LTS "Time step too small" error

[alec_t]

Rapid switching, particularly in sim circuits involving inductance or zeners or varistors, often gives me this dreaded error message and the sim grinds to a halt. The ad hoc fix is sometimes just a matter of tweaking a component value slightly. Is there a generic solution, e.g. by setting some value in Control Panel / Spice, or by adding some directive on the schematic? I know there is the option to set a maximum time step (though I haven't fathomed what effect this actually has on the sim), but how about a minimum?

 

[ericgibbs]

hi alec,
Any chance of seeing a LTS asc file with this problem.? I will give it a run.

E.

 

[simonbramble]

Try going into Simulate -> Control Panel -> Select the SPICE tab then set the Solver to alternate. this sometimes works. Failing that, look for obvious errors in the circuit (currents trying to get to millions of amps, voltages trying to get up there too).

 

[ericgibbs]

hi alec,
I use the same method as Simon, also I was also advised to use Control Panel/Spice/Trtol = 7 rather than 1

E.

 

[alec_t]

Thanks for the tips, guys. As you say, sometimes the Alternate solver does the trick.
I was hoping something like tinkering with Trtol (whatever that does!) might be the answer, but sadly not in the attached example, where I'm trying to sim the effect of switching off the supply to an AC motor. If the switch-off is slow (>3.5ms) it works ok-ish; but that's not very realistic. A real world limit-switch, for example, would open much quicker. (You'll need MLseries_ltspice.lib to run the sim).

 

[ericgibbs]

hi alec,
For the circuit you posted.

Using Normal and Ttrol at 7 I can get fast results for switch time rise/fall of 1uSec.
[tried steps of 100u and 10u] OK

These results are with 1uS Trise/Tfall
With Alternate, Ttrol = 1 , it works but is slow,,, Ttrol=7 fast.
With Normal,Ttrol = 1, works but slow.... Ttrol=7 fast.

Two images with .tran and .dc both fast.

Conclusion:
The Ttrol at 7 rather than 1 does speed up sim [ as per Helmutt]

E.