[SOLVED] LTSpice an Astable multivibrator startup difference

[DanyR]

Hi, I've see something strange when I use LTSpice to simulate (transient analysis) an astable multivibrator. The startup behaviour (actually the delay before oscillation begins) differs when I use a 50 ms stop time versus 500 ms stop time. In the first case I see oscilation beginning after a few ms, in the second case I see oscillation only beginning after 115ms.

I do expect that oscillation start can have some delay (due to the symmetry of the circuit), but should not be the same delay be there using both transient analysis stop times?

Thanks in advance!

.tran 50m
View attachment AS_50ms.bmp

.tran 500m
View attachment AS_500ms.bmp

 

[KlausST]

Hi,

Your circuit is absolutely symmetric. But the startup needs sone unsymmetry. In real world every capacitor differs from the other, every transistor differs from the other...

You can give your circuit a kick start with a pulse or just make it a little unsymmetric..

Try it and tell us if you see any difference.

Klaus

 

[DanyR]

Thanks.
Yes, making the circuit even a little assymetrical makes it starting up right away. But that was not the question, it was:

I do expect that oscillation start can have some delay (due to the symmetry of the circuit), but should not be the same delay be there using both transient analysis stop times?

 

[FvM]

Assume that LTspice is calculation transient analysis step size depending on the stop time.