Convergence Assistant (Op-Amp error)

[DAPortman]

Greetings all,

this is my first post here and I'm familiar with Multisim on a basic level. I created my sim file based on a schematic that I did not design. I am getting the error:

A simulation error has occurred. Would you like to run the Convergence Assistant to attempt to resolve this problem automatically? 

Most everytime I hit yes, the convergence assistant is able to solve the problem, but it is unable to fix this circuit. I believe the problem is with the op-amps and have been trying to tinker around to fix.

I've included:

1. JPG of sim file
2. GIF of source schematic
3. multisim file

Any ideas? Thanks. 

By the way, this is a pulse width modulator used to keep the rms current constant.

[NatashaB · Accepted solution]

Hi,

I took a look at your circuit, and it looks like the convergence issue lies in the model you are using.  The model is a 3-terminal opamp; that is, only the input and output pins are modelled, and not the power supplies.  This causes issues in your particular circuit because you have some opamps in open-loop configurations expecting that the output will be clipped at the supply voltages.  But because the power supplies are not modelled, the output is not being clipped, and therefore very large voltage values are occurring at the outputs.

I replaced the model you are currently using in the circuit (IIT/LM324A) with another one that does model the power supplies (IIT/LM324A_2).  Looks like this solved the convergence issue.  The modified file is attached.  There are also vendor-specific LM324 models in the database in case you are using a vendor specific component (depending on your version of Multisim).

Let us know if this solves the issue.

[DAPortman]

Thanks a lot! I'm still having trouble with tuning the waveform with the pots and built in oscilloscopes, but I should be able to figure something out - at least it is working now!