pspice / orcad / Capture very slow simulation

I used Capture to draw my circuit, and I want to do a transient simulation with a time period of a few hours. The simulation takes forever. If I simulate .1 seconds, it takes 10 minutes, so simulating an hour would take way longer than I want to wait.

In the simulation profile, I've increased the maximum step size, but it doesn't seem to speed up the time required for the simulation.

This has been bugging me for a while now, because I'm sure I'm missing something elementary. Thanks for any help you can give!

Ben

What are you simulating? What frequencies are involved in the circuit? Usually simulations take longer than real time e.g. a simulation of a circuit which spans 1ms might take several seconds. So a simulation of something over several hours will take weeks! The exception would be if nothing much is happening during the simulation. For example a 1,000,000 second simulation of a very large capacitor being charged by a very large resistor only takes a fraction of a second on my simulator.

I think we need to see your circuit and simulation settings,

Keith.

That's a good point. I'm trying to model how long a capacitor will keep its charge in a circuit that charges it, then disconnects the cap from the DC power source, and measures the voltage, let's say, an hour later using the analog input of a PIC microcontroller. The highest frequency is the sampling rate of the PIC analog input.

Does OrCAD automatically compensate for aliasing? That would explain why the simulation is so slow -- if it know's it needs to pick timesteps small enough to prevent aliasing. I figured there should be some setting where I could manually adjust the sample rate, but it seems I can only set the maximum sample rate.

My computer is being quite lame right now, but I'll try playing with different frequencies soon, and let you know how that goes.

I'm used to using an old student version, and now I'm on version 16.3, and am trying to get used to it.

Thanks again for the help

Sounds like a problem, that can be better handled with pencil and paper method. If there's an actual need of circuit simulation to evaluate the circuit behaviour, you should scale component values (e.g. time constants) in a reasonable way.

Generally, increasing the maximum time step in SPICE simulation may be inappropriate, becauses it introduces an inacceptable error in some cases, but it still won't keep the simulator from choosing the time step based on the the circuit time constants and stimulating waveforms. Another important point for simulation speed is the complexity of your circuit.

As FvM said - probably a pen & paper solution is the easiest. If there is something periodically going on that is complex then use Pspice to help calculate that short period of time & use the result in your hand calculations. Again, as FvM said - change some values to speed it up & extrapolate the real results.

You cannot make the maximum time step very long if something happens occasionally but with high speed. At best the results will be innacurate, at worst it could miss the event completely.

Pspice doesn't compensate for aliasing. It will vary the timestep based on the results it gets - reducing it to get accuracy then increasing it when things don't seem to be changing much.

Keith.

I've done some rough pen and paper calculations, just estimating leakage currents, and counting coulombs. I suppose I should just build it on a breadboard and see if it works.

Thanks for the help.

I suppose I should just build it on a breadboard and see if it works

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Absolutely. The simulation model assumptions may reveal incorrect anyway.