Simulation startup weirdness.

[AJollet]

Hi, I'm having a problem simulating a simple circuit using Proteus ISIS.
If I connect the circuit to the power rails first and then launch the simulation, the output seems to be initialized at exactly half the supply voltage and so my Schmitt trigger based oscillator stays in that "unstable equilibrium" and never starts oscillating.
But now if I add a switch to be able to launch the simulation first and then connect the circuit to the power rails during simulation, it starts oscillating as intended as soon as I close that switch.

Is there a simulation setting somewhere that I need to change to get rid of this problem ?

Thanks

[T3sl4co1l]

Well, SPICE is SPICE, it'll always simulate as stable if it's set to analyze it as stable...

To get oscillator startup conditions, try setting "set to zero" in the SPICE options.  I don't know where this is in Proteus, but there should be a setting or dialog somewhere.

There are also a lot of settings which work better for transient simulation, besides defaults.  I typically use somewhere around these,
http://seventransistorlabs.com/Images/SimSettings1.png
http://seventransistorlabs.com/Images/SimSettings2.png
http://seventransistorlabs.com/Images/SimSettings3.png

Tim

[AJollet]

Sure.  Start using the free LTspice.  That is what most people use and you will be able to get better support from the community.  Not that many people use "Proteus ISIS", and you will have a hard time getting support for that.

Probably, there is a setting to alter startup behaviour [as there is in LTspice], but it beats me what that might be, as I've never used the "Proteus ISIS" simulator.

The thing is I'll need a PCB design tool as well and Proteus seems to make it quite easy going from circuit drawing and simulation to PCB layout.

[Warhawk]

Person usually analyses only crucial parts of the complex system and right after the design is verified a circuit diagram is being created by a separate tool.
LTSpice & OrCad are the industry standards these days. So try to switch to LTspice as suggested before. These universal (SCM+PCB+SIM ...) tools are usually disappointing.
Good Luck !

PS: Try to find a simulation setting called "skip initial bias point calculation"