Hierarchical block works alone but not with circuit

[Johnsson]

Here is the problem I am having. I created an AC to DC converter using the hierarchical block system. The converter works on its own. If I create a blank page to place the converter on and connect an O-scope to the output I get the 12-14VDC that I am looking for.

I have a thermal circuit I made. R2 represents a thermistor (to make things clear). When I use the Place Component>>Power Source>>12VCC, it works. All goes well.

Now that all of that is out of the way. If I place the converter on the same page as the thermal sensor, and send a conductor from the converter's output to where I normally place VCC, I get a convergence error (I'm still not sure what that error means either) that multisim tries to resolve. I always get "convergence failed etc...", but that's not the point. The point is that I get an error to begin with.

I've tried making the converter with two inputs for 120AC and doing it that, still the same thing. Everything I try, I can get the thermal circuit to work on it's own, the converter to work on it's own, but not together. Any help on this would be very appreciated.

[yyao]

Hi Johnsson,

This symptom appears when two parts of your circuit are operating at very different time scales. Your thermal sensor circuit is quite complex and is taking a while to simulate. A few seconds on the wall clock corresponds to 1us simulation time on your thermal sensor circuit. However, your AC/DC simulates much faster, which is why you are able to read the rectified voltages interacively as they simulate. For the AC/DC converter to simulate in conjunction with your thermal sensor circuit, Multisim will simulate both at the same time which is even more computationally intensive and takes a very long time. This is why you don't see your AC/DC converter running in the thermal sensor circuit. If you left it running long enough, you will see it update.

As for the convergence error, these appear sometimes due to short circuits, open circuits, positive feedback or other scenarios with no circuit solutions. Other times, playing around with the simulation settings may help converge on a solution. Try this:

1. Click Simulate>>Interactive Simulation Settings
2. Click on the Analysis options tab
3. Click Use custom settings
4. Click Customize. The dialog you now see contains all the settings that you can play with to help Multisim simulate your circuit.

My favourite settings to modify are:

• RSHUNT - decreasing the value usually helps simulation, but can dramatically decrease the accuracy of sensitive parts of your circuit
• METHOD - Using Gear method generally gives more accurate results, but takes a bit longer to simulate

Hope that helped.