Author Topic: circuit simulation  (Read 941 times)

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Offline yalectTopic starter

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circuit simulation
« on: April 20, 2020, 06:37:21 am »
Hi,
I would like to ask you that I have problem with some circuits like this (attached picture) sometimes when I simulated it with Proteus 8 it shows me the message ([SPICE] TRAN : timestep too small; timestep =0.00133113: trouble with instance Q1 and simulation stoped]).
how to resolve or set that problem? timestep too small
thank you
 

Offline JoeyG

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Re: circuit simulation
« Reply #1 on: April 20, 2020, 06:49:58 am »
I'm not familiar with this simulator,  but where is the Voltage (power source)?
 

Offline Ian.M

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Re: circuit simulation
« Reply #2 on: April 20, 2020, 07:23:26 am »
Its more likely to be an issue with the switch in series with C2.   When it initially closes it will cause a step change in Q2 Vbe, which may be problematic if the Q2 BC547 model is a macromodel.
 

Online SiliconWizard

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Re: circuit simulation
« Reply #3 on: April 20, 2020, 02:04:53 pm »
I don't know much about Proteus, I'm assuming it's a Spice-based simulator. In particular, I don't know about the base parameters for base parts.

I would try adding some small ESR to C2 (if there is an ESR parameter for capacitors), or manually add a small resistor in series (like 1-2 ohm) with C2, and see how that goes.
 

Offline Wimberleytech

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Re: circuit simulation
« Reply #4 on: April 21, 2020, 01:52:02 pm »
I suggest that you reduce the cap to 1uf or less and run the simulation. 

Is your switch starting in the on or off position?
What is the on resistance and off resistance of the switch?
 

Offline bson

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Re: circuit simulation
« Reply #5 on: April 21, 2020, 06:27:40 pm »
Is that thing that says "+88.8" supposed to be voltage source or a volt meter?  Is R1(1) a measurement, or a power rail?

'Time step too small' in Spice usually implies it's unable to converge while determining the DC operating point.  With the capacitor open looped it would have to derive this from the BJT, but its internal model may not allow it to do that, or there may be a discontinuity - for example around where the base current transitions from zero to non-zero.  It may be a model parameter, but it can't be found iteratively.  If so the easiest workaround will likely be to put a 1G resistor between the base and ground.
 

Offline Wimberleytech

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Re: circuit simulation
« Reply #6 on: April 21, 2020, 06:51:23 pm »

'Time step too small' in Spice usually implies it's unable to converge while determining the DC operating point.


No, that is an error during transient analysis.  For dc operating-point problems there is a different error message...but I cannot recall what it is because I have not had a convergence issue recently.
 


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