I'm having trouble getting reasonable results modeling circuits with long time constants. For example, 1 10M resistor and a 10uF capacitor ends up with a far shorter TC than estimated with a calculator or observed on a breadboard. I'm by no means a PSPICE expert, but if I had to guess, it would be one of two things: (1) I'm doing something wrong so mea culpa, or (2) extended precision math results are being rounded or truncated. Any ideas? Thanks, --jim |
by audiomath
June 29, 2015 |
As you have not shared an example of the circuit in question here is an example that demonstrates that CL can adequately simulate an RC time constant of 10MOhms*10uF: Have a look at how the Time Domain Simulation is set up and also have a look at: https://www.circuitlab.com/docs/the-basics/#time_domain_simulation Please note that CL does not use SPICE. and also see: :) |
by signality
June 30, 2015 |
Thanks for the reply. Here's a link to an example of the behavior I was talking about. If you increase the value of the resistor >= 1m, the simulation appears to me at least to fail. I'll review the links you provided and see if I'm doing something obviously wrong. |
by audiomath
June 30, 2015 |
If you have been entering resistances as CL is not SPICE: it is case sensitive and uses
and
See: https://www.circuitlab.com/docs/the-basics/#human_friendy_inputs for more. Also, your simulation is not well conditioned at T=0. It is not clear and not documented exactly what CL does for T<= 0. If you look closely at your sim, you'll see that it does not start at exactly zero in the time or voltage scale. If you want to start a voltage source at a specified time and be sure of it's behaviour prior to that it is best to use the USTEP(T-1f) trick I used in my example. In CL, sometimes USTEP(T) or USTEP(T-0) and similar tricks works but sometimes it does not. Starting a source from zero or some defined value at T>0 is much safer in CL. You can also use the PWS() source: https://www.circuitlab.com/docs/expressions/ :) |
by signality
June 30, 2015 |
Aha! As it turns out, it was my error, as I suspected. That circuit with a resistance of 1 milliohm would not have a very long time constant... ;-) Thank you for your time. --jim |
by audiomath
June 30, 2015 |
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