mention what kind of analysis you learned already and what math you learned up to now.
We can do a couple example circuits if you like but i need to know what you have studied so far first.

• atferrari and Marasha
Thread Starter

#### Marasha

Joined Feb 6, 2018
10
Hello,

If you are 'horrible' at circuit analysis that is probably because you never learned a general analysis method. If you were to learn something like that you would suddenly be very good at it, as long as you have some math to back it up. As a prerequisite algebra, simultaneous equations, and complex numbers and complex algebra.
If you dont have much algebra such as simultaneous equations behind you then you are going to have a difficult time so i would suggest learning that first. After that, get proficient at DC circuits with DC sources and resistors. After that start getting into purely AC circuits (like you have here). In all these cases you can use Nodal Analysis because it is very general.
So here is a list:
1. Algebra including simultaneous equations.
2. Complex numbers and algebra.
3. DC circuits.
4. AC circuits.

For an example, for the first circuit you show, i would first convert the inductors into their impedances, then proceed almost as if it was a DC circuit but using the complex quantities.

For the second circuit, do the same thing, and if the two sources are in phase then just proceed as you would with a DC circuit but if the two sources are not of the same phase then convert each to their complex form first, then proceed almost as you would in a DC circuit except using complex numbers and/or variables.

So you see DC circuit analysis is important, and a general method like Nodal Analysis allows you to approach each and every circuit in the same way basically.

Maybe you should mention what kind of analysis you learned already and what math you learned up to now.
We can do a couple example circuits if you like but i need to know what you have studied so far first.
I have no problem with physics, math or other "difficult" fields. I just can't wrap my head around circuit analysis. Over the years, I have studied them repeatedly, even had tutoring by professors but still can't understand them. To be honest I am just trying to get a passing grade and work on other subjects I am good at.

On the circuits I posted, we have to work only with KVL and complex resistances. Using capacitance or inductance wont be acceptable. Phase is not mentioned, nor needed. The only values we are given are the Volts and Ωms.

So far we know only how to analyze circuits using KVL and KCL so the solution has to be KCL. Since this homework is very early on the class, that's basically all we have been taught.

#### BobaMosfet

Joined Jul 1, 2009
1,776
Greetings,

I really need help with some assignments. I am horrible with circuit analysis, so please keep it as simple as possible.

If you have the time, please explain to me, preferably step-by-step how you would solve these.

Circuit 1:
View attachment 240684
We need to find the current running through the equivalent R1+L1 resistor using KVL. If it even has current, how does the Voltage transfer from the source to R1+L1? Can I apply KVL when there are inductors in the circuit, or do I have to turn them into an equivalent resistor?

Circuit 2: Again, we need to use KVL. I started with Superposition but got stuck. Can I use KVL with a capacitor and an inductor in play or do I need to combine them in equivalent resistors?
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

• Marasha
Thread Starter

#### Marasha

Joined Feb 6, 2018
10
Start with a simple circuit.
Can you solve this, i.e. find the voltage across R1?

View attachment 240714
Yes I can. I think I am not clear on my questions, my apologies.

In the 1st circuit, what confuses me is the voltage source in the middle. So far I have only seen examples where the source is on the side, so the methodology is pretty linear. To use KVL, do I have to make equivalent resistors of R2//L2 and R3//L3? If so, do I place the voltage source between them?

2nd circuit basically same question. Should I make equivalent resistors or does KVL "work" with capacitors and inductors in the circuit?

Thread Starter

#### Marasha

Joined Feb 6, 2018
10
Hello,

If you are 'horrible' at circuit analysis that is probably because you never learned a general analysis method. If you were to learn something like that you would suddenly be very good at it, as long as you have some math to back it up. As a prerequisite algebra, simultaneous equations, and complex numbers and complex algebra.
If you dont have much algebra such as simultaneous equations behind you then you are going to have a difficult time so i would suggest learning that first. After that, get proficient at DC circuits with DC sources and resistors. After that start getting into purely AC circuits (like you have here). In all these cases you can use Nodal Analysis because it is very general.
So here is a list:
1. Algebra including simultaneous equations.
2. Complex numbers and algebra.
3. DC circuits.
4. AC circuits.

For an example, for the first circuit you show, i would first convert the inductors into their impedances, then proceed almost as if it was a DC circuit but using the complex quantities.

For the second circuit, do the same thing, and if the two sources are in phase then just proceed as you would with a DC circuit but if the two sources are not of the same phase then convert each to their complex form first, then proceed almost as you would in a DC circuit except using complex numbers and/or variables.

So you see DC circuit analysis is important, and a general method like Nodal Analysis allows you to approach each and every circuit in the same way basically.

