AC Circuit Analysis Question - Page 1
Forum section: electronics / beginners

author: tec5c
author_rank: Frequent Contributor
timestamp: June 08, 2014, 07:27:33 am
content: Hi all,I'm having some trouble wrapping my head around some concepts with AC circuit analysis (2nd year EE degree). I'm not asking for someone to do my "homework" for me, I just want some help to understand where I'm getting stuck. I'm pretty sure that I'm getting confused with my complex number arithmetic.I have attached the given circuit and my workings to get the impedances and phasors for the circuit. In reference to the image, I am trying to find the voltage, V(t).My approach to find V(t) was to do a nodal analysis to get one equation and one unknown. I am able to post up my node equation but this is where I get stuck.Thank you in advance for any help.

author: tec5c
author_rank: Frequent Contributor
timestamp: June 08, 2014, 07:32:31 am
content: Attached here is my working for the node equation. I'm not sure if my approach in using node analysis is the correct one, but this is where I'm up to.

author: jimjam
author_rank: Regular Contributor
timestamp: June 08, 2014, 07:46:59 am
content: I can't help, but gosh this brings back memories

author: Creep
author_rank: Regular Contributor
timestamp: June 08, 2014, 08:10:43 am
content: I can't quite read the very beginning of the last line, but aside from that, everything seems fine.Next you just want to multiply the entire equation by 5200 (or whatever the denominator value was, can't remember) and do the basic arithmetic in the left side. Next go from algebric form to the exponential form (the one with the phase angle) and divide the whole equation by the multiplier next to V1. Remember that you can always check if your doing it right with some math software (octave, matlab, heck, even python can work with complex numbers with ease) or even wolframalpha for that matter.If still not sure of yourself, model the circuit on some SPICE software and check to see if you got it right.

author: tec5c
author_rank: Frequent Contributor
timestamp: June 08, 2014, 09:05:12 am
content: Thanks for the replies.@Creep, this was the approach I initially took. I had even already modeled the circuit in LTSpice (Another thing I'm a noob with)I've attached the previous page with continued work in order to solve for V1...As I mentioned, I'm not too experienced with the whole simulation command of LTSpice, but when I did manage to get a voltage reading, it was 4.8V. There's a good chance that I don't have the settings correct for the measurement in spice though.

author: Creep
author_rank: Regular Contributor
timestamp: June 08, 2014, 09:33:22 am
content: My guess is you just have trouble modeling the simulation. I just ran the circuit through PSpice and the result matched your calculations. Though I'm not familiar with LTSpice so sadly I wont be of any help in that regard.

author: tec5c
author_rank: Frequent Contributor
timestamp: June 08, 2014, 09:40:07 am
content: Well, thanks for the help Creep. Most appreciated.Now I need to get some more practice with spice software

author: Rudane
author_rank: Regular Contributor
timestamp: June 08, 2014, 04:46:03 pm
content: Here's my simulation assuming the values you gave were peak and not RMS.

author: johnwa
author_rank: Frequent Contributor
timestamp: June 09, 2014, 02:30:20 am
content: You might want to consider doing a Thevenin transform on the current source and capacitor. Then it is just a simple series circuit.

author: tec5c
author_rank: Frequent Contributor
timestamp: June 09, 2014, 03:50:34 am
content: @Rudane, thanks very much for taking the time to run the simulation. I realised where I was going wrong... I didn't make the current source an AC signal. After fixing that up I now get a pretty similar result when comparing the simulation to the calculated values.Thanks again for the replies.

author: tec5c
author_rank: Frequent Contributor
timestamp: June 10, 2014, 01:01:23 am
content: @Rudane. Just a question in regards to your simulation setup. Can you please explain how come you have the frequency of the sine wave to be 636.6Hz. Is there a way to calculate this value that I'm not aware of?Edit: Ah, disregard the above questions, I figured it out... As w = 2*pi*f, then f = w/(2*pi) which gives 636.62Hz...