diode analysis failure

[John Loomis]

I was trying to devise a problem for student analysis - and tried two diodes in parallel - but with resistors

above and below one of the diodes (so diode is not at ground potential on either side).

Multisim failed to produce a DC solution.

LTSpice worked fine.

So ... how do I get Multisim to produce correct results?

[yyao]

Hi John,

What exactly do you mean by producing correct results? It looks like Multisim can converge on a DC solution and it is displaying it in the PDF that you have attached.

Is the question why Multisim's results are different from LTSpice? LTSpice has a different SPICE model for the 1N4148 than Multisim. In fact, Multisim has two SPICE models for this particular diode! Using different models will yield different results.

Please clarify what the problem is.

[John Loomis]

Look at the probes:   4.14 mA flows into the diode and 37.4 mA flows out!

The LTSpice solution is internally consistent AND matches the model that says the currents should be approximately that of the

underlying current divider.

[John Loomis]

Whoops - I pressed solution (which accepted the solution) and now I can't undo it!

[yyao · Accepted solution]

Oh my! I didn't see that.

I know what your problem is. The two wires which you have named N3 and N4 in LTSpice are both named 2 in Multisim. These two wires will behave as if they are connected. We call this a virtual connection. To see the net name, double click on the wire. I would also recommend that you turn on visibility to all net names for debugging:

1. Click Options>>Sheet properties
2. In the Circuit tab, click Show all in the net names option box

You are not the first one to fall victim to this not-so-obvious behaviour. We addressed this issue in our latest version, Multisim 11. Virtually connected nets are explicitly drawn with connectors now and it is a lot harder to oversee this sort of problem.

In the mean time, if you don't want to upgrade to Multisim 11, it is worth your time to upgrade your Multisim to at least 10.1.0.1. There was a bug in 10.1 that we fixed for 10.1.0.1 which could cause this strange behaviour. The 10.1.0.1 update is available for free through NI Update Service.

Hope that helps.

[John Loomis]

Attached is the corrected version.  Most of my students are using 10.1 so I will stay there until

the end of the semester and then switch to 11.x

Thanks!