Maybe you should mention what kind of analysis you learned already and what math you learned up to now.
We can do a couple example circuits if you like but i need to know what you have studied so far first.
Same reply as on another post. I am not a native English speaker and I am studying circuits in another language, so maybe I have not made my questions clear, my apologies.

In the 1st circuit, what confuses me is the voltage source in the middle. So far I have only seen examples where the source is on the side, so the methodology is pretty linear. To use KVL, do I have to make equivalent resistors of R2//L2 and R3//L3? If so, do I place the voltage source between them?

2nd circuit basically same question. Should I make equivalent resistors or does KVL "work" with capacitors and inductors in the circuit?

• Erick Serdahl

#### ericgibbs

Joined Jan 29, 2010
12,932
hi Mara,
This is your Fig 1, rotated 90 deg.
The R1 and L1 have been removed.
Assuming R2=R3 and L2 =L3, what does this configuration remind you of.??
Think Bridge..

So what is the voltage across R1 and L1.??

E

#### Attachments

• Marasha

#### RBR1317

Joined Nov 13, 2010
633
If required to use KVL, then you will use the fact that the sum of voltages around each loop must equal zero. However, those voltages are produced by currents flowing between nodes. So the first step is to identify the currents involved while also realizing that the currents must obey KCL. Attached figure shows one possibility to identify the currents. (3 currents & 3 loop equations) With all that said, this remains a less than ideal approach for analyzing circuits (although it does require that you demonstrate a knowledge of Kirchoff's laws.) Nodal Analysis is the only method that I normally employ.

#### Attachments

• Marasha

#### MrAl

Joined Jun 17, 2014
8,251
Same reply as on another post. I am not a native English speaker and I am studying circuits in another language, so maybe I have not made my questions clear, my apologies.

In the 1st circuit, what confuses me is the voltage source in the middle. So far I have only seen examples where the source is on the side, so the methodology is pretty linear. To use KVL, do I have to make equivalent resistors of R2//L2 and R3//L3? If so, do I place the voltage source between them?

2nd circuit basically same question. Should I make equivalent resistors or does KVL "work" with capacitors and inductors in the circuit?
Ok, so with the circuit redrawn with the source on the left, can you analyze that circuit then?
See attachment.

#### Attachments

• Marasha
Thread Starter

#### Marasha

Joined Feb 6, 2018
10
hi Mara,
This is your Fig 1, rotated 90 deg.
The R1 and L1 have been removed.
Assuming R2=R3 and L2 =L3, what does this configuration remind you of.??
Think Bridge..

So what is the voltage across R1 and L1.??

E
Honestly, we have not learned about bridges but I assume you find the voltage in the bridge and then the whole circuit. That won't work since I am required to use KVL on each of the 3 loops.

#### ericgibbs

Joined Jan 29, 2010
12,932
Hi Mara,
V=I*(L1+R1)
How can that equation give voltage across R1.??
E

• Marasha
Thread Starter

#### Marasha

Joined Feb 6, 2018
10
If required to use KVL, then you will use the fact that the sum of voltages around each loop must equal zero. However, those voltages are produced by currents flowing between nodes. So the first step is to identify the currents involved while also realizing that the currents must obey KCL. Attached figure shows one possibility to identify the currents. (3 currents & 3 loop equations) With all that said, this remains a less than ideal approach for analyzing circuits (although it does require that you demonstrate a knowledge of Kirchoff's laws.) Nodal Analysis is the only method that I normally employ.
Using KVL, shouldn't I do this? Last edited:
Thread Starter

#### Marasha

Joined Feb 6, 2018
10
Hi Mara,
V=I*(L1+R1)
How can that equation give voltage across R1.??
E
My apologies, I read the question wrong.

V - V(L) = V(R)

#### Ian0

Joined Aug 7, 2020
2,252
In the 1st circuit, what confuses me is the voltage source in the middle. So far I have only seen examples where the source is on the side, so the methodology is pretty linear. To use KVL, do I have to make equivalent resistors of R2//L2 and R3//L3? If so, do I place the voltage source between them?
In that case, turn the paper around by 90°, with R1/L1 at the top. Sometimes that's all it takes to be able to see what is going on.
Then, ignoring R1/L1, don't you see two similar circuits loading a sinewave generator?

• Marasha

#### ericgibbs

Joined Jan 29, 2010
12,932
hi M,
V - V(L) = V(R)

L
is a inductive symbol, not the impedance of the L component.

Do you know how to calculate the Inductive impedance of an Inductor.?

E

• Marasha

#### ericgibbs

Joined Jan 29, 2010
12,932

#### Ian0

Joined Aug 7, 2020
2,252
Similar threads