# RLC in parallel, resonance and multisim

Discussion in 'Homework Help' started by ninjaman, Dec 16, 2014.

1. ### ninjaman Thread Starter Member

May 18, 2013
306
1
hello

I have this

the values are
1uF
151mH
4700 ohm

I put this together on multisim to find resonance using the ac analysis option but all that happened was a high pass filter. does the capacitor short circuit as the frequency increases. I have to calculate the resonant frequency and im not sure how to do that.

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109
Why do you need to put it togerher on multisim to find the resonant frequency? If associative learning kicked in, you already know the resonant frequency.

Start your bode plot at 1 hz or 0.1 hz

If your starting it higher it will look like low pass filter.

You have another fine example of low Q circuit.

Last edited: Dec 16, 2014
3. ### ninjaman Thread Starter Member

May 18, 2013
306
1

thanks Joe!
what I meant to say was, I have a nice picture from the AC analysis function on multisim that shows bandwidth and resonant frequency. I cant get the bandwidth or resonant frequency. I know that at resonant the impedance will be maximum so it should look the opposite of what I have for the series circuit. I think I know how to calculate the resonant frequency. I already know the values, I think the formula is different than series but there is a resistor in there. I am thinking, work out the RL circuit first, then that in parallel with the capacitor?

cheers!!!

simon

4. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109
The current will be minimum at resonace in this circuit. In my youth, this was an antiresonant circuit while the series RLC was called resonant .... With maximum current flow.

5. ### ninjaman Thread Starter Member

May 18, 2013
306
1

hello Joe

thanks for your help!

I have put this in multisim and tried to get an AC analysis, this is what I get. so this is a low pass filter?

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,936
1,088
Please start your AC analysis from 0.1Hz

7. ### crutschow Expert

Mar 14, 2008
12,536
3,071
That circuit has a damping factor much greater than 1 so will show no resonant peak. R must be less than 777Ω to reduce the damping below 1, to where you can see a peak (reduction) in the generator current at resonance.

8. ### ninjaman Thread Starter Member

May 18, 2013
306
1
hello

tried 0.1Hz and 0.001Hz, the same result
change resistance to less than 777 ohms and same problem

shouldn't it look like a "U" shape when I have magnitude and frequency?

9. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
Here is the "resonance" of your network as a function of the values of R1 = 10, 100, 1000, and 4700Ω. What happens to the Q between 100Ω and 1000Ω?

• ###### 245.gif
File size:
39.1 KB
Views:
90
Last edited: Dec 16, 2014
10. ### crutschow Expert

Mar 14, 2008
12,536
3,071
Are you looking at the source current?

11. ### WBahn Moderator

Mar 31, 2012
17,446
4,699
You haven't given any indication of what you are using as the parameter that you are trying to find the resonance for? Is it a voltage? If so, what voltage? Is it a current? If so, what current?

12. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109
One thing, by experiment, your oscillator or signal generator will have an impedance ... probably 50 ohms.

Attached is the sweep using a signal generator with an impedance of 50 ohms and the inductor has zero resistance.

File size:
33.7 KB
Views:
79
13. ### ninjaman Thread Starter Member

May 18, 2013
306
1

hello joe

would I have to include the 50 ohms in the calculations then. I seem to get voltage readings that are a little off. I put this down to poor test equipment.
is the 50 ohms in series with the circuit?

thanks

simon

14. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,316
1,109
You need to publish in the forum your recorded readings. It would be best if you annotated the schematic and indicate your reference point (i.e. ground or across the component.)