# Need some help finding phase shift and magnitude on a RL circuit

## Need someone that help me finding phase shift and magnitude of an RL circuit

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#### Jhon Molano

Joined Nov 8, 2017
2
Hello everyone,

I am a newby at electronics and i am doing a homework, i am stuck and can not find the answer for the following problem:

"Refer to the RL circuit of Figure 9.36. If 1 V is applied, find the magnitude and the phase shift produced at 5 kHz. Specify whether the displacement from phase go foward or backward"

In the attached doc file is what i have made until now viewing videos and some books, please, i really need some help with this i am out of choices.

Regards, Jhon.

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#### Øivind Bakke

Joined Aug 22, 2017
12
Hi Jhon,

your analysis seems really good, but you have a small mistake in the beginning that causes the error. (at least what I can see at first glance) Take a look at your Z_in and try to solve it again. Also, when it comes to phase shift magnitude I learned a rule a few years ago, that still sticks with me. Key words: ICE and ELI. Current I leads voltage E in a capacitor & voltage E leads current I in an inductor.

Regards

#### The Electrician

Joined Oct 9, 2007
2,778
Why have you calculated Zin? The problem appears to be asking for Vo/Vi

I get the same result for Zin, although you should probably carry a couple more digits in your calculations.

#### Øivind Bakke

Joined Aug 22, 2017
12
The way he goes about this is correct and probably the way they were told to do it by the prof. (I had to do the same). I don't want to give the answer right away in order for him learn from it, but if you take another look at Z_in, you'll see a tiny mistake that messes up the calculations further.

#### The Electrician

Joined Oct 9, 2007
2,778
The way he goes about this is correct and probably the way they were told to do it by the prof. (I had to do the same). I don't want to give the answer right away in order for him learn from it, but if you take another look at Z_in, you'll see a tiny mistake that messes up the calculations further.
I see no mistake. His final result is 8.35 + j3.31 which is the same result I get except I carried more digits; I got 8.3514 + j3.3142

How will he use Zin to find Vo/Vi? Wouldn't it be more straightforward to treat the problem as a cascaded pair of voltage dividers?

#### Øivind Bakke

Joined Aug 22, 2017
12
Alright, I realize I've formulated myself poorly. Here is how I would've done it:
Add Z1,Z2 and R1 to one total impedance and do voltage divider with Vo=voltage across R2.

Last edited:

#### Jhon Molano

Joined Nov 8, 2017
2
Thank you all for the comments...

you will see. I do not know what else i need to do from this point, some people told me that i need to find Vo/Vi in order to find the phase shift and the magnitude, and that seems the way also on the steps on the book i am reading. but i don't know how to do it. i was following the problem that appears on the book (i attached the page of it), but if i do the same procedure of the example 9.14 on the practice exercise 9.14 which is almost equals that the one i am working (the H value of L1 and L2 change) i can not get the same result that the book said. so in some point i am missing something and i do not know what to do. in theory, the phase shift that i got for my exercise is 0.256333882/69.26687064deg

I also send the excel in which I am working with.

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#### The Electrician

Joined Oct 9, 2007
2,778
Thank you all for the comments...

you will see. I do not know what else i need to do from this point, some people told me that i need to find Vo/Vi in order to find the phase shift and the magnitude, and that seems the way also on the steps on the book i am reading. but i don't know how to do it. i was following the problem that appears on the book (i attached the page of it), but if i do the same procedure of the example 9.14 on the practice exercise 9.14 which is almost equals that the one i am working (the H value of L1 and L2 change) i can not get the same result that the book said. so in some point i am missing something and i do not know what to do. in theory, the phase shift that i got for my exercise is 0.256333882/69.26687064deg

I also send the excel in which I am working with.