# LT SPICE and resonance

Discussion in 'Homework Help' started by Nelson2001, Sep 26, 2012.

1. ### Nelson2001 Thread Starter Member

Nov 10, 2011
27
0
Hi, I'm trying to graph the series in LT SPICEresonance, making a AC sweep analysis.

SIMPLE RCL circuit in series
*
VS 1 0 SIN(0 10 166.81) AC 100
*
RESISTOR 1 2 4
CAPACITOR 2 3 10uF
Linduc 3 0 100mh
*
* ANALYSIS
.AC DEC 9 10 1MEG
*.TRAN 10s 10.02s 10s
* VIEW RESULTS
*.PRINT VP(2) vm(1,2)
*.PLOT AC VM(2) VP(2)
*.PRINT TRAN V(1) V(2)
*.PLOT TRAN V(1) V(2)
#autoplot 1 v(1,2) v(2,3) v(3)
#autoplot 2 i(RESISTOR) i(VS)*-1
#autoplot 3 vp(1,2) vp(2,3) vp(3)
.probe
.END

My question is : at resonance the current must be of I=100v/4Ω≈25A. So the voltage in resistance= 25A*4Ω=100volts, capacitor or inductor should be ≈3900 Volts.
The resistor is not at 0° so there is no 100Volts.
The link example is below, and I would aprecciate your help. Guess LT SPICE have a bug?. Don`t think so.
Below is the link with example the example. I make the math again & is correct!. SO why lt spice is not showing the correct graph?.
THANK YOU VERY MUCH
http://www.electronics-tutorials.ws/accircuits/series-resonance.html

2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
Does this look like what you expect?

File size:
50.3 KB
Views:
206
• ###### resonance.asc
File size:
742 bytes
Views:
80
Nelson2001 likes this.
3. ### Nelson2001 Thread Starter Member

Nov 10, 2011
27
0
Thanks a lot, so I have another problem with LTSPICE when performing transient analisys. In your example transient analisys works very well, the sin function I put works (the 25A mirros in both analisys)- But in this example (new resonance circuit), the sweep analisys shows 25A in resonance, when in transient analisys it shows only 15A.
I send you all-
THANK YOU VERY MUCH AGAIN!

View attachment resonance2.asc

View attachment resonance at 50hz.zip

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
I didn't open the zip file, but I modified your simulation. Run the .asc file.
Note that the sim runs for 2.5 sec, because the Q of the circuit (XL/R) prevents the current from reaching full amplitude immediately.
Also note that the maximum timestep is 10uS. This gives more accuracy.
I also ran your .AC sim over a single octave, because I was too lazy to calculate the resonant frequency. I modified your transient sine wave frequency to 49.96Hz.

File size:
74.7 KB
Views:
114
• ###### resonance2.asc
File size:
817 bytes
Views:
65
Nelson2001 likes this.
5. ### Nelson2001 Thread Starter Member

Nov 10, 2011
27
0
excellent!, very clear, thank you very much