I just simulated your circuit in my Multisim 2001 Power Pro and here are my findings. Your AC source doesn't have a ground. This is what is causing your error. What I like to do when dealing with AC sources connected to bridge rectifiers is to use the standard ground on the source and use the VGRND or Virtual Ground on the output side of the rectifier. This separates the DC ground from the AC ground. You will find this used frequently in many schematics. The other thing I noticed is that your O-scope channels are not connected to a ground. This might not cause an error but could cause you not to see a waveform or an inaccurate waveform if you do see one.
Once I corrected the errors the circuit simulated just fine on my Version. I hope I was able to help.
unfortunately I can't test your adwise right now. I will test it later on this day.
But I'm a little bit confused about your last answer. I understand your ideawith 2 different grounds. But if I use the same ground both for DC and AC, then I will shorten 2 of the diods in the bridge rectifirerectifierWouldn't that be the same as draw a wire between AC(-) and DC-gnd?
Thanks for your help.
Ideally, my first post is usually the way to do this, but when I tested the circuit my results were goofy. I tried using the same ground type for both sides and the results were what I was looking for. My O-scope has only one ground terminal for both channels (This is an older version of Multisim). Therefore, when I grounded using the AC ground, my DC side reading were off. Using the DC side ground caused the AC reading to be off.
To actually answer your question. I think the simulator differentiates internally between the grounds even though they are the same and therefore doesn't actually produce a short circuit. At least this is the way it worked for me. If you like the Dual Ground system better and your results are OK, then I would recommend you use it if you want to